Cymbasoma sp.

Floating amidst the ocean's plankton is a tiny monster, it has no mouth and it must mate, after which it will give birth to a new generation of little monsters that will grow within the bodies of worms. Everything about this tiny crustacean sounds like a science fiction monster, starting with the group's name - Monstrilloida, meaning "tiny monster" - coined by a scientist who found their life cycle and appearance to be delightfully bizarre.

Left: Copepodid stage of a female Cymbasoma dissected from a Haplosyllis worm, Right: Adult stage of a female Cymbasoma 
Photos from Fig. 2 and Fig. 4 of the paper

Adult monstrilloid are free-swimming and they don't feed, but as juveniles, they live as parasites that can grow inside various marine invertebrates including snails, mussels, and polychaete worms. In polychaete worms, they can grow pretty large in relation to their host, and when they reach adulthood, they bust out of the host like it's a novelty birthday cake. In that way, their life cycles are comparable to the hairworms that parasitise crickets and mantids. 

Unlike other planktonic copepods that often swim by flicking their long antennae, the antennae of adult monstrilloids are fixed, so instead they have powerful swimming legs that allow them to kick their way through the water.  And while the adult stage of these weird little crustaceans are sometimes found in plankton trawl samples, their juvenile stage are much more elusive. Out of the 195 known species of monstrilloids the parasitic juvenile stage has only been identified for seven species, since they are hidden away in the bodies of their hosts. As a result, it has been over a century since anyone has investigated those parasitic juveniles in detail.

In this study, scientists in Japan were examining pieces of sponge that had been washed up on Tancha Beach at Okinawa Island. Those sponges turned out to be home for hundreds of Haplosyllis polychaete worms, but the worms themselves were also occupied by monstrilloids. This was also the case for sponge worms from Diamond Beach on another part of the island, which turned out to be an absolute haven for the little monsters, with over half of the worms hosting monstrilloids. This abundance of monstrilloids at Okinawa Island presented an amazing opportunity for scientists to get a better look at the parasitic stage of these copepods. 

In order to find out more about these enigmatic crustaceans, scientists first had to coax the host worms out of their spongey home, and they did that by taking chunks of the sponges and kept them in water without aeration. As oxygen level dropped, the worms were forced to abandon their sponge to seek more oxygenated water, at which point they could be collected and examined under the microscope. Monstrilloids are relatively large and highly visible as the bulk of the copepod stretches out the worm's body wall to transparency. 

Among these sponge-dwelling polychaete worms, the scientists found the larvae of two monstrilloid genera - Cymbasoma and Monstrilla, the former is coloured pale pink while the latter is teal green, but only the female copepods are so eye-catching due to their ovaries. The males are colourless and transparent. These larvae also live up to the monstrilloid name - they are banana-shaped, with a single eye, enclosed in a translucent sheath, and have a pair of long feeding tubes which it uses to slurp up nutrients from the host's body. When they reach maturity, the copepod uses those same tubes to make its exit by tearing a hole through the worm's body wall. Once free of the host's body, the monstrilloid shrugs off its juvenile exoskeleton to transform into an adult and takes its place among the zooplankton. 

In order to complete its life cycle, monstrilloids have to survive in three very different environments - the open ocean as adults, the sea floor (briefly) as nauplii, and inside the body of animals as juveniles. In the words of one of the scientists who study these little monsters, they are simply an awesome group of crustaceans.

Reference:

Emelianenko, V., Savchenko, A. S., Husnik, F., & Huys, R. (2025). Live observations of endoparasitic stages of Cymbasoma sp. and Monstrilla sp.(Copepoda: Monstrilloida) utilizing sympatric sponge-associated Haplosyllis spp.(Polychaeta: Syllidae) in Okinawa. Plankton and Benthos Research 20(Spec): s235-s241.

Dracunculus insignis

The Guinea Worm, Dracunculus medinensis, is an agonising parasite for those who have to endure its wrath. The female worm can grow up to 80 cm long and when it comes time for it to release its offspring, it does so by poking its body partially out of the host's arms or legs, all while causing a fiery pain that forces the host to immerse their limbs into the water, allowing the worm to release its larvae. This parasite has afflicted humans since antiquity, with description of pathologies and treatment associated with the worm dating from ancient Egypt, and depiction of the parasite in a 15th century painting. 

In the modern era, the Guinea worm has been the subject of an eradication effort by the World Health Organization (WHO) since the 1980s. An obituary was even written about this parasite in 2013. But while this campaign has been largely successful, the effort to completely eradicate the Guinea worm has hit an obstacle in some regions as the worm has taken to using dogs as alternative hosts in place of humans.

Left: Large bundle of Dracunculus insignis in the paws of a river otter (Lontra canadensis), Right: A Dracunculus worm being removed from a river otter (Lontra canadensis)

But aside from the infamous Guinea worm, there are many other species of Dracunculus out there which are found in a wide range of animals, many of which are actually reptiles. Of those, Dracunculus insignis is considered the most important because in addition to parasitising many species of wildlife,

it can also parasitise cats and dogs. The female worm can grow to 30 cm long, and about 300 days after the initial infection, the mature worm - now loaded with larvae - will migrate to the extremities and exit through a lesion, to explosively release a load of baby worms to begin the cycle anew.

This study looked at Dracunculus worms in river otters from North America. The worms the researchers examined came from various sources, including wildlife parasite surveys, as well as dead otters which were obtained from trappers. In addition, they also collected some worms from an otter in Florida that was recovering in a rehabilitation centre after being struck by a car. During its stay in rehab, worms started emerging on their own out of the otter's body. It was just one thing after another for that unlucky otter.

The worms dwelled in swollen abscesses under the skin on the otter's back, and examination of dead otters obtained from trappers revealed that some of the worms were also located in swellings deep in the limb joints or in the otter's paws, particularly D. insignis. In total, the researchers found four different Dracunculus species in the otters - alongside D. insignis, there was also D. lutrae, as well as two other unique lineages of Dracunculus, one of which was first discovered in a Virginia opossum. It seems that otters are just a cornucopia of different Dracunculus species, some of which are currently undiscovered. Just last year, another newly found species of Dracunculus - D. jaguape - was described from neotropical otters (Lontra longicaudis).

Like other Dracunculus, those worms have larvae that stowaway in tiny crustaceans called copepods where they moult and grow. But unlike the Guinea worm which usually infect people when they drink from stagnant water that contains parasitised copepods, in order to get inside otters, the larval Dracunculus would need to take a detour up the food chain into larger aquatic animals such as a fish or amphibians which are on these otters' menu. Incidentally, that is also the suspected route through which the Guinea worm is infecting dogs in places like Chad, because in contrast to how humans drink water, the way that dogs lap water with their tongue means they are unlikely to end up swallowing infected copepods.

While most of the research on Dracunculus have focused specifically on the Guinea worm and its "classical" route of transmission through infected copepods, this has blinded us to the other potential ways that these parasites can circulate in the environment. Understanding how D. insignis and other wildlife-borne Dracunculus complete their life cycles can provide insight into the different ways that these parasites reach their hosts, which in turn can help us better understand how to control the Guinea worm in affected communities.

Reference:

Yabsley, M. J., et al. (2024). Otterly diverse-A high diversity of Dracunculus species (Spirurida: Dracunculoidea) in North American river otters (Lontra canadensis). International Journal for Parasitology: Parasites and Wildlife 23: 100922.

Diexanthema hakuhomaruae

The study in this post takes us to one of the darkest corners of the deep sea, over 7000 m below sea level in the Kuril-Kamchatka Trench, located in the northwestern Pacific. Living in this dark and oppressive environment are isopods called Eugerdella kurabyssalis. And despite the crushing pressure, these crustaceans like it just fine, in fact they are the most abundant isopod down in those depths. But such success and abundance can also attract the attention of parasites, and this post is about a newly described parasitic copepod called Diexanthema hakuhomaruae.

Left: Diexanthema hakuhomaruae (indicated by white arrow) attached to the leg of its Eugerdella kurabyssalis isopod host. Right: Close-up of D. hakuhomaruae, the arrow indicating the copepod's ovaries. Photos from Figure 1 of the paper

Those who are familiar with this blog would know that parasitic copepods come in all kinds of shapes  that would defy most people's idea of what a crustacean is "supposed" to look like. And D. hakuhomaruae is no different - its tiny body is ROUND and if anything, it looks almost like a legless tick. And much like a tick, D. hakuhomaruae attaches itself stubbornly to the leg of its host.

Diexanthema hakuhomaruae belongs to the Nicothoidae family, a group of parasitic copepods that contains about 140 known species. They live on a variety of crustacean hosts, including tanaidaceans, ostracods, amphipods, cumaceans, mysid shrimps, and lobsters. Most of them have a rotund, almost spherical body, greatly reduced or no legs at all, and a specialised mouthpart that ends in a sucker with syringe-like mandibles. And much like the ticks that they resemble, these copepods feed by stabbing their mouth syringe into their host's body and sucking up that crustacean blood (hemolymph) on tap. Some species such as Choniomyzon infaltus are specialised egg parasites - their balloon-shaped bodies allow them to hide amidst broods of their hosts and feed on their eggs without being discovered.

There are currently six other known species of Diexanthema, all of them are parasites of deep sea isopods. And Diexanthema is not alone in its preference - there are other nicothoid copepods that have also been found parasitising deep sea isopods. What makes D. hakuhomaruae special is that it is the first to be found from the Hadal Zone. All other Diexanthema species have been reported from depths of 1300 to 3500 metres below sea level, but none of them had gone down as deep as D. hakuhomaruae.

It is unknown whether D. hakuhomaruae feeds on the host's fluid or if it is an egg parasite, or how it even completes itself life cycle in the hadal zone - as you can imagine, discovering and describing such parasites in an environment like the deep sea is challenging enough as it is. Studying the life style and ecology of these deep sea parasites with current technology is next to impossible. Even so, this description shows that parasitism is indeed ubiquitous on this planet, and wherever you find life, you can be sure that some of them will be parasites

Reference:

Kakui, K., Fukuchi, J., & Ohta, M. (2023). Diexanthema hakuhomaruae sp. nov.(Copepoda: Siphonostomatoida: Nicothoidae) from the Hadal Zone in the Northwestern Pacific, with an 18S Molecular Phylogeny. Acta Parasitologica 68: 413-419.

Sarcotaces izawai

Parasitic copepods are a weird bunch, and many of them look nothing like what most people would recognise as a crustacean. But even among those weirdos, Sarcotaces stands out, because during the course of its evolution, it has turned into a big teardrop-shaped blob living inside a fish's body.

Top left: Staining of the fish body due to Sarcotaces, Bottom left: Sarcotaces extracted from fish flesh
Right: A Female Sarcotaces specimen (about 4 cm in length)
Photos from Fig. 1 and Graphical Abstract of the paper.

There are seven known species of Sarcotaces, all of which are parasites that dwell in fleshy galls embedded in the muscles of fish. The female of the species can grow up to about 5 cm long. They belong to a family of copepods called Philichthyidae which all specialise in living within nooks and crannies of a fish's body, including their skull, sensory canals, or inside galls just beneath the fish's skin. The study featured in this blog post described a newly discovered species of Sarcotaces - Sarcotaces izawai.

Specimens of S. izawai were retrieved from a consignment of frozen fish which were originally destined for the fish market, but were redirected to researchers when the County Veterinary Inspector of Szczecin noticed signs of infection in some of the fish. In total, 29 fish were taken to the University of Szczecin for further examination. Nine of those fish were found to harbour the gall of Sarcotaces - where there was once fish muscle had been turned into a black void, a dark fleshy cavern where the female Sarcotaces resided alongside her tiny males and microscopic larvae.

The black liquid associated with this copepod is what gave Sarcotaces its German name - "Tintenbeutel" which means "ink bag", and why in parts of Australia, they're called "Iodine Worms". Even in the other fish where no Sarcotaces were found, the fish's flesh were tainted with an ink-stained void, which most likely meant a Sarcotaces had once lived there, but was inadvertently removed when the fish were being processed. While the presence of this parasite does not pose any health hazards to any would-be consumers, the inky stain in the fish's flesh do render them off-putting to any would-be buyers on the market. But, because of this, the researchers were given an opportunity to conduct detailed scanning electron microscopy on the copepod, and provided the first DNA barcode for this unique genus of parasite based on its COI gene.

While the female S. izawai is very distinct and noticeable, the male is rather inconspicuous - they grow to about 3 mm in length, and are comparatively tiny and fairly nondescript. In comparison, the female is shaped like a knobbly radish, and grows to 2.5 to 5 cm in length or 10-20 times the length of the male. This size difference is comparable to that of a human and a sperm whale. It also means that a single female could be accompanied by multiple males. Indeed, the researchers found one female who was accompanied by 18 suitors in her flesh gall.

While very little is known about how the microscopic, free-swimming larvae of Sarcotaces gets into a fish in the first place,  it seems that the growth and development of the female Sarcotaces takes place entirely within the sac-like gall. This flesh bag has a tiny opening to the outside world that the copepod usually keeps plugged using the pointy tip of her body, and unplugs to release larvae into the surrounding waters. Because of this, the researchers consider Sarcotaces as a "mesoparasite", because while they largely live within the fish's body, they still maintain some contact with the outside world with the tip of the body plugging up that hole.

As an added layer to that study, the consignment of frozen fish that the researchers examined have been been frozen and thawed multiple times, and were "pan-dressed" - in that their head, fins, and the guts have been taken out - this might be why some of the fish had the characteristic inky stain of Sarcotaces even though the parasite was absent. This made the identification of those fish rather difficult simply through visual inspection. While the consignment of fish were labelled as Pseudophycis bachus - red codling - from "The Falklands", the researchers found this to be a case of seafood identity fraud.

When they did some DNA analyses they found that the fish were actually Mora moro - a species of deep sea cod which is found in temperate seas across many parts of the world, but has not been recorded from the Falklands. It is likely that the fish wholesalers were trying to use the mislabelling to bypass regional quotas or conceal catches from restricted waters.

This type of seafood mislabelling is very common around the world, and presents problems for consumer protection, food safety and supply, fisheries regulations, and conservation. In this case, not only did the ink-stained fillets of these Sarcotaces-infected fish provide scientists with an opportunity to examine a poorly-understood parasite, the presence of this tubby copepod also helped draw attention to a case of seafood identity fraud.

Reference:

Piasecki, W., Barcikowska, D., Panicz, R., Eljasik, P., & Kochmański, P. (2022). First step towards understanding the specific identity of fish muscle parasites of the genus Sarcotaces (Copepoda: Philichthyidae)—New species and first molecular ID in the genus. International Journal for Parasitology: Parasites and Wildlife 18: 33-44.

Pennella instructa

Swordfish are one of the top predators of the ocean. They can swim through the sea at blistering speed, and slash at their prey with their long, flat bill. But no matter how fast you are, there's one thing you can never swim away from - and that's parasites. This is especially the case for big animal like swordfish as their anatomy provides a wide range of different habitat for all kinds of parasites.
They range from sea lice (caligid copepods) that cling to the swordfish's face, to tapeworm larvae which dwell in their muscle, to roundworms that lay eggs under their skin - just to name a few.

Pennella instructa adult with a cyst. From Fig. 4 of the paper

This post will be focused on a study that reported on the occurrence a parasitic copepod - Pennella instructa - on swordfish caught from the north-eastern Atlantic. The researchers in this study visited the fish auction market at Virgo, Spain, during March to September 2011, looking for the presence of P. instructa on swordfish which were brought in by Portuguese and Spanish long line fish boats over that period.

Even though P. instructa is classified as a crustacean, those who are familiar with this blog (and my Twitter feed) would know that when it comes to parasitic copepods, one should abandon any and all preconceptions they might have of what a crustacean is "supposed" to look like. Pennella instructa is shaped vaguely like a toothbrush - a long narrow body that ends with an abdomen covered in a brush-like plume. The adult parasite can grow to about 20 centimetres (or 7 inches) long. It spends its adult life with the lower half of the body protruding from the swordfish, while the front half is anchored deeply in the host's tissue.

Having a parasite that is half-buried in its host's flesh sounds gruesome enough, but P. instructa does something else which elevates it to Cronenberg-level body horror. See, the parasite has not merely stuck its head into the swordfish's flesh and sucking its blood, it is also wrapped in a kind of meat cocoon that the parasite has crafted out of the host's own tissue. Essentially this parasite has sculpted a cosy little bag for itself out of swordfish meat. This parasite-induced cyst is similar to what some other fish parasites, like the fluke that lives on sunfish (Mola mola) gills, can do with their host.

Of the 1631 swordfishes that the researchers looked at, 167 were found to have visible P. instructa infections, though they only occurred in low numbers on each fish, with the most heavily infected fish carrying 4 individual copepods. But being the kind of parasite that it is, even a single P. instructa can have some significant impact on the swordfish's overall health, depending on where it is located. Aside from drinking the host's blood, the meaty cyst that P. instructa forms around itself can put pressure on the surround tissues and organs. The researchers found that while P. instructa can be found all over the swordfish's body, for whatever reasons, most of them prefer the posterior part of the swordfish, mostly in the thick, meaty part of the tail.

It could be that those sturdy tail muscles provide the parasite with a good site to anchor itself in place. Furthermore, that part of the fish's body is made of the powerful muscle which allows the swordfish to propel itself so quickly through the water, thus they'd be constantly supplied with a steady flow of blood which P. instructa can drink from. But this comes at a significant cost to the host, because if the parasite's cyst is located near the vertebrate column - as they would be if they are embedded in the tail - it may affect the fish's nervous system and compromising its swimming ability.

While P. instructa doesn't infect or cause any health issues in humans, a piece of swordfish steak with a big hole through it and a weird worm thing dangling out the side would probably be off-putting to any would-be customers. But perhaps we might want to consider adding P. instructa to the menu?
Pennella balaenopterae - a related copepod which infect whales - is considered to be gastronomic treat by the Inuit people of the Canadian arctic. So instead of seeing them as a pest, perhaps Pennella might be reconsidered as added garnish for your swordfish steak?

Reference:
Llarena-Reino, M., Abollo, E., & Pascual, S. (2019). Morphological and genetic identification of Pennella instructa (Copepoda: Pennellidae) on Atlantic swordfish (Xiphias gladius, L. 1758). Fisheries Research 209, 178-185.

Balaenophilus manatorum (revisited)

At some stage of their lives, parasites need to move from one host to another - some move around a lot throughout their lives, staying just briefly on a given host before moving onto another. While others only do it once during their larval stage - once they reach their host, they are there for life. Either way, they still need to make a perilous journey to their host.

Top right: newly hatched nauplii, Top left: Copedpodite V stage, Bottom: Adult female with eggs
Image composited from photos from Fig.1, 5, and 6. of the paper

This post is about study on Balaenophilus manatorum - a tiny parasitic copepod that lives on sea turtles. How does a tiny crustacean like that manage to find their way onto a turtle in the wide expanse of the sea? Do they jump on board when the turtle come into contact with each other, or can the larval stage swim on their own? Obviously they have managed to find a way because this copepod is very common among the juvenile loggerheads in the western Mediterranean, with over 80 percent of loggerhead turtles infected with B. manatorum. Given how small they are (the adult copepod is only about a millimetre long), it seems as if they would be barely a nuisance to their host. But when they occur in large numbers, they can be an serious menace.  And they seem to have a very particular taste. It was thought that B. manatorum feed mostly (if not exclusively) on sea turtle skin.

To find out more about how B. manatorum infect their hosts and what they feed on, a team of scientists did a series of studies on some B. manatorum which were removed from a batch of sea turtle hatchlings. These hatchlings were being reared at the Sea Turtle Rescue Centre (ARCA del Mar) - a rescue and rehabilitation for sea turtles in Spain. They came from a brood of eggs that was removed from a beach frequent by tourist to ensure their safety, but during their stay at the centre, many of them develop symptoms of infestation by B. manatorum, each of them infected with about 300 B. manatorum and one unlucky turtle was hosting over 1400 copepods. While removing the copepods from the turtles, the research team collected some of the egg-bearing female copepods that were on the turtles, and reared them until their eggs hatched into larvae for the further study.

In the feeding trials, the copepods were offered a menu selection consisting of: flakes from the baleen plates of a fin whale, fish skin (from a blue whiting), green alga, loggerhead turtle skin flakes (from some hatchlings that had succumbed to B. manatorum infestation). All those items were dyed with a stain to track if they get ingested. They confirmed that these copepod only ate turtle skin flakes and didn't touch the other items on the menu. Other species of Balaenophilus have been recorded from the baleen plates of whales, but B. manatorum feed exclusively on turtle skin. From the moment it is born, B. manatorum is equipped with mouthparts which are well-suited for scrapping flakes from hard flat surfaces, such as the skin of a turtle. So it is no wonder heavy infestations of B. manatorum can cause severe lesions and skin erosions in turtles, especially for the more vulnerable hatchlings

But B. manatorum still need to reach the turtle in the first place. When placed in a dish of seawater, newly hatched copepods (called nauplii) seemed rather helpless, only able to crawl around. But if they manage to survive to grow into the subsequent stages called copepodite, they will develop legs that would allow them to swim for a bit - just barely, and once they grow past a stage call Copepodite IV, they can swim well enough to reach another turtle on their own. It seems that this parasite relies mostly on the social behaviour of the turtle for transmission. Newly hatched B. manatorum nauplii cannot swim and would have to wait for the turtles to touch each other (for example during mating) to climb onboard another host (rather like how human lice are transmitted), whereas the copepodites and adults can just swim across if another turtle comes close enough

Therefore, these parasitic copepods may present as a kind of social cost to these turtles, since not only is a social communicable parasite, it can also be a sexually transmitted infection. For B. manatorum, their entire world really is found on the back of a turtle.

Domènech, F., Tomás, J., Crespo-Picazo, J. L., García-Párraga, D., Raga, J. A., & Aznar, F. J. (2017). To Swim or Not to Swim: Potential Transmission of Balaenophilus manatorum (Copepoda: Harpacticoida) in Marine Turtles. PloS One 12(1), e0170789.

Pennella balaenopterae

Photo of Pennella balaenopterae embedded on
the side of the porpoise's peduncle (from Fig 2 of the paper)

Most people usually think of copepods as tiny crustaceans which live as zooplankton near the, and for most part that is true. But it might be a surprise to some of you that over a third of all known copepods are actually parasitic and they live on/in all kinds of aquatic animals. One particularly successful family of such copepods is the Pennelidae - not that you would necessarily recognise them as crustacean if you are to ever see one. While most species in this family live on fish, the parasite that we are featuring today has evolved to be a bit different. Instead of infecting fish, it has managed to colonised aquatic mammals - specifically cetaceans (whales).

Whales are among the largest known animals to have ever lived, and P. balaenopterae also happens to be the largest known copepod (most free-living copepod are tiny zooplankton measuring a few millimetres in length). As its name indicates, this parasite was initially found on baleen whales, such as fin whales, but it has been reported from different species of toothed whales as well. Despite being known to science since the 19th century, there is very little information about the biology of this peculiar parasite.

The cephalothorax or the "head" of Pennella balaenopterae
which is deeply buried in the host's blubber

The paper we are featuring today reports this parasite infecting harbour porpoises (Phocoena phocoena relicta) in the Aegean Sea. These parasites each measured over 10 centimetres long and most of it is buried deep in the blubber. In this study, Pennella balaenopterae were mostly found on the porpoises' back and abdominal area, probably because those areas are rich in easily accessible blood vessels that the parasite can tap into.

Even though technically it is an ectoparasite (external parasite) as it can be found dangling on the host's external surface, a significant portion of its body is actually deeply buried in the porpoises' tissue (not unlike the shark-infecting barnacle Anelasma squalicola which was featured last year). Hence some parasitologists call them "mesoparasites"; they are not strictly internal parasites (endoparasites) such as many parasitic worms, but they do interact with the host's internal tissues in some major waya.

Species like P. balaenopterae shows that over evolutionary time, some parasites can make rather radical shifts in their preferred host if given the opportunity to do so. Last year I wrote about an elephant blood fluke which has colonised rhinos because both of its mammalian host share the same habitat. Indeed, both whales and fish that are infected other pennelid copepods are both marine animals, so there have been many opportunity for such a host jump to occur.

However, it is one thing to jump from one large, terrestrial mammal into another, it is quite another to branch off and infect an entirely different class of animal which has a very different anatomy and physiology to the ancestral host. More studies will be needed to find out what makes P. balaenopterae different from its related species, as well as when and how it made the leap from living on scale-covered bony fishes, to burying themselves in the tissue of air-breathing blubbery whales.

Reference:
Danyer, E., Tonay, A. M., Aytemiz, I., Dede, A., Yildirim, F., & Gurel, A. (2014) First report of infestation by a parasitic copepod (Pennella balaenopterae) in a harbour porpoise (Phocoena phocoena) from the Aegean Sea: a case report. Veterinarni Medicina, 59: 403-407.

Ismaila sp.

Those who have been reading this blog for a while might recall that this time last year, I featured some guest posts written by students from my Evolutionary Parasitology  (ZOOL329/529) class. Well, it is happening again for this year! For those who are unaware of this, one of the assessment I set for the students is for them to summarise a paper that they have read, and write it in the manner of a blog post, much like the ones you see on this and other blogs. 

I also told them that the best blog posts from the class will be selected for re-posting (with their permission) here on the Parasite of the Day blog. I am pleased to be presenting these posts from the ZOOL329/529 class of 2014. To kick things off, here's a post by Courtney Waters on a paper published in 2002 that documented the diversity of parasitic copepods that live inside sea slugs off the coast of Chile (see also this post from June this year).

Picture of infected sea slug from the paper

Bright colourful sea slugs are every diver’s ultimate find. Imagine getting up close to it with that macro lens and... wait, what's that protruding from the slug's side? They appear to be the egg sacs of an endoparasitic copepod - small crustaceans, which parasitises the insides of these soft‐bodied molluscs. The aim of the study I am writing about for this post was to expand existing knowledge about these endoparasites, particularly the genus Ismaila from the family Splanchnotrophidae. This particular genus is characterised by the presence of a pair of well-developed first appendages which are absent in related genera.

The six year study was based mainly in Chilean waters where different sea slug species were collected and examined for parasite infection. This was done simply by examining the sea slug externally without dissection as the egg sacs of the adult parasite protrude conspicuously from the abdominal wall of the host (see the accompanied figure). Over 2000 specimens from 47 species of sea slug were examined in such a manner and only 8 species of slugs were found to be parasitised by those copepods. These parasites are very host specific and each parasite species is only found in one host species. The overall infection rate was 13% which is the highest infection prevalence documented. Fortunately, these parasites only like the soft innards of our mollusc friends - otherwise I would not be so jealous of the scuba divers who were doing the collecting!


Obvious differences were seen between the infection rates of different host species, with some parasitised more than others. For example, in several species of hosts, only one individual was observed to be infected, whereas for other species the infection rate was almost 90%. The infection frequencies for two of the main sea slug host species did not vary much between years and seasons, though this would need to be verified with further studies. An additional result of the study was information on the evolution of these parasites. The disjunct distribution of the copepods along with their host groups suggest that these parasites had evolved from an ancestor that was not very host-specific, but as different populations became isolated, they evolved to be very specific to their hosts. This resulted in scattered pockets of area with high parasite abundance. As for why they have not spread out to wherever appropriate hosts are available, this is likely due to other life-cycle requirements of the parasite which are currently unknown.

In summary, the study found 4 new species of host for splanchnotrophid copepods, taking the world total to 47 host species (at least as of 2002 when this paper was published), with 12 of which being found in Chilean waters and 9 of them being host to copepods in the Ismaila genus. This means the waters of Chile have over a quarter of all known splanchnotrophid species. Additionally, the percentage of infected sea slug in Chile is ten times higher than anywhere else in the world - a fact that, if I was a sea slug in those waters, would probably give me the chills...

Reference:
Schrödl, M. (2002). Heavy infestation by endoparasitic copepod crustaceans (Poecilostomatoida: Splanchnotrophidae) in Chilean opisthobranch gastropods, with aspects of splanchnotrophid evolution. Organisms Diversity & Evolution, 2: 19-26.

This post was written by Courtney Waters

Ismaila belciki

Photo of infected Janolus fuscus
used with permission from Jeff Goddard

If you ever find yourself down by the sea, you may come across some very flamboyant sea slugs call nudibranchs. But beneath their colourful exterior, some of them are harbouring a dark secret in the form of a very strange looking parasite. These parasites live hidden inside the main body cavity of their molluscan host, so if you are unfamiliar with this particular critter, you might not even notice it. The main thing that gives away their presence are a pair of egg sacs poking out of the sea slug (see photo on the right). Those egg sacs belong to a parasite call Ismaila belciki - it is a crustacean, though it looks more like one of Cthulhu's lovechild or something out of Men In Black.

Ismaila and other copepods of the Splanchnotrophidae family are specialist parasites of sea slugs and they can get pretty big in comparison with their host, taking up substantial room and resources. Ismaila belciki infects Janolus fuscus, a nudibranch found along the west coast of North America from Alaska to California, as well as the shores of northern Japan. In some areas, such as Coos Bay, Oregon where the study we are featuring today took place, up to 80% of the slugs are infected with this odd creature. Having such a big parasite sitting in the middle of slug's body soaking up nutrient obviously carries some kind of cost - but just how much?

Photo of a female Ismaila belciki with an
embraced dwarf male front and centre.
Photo by and used with permission
from Maya Wolf

A pair of researchers from University of Oregon decided to find out just how costly this parasite is to its host. They compared the growth, survival, and reproductive capacity of infected and uninfected J. fuscus, and measured how much resources the parasite takes up.

While I. belciki did not seem to interfere with sea slug's growth, infected slugs do have a lower survival rate. Additionally, they have shrunken gonads that are only capable of producing about half as many eggs as healthy slugs. But the reproductive capacity of those afflicted sea slug suffers not just in terms of quantity, but in quality as well. In addition to producing fewer eggs, infected slugs also produced eggs that were smaller, and the baby slugs that hatch out of them also have lower survival rates.

So it seems I. belciki can be very harmful indeed, but it cause even greater harm if the parasite itself is breeding. The researchers noted that I. belciki bearing developing egg sacs exert a greater toll on the host than egg-free parasites. A female I. belciki is an egg-laying machine that can churn out over 88000 embryos per month and all the expenses for that are paid for by the host. To fuel the development of its eggs, I. belciki draws from the same pool of resources that the host normally use for its own egg production. Slugs with brooding I. belciki produce even fewer eggs than those that are "just" stuck with an egg-free parasite.

It is as if the sea slug is a factory that has been retooled from solely making slug babies into one which now has to divert some of its attention and raw material to making parasite babies too, via a proxy in the form of a female I. belciki. Given that Janolus fuscus usually only live for five months, by shorten their lives and severely reducing their reproductive capacity, I. belciki might actually be putting a natural check on the population growth of these flamboyant nudibranchs.

Reference:
Wolf, M., & Young, C. M. (2014). Impacts of an endoparasitic copepod, Ismaila belciki, on the reproduction, growth and survivorship of its nudibranch host, Janolus fuscus. International Journal for Parasitology 44: 391-401.

P.S. I will be attending the annual Australian Society for Parasitology annual conference in Canberra, Australia between 30th June to 3rd July. So watch for tweets about highlights from conference at my Twitter @The_Episiarch! Meanwhile, I have written a article for The Conversation about the crab-castrating barnacle Sacculina carcini - you can read it here.

Octopicola superba

When it comes to reproduction, most living things can be classified along a scale. At one end, you have the r-strategists (many insects and molluscs) that produce a prodigious number of offspring but few survive to adulthood. And on the other end are the K-strategists that produces only a few progeny, but to invest a lot of resources into each to ensure they are more likely to reach maturity (for example, elephants, humans, etc).

SEM photo of female
Octopicola superba from here

There is a cost/benefit trade-off inherent with being on either side of the scale because as a r-strategist, you might be producing a lot of progeny, but most of them will probably die before they get to reproduce themselves. While on the K-strategist end, by investing so much resources into each individual young, you can only afford to produce a few of them. The reproductive strategy of different organisms all fall somewhere along that continuum between low quality mass production or high quality but infrequent output, and different circumstances call for different strategies.

Textbook often use parasites as key examples of r-strategists, as a model of organisms that producing prodigious number of offspring. Indeed some internal parasites are well-known for their reproductive capacity - for example, the female blood fluke Schistosoma mansoni produces 300 to 3000 eggs per day, while tapeworms like Diphyllobothrium dendriticum can produce tens of millions of eggs per day. But not all parasites opt for quantity over quality.

The study we are featuring today examined the reproductive capacity of the parasitic copepod Octopicola superba, which, as its name indicates, lives in the common octopus. As far as a parasite goes, this crustacean seems rather innocuous and does not really cause much harm to its host. Octopicola superba can be found all over the body of the octopus but most of them are located on the skin and gills. Even though it is a parasite, it has a reproductive strategy which brings it closer to being a K-strategist.

Each female O. superba produces a clutch of only a few dozen eggs per season; if a female was to produce more than about forty eggs in a clutch, she starts reaching the upper limit of her reproductive capacity and the size of each egg (which reflects how much resources is invested into it) begins to shrunk as the brood imposes too much of a drain. This reproductive capacity varies considerably between individual; the most productive copepods are able to produce over twice as many eggs as the least productive ones, while some produced eggs that were almost twice as big as those produced by others.

Octopicola superba's reproductive strategy also shifts during different seasons; in winter, they produced a larger clutch of smaller eggs, whereas in summer they produce a smaller clutch of bigger eggs. Such season shifts has been observed in other parasitic copepods, though for O. superba, the reason for them doing so remains unknown. Despite these seasonal and individual differences, overall O. superba is certainly low-key when it comes to reproduction - even the most fecund female had just above sixty eggs in a clutch and the rest mostly produced between thirty to forty eggs.

So why has this parasitic copepod evolved to produce so few eggs compared with parasites like tapeworms and blood flukes that pump out thousands or even millions of eggs on a daily basis? It might have something to do with the habits of its host.

Octopus tend to be territorial homebodies that likes to stay in their little corner of the sea. Previous analyses indicate that hosts with such sedentary habits tend to select for parasitic copepods that produce larger eggs. Unlike one infecting more mobile animal (like a fish), parasites of sedentary animals cannot rely upon their host's routine daily movement to bring them into contact with new hosts. Therefore, they must do so under their own steam. By investing more into each egg, the female O.superba ensures each of her babies are better equipped for the long journey to find a new home, even if it means she can only produce just a few dozen of them at a time.

With offspring, you can only invest so much into them - at some point, they are on their own

Cavaleiro, F. I., & Santos, M. J. (2014). Egg number-egg size: an important trade-off in parasite life history strategies. International Journal for Parasitology 44:173-182

Choniomyzon inflatus

Photo of C. inflatus from the paper

I guess you could say that the parasite we are featuring today is a "balloon animal" and indeed its name refers to that property. According to the paper that described and named this copepod - Choniomyzon inflatus - "The specific name of the new species is a reference to its swollen prosome, which resembles a balloon."

But you won't be finding this odd little crustacean at any kid's party, instead it is usually attached to the egg masses of smooth fan lobsters (Ibacus novemdentatus) on the coast of western Japan. It is the third species from the genus Choniomyzon to have ever been described. The other two known species are C. panuliri, which are found on spiny lobsters from India, the British Solomon Islands and the Great Barrier Reef, and C. libiniae, which live on spider crabs from São Sebastião Island, Brazil. All three species attach themselves to the external eggs masses of their respective hosts.

SEM photo of C.inflatus
from the paper

So why do they look like a miniature hopper ball toy? Well, that relates to where they live and what they feed on. Chioniomyzon inflatus belongs to a family of copepods called the Nicothoidae and the reason they do this Humpty Dumpty impersonation is so that they can insinuate themselves amidst the eggs masses of larger crustaceans.

Normally the host crustaceans would remove any foreign particles or organisms that get caught up in their brood pouch or egg mass, but by disguising themselves as an egg, C. inflatus and their relatives can stay there undisturbed. And while the appearance seems comical to us, it is seriously bad news for its host because nicothoid copepods are egg-eaters - they have a syringe-like mouthpart with which they puncture their host's eggs and suck out their contents.

So C. inflatus masquerades as just another egg in the brood to avoid being expelled meanwhile munching on the actual eggs around it. This strategy is rather reminiscent of another creature that we featured during the first year of the Parasite of the Day blog - the cuckoo catfish which hides its eggs amongst that of mouth-brooding cichlids. You can read more about the cuckoo catfish here.

Reference:
Wakabayashi, K., Otake, S., Tanaka, Y., & Nagasawa, K. (2013). Choniomyzon inflatus n. sp.(Crustacea: Copepoda: Nicothoidae) associated with Ibacus novemdentatus (Crustacea: Decapoda: Scyllaridae) from Japanese waters. Systematic parasitology 84: 157-165.

Tracheliastes polycolpus

Photo of adult T. polycolpus from here

Tracheliastes polycolpus is a parasitic copepod that lives on freshwater fish and does so by attaching to the fins of its host, grazing on mucus and epithelial cells. While T. polycolpus can infect a handful of different freshwater fishes, it is primarily found on the beaked dace (Leuciscus burdigalensis). When they occur in large numbers, their feeding activities can severely erode the fins of their hosts, so much that in some fish the fins are gnawed down to mere nubs (see the photo below of a heavily parasitised dace, with outlines showing the missing fin tissue).

So when it gets crowded on this parasite's usual, preferred host, some T. polycolpus find a home elsewhere and start parasitising other species of fish living in the same area. Even though T. polycolpus is considered to be a host generalist and can infect multiple species of fish, not all fish are considered equally habitable for this parasite and it does have a predilection for certain species over others. So what determines which other fish end up acquiring these parasitic copepods?

A group of scientists from France conducted a study looking at T. polycolpus population on freshwater fish in two French rivers, focusing on the 10 most abundant fish species in those rivers. Of the fish that they examined, eight of them were cyprinids (the family of fish that include dace, roach, and carp) while the two remaining species were the stone loach and brown trout.

Photo of parasitised dace with missing fin tissue from this paper

Only cyrpinids were found to be infected with T. polycolpus and of those only four species (dace, nase, gudgeon, minnows) were found to be consistently infected across both study sites. It turns out that next to the beaked dace, the second most preferred host for T. polycolpus is Parachondrostoma toxostoma, also known as South-west European Nase. After the beaked dace, it was the most commonly infected fish, especially in the Viaur river where there was generally higher abundance of the parasite.

It just so happens that out of all the fishes in those rivers, the nase is most similar to the dace in terms of its general body size, feeding style and habitat, making it the ideal second choice for T. polycolpus. On the flip side, it seems that minnow is the worst host for T. polycolpus - it hosted the least parasites out of the four fish species that were found with T. polycolpus and the parasites that were found on minnows were smaller and produced less eggs than those found on the other fish species. This is probably due to the minnow being a smaller fish than the beaked dace or the nase, so it does not produce as much mucus for T. polycolpus to graze on.

So even when generalist parasites do infect other hosts, they prefer some familiarity. The more similar you are (physiologically and/or ecologically) to the parasite's preferred host, the more likely that you will be next in line to get infected should the parasite's preferred host become too heavily parasitised.

But here's an added to layer to this story which you might want to consider - the South-west European nase is actually listed as a vulnerable species - its population has declined by at least 30% in the past 10 years due to habitat destruction and hybridisation with introduced species, so if the number of nase continues to decline, what does this mean for T. polycolpus? Would this result in increased parasite pressure on other fish species as they find themselves soaking up the "excess" T. polycolpus? Or will the the beaked dace experience even more exacerbated pathology as T. polycolpus are left with less alternative hosts to infect?

Reference:
Lootvoet, A., Blanchet, S., Gevrey, M., Buisson, L., Tudesque, L., & Loot, G. (2013). Patterns and processes of alternative host use in a generalist parasite: insights from a natural host–parasite interaction. Functional Ecology 27: 1403-1414

Urogasilus brasiliensis

While most people who have some passing familiarity with copepods would know them as tiny zooplankton crustaceans, a large number of them are actually parasitic. In fact, about a third of all known species of copepods are parasites and with about 13000 known species of copepods in total, that is a lot of parasitic species. These parasitic copepods infect a wide variety of aquatic animals and come in all kinds of weird shapes.

Photo composed from Fig 4 and Fig 5 of the paper

Naturally, many of them are fish parasites as fish are such an abundant and diverse group of aquatic animals. But while most parasitic copepods of fish usually infect the skin or gills of their host, today's parasite stands out from the crowd as it inhabits the fish's urinary bladder and is the first parasitic copepod ever known to live in that organ.

Now that is not to say that a fish's bladder is a parasite-free zone - far from it. You wouldn't think that an organ that gets periodically filled up with urine and metabolic waste would be prime real estate, but there are all kinds of parasites that call it home ranging from single-cell eukaryotic parasites, to myxozoans, parasitic flatworms like monogeneans and digenean flukes - some of them are even found exclusively in the urinary bladder. However it is an unusual habitat for a parasitic copepod seeing how, as mentioned above, most live on the fish's skin or gills

Today's featured parasite, Urogasilus brasiliensis, is a newly described species that has been found in some freshwater fish living in the Cristalino River, a tributary of the Araguaia River in Brazil. The known hosts to this parasite include the tiger fish and two species of peacock bass. Much like other parasites that infect many different species of hosts, some hosts are just better than others and that is the case for U. brasiliensis too. This copepod tends be more common in the tiger fish and grows to a larger size in that host, indicating that it is possibly a better host for the parasite than the peacock bass. But while U. brasilensis is not particularly picky about what species of fish it infects, it is picky about where it lives within that fish - it is always found in the bladder.

Living in the urinary bladder does present some physiological challenges - as mentioned above, it is an organ that regularly alternates between being empty and being full of urine. Such periodic shifts in the concentration of fluid surrounding U. brasiliensis would cause severe osmotic stress like those experienced by animals that regularly migrate between freshwater and marine habitats. Presumably U. brasiliensis has overcome this particular obstacle and in doing so has been able to colonise an otherwise fairly vacant niche not occupied by other parasitic copepods.

Urogasilus brasiliensis is one of the few parasitic copepod that has evolved into an endoparasite (internal parasite) as opposed to being an ectoparasite (external parasite). But it is not alone - a few other species of copepods have also evolved to conquer that frontier, some of which we have featured on this blog such as one that lives in the cephalic canal of fish in Australia and another species lives in the rectum of rockfish.

Reference:
Rosim DF, Boxshall GA, Ceccarelli PS. (2013) A novel microhabitat for parasitic copepods: A new genus of Ergasilidae (Copepoda: Cyclopoida) from the urinary bladder of a freshwater fish. Parasitology International 62: 347-354

Lepeophtheirus acutus

Today, we are featuring a paper which reported on a grey reef shark (Carcharhinus amblyrhynchos) at Burger's Zoo in the Netherlands that had to be euthanized. "Wait a sec!" you think, "Isn't this supposed to be a blog about parasites? I didn't come here for dead sharks!" Well, just calm down before you close your browser tab in outrage. This particular shark actually succumbed due to a heavy infection of today's parasite - Lepeophtheirus acutus. This parasite is in the same genus as other fish lice that we have previously featured on this blog, but very little is known about this particular species. Prior to this incident, it has only been reported once from the wild, and it was found on the back of a ribbon-tailed stingray (Taeniura lymma), not a shark and certainly nothing was known about how harmful it can be to its host.

From what the staff at the aquarium could work out, this deadly little crustacean was introduced to the facility by an infected male zebra shark (Stegostoma fasciatum) collected off Cairns, Australia on the Great Barrier Reef, which appeared perfectly healthy at the time and passed quarantine. However, about 2 weeks after he was introduced into the aquaria with the other fishes, he started acting weird. At the same time, the grey reef shark mentioned at the start of this post became lethargic and ceased to eat regularly, and about a month after that, both sharks were afflicted with swollen and opaque eyes. Despite the best efforts of the staff to put the infected sharks in quarantine, filter the water with activated carbon, and give them anti-parasite drugs, they were unable to save the grey reef shark, by which time it was swimming with its mouth wide open, not eating at all, and its eyes had deteriorated even further, so the decision was made to euthanize the long-suffering shark.

A necropsy revealed the identity of the killer - a parasitic copepod - most of which were found around the shark's eyes which caused them to become swollen and covered in mucus, and the mouth which led to bleeding gums. The parasite was also found on a female zebra shark and a shovelnose ray (Glaucostegus typus) which shared the aquaria with the deceased grey reef shark. Notably, the blacktip reef shark (Carcharhinus melanopterus) and blacktip sharks (Carcharhinus limbatus) which swam in the same water alongside those infected sharks did not become infected, nor did the many different species bony fishes sharing the same tanks and water. This indicates that L. acutus does display some selectivity in the type of host it infects, with a particular preference for elasmobranchs (sharks and rays), and even then only certain species within that group.

Other than the dead grey reef shark, the other infected sharks survived and recovered fully after treatment. However, this incident shows how outbreaks of infectious diseases can be a big problem for animals in the confined conditions of captivity. In the case of L. acutus, its small size, semi-transparent body, its tendency to infect parts of the host that are difficult to inspect (for example, inside the mouth), and the fact that nothing is know about its ecology meant that the staff had not anticipated such an outbreak. It was the first documented case of infection by a parasitic copepod that led to a shark dying in captivity. This case also illustrates the importance of thorough quarantine procedures, especially when introducing new animals into any facility, as captive conditions can seriously alter the transmission dynamics and pathology of relatively harmless parasites.

Image from figure in the paper.

Reference:

Kik, M.J.L., Janse, M., Benz, G.W. (2011) The sea louse Lepeophtheirus acutus (Caligidae, Siphonostomatoida, Copepoda) as a pathogen of aquarium-held elasmobranchs. Journal of Fish Diseases 34: 793-799,

Nicothoë astaci

The parasite we are featuring today is Nicothoë astaci, the "lobster louse." Despite its name, it is not a "louse" (true lice are insects) as such, but rather a copepod (a type of crustacean), just like the salmon lice we have previously featured on this blog. But whereas salmon lice are well-studied due to their economic impact on salmonid fisheries (especially on farmed fishes), far less is know about the lobster louse. Despite having been recorded on the European lobster (Homarus gammarus) since the 1950s, to this day there is very little known about this parasite, including the type of pathology it causes, its complete life-cycle, or even what the male of the species looks like (parasitic copepods often have cryptic or dwarf males which are very elusive).

The paper we are looking at today is taking the first step to rectifying that situation. The photo (from the paper itself) depicts larval stages of N. astraci, with the arrows indicating the oral cone,the structure this parasite uses (along with its front pairs of legs) to attach itself to the host's gill filament and feed on its blood. While the larval stage looks like a rather ordinary copepod, as it matures into an adult, it morphs into what looks like a miniature boomerang with a pair of stretched out "wings" on either side, and a pair of bulbous egg sacs dangling from its rear end. The attachment and feeding activity of the lobster louse can cause pronounced physical damage to the lobster's gill filaments.

As with any kind of infection, you would expect to see some kind of cellular response. While the innate immune systems of invertebrates like lobsters are not as sophisticated as the adaptive immune system of vertebrate animals such as ourselves, they can present a formidable challenge to any would-be intruder (to see an example of what the cellular defence of a crustacean can do to a parasite, click here). Basically, the crustacean's equivalent of blood cells wrap themselves around the parasite or pathogen and initiate the process of melanization, where the intruder becomes entombed in a hardened capsule of melanin (the pigment which determines our skin colour). The researcher did find signs of melanization and other cellular disruption throughout the gills of infected lobsters, but none of it was near the lobster louse's attachment point.

So the lobster's immune system recognizes the presence of an intruder, but is unable to pinpoint and focus its wrath on the parasite. The authors of this paper suggest that this indicates the lobster louse is able to somehow interfere with the lobster's defensive mechanism so that it can blood-feed in peace. The mechanism through which the lobster louse disrupts this particular aspect of host physiology is yet to be uncovered, along with much of the parasite's ecology and life-cycle. Hopefully, with further research on this host-parasite system, this situation will change in the future.

Image from the paper.

Reference:

Wootton EC, Pope EC, Vogan CL, Roberts EC, Davies CE, Rowley AF. (2011) Morphology and pathology of the ectoparasitic copepod, Nicothoë astaci ('lobster louse') in the European lobster, Homarus gammarus. Parasitology 138:1285-1295.

Chondracanthus parvus

Chondracanthus parvus is a parasitic copepod that parasitises the smooth-cheek sculpin, Eurymen hyrinus, by attaching itself to the inner side of the fish's operculum (the flap covering the fish's gills). Chondracanthus parvus belongs to a family of parasitic copepods known as the chondracanthids, which contains 160 species, all of which are parasites of marine fishes. Phylogenetic studies of the chondracanthids indicate that these copepod have consistently co-evolved with their hosts, and their phylogeny closely reflects the evolutionary history of the fish that they infect. Such parasites are like heirlooms of the evolutionary past and phylogenetic studies conducted on these living markers can in turn shed light on the evolutionary history of their hosts.

Picture from Ho et al. (2006).

References:

Paterson, A.M. and Poulin, R. (1999) Have chondracanthid copepods co-speciated with their teleost hosts? Systematic Parasitology 44:79-85.

Ho, J-s., Kim, I-H., and Nagasawa, K. (2006) Copepod parasites of the fatheads (Pisces, Psychrolutidae) and their implication on the phylogenetic relationships of Psychrolutid genera. Zoological Science 22:411-425.

Herpyllobius vanhoeffeni

Regular readers of this blog will no doubt be familiar with the wonderfully weird and twisted morphology of parasitic copepods. However, this is probably the weirdest we have featured yet. Herpyllobius vanhoeffeni is a spooky-looking parasitic copepod which has all the trappings you might associate with an Lovercraftian horror tale. They are found in the Antarctic Penninsula, in waters 666-673m deep, and they parasitise a polychaete worm, Eulagisca corrientis.

The top picture shows a pair of females attached to the ventral surface of their host; note that the lower individual has a pair of lobe-shaped egg sacs extending from its side like wings. The bottom picture shows a specimen that has been dissected from the host, showing the rest of the copepod, which is usually embedded in the host. Overall, the whole parasite looks not unlike a bulbous skull resting atop a twisted stalk of a body.

Reference:
López-González, P.J. and Bresciani, J. (2001) New Antarctic records of Herpyllobius Steenstrup and Lütken, 1861 (parasitic Copepoda) from the EASIZ-III cruise, with description of two new species. Scientia Marina 65:357-366

Colobomatus sillaginis

Colobomatus sillaginis is a parasitic copepod that lives in the head of two species of fish (commonly known in Australia as "whiting") in the genus Sillago (Sillago maculata and Sillago analis). This copepod dwells in the system of cephalic canals in the head of the fish. Interestingly, while the gut tracts of males and juvenile females are bright green, the gut of mature female copepods are usually coloured red or black. Living in the cephalic canal alongside C. sillaginis are small ciliates that are bright green due to the symbiotic algae living within them. These ciliates can be so numerous that some fish have a greenish tinge around front of the head. The male and juvenile female copepods graze upon this turf of abundant food. However, once they become mature, the female takes to feeding on blood, probably due to the physiological demand of egg production, rather like a female mosquito which normally feeds on nectar, but needs to obtain a blood meal for egg development.

Reference:
West, G.A. (1983) A new philichthyid copepod parasitic in whiting (Sillago spp.) from Australian waters. Journal of Crustacean Biology 3: 622-628.

Contributed by Tommy Leung.

Arthurhumesia canadiensis

Parasites come in all kinds of bizarre shapes and you don't get much more bizarre than today's parasite - Arthurhumesia canadiensis. This species is a parasitic copepod that lives inside the intestine of the compound ascidian (sea squirt) Aplidium solidum. The diagram shows a female specimen, with a pair of lobe-like egg sacs attached. And if you are wondering "what's the weird little blob the arrow is pointing at?", well that's the male copepod. This weird little crustacean is named after Arthur Humes - a very prolific taxonomist. Over the course of 60 years, he was responsible for describing over 700 new species of parasitic copepods. So it's only right that a copepod named after him would appear on a blog which is about parasite biodiversity!

Reference:
Bresciani, J. and López-González, P.J. 2001. Arthurhumesia canadiensis, new genus and species of a highly transformed parasitic copepod (Crustacea) associated with an ascidian from British Columbia. Journal of Crustacean Biology 21(1): 90-95.

Contributed by Tommy Leung.