By the Light of the Pony

Light-emitting organs have evolved in many different species of marine fish. For the greater part, they are associated with inhabitants of the deep sea, the twilight and midnight zones beyond the reach of celestial light. Light production by species found in shallow waters is much less common. Nevertheless, one particularly notable radiation of near-surface glowers is the ponyfishes of the family Leiognathidae.

Leiognathus equulus, copyright Sahat Ratmuangkhwang.

Ponyfishes are small, mostly silvery fishes found in coastal and brackish waters in tropical regions of the Indo-West Pacific. The largest ponyfishes grow to about 25 cm in length but most species are much smaller (Woodland et al. 2002). They live in large schools that forage near the surface at night, descending close to the bottom sediment during the day. Why these animals are referred to as 'ponyfishes', I have no idea (perhaps the head is meant to look a bit pony-like?) An alternative vernacular name of 'slipmouth' makes a lot more sense as these fish have highly extensible jaws that can be used to snipe prey out of the water. A groove along the top of the skull allows for reception of a long, mobile premaxilla, supporting the mouth as an elongate tube when extended. Most ponyfishes are planktivores with simple, minute teeth in the jaw and the mouth extending horizontally. Species of the genus Deveximentum have the mouth tilted obliquely at rest so that it stretches upwards when extended. Members of the genus Gazza are piscivores when mature, feeding on other fish, and possess a pair of large caniniform teeth in each of the upper and lower jaws to hold their prey (James 1975).

Ponyishes are also notable for their elaborate light-producing organs. In most bioluminescent fishes, the photophores sit on or close to the skin surface but in leiognathids it is an internal outgrowth of the gut. A cavity around the end of the oesophagus houses colonies of bioluminescent bacteria, usually the species Photobacterium leiognathi. This light organ sits alongside or projects into the gas bladder which has a reflective internal coating. In many species, patches of scale-less, translucent skin allow the transmitted light to shine forth brightly. Muscular 'shutters' associated with the light organ allow the fish to control light transmission more directly (Woodland et al. 2002).

Photopectoralis bindus, copyright D. G. R. Wiadnya.

In a review of ponyfish taxonomy by James (1975), no mention was made of the light-emitting organ or many of its associated structures (though reference was made to the absence of scales on certain parts of the body). With the exceptions of the distinctive genera Gazza and Deveximentum, ponyfishes were assigned to a broad genus Leiognathus. Since then, variations in the structure of the light organ have been recognised as taxonomically significant, allowing the recognition of several genera divided between two subfamilies Leiognathinae and Gazzinae (Chakrabarty et al. 2011). Leiognathinae is defined by plesiomorphic characters and is likely to be paraphyletic to Gazzinae (Sparks & Chakrabarty 2015).

Because of the nocturnal habits of ponyfish and the delicacy of the light-emitting structures, our understanding of how light production functions in Leiognathidae remains somewhat limited. In Leiognathinae and females of Gazzinae, the light organ is relatively small and the external body surface lacks translucent patches. For the most part, light is expressed in these individuals as a uniform ventral glow that probably functions as counter-illumination (the light from the venter prevents the fish from appearing as a silhouette against light from the water surface to predators swimming below). Alternatively, light may be flashed to warn school-mates of danger. In males of Gazzinae, conversely, the light organ is enlarged relative to females and associated with translucent 'windows'. The shape of the organ and the arrangement of the 'windows' is a primary factor in distinguishing genera. Rhythmic flashing of light has been observed in males of many gazzine species and is probably characteristic of the group as a whole. Woodland et al. (2002) observed a school of several hundred Eubleekeria splendens flashing their lights synchronously shortly after nightfall. The exact function of such displays is uncertain, whether in courtship displays, co-ordinating school movements, attracting prey or dissuading predators. The sexually dimorphic nature of the light organ system, together with its species-specific expression, might seem to favour the first of these options but it should be noted that they are not all mutually exclusive.

Despite their small size, ponyfishes are often significant food fish for people living in areas where they are found. Thanks to their schooling behaviour, they are often a major component of dredge catches. In the Philippines, they are used for making bagoong, a fermented fish paste. In other places, they may be cooked whole after cleaning. The glow, sadly, does not survive the process.

REFERENCES

Chakrabarty, P., M. P. Davis, W. L. Smith, R. Berquist, K. M. Gledhill, L. R. Frank & J. S. Sparks. 2011. Evolution of the light organ system in ponyfishes (Teleostei: Leiognathidae). Journal of Morphology 272: 704–721.

James, P. S. B. R. 1975. A systematic review of the fishes of the family Leiognathidae. J. Mar. Biol. Ass. India 17 (1): 138–172.

Sparks, J. S., & P. Chakrabarty. 2015. Description of a new genus of ponyfishes (Teleostei: Leiognathidae), with a review of the current generic-level composition of the family. Zootaxa 3947 (2): 181–190.

Woodland, D. J., A. S. Cabanban, V. M. Taylor & R. J. Taylor. 2002. A synchronized rhythmic flashing light display by schooling Leiognathus splendens (Leiognathidae: Perciformes). Marine and Freshwater Research 53: 159–162.

Steatocranus gibbiceps, the Rapid River Bumphead

The cichlid fishes of the Great Lakes of Africa are rightly renowned as one of the world's most spectacular species radiations. Hundreds of species, occupying a wide range of ecological niches, have evolved in what is, geologically speaking, a short period of time. However, cichlids in Africa are not a phenomenon of the Great Lakes alone and many interesting species may be found in other parts of the continent, some of them belonging to local radiations of their own. Consider, for instance, the Congo River rapids endemic Steatocranus gibbiceps.

Male Steatocranus gibbiceps, copyright Polypterus.

The Congo is one of the largest African rivers with a drainage basin covering one-eighth of the continent (Schwarzer et al. 2011). Downstream of Kinshasa, the river gets funneled into an intermittently deep, narrow channel for a distance of some 300 km before broadening as it approaches the sea. The result is the world's longest stretch of river rapids. Many fish species are found only in this unique region of fast-flowing waters, among them multiple species of the cichlid genus Steatocranus including S. gibbiceps. The genus as a whole is restricted to the Congo basin; a single species previously recognised from the Volta River has since been transferred to its own genus (Weiss et al. 2019). The names Steatocranus and gibbiceps both basically mean the same thing: 'fat head', in reference to a fleshy swelling atop the fish's noggin. The exact size of this swelling varies between individuals, being most prominent in large males. Vernacular names given to Steatocranus species generally reflect this feature, such as bumphead cichlid or buffalo-head cichlid. Half a dozen species have been named within Steatocranus with several more being recognised but not yet formally described, most of them belonging to the radiation within the rapids. Schwarzer et al. (2012) found evidence for extensive historical cross-breeding between species and suggested that hybridisation may have been a significant factor in the genus' diversification.

Steatocranus gibbiceps is the largest species in this genus of moderately-sized fishes, growing up to about nine centimetres in length (Roberts & Stewart 1976). Its fast-current habitat is reflected in its slender body form. It is olive brown in coloration with the scales being light in colour at the centre and darker around the margins. Steatocranus gibbiceps is most clearly distinguished from other described species in its genus by its teeth: the front teeth of both the upper and lower jaws are conspicuously large and truncate. It also has a shorter gut than its congeners. This species appears to be specialised in feeding on freshwater snails which it scoops up and swallows whole, though it will take a broader range of food in captivity. Other species of Steatocranus mostly feed on algae.

Steatocranus species are not buoyant and tend to sit at the bottom of the water (Chase Klinesteker describes their behaviour as 'hopping around the bottom like a goby'). They escape the current by spending time in the hollows and crevices among rocks. Breeding happens within such hollows with dedicated pairs forming and females affixing their eggs to the rocks. Like many other cichlids, Steatocranus gibbiceps are dedicated parents after the eggs hatch. Tending of the fry is mostly the responsibility of the female while the male patrols the territory on the watch for danger. In this way, the baby bumpheads are given the best possible start at life.

REFERENCES

Roberts, T. R., & D. J. Stewart. 1976. An ecological and systematic survey of fishes in the rapids of the lower Zaïre or Congo River. Bulletin of the Museum of Comparative Zoology 147 (6): 239–317.

Schwarzer, J., B. Misof, S. N. Ifuta & U. K. Schliewen. 2011. Time and origin of cichlid colonization of the lower Congo rapids. PLoS One 6 (7): e22380.

Schwarzer, J., B. Misof & U. K. Schliewen. 2012. Speciation within genomic networks: a case study based on Steatocranus cichlids of the lower Congo rapids. Journal of Evolutionary Biology 25: 138–148.

Weiss, J. D., F. D. B. Schedel, A. I. Zamba, E. J. W. M. N. Vreven & U. K. Schliewen. 2019. Paragobiocichla, a new genus name for Gobiochromis irvinei Trewavas, 1943 (Teleostei, Cichlidae). Spixiana 42 (1): 133–139.

Of Hawks and Marble

The acanthomorph fishes (a major clade of fishes mostly characterised by the presence of spines at the front of the dorsal fin) have long been recognised as a particularly thorny problem for higher-level systematics. Morphological relationships between many of the large number of families recognised in this clade have been almost impossible to unravel, and it is only in recent years that molecular analyses have been able to start making sense of the rapid divergences. Nevertheless, there are some subgroups of the acanthomorphs that have been recognised for a long time and which recent analyses have continued to support. One such group is the cirrhitoids.

Spottedtail morwong Goniistius zonatus, copyright Joi Ito.

Variously referred to in recent sources as the Cirrhitoidea, the Cirrhitoidei, or the Cirrhitiformes, the cirrhitoids include about eighty known species usually divided between five families. These are the hawkfishes of the Cirrhitidae, the trumpeters and morwongs of the Latridae, the Cheilodactylus fingerfins, the Chironemus kelpfishes and the Aplodactylus marblefishes (the morwongs were historically placed with the fingerfins in the Cheilodactylidae but have recently been transferred based on molecular data—Ludt et al. 2019). The largest cirrhitoid is the dusky morwong Dactylophora nigricans of western and southern Australia, growing to 1.2 metres in length, but most species are only a fraction of this size. Some of the largest species are of note to fisheries. Cirrhitoids are generally inhabitants of reefs, mostly feeding on benthic invertebrates such as crustaceans. They have long been recognised as a coherent group owing to their distinctive fin structure. The lower rays of the pectoral fins are not branched, and in a number of species they are thickened and protrude past the fin membrane (observant readers of this post may have already noticed a theme in many of the genus names given to cirrhitoids, relating to this feature). The pelvic fins are set well behind the pectoral fins. Other notable features of the clade include a relatively high number of vertebrae, a relatively low number of rays in the caudal fin, and the presence in juveniles of a fatty sac running along the fish's underside (Greenwood 1995).

Coral hawkfish Cirrhitichthys oxycephalus, copyright Aquaimages.

Both morphological and molecular studies have agreed that the hawkfishes of the Cirrhitidae represent the sister clade to the remaining cirrhitoids. Hawkfishes are brightly coloured inhabitants of the tropics, usually well under a foot in length. They are distinguished by bundles of trailing filaments emerging from the ends of the spines on the dorsal fin. Perhaps the most familiar member of the group is the longnose hawkfish Oxycirrhites typus, a regular in marine aquaria. However, this is also perhaps the most atypical member of the family as other species do not have the elongate snout. Hawkfishes commonly perch atop corals on the uppermost part of the reef, protected by the coral's sting and able to maintain a clear view of their surrounds. Wikipedia suggests that this behaviour is the inspiration for the name of 'hawkfish', but I'm not sure I buy this. I mean, it sounds plausible, but it also sounds like the sort of thing you would have to be diving below the reef to see. Vernacular names for fish tend to more often refer to things you might observe while hauling them onto a boat.

Marblefish Aplodactylus arctidens, copyright Peter Southwood.

The remaining cirrhitoids are all found in cooler waters, mostly in the Southern Hemisphere. Two species of Latridae, the redlip morwong Goniistius zebra and the spottedtail morwing G. zonatus, are found in the northern Pacific off the coast of eastern Asia (the kind of distribution shown by the genus Goniistius, where species are found in northern and southern temperate waters but not in the intervening tropics, is known as 'anti-tropical' and it's an interesting question how such a distribution would come to be). They are mostly found among rocky reefs, with the kelpfishes Chironemus and marblefishes Aplodactylus being particularly associated with patches of seaweed. The marblefishes feed on algae (particularly reds) as well as on some invertebrates and are characterised by a transverse mouth that is little or not protractible (Regan 1911). As noted above, the family Latridae has been inflated recently by the inclusion of most of the species previously included in the Cheilodactylidae. Cheilodactylus itself is now restricted to two species found around southern Africa. They differ from the remaining species in the latrids by the absence of a gas bladder as well as by elements of the skeleton. Many of the latrids are favourites of anglers, being well regarded as eating fish. By contrast, the herbivorous marblefishes are maligned as very poor fare and avoided. There's something to be said for eating your greens.

REFERENCES

Greenwood, P. H. 1995. A revised familial classification for certain cirrhitoid genera (Teleostei, Percoidei Cirrhitoidea), with comments on the group's monophyly and taxonomic ranking. Bulletin of the Natural History Museum of London (Zoology) 61 (1): 1–10.

Ludt, W. B., C. P. Burridge & P. Chakrabarty. 2019. A taxonomic revision of Cheilodactylidae and Latridae (Centrarchiformes: Cirrhitoidei) using morphological and genomic characters. Zootaxa 4585 (1): 121–141.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5^th ed. Wiley.

Regan, C. T. 1911. On the cirrhitiform percoids. Journal of Natural History, series 8, 7: 259–262.

Tuskfish

The reefs of the Indo-west Pacific Oceans are one of the most species-rich regions of the entire marine environment. A complex geological history and high geographical complexity have contributed to drive speciation, resulting in a number of local radiations. One such radiation is the tuskfishes of the genus Choerodon.

Orange-dotted tuskfish Choerodon anchorago, copyright Bernard Dupont.

Choerodon is a genus of the wrasse family Labridae, most diverse around the islands of south-east Asia and northern Australasia where they inhabit coastal reefs or sea-grass beds. A revision of the genus by Gomon (2017) recognised 27 species, varying in size from a little over ten centimetres in length to half a metre or more. LIke other members of the wrasse family, they are often brightly coloured, with juveniles in particular of a number of species being patterned with bold vertical stripes. The vernacular name of 'tuskfish', as well as the zoological name of the genus (which translates as 'pig-tooth'), refers to the possesion of a pair of prominent, protruding incisors at the front of each of the upper and lower jaws. Other characteristic features of Choerodon include a dorsal fin with twelve spiny rays and eight soft rays, or thirteen spines and seven soft rays, and a lack of scales on the lower part of the cheek and lower jaw. Choerodon species, like most other wrasses, are protogynous hermaphrodites, starting their lives as females before eventually transforming into males.

Baldchin groper Choerodon rubescens, copyright Katherine Cure.

Diet-wise, tuskfishes are predators, feeding on animals such as crustaceans or mollusks. Larger species may even take other vertebrates. A kind of tool use has been observed for the genus, with difficult prey such as clams (Jones et al. 2011) or young turtles (Harborne & Tholan 2016) being grasped in the mouth and hammered against rocks to subdue them and/or break open shells. Multiple species of tuskfish may be found in close proximity though they will often differ in their preferred habitat. A study of five Choerodon species found around Shark Bay in Western Australia by Fairclough et al. (2008) found that the baldchin groper C. rubescens was found only on exposed marine reefs whereas the other four species preferred more sheltered habitats further inside the bay. The blue tuskfish C. cyanodus and blackspot tuskfish C. schoenleinii were both found in a range of habitats in this region but C. cyanodus was most abundant along rocky shores whereas C. schoenleinii preferred coral reefs (C. schoenleinii also differed from other species in the region in constructing burrows at the base of reefs that it used as a retreat). The purple tuskfish C. cephalotes was almost exclusively found among seagrass meadows. Finally, the bluespotted tuskfish C. cauteroma spent the early part of its life among seagrasses but moved onto reefs as it matured to adulthood.

Tuskfish and other wrasses are highly prized as eating fishes. However, it would be remiss to refer to the reefs of the Indo-west Pacific without mentioning that many of them are highly endangered. Heavy fishing, often using destructive methods, have combined with the effects of changing climate to cause a dramatic reduction in reef cover in recent decades. Should the decline continue at current rates, the lives of millions of people stand to be dangerously impacted.

REFERENCES

Fairclough, D. V., K. R. Clarke, F. J. Valesini & I. C. Potter. 2008. Habitat partitioning by five congeneric and abundant Choerodon species (Labridae) in a large subtropical marine embayment. Estuarine, Coastal and Shelf Science 77: 446–456.

Gomon, M. F. 2017. A review of the tuskfishes, genus Choerodon (Labridae, Perciformes), with descriptions of three new species. Memoirs of Museum Victoria 76: 1–111.

Harborne, A. R., & B. A. Tholan. 2016. Tool use by Choerodon cyanodus when handling vertebrate prey. Coral Reefs 35: 1069.

Jones, A. M., C. Brown & S. Gardner. 2011. Tool use in the tuskfish Choerodon schoenleinii? Coral Reefs 30 (3): 865.

Strike up the Bandfish

The diversity of fishes can be absolutely overwhelming and, as a result, there a some distinctive groups that fail to get their time in the spotlight. For this post, I'm briefly highlighting one of the lesser-known fish families, the bandfishes of the Cepolidae.

Australian bandfish Cepola australis at home in its burrow, copyright Rudie H. Kuiter.

Cepolids are small fish (growing to about 40 cm at most with many species much smaller) that are widespread in the eastern Atlantic and the Indo-Pacific but nowhere common. They have a laterally compressed, tapering body and a lanceolate caudal (tail) fin. They have an angled mouth that is relatively large compared to their size and pelvic fins with a single spine and five segmented rays, four of which are branched (Smith-Vaniz 2001). Two subfamilies are recognised, the Cepolinae and Owstoniinae. The Cepolinae are particularly elongate in body form and have the dorsal and anal fins connected by membranes to the caudal fin; these three fins are all distinctly separate in the Owstoniinae. Cepolines are divided between two genera: Acanthocepola species have scaly cheeks and spines on the preopercular margin whereas Cepola have naked cheeks and no such spines. Classification of Owstoniinae has been a bit less settled. A recent revision of the subfamily recognised only a single genus Owstonia (Smith-Vaniz & Johnson 2016), synonymising the genus Sphenanthias previously distinguished by features of the lateral line. As an indication of how little-known cepolids are, Smith-Vaniz & Johnson's revision more than doubled the number of known species of owstoniine from fifteen to 36 .

Male Owstonia hawaiiensis, from Smith-Vaniz & Johnson (2016).

Cepolids are most commonly found in relatively deep water, up to about 475 m. They are not targeted by any significant fisheries though Wikipedia claims that the oldest known recipe from a named author is for the cooking of bandfish. Cepolinae live on sandy or muddy bottoms on continental shelves where they excavate burrows in which they insert themselves with the head protruding above the substrate. Owstonia species are free-swimming, more commonly found near rocky bottoms on upper slopes or around atolls. The diet, where known, appears to be composed of zooplankton though Smith-Vaniz & Johnson (2016) suggested on the basis of tooth morphology that Owstonia were detritivores for at least part of their life cycle.

REFERENCES

Smith-Vaniz, W. F. 2001. Cepolidae. Bandfishes. In: Carpenter, K. E., & V. H. Niem (eds) FAO Species Identification Guide for Fishery Purposes. The Living Marine Resources of the Western Central Pacific vol. 5. Bony fishes part 3 (Menidae to Pomacentridae) pp. 3331–3332. Food and Agriculture Organization of the United Nations: Rome.

Smith-Vaniz, W. F., & G. D. Johnson. 2016. Hidden diversity in deep-water bandfishes: review of Owstonia with descriptions of twenty-one new species (Teleostei: Cepolidae: Owstoniinae). Zootaxa 4187 (1): 1–103.