Invertebrate Endocrine Systems
Advances in the study of invertebrate endocrine systems have lagged behind those in vertebrate
endocrinology, largely due to the problems associated with adapting investigative techniques that
are appropriate for large vertebrate animals to small invertebrates. It also is difficult to maintain and
study appropriately some invertebrates under laboratory conditions. Nevertheless, knowledge about
these systems is accumulating rapidly.
All phyla in the animal kingdom that have a nervous system also possess neurosecretory neurons.
The results of studies on the distribution of neurosecretory neurons and ordinary epithelial
endocrine cells imply that the neurohormones were the first hormonal regulators in animals.
Neurohemal organs appear first in the more advanced invertebrates (such as mollusks and annelid
worms), and endocrine epithelial glands occur only in the most advanced phyla (primarily
Arthropoda and Chordata). Similarly, the peptide and steroid hormones found in vertebrates are
also present in the nervous and endocrine systems of many invertebrate phyla. These hormones
may perform similar functions in diverse animal groups. With more emphasis being placed on
research in invertebrate systems, new neuropeptides are being discovered initially in these animals,
and subsequently in vertebrates.
The endocrine systems of some animal phyla have been studied in detail, but the endocrine systems
of only a few species are well known. The following discussion summarizes the endocrine systems of
five invertebrate phyla and the two invertebrate subphyla of the phylum Chordata, a phylum that
also includes Vertebrata, a subphylum to which the backboned animals belong.
Phylum Nemertea
Nemertine worms are primitive marine animals that lack a coelom (body cavity) but differ from
other acoelomates (animals that lack a coelom) by having a complete digestive tract. Three
neurosecretory centres have been identified in the simple nemertine brain; one centre controls the
maturation of the gonads, and all three appear to be involved in osmotic regulation.
Phylum Annelida
The cerebral ganglion (brain) of Nereis, a marine polychaete worm, produces a small peptide
hormone called nereidine, which apparently inhibits precocious sexual development. There is a
complex just beneath the brain that functions as a neurohemal organ. The epithelial cells found in
this complex may be secretory as well, but this has not been proved. Neurohormones are released
from the infracerebral complex into the coelomic fluid through which they travel to their targets. In
the lugworm, Arenicola, there is evidence for a brain neuropeptide that stimulates oocyte
maturation.
Phylum Mollusca
Within the phylum Mollusca, the class Gastropoda (snails, slugs) has been studied most extensively.
The cerebral ganglion (brain) of several species (e.g., Euhadra peliomorpha, Aplysia californica, and
Lymnaea stagnalis) secretes a neurohormone that stimulates the hermaphroditic gonad (the
reproductive gland that contains both male and female characteristics); hermaphroditism is a
common condition among mollusks. This gonadotropic peptide hormone (a hormone that has the
gonads as its target organ) is stored in a typical neurohemal organ until its release is stimulated. For
example, phototropic information detected by the so-called optic gland (located near the eye) can
direct the release of the gonadotropic hormone. The gonadotropic hormones that cause egg laying
in Aplysia and Lymnaea have been isolated, and they are very similar small peptides. The
hermaphroditic gonad of Euhadra secretes testosterone (identical to the vertebrate testosterone),
which stimulates formation of a gland that releases a pheromone for influencing mating behaviour.
The optic gland of the octopus (of the class Cephalopoda) influences development of the
reproductive organs on a seasonal basis. It is not known, however, whether any neurohormones are
involved or whether this is purely a neurally controlled event.
Phylum Arthropoda
The arthropods are the largest and most advanced group of invertebrate animals, rivaling and often
exceeding the evolutionary success of the vertebrates. Indeed, the arthropods are the most
successful ecological competitors of humans. There are several major subdivisions, or classes, within
the phylum Arthropoda, with the largest being Insecta (insects), Crustacea (crustaceans, including
crabs, crayfishes, and shrimps), and Arachnida (arachnids, including the spiders, ticks, and mites).
Even within these major classes, few species have been studied. Those that have been studied are
large insects (e.g., cockroaches, grasshoppers, and cecropia moths) and crustaceans.
The organizations of arthropod endocrine systems parallel those of the vertebrate endocrine
system. That is, neurohormones are produced in the arthropod brain (analogous to the vertebrate
hypothalamus) and are stored in a neurohemal organ (like the vertebrate neurohypophysis). The
neurohemal organ of insects may have an endocrine portion (like the vertebrate adenohypophysis),
and hormones or neurohormones released from these organs may stimulate other endocrine glands
as well as nonendocrine targets. A general description of the endocrine systems of insects and
crustaceans is given below.
Class Insecta
Neurosecretory, neurohemal, and endocrine structures are all found in the insect endocrine system.
There are several neurosecretory centres in the brain, the largest being the pars intercerebralis. The
paired corpora cardiaca (singular, corpus cardiacum) and the paired corpora allata (singular, corpus
allatum) are both neurohemal organs that store brain neurohormones, but each has some endocrine
cells as well. The ventral nerve cord and associated ganglia also contain neurosecretory cells and
have their own neurohemal organs; i.e., the multiple perisympathetic organs located along the
ventral nerve cord. The insect endocrine system produces neurohormones as well as hormones that
control molting, diapause, reproduction, osmoregulation, metabolism, and muscle contraction.
Molting
A peptide neurohormone that controls molting is secreted by the pars intercerebralis and is stored
in the corpora cardiaca or corpora allata (depending on the group of insects). This brain
neurohormone is known as the prothoracotropic hormone (PTTH), and it stimulates the prothoracic
glands (also called ecdysial or molting glands). In turn, the prothoracic glands release the steroid
ecdysone, which is the actual molting hormone. Ecdysone initiates shedding of the old, hardened
cuticle (exoskeleton).
In the 1940s Sir Vincent (Brian) Wigglesworth discovered that distention of the abdomen of the
blood-sucking hemipteran bug Rhodnius prolixus following consumption of a blood meal sends
neural impulses to the brain and triggers the release of PTTH. A similar mechanism has been found
in a herbivorous (plant-eating) hemipteran as well. Size seems to trigger molting in lepidopterans
(moths, butterflies), although the mechanism is not understood. Each molt is aided by a small
amount of juvenile hormone (JH) secreted by endocrine cells of the corpora allata. Without JH
during a critical time of the intermolt period of the last larval stage, either a pupa stage (diapause, or
a resting state) or an adult stage is achieved. Juvenile hormone also keeps the epidermis in a larval
state and causes it to secrete larval cuticle. Without JH, the epidermis changes and secretes the
adult cuticle type. Three different closely related forms of JH have been isolated from seven major
insect orders.
Diapause
Some insects enter diapause during development. Diapause is characterized by cessation of
development or reproduction, decrease in water content (dehydration), and reduction in metabolic
activities. It usually is preceded by an accumulation of nutrients resulting in hypertrophy of the fat
bodies. Environmental factors (such as temperature, photoperiod, and food availability) cause
storage of neurohormones, and the corpora allata become inactive. Termination of diapause can be
brought about by reversing the environmental conditions that induced the diapause. Although
juvenile hormone can terminate diapause, it triggers diapause in some insects. The stage of the life
history may be important in determining the role of JH. For example, in imaginal diapause
(characterized by cessation of reproduction in the imago, or adult), the absence of JH initiates
diapause. In lepidopterans, a peptide that initiates diapause has been isolated from the
subesophageal ganglion.
Reproduction
In some insects the pars intercerebralis secretes a neurohormone that stimulates vitellogenesis by
the fat body (vitellogenesis is the synthesis of vitellogenin, a protein from which the oocyte makes
the egg proteins). This neurohormone is stored in either the corpora cardiaca or the corpora allata,
depending on the species. Uptake of vitellogenin by the ovary is enhanced by JH. In most insects, JH
also stimulates vitellogenin synthesis by the fat body. There is evidence that other neurohormones
secreted by the pars intercerebralis also influence reproduction. Some induce other tissues to
secrete pheromones that influence reproductive behaviour of other individuals. In the live-bearing
tsetse fly, Glossina, a neurohormone released from the corpora allata stimulates milk glands that
provide nourishment to the developing larvae.
Osmoregulation
All insects produce a diuretic hormone and many produce an antidiuretic hormone as well. Insects
feeding exclusively on a liquid diet (such as plant sap or blood) have only the diuretic hormone that
allows them to eliminate excess fluid and salts through the malpighian tubules (the insect kidney).
These osmoregulatory neurohormones are produced both in the brain and in the ventral nerve cord.
Myotropic and metabolic factors
One or more small peptide neurohormones are produced in the brain and ventral nervous system
and are stored in the corpora cardiaca and perisympathetic organs, respectively. These myotropic
factors stimulate heart rate as well as contractions of the kidney tubules and digestive tract. The
corpora cardiaca were named for the heart-stimulating action produced by extracts of these organs.
The glandular portion of the corpora cardiaca is thought to secrete the hyperglycemic hormone that
causes a rapid increase in blood levels of trehalose, the blood sugar of insects. It is sometimes
called the hypertrehalosemic hormone. This hypoglycemic hormone apparently is identical to the
myotropic factors in at least one species, the American cockroach. An adipokinetic neurohormone
released from the orthopteran corpora cardiaca (locusts, grasshoppers) causes the release of
diglycerides into the blood during, and immediately after, flight. It is a peptide similar to the
myotropic factors.
Class Crustacea
Among the crustaceans, the major neuroendocrine system consists of the neurosecretory X-organ
and its associated neurohemal organ, the sinus gland. Both an X-organ and a sinus gland are located
in each eyestalk, and together they are termed the eyestalk complex. Two endocrine glands are well
known: the Y-organ and the androgenic gland. As in insects, hormones and neurohormones of the
crustacean regulate molting, reproduction, osmoregulation, metabolism, and heart rate. In addition,
the regulation of colour changes is well developed in crustaceans, whereas only a few insects exhibit
hormonally controlled colour changes.
Molting
The steroid ecdysone secreted from the Y-organ stimulates molting. After it is released into the
blood, ecdysone is converted to a 20-hydroxyecdysone, which is the active molting hormone.
Secretion of ecdysone is blocked by a neurohormone called molt-inhibiting hormone, produced by
the eyestalk complex. The existence of several additional molting factors has been proposed from
experimental studies, and the regulation of molting may be much more complicated than suggested
here.
Reproduction
The eyestalk complex appears to produce a neurohormone that inhibits vitellogenesis by the fat
body and blocks vitellogenin uptake by oocytes in the ovary. Older follicles in the ovary, however,
may secrete a vitellogenin-stimulating hormone that overrides the effects of the eyestalk
neurohormone. In shrimps and other crustaceans that exhibit sequential hermaphroditism, the
androgenic gland produces a peptide hormone that is necessary to masculinize the gonad. These
animals function first as males, and later with the degeneration of the androgenic gland they
become females. Surgical removal of the androgenic gland causes a precocious change of a male to a
female.
Osmoregulation
There are four known sources of factors that influence water and ionic balance (osmoregulation) in
crustaceans. The brain factor is known to regulate function of the antennal glands (paired kidneys
located at the base of each antenna), the intestine, and the gills. The thoracic ganglion factor affects
the stomach, intestine, and gills. Both the antennal glands and the gills are affected by a factor from
the eyestalk complex. Finally, the pericardial organs (neurohemal glands located in the pericardial
cavity) influence salt and water metabolism by heart muscle and gills.
Myotropic factor
Heart rate is accelerated in crustaceans by a factor released from the pericardial organs. It is not
known if this factor is the same one that has osmoregulatory actions mentioned above. There is
evidence to suggest that the crustacean cardioacceleratory factor is identical to one of the insect
cardioacceleratory factors.
Colour changes
Several neurohormones that regulate colour changes (chromatophorotropins) by pigment cells
(chromatophores) have been found in extracts of the eyestalk complex. The best known are the
light-adapting hormone and the red-pigment-concentrating hormone. This latter peptide is
chemically similar to the insect adipokinetic and myotropic factors. Regulation of the
chromatophores allows an animal to adapt to different backgrounds by changing colours or by
becoming lighter or darker.
Phylum Echinodermata
Female sea stars (starfishes) are the only echinoderms that have been studied extensively. A
neuropeptide called the gonad-stimulating substance (also called the gamete-shedding substance) is
released from the radial nerves into the body cavity about one hour before spawning. Gonad-
stimulating substance has been reported in more than 30 species of sea star. This neuropeptide
contacts the ovaries directly and causes formation of 1-methyladenine, an inducer of oocyte
maturation and spawning. This same hormone has been demonstrated in the ovaries of the closely
related sea urchin, where it also promotes maturation of the oocyte.
Phylum Chordata
The phylum Chordata is separated into three subgroups (or subphyla). The invertebrate subphylum
Tunicata consists of the marine tunicates, including the ascidians and salps. The invertebrate
subphylum Cephalochordata includes the fishlike amphioxus (or lancelet). Amphioxus is a small
marine animal that closely resembles the larva of the jawless fishes (class Agnatha). The subphylum
Vertebrata is the largest chordate subgroup.
Subphylum Tunicata
The ascidians (also called sea squirts) have a tadpolelike larva that lives free for a short period. The
larva eventually attaches itself to a solid substrate and undergoes a marked metamorphosis into the
sessile adult sea squirt. The larva and adult have a mucus-secreting gland, the endostyle, that is
believed to be the evolutionary ancestor of the vertebrate thyroid gland. Metamorphosis in
ascidians can be induced by application of thyroid hormones.
Neurosecretory neurons in the cerebral ganglion (brain) contain the vertebrate peptide
gonadotropin-releasing hormone (GnRH). Directly adjacent to the brain is the neural (or subneural)
gland that may be the forerunner of the vertebrate pituitary gland. Extracts prepared from ascidian
neural glands stimulate testicular growth in toads, demonstrating the presence of a gonadotropic
factor in the neural gland. A protein similar to human prolactin has been found in the neural gland of
Styela plicata.
Subphylum Cephalochordata
The cephalochordate brain contains neurosecretory neurons that possibly are related to a structure
called Hatscheks pit, located near the brain. Hatscheks pit appears to be related to the neural gland
and hence to the vertebrate pituitary gland. Treatment of amphioxus with GnRH or luteinizing
hormone (LH) reportedly stimulates the onset of spermatogenesis in male gonads. Furthermore,
extracts prepared from Hatscheks pit can stimulate the testis of a toad. Amphioxus has a mucus-
secreting endostyle like that of the ascidians. and studies have shown that the cephalochordate
endostyle can synthesize thyroid hormones, too. Thus, the basic organization of the vertebrate
endocrine system appears to show its early beginnings in the simple organs of these invertebrate
chordates.
David O. Norris
Phylum Echinodermata
Bintang laut betina (starfishes) adalah satu-satunya echinodermata yang telah dipelajari secara
ekstensif. Neuropeptida disebut zat stimulasi gonad (juga disebut substansi penumpukan gamet)
dilepaskan dari saraf radial ke dalam rongga tubuh sekitar satu jam sebelum pemijahan. Zat stimulan
ganas telah dilaporkan di lebih dari 30 spesies bintang laut. Neuropeptida ini kontak dengan ovarium
secara langsung dan menyebabkan pembentukan 1-methyladenine, sebuah inducer pematangan
oosit dan pemijahan. Hormon yang sama ini telah ditunjukkan di ovarium bulu babi yang terkait erat,
di mana ia juga mempromosikan pematangan oosit.
Phylum Chordata
Filum Chordata dipisahkan menjadi tiga subkelompok (atau subphyla). Subfilium invertebrata
Tunicata terdiri dari tunicates laut, termasuk ascidia dan salps. Subfilium invertebrata
Cephalochordata mencakup amphioxus seperti ikan (atau lancelet). Amphioxus adalah hewan laut
kecil yang sangat menyerupai larva ikan tanpa rahang (kelas Agnatha). Subgrilum Vertebrata adalah
subkelompok chordate terbesar.
Subfilum Tunicata
Orang-orang ascidia (juga disebut penyiraman laut) memiliki seekor larva tadpolelike yang hidup
bebas dalam waktu singkat. Larva tersebut akhirnya menempel pada substrat padat dan mengalami
metamorfosis yang ditandai ke dalam semprotan laut dewasa sessile. Larva dan orang dewasa
memiliki kelenjar yang mengeluarkan lendir, endostyle, yang diyakini sebagai nenek moyang
evolusioner kelenjar tiroid vertebrata. Metamorfosis pada ascidia dapat diinduksi dengan penerapan
hormon tiroid.
Neurosecretory neurons di ganglion serebral (otak) mengandung hormon peptida gonadotropin-
releasing vertebrata (GnRH). Berbatasan langsung dengan otak adalah kelenjar syaraf (atau
subneural) yang mungkin merupakan cikal bakal kelenjar pituitary vertebrata. Ekstrak yang disiapkan
dari kelenjar tiroid ascidian merangsang pertumbuhan testis pada kodok, menunjukkan adanya
faktor gonadotropik pada kelenjar syaraf. Protein yang mirip dengan prolaktin manusia telah
ditemukan di kelenjar saraf Styela plicata.
Subfilum Cephalochordata
Otak cephalochordate mengandung neurosecretory neurons yang mungkin terkait dengan struktur
yang disebut pit Hatschek, terletak di dekat otak. Lubang Hatschek nampaknya berhubungan dengan
kelenjar syaraf dan karenanya kelenjar pituitary vertebrata. Pengobatan amphioxus dengan GnRH
atau luteinizing hormone (LH) dilaporkan menstimulasi timbulnya spermatogenesis pada gonad pria.
Selanjutnya, ekstrak yang disiapkan dari pit Hatschek dapat merangsang testis kodok. Amphioxus
memiliki endostyle yang mengeluarkan endapan lendir seperti pada ascidia. dan penelitian telah
menunjukkan bahwa endostyle cephalochordate dapat mensintesis hormon tiroid juga. Dengan
demikian, organisasi dasar sistem endokrin vertebrata tampaknya menunjukkan permulaannya yang
awal dalam organ sederhana chordat invertebrata ini.
David O. Norris
