Endocrine systems of invertebrates

The endocrine systems of many invertebrates are nearly as complicated as vertebrate endocrine systems. The principal source of hormones in most invertebrate phyla is neurosecretory cells. These hormonal mechanisms have been found in arthropods, annelid worms, mollusks, and echinoderms. The physiological processes that are affected are generally fundamental, long-term ones that include such biological phenomena as growth, regeneration, reproduction and development, and certain metabolic processes.

The subkingdom of animals made up of all vertebrates and most invertebrates (protozoa and sponges are not included) is called the Metazoa, which is defined by the presence of nervous and endocrine systems in the animals in the group. These systems coordinate the activities of the animal so it can function as a whole. The nervous system is important in rapid communication, such as contraction of muscles for movement, while the endocrine system controls long-term processes within the body, such as the growth of organs or maintenance of appropriate metabolic concentrations. Chemical messengers released by the endocrine systems have to travel to specific target organs to exert their effects. The means of travel is the circulatory system. Because it takes time for the chemicals to accumulate to effective concentrations, they must be stable enough to remain in the body without undergoing chemical changes and without being excreted. These chemical messengers—hormones—are, then, well suited to working over long periods. However, the nervous and endocrine systems do not work independently, and most animals’ central nervous systems are often strongly affected by hormones.

Neurosecretory Cells

In 1928, German chemist Ernst Scharrer hypothesized that certain nerve cells have qualities of both nerve and endocrine gland cells. These neurosecretory cells are neurons which are cellularly like gland cells but are widespread within the invertebrate body. They receive nervous impulses, but rather than communicating through synapses with other neurons or effector cells, they terminate close to the circulatory system and release substances which travel to act on organs or upon endocrine glands. These neurosecretory substances, therefore, are themselves hormones. Neurosecretory cells are usually found in clusters within the central nervous system. Extending from the cell bodies are axons that terminate in swollen knobs associated with blood spaces. Terminals that are aggregated into a body are called neurohemal organs. Neurosecretory material is produced by the cell bodies, transported down the axons, and stored in the swollen knobs. Release is accomplished by exocytosis.

To be classified as a neurosecretory cell, three criteria must be met: The cell must have the structural features of a neuron (cell body and axonlike fibers); the axons must not synapse with other cells but end in close association with an area of body fluid (generally a blood vessel or sinus, the combination making a neurohemal organ); the neuron must contain membrane-bound vesicles within the cytoplasm. There are, in addition, two physiological criteria: Destruction of these clustered cells, or the areas or organs where they are found, produces an alteration of existing internal conditions within the organism that can be restored by replacing the removed organ or injecting an extract from it; implantation of an organ thought to be neurosecretory into a normally functioning animal brings about a change in internal state by either prompting or inhibiting the occurrence of certain events.

Neurosecretions in invertebrates may influence behavior or target another endocrine gland by trophic neurosecretions or trophic hormones. For example, in many insects, neurosecretions from neurosecretory cells in the brain exert a trophic effect on the prothoracic glands, which then produce and release the hormone ecdysone that controls molting, the developmental sequence of insects. There are other examples of how hormonal release is dependent on and dictated by the nervous system. Most animals respond developmentally to environmental changes, such as seasonal variations throughout the year. If unfavorable conditions develop, the animal may compensate by going into dormancy or migrating or may overcome the conditions by other changes in habit or physiology. Even brief fluctuations, such as a temporary shortage of food or the absence of suitable mates, may dramatically affect development. The mating act in a female insect may speed up the development of her eggs; the changing day length may control when metamorphosis begins in annelids. Stimulation of sense organs sets up nervous messages that result in changes in the amounts of circulating hormones, which generate these “new” responses.

The Roles of Invertebrate Hormones

Invertebrate hormones play as many roles as there are invertebrate phyla. In the less highly organized invertebrates, endocrine glands are apparently absent; hormonal coordination depends on neurosecretions. Hormones released in the plant hydra are believed to come from the hypostome region (the nerve ring around the oral opening) and from actively growing areas and are thought to regulate growth, regeneration, and the development of sexuality. Little is known about substances termed “wound hormones” in planarians, but their presence in wounded tissues has been inferred, even though their site of production is unclear.

All annelids possess neurosecretory cells in the brain that control growth, reproduction, and maturation. In nereids, reproductive body forms releasing eggs and sperm are controlled by at least one brain neurohormone, and normal reproductive development appears to depend on the gradual withdrawal of brain neurohormones with increasing age. Regeneration, too, is probably controlled by neurohormones.

In starfish and sea urchins, spawning of eggs is preceded by release of a “shedding hormone” found only in the radial nerves. This hormone, known as gonad-stimulating substance, also stimulates the manufacture and secretion of a second substance by the gonads called meiosis-inducing substance (MIS). MIS causes the follicle cells to pull away from the gametes so the gametes can be expelled more easily; it induces meiosis within the oocytes, and, after diffusing into the coelomic fluid, stimulates muscle contractions which cause spawning.

Many mollusks have neurons resembling neurosecretory cells that change their apparent secretory activity with conditions such as reproductive state. In a few cases, evidence has been found for neurosecretory control of reproduction, water balance, or heart function. Cephalopods such as squid, however, are among the animals with endocrine glands. The cephalopod brain is connected to the optic lobes by short optic stalks bearing optic glands. As the animal matures, the size of these glands increases. These glands function in the control of reproductive development. Glands on the gills, called mesodermal branchial glands, are also endocrine organs and are thought to function similarly to vertebrate adrenal glands.

The best-studied invertebrate system is that of insects. Insects possess discrete clusters of neurosecretory cells, well-developed neurohemal organs, and even nonneural endocrine glands. The insect endocrine system has four major components: the corpora cardiaca, a group of neurosecretory cells in the brain, the corpora allata, and the thoracic glands. The corpora cardiaca, closely associated with the heart, store and secrete hormones from the brain as well as produce their own inherent hormones. Along with the brain’s neurosecretory cells, they compose the cerebral neurosecretory system. Molting is controlled by hormones called ecdysteroids produced under the brain’s direction. Secretion of these ecdysteroids stimulates the release of ecdysone from the prothoracic gland. Ecdysone, also called the molting hormone, stimulates the development of adult structures but is held in check by juvenile hormone (JH), which favors the development of juvenile characteristics. During juvenile life, JH predominates, and each molt yields a larger juvenile. High levels of JH are released by the corpus allatum during early stages of life. Its major function, then, is to ensure that when molting is triggered by ecdysone secretion, the next larval stage results. When the final stage is reached, ecdysone production dramatically falls, but sufficient levels are produced to induce a molt that will result in the adult stage. Similar systems are found in the crustaceans.

Because invertebrates make up about 95 percent of the species in the animal kingdom, one might anticipate a great diversity of invertebrate endocrine mechanisms. Eventually, this expectation may be confirmed; but knowledge of endocrine systems in many invertebrate groups is, for the most part, incomplete. What is known is that in most groups of invertebrates, neurosecretory systems are distinctly more prominent than nonneural endocrine glands, which occur in very few cases.

Refinement of Study Techniques

Until the 1960s, the search for hormonal regulators in invertebrates was largely unsuccessful because early experiments on gonad transplantation from insects of one sex to those of the other and injection of vertebrate hormones into invertebrates yielded negative results. Strides made in the last twenty-five years are mostly the results of refinements of microscopic, operative, and analytical techniques, both chemical and physical. Arthropods have provided the most accessible material for study, and more is known about the phenomenon in crustaceans and insects than in any other group. The range of investigation is expanding, however, and neurosecretion in invertebrates is not only accepted but recognized as widespread among them.

Many problems are generally associated with determining the functions of the neurosecretory system. The classical experimental method involves removal of the suspected endocrine gland and then reimplanting it at another location in the body. If the effects of removal are reversed and normal conditions return when the organ is relocated, then a hormonal mechanism is probably involved. The problem arises, however, because removing a neurohemal organ will leave behind the cut ends that may continue to release hormones, perhaps in an uncontrolled manner. A new neurohemal organ may be rapidly regenerated so that the effects of lessened amounts of neurosecretory hormones cannot be observed. In addition, reimplanting the organ may produce several hormones in the animal in abnormal concentrations or proportions. Hormonal deficiency may not be obvious immediately because hormones stored outside the neurohemal organ may be secreted or leached out for some time after the organ’s removal. Yet another problem is the lack of distinct neurohemal organs; instead, scattered neurosecretory cells may be found throughout the nervous system. It is, therefore, difficult to determine the exact function of the mechanisms because of the virtual impossibility of removing and testing these individual cells. In these cases, the neurosecretory nature of the cells is deduced based on their structural and chemical similarity to those whose function has already been verified in other animals.

Typical of the early work on insect growth hormones was the work done in the 1930s in England by Vincent B. Wigglesworth on the metamorphosis of a bloodsucking insect named Rhodnius. This insect goes through five immature stages, each separated by a molt, until it reaches adulthood. During each of these stages, it engorges and stretches its abdomen by ingesting a blood meal. This filling meal apparently stimulates the release of hormones that cause molting at the end of a specified time interval following the meal. Usually, the final molt (to adulthood) occurs about twenty-eight days after the blood meal. If the insect is decapitated during the first few days after its meal, molting does not occur, even though the animal may live for several months longer. Decapitation more than eight days after the blood meal does not interfere with molting, although a headless adult is produced. Wigglesworth further showed that joining the circulatory system of a later-decapitated insect to that of an earlier-decapitated insect allows both to molt into adults. It appears obvious that some stimulus passes via the blood from one insect to the other and induces molting; it is assumed that the stimulus is a hormone that is secreted about eight days after the blood meal.

Since Wigglesworth’s time, studies typical of his work have shown evidence of hormonal activity and control of many other invertebrate phyla. Experimentation with insects still outweighs all other studies, however, since they are so available and easy to work with because of their size.

Comparing Vertebrate and Invertebrate Hormones

The study of invertebrate hormones began as an attempt to draw parallels between invertebrate hormones and known vertebrate hormones. Experimenters were virtually forced to look for these similarities because of legal restrictions placed on the use of vertebrates for experimental study. The end results, however, have shown that invertebrate hormones share little with vertebrate hormones.

One of the few similar hormones, in structure at least, is prothoracicotropic hormone (PTTH), isolated from the heads of adult silkworms. (PTTH stimulates the prothoracic gland to release ecdysone, which then regulates molting and growth.) Though structurally similar to vertebrate insulin, insect insulin has no functional link with vertebrate insulin.

The interaction of neurohormones and the nervous system has been studied using lobster, tying the release of neurohormones to its behavior. By introducing neurohormones via injection, one can induce behavioral changes, such as increased aggression. By working with these crustaceans, the apparent relationship among neurohormones, the nervous system, and behavior modification may be used in observing and controlling animal behavior.

Endocrine disruption

Chemicals called endocrine disruptors (EDs) have become a focus of scientific study in the twenty-first century and a growing concern for the public. Evidence of the negative impact of EDs on the animal kingdom is growing, but the known impacts among invertebrates include reproductive and sexual function abnormalities and the increased occurrence of tumors. Invertebrates, like mollusks, echinoderms, and arthropods, are often used in laboratories to test toxic chemicals and study the endocrine system. As researchers better understand the repercussions of EDs on invertebrates, they may be better able to prevent or remedy the impacts on vertebrates.

The United States Environmental Protection Agency (EPA) lists several EDs linked to negative animal outcomes, including organochlorine compounds, ethane (DDT) and its metabolite dichorodiphenyldichloroethylene (DDE), and polychlorinated biphenyls (PCBs). Among invertebrates, antidepressants, Tributyltin, and Chlordecone, which is called Kepone in the United States, pose the most potential for harm.

Principal Terms

Budding: A form of asexual reproduction that begins as an outpocketing of the parental body, resulting in either separation from or continued connection with the parent, forming a colony

Diapause: A resting phase in which metabolic activity is low and adverse conditions can be tolerated

Molt: The process of replacing one exoskeleton with another

Neurons: Cells specialized for the conduction of electrical signals and the transmission of information (nerve cells)

Neurosecretory Cells: Specialized neurons capable of manufacturing and releasing hormones (neurosecretions or neurosecretory hormones) and discharging them directly into circulation

Photoperiod: The measure of the relative length of daylight as it relates to the potential physiological responses that exposure to daylight evokes

Target Organ: A specific body part that a particular hormone directly affects

Trophic Hormone: A hormone that stimulates another endocrine gland

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