Physiology of Finfish and Shellfish (2+1)

5.2.24. Other Invertebrate Hormones or Molluscan Hormones

Hormone production is not well documented in mollusks other than gastropods and cephalopods. Antagonistic neurohormonal control of reproductive activity and metabolic processes is performed in the gastropods through cerebral dorsal bodies and lateral lobes or juxtaposed organs. There is a pair of salivary glands line the crop or esophagus, a large digestive gland called the liver empties into the stomach and a gland secreting albumin.

Experimental studies indicate an endocrine relationship in gastropods between the gonad (ovotestis) and possible neurosecretory cells in the tentacles and the brain; one ganglion of the gastropod Lymnaea may secrete a neurohormone with a diuretic (urine producing) action.

Epithelial glands in mollusks are important; in the cephalopods, which are the most advanced invertebrates in some respects, optic glands on the optic stalks (eyestalks) secrete a hormone that promotes development and maturation of the gonads. In immature cephalopods, the activity of the glands is inhibited by the central nervous system, apparently by a chemical mediator that diffuses from nerve fibres. The anterior salivary gland secrets the clear liquid (of water, mucin, protein and enzymes) into the mouth. It moistens food and starts the breakdown of starches. The posterior salivary gland (poison gland) produces venom to paralyze prey. The oviductal gland surrounding the end of the primary oviduct is responsible for secreting some of the external coatings over spawned eggs. In octopuses, it also acts as a spermatheca. In some cephalopods, the hormones also affect death by starvation after the mollusk has deposited its eggs or has mated.

The nerve net, which constitutes the very primitive nervous system of the coelenterates, probably the most primitive multicellular animals, apparently contains neurosecretory cells; indirect but convincing evidence suggests that the cells release a secretion that promotes growth and inhibits sexual reproduction.

Neurosecretions by cells outside the nerve cell bodies (ganglia) have been described in gastropods and cephalopods, the released hormones diffusing through the tissues rather than being concentrated in special organs.

Unlike insects, crustaceans have an androgenic gland, which typically is located on the genital duct (vas deferens) of the male. The androgenic gland secretes a hormone, possibly steroid in nature, which controls both the differentiation of the gonad of the male into a testis and the male characteristics of its limbs. The absence of the androgenic gland in the female results in the formation of an ovary, which subsequently synthesizes one or more hormones that, in female amphipods, promote the development of brood chambers (in which the young are hatched) and other structures associated with reproduction.