Endocrine influences in annelids

Abstract

The only certain source of hormones in annelids is the central nervous system, most importantly the supraesophageal ganglion. Neurosecretory cells are common and widely distributed in the nervous system and presumably secrete hormones. As in other nonarthropodan invertebrates, histological and cytological investigations of annelid neurosecretory systems have outpaced experimental endocrinological studies, and since the interpretation of the histological demonstration of neurosecretory material is uncertain, it is important to distinguish between what has been established by controlled experiment and what is only suspected. Annelid hormones are concerned with the control of sexual maturation and reproduction, normal and regenerative growth and, in a few worms, with color change. So far as the scattered information permits generalization, annelid control systems appear to vary widely. In nereid polychaetes, sexual maturation is inhibited by a juvenile hormone secreted by the supraesophageal ganglion. As the hormone level falls in response to an environmental stimulus, maturation processes begin, and they are completed in the total absence of the hormone. A similar control system probably exists in the closely related Nephtyidae. In the polychaete Arenicola, the oligochaete Eisenia, and probably in leeches, on the other hand, a cerebral hormone acts as a stimulant of sexual maturation. Whether or not a gonadotropic hormone is additional to a juvenile hormone is uncertain, but there is no good evidence that a juvenile hormone exists in these worms. In nereid polychaetes, both normal and regenerative growth require the presence of a hormone secreted by the supraesophageal ganglion. Possibly the growth hormone is identical with the juvenile hormone, since growth and regeneration fail when the juvenile hormone ceases to be secreted. In lumbricid oligochaetes and many polychaetes, regeneration proceeds in the absence of the anterior nervous system so that if growth is controlled by hormones they are not cerebral in origin. Whatever the control system, growth is generally incompatible with reproduction in annelids (syllid polychaetes are exceptional in that a high somatic growth rate is an essential concomitant of sexual reproduction), and there is some evidence that the prime effects of annelid hormones are on the mitotic cycle and on metabolic pathways, switching the animal into either a growth regimen or a gametogenic regimen, so avoiding competition between the activities with a high energy demand in animals lacking nutritional reserves.