Anal and Oral Intake of Water by Crustacea

Abstract

In the majority of small and transparent Crustacea water can be seen to be pumped into the alimentary canal through the anus by rhythmic antiperistaltic swallowing movements of the rectum. This anal drinking is continuous in small species and in the young of larger species, but occurs in intermittent bursts in the adults of larger species. In prawns the intermittent anal intake of water acts as an enema, for it occurs only at the time of defaecation, which is preceded by one or two dozen rapid rectal gulps of water. The continuous anal intake of water by smaller Crustacea acts likewise as an enema, being continuous because of the more frequent defaecations, due to the higher metabolism and therefore greater food requirements of small animals. The water acts as an enema as in man, stretching the gut-wall muscles until they contract. In prawns the rectal swallowing of water initiates and maintains intestinal antiperistalsis, which moves the swallowed water forwards in the intestine towards the thorax. A further function of the anal intake of water is thus to stretch the gut-wall muscles until they contract antiperistaltically. This is comparable with the initiation and maintenance of the heart beat in molluscs by hydrostatic pressure. In the past it has been thought that the rectal swallowing of water by Cladocera is respiratory. This opinion was apparently strengthened by experiments showing that a deficiency of dissolved oxygen increases the rate of rectal swallowing movements. These experiments have not been confirmed, and other reasons are given which make a respiratory function unlikely. Two further possible functions of the intake of water through the anus, namely the collection of salts necessary for osmoregulation, and a hydrostatic maintenance of body turgor, are discussed, tested and rejected. Water can be seen to be swallowed more or less continuously through the mouth, by rhythmic peristaltic movements of the gullet, in the majority of small and transparent Crustacea. A function of this oral drinking appears to be the same as that of anal drinking, namely to stretch the muscles of the gut wall. In Daphnia the antiperistaltic contractions of the midgut wall, which mix food and digestive enzymes, seem to be maintained by the hydrostatic pressure of water pumped into the gut by both gullet and rectum, defaecation occurring when this pressure rises to a certain value. A second function of the rhythmic oral drinking by Daphnia and perhaps other Crustacea is to force the food in the midgut back towards the rectum. The gullet and rectum of Crustacea have dilator muscles inserted into the exoskeleton. These muscles suck in water through mouth and anus, and by stretching the circular muscles they enable the latter to pump the water into the gut. Much more water is taken into the gut of Crustacea than makes its exit at defaecation. Evidence is given that this water passes through the gut wall into the blood and out of the body by way of the excretory organs.