The diurnal rhythm and the cycle of feeding and digestion in the slug Deroceras caruanae

Abstract

Deroceras caruanae (Pollonera 1891) possesses an endogenous cycle of activity and inactivity entrained by the cycle of night and day. D. caruanae remains inactive on the undersurface of experimental feeding panels during daylight hours and as dusk approaches undertakes a phase of crawling bringing it into contact with food on the upper surface of the feeding panel at night. As dawn approaches, the slugs retire to the undersurface of the panel. In total darkness, the slugs lose their normal rhythm of light and dark regulated phases of activity and inactivity. The rhythm is retained (albeit out of phase with slugs experiencing a normal light‐dark sequence) in constant illumination. The relative feeding rates of D. caruanae have been correlated with this cycle of activity and inactivity.Structural and functional changes in the organs of digestion, notably the digestive diverticula, have been correlated with the cycle of night and day, the cycle of activity and inactivity and feeding. The digestive tubules undergo a sequence of cytological changes over the course of 24 hours that primarily involves the digestive cells. These cells absorb and intra‐cellularly digest food in the hours of daylight and in late evening commence–and at night complete–a process of breakdown releasing fragmentation spherules and excretory concretions into the stomach. The fragmentation spherules, by the release of unused intra‐cellular enzymes, possibly assist in the primary extra‐cellular digestion of food in the crop on the next feeding cycle.The digestive diverticula comprise three cell types; digestive cells, calcium cells and thin cells. The function of these cell types is discussed and the structure of the digestive diverticula of D. caruanae compared with that of other pulmonates.Finally a co‐ordinated scheme of the processes of activity and inactivity, and feeding and digestion is constructed showing how such processes are inter‐related and affected by environmental variables; most notable among these is the cycle of night and day.