A Study on the Digestive Physiology of a Marine Polychaete (Eulalia viridis) through Microanatomical Changes of Epithelia During the Digestive Cycle

Abstract

As for many invertebrates, the gut of marine polychaete species has key physiological functions. However, studies integrating microanatomical descriptions with physiological processes are scarce. The present investigates histological, histochemical and cytological changes in the alimentary canal during the digestive cycle of the marine annelid Eulalia viridis, a species that combines opportunist scavenging, predation and cannibalistic behavior. The gut is comprised of an eversible pharynx, esophagus, intestine and rectum. Three main phases of digestion were identified, namely, resting/secretory, absorptive and excretory. The intestinal epithelium is complex and exhibited the most significant changes regarding intracellular digestion, excretion and storage. Conversely, the pharynx and esophagus were chiefly important for enzyme secretion. The results also indicate the existence of two distinct types of secretory cells in the intestine, with likely distinct physiological roles. Some similarities have been found between the intestinal epithelia and the molluscan (especially cephalopod) digestive gland, as, for instance, the shedding of apical corpuscles by digestive cells at posterior stages of digestion. The findings indicate that the digestive process in this worm is complex and related to the many physiological roles that cells need to play in the presence of reduced organ differentiation.

A morphoanatomical approach to the adaptive features of the epidermis and proboscis of a marine Polychaeta: Eulalia viridis (Phyllodocida: Phyllodocidae)

“…They possessed heavily electron‐dense granules that remained green regardless of histological dye, in both paraffin and resin sections, as previously described by Rodrigo et al. (). The pigment cells were compressed between the remaining epidermis cells, to which they were bound with ‘cuff’ links.…”

“…Even though serous cells lining the epithelium of the pharynx have been associated to the production of digestive enzymes (Rodrigo et al. ), these cells, especially those located in the vicinity of tentacles, are thus likely associated to toxin secretion (see Cuevas et al. ).…”

“…Even though the nature of the green substances has not yet been disclosed, they most likely result from haem breakdown (Lederer, ; Kennedy, ; Rodrigo et al. ). Another possible function of these pigments can be aposematism, i.e.…”

“…; Rodrigo et al. ). Resin sections required permeabilisation using a saturated solution of potassium hydroxide in ethanol 100% for 5 min followed by 2 min of 2% m/v borax (sodium borate) just before staining.…”

“…Pigment cells in E. viridis were noticed in sensorial papillae and the pharynx (Michel, ), and the presence of pigments and their transformation processes in the intestine have been investigated (Rodrigo et al. ). However, the function of pigments cells and sensory organs in the epidermis of this worm remains, to date, conjectural.…”