OBSERVATIONS ON THE FEEDING MECHANISM, DIET AND DIGESTIVE PHYSIOLOGY OFHISTRIOBDELLA HOMARIVAN BENEDEN 1858: AN ABERRANT POLYCHAETE SYMBIOTIC WITH NORTH AMERICAN AND EUROPEAN LOBSTERS

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Abstract
1. The aberrant annelid Histriobdella homari (Polychaeta:Eunicida) lives in the branchial chambers of the marine lobsters Homarus americanus and H. vulgaris where it feeds on the rich microflora of bacteria, blue-green algae and related organisms which grow on the inner surface of the branchial chamber, the setae fringing the edges of the carapace, the gill filaments and, especially, the surfaces and setae of the epipodite plates between the gills. H. homari, therefore, is to be regarded as an epizoic microphagous cleaning symbiote of the lobsters. 2. The alimentary canal consists of mouth, buccal cavity, oesophagus, proventriculus stomach, intestine and anus. The much-modified proboscis lies ventrally below the oesophagus and proventriculus, with its anterior portions protruding into the rear of the buccal cavity. 3. The proboscis consists of two fixed parallel mandibles, a transverse carrier which slides upon the mandibles and to which is attached, posteriorly, a median flexible dorsal rod and, anteriorly, four pairs of movable articulated maxillae, paired external and internal retractor muscles and various tensor, flexor and extensor muscles. 4. Contraction of the retractor muscles withdraws the carrier and maxillae posteriorly, causing bowing of the dorsal rod which is fixed at its posterior end. Relaxation of the muscles allows the rod to straighten and, thus, causes protraction of the carrier and protraction and lateral expansion of the maxillae. Contraction and relaxation of the retractor muscles are supplemented by appropriate changes in the other muscular components of the proboscis. 5. During feeding the serrated anterior ends of the mandibles are applied to the food, the maxillae are fully expanded and then drawn ventro-posteriorly toward the mid-line by contraction of the retractor muscles in the effective movement of the feeding mechanism. This draws the food organisms across the anterior ends of the mandibles, detaching them from the substratum and allowing their ingestion by ciliary action. The first pair of maxillae are also capable of independent action and can be used while the remainder of the proboscis apparatus is held in the protracted position. 6. Detached microorganisms are entangled in a sticky mucous secretion from the salivary glands; other salivary secretions provide a transport medium for the clumped particles and a third set contain C-esterases which initiate digestion. 7. Ingested food is held briefly in the proventriculus, then passed to the stomach where gland cells secrete A- and C-esterases which continue and extend the digestion initiated by the salivary C-esterases. 8. Some soluble products of gastric digestion are taken up by absorptive cells in the stomach wall and their digestion is completed intracellularly by enzymes which include β-glucuronidase. Others pass into the intestine for absorption and completion of digestion by cells similar to the gastric absorptive cells but which lack β-glucuronidase. Insoluble residues of intracellular digestion accumulate in the stomach and intestinal cells as pigmented granules; residues of extracellular digestion aggregate in pellets and are voided through the anus. 9. Lipid forms the principal food reserve and is laid down in the absorptive cells of the stomach and intestine. 10. The histriobdellid-crustacean type of symbiosis, as exemplified by H. homari and its lobster hosts, is compared with the temnocephahid (Platyhelminthes: Turbellaria)-crustacean type. Basic similarities in the relative lack of specialization in the nutritional physiology of the annelid and platyhelminth symbiotes, when compared with that of their free-living relatives, are discussed, as also are the implications of known similarities in their life histories and geographical distributions.

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