Observations on the occurrence, nutritional physiology and respiratory pigment of three species of flatworms (rhabdocoela: pterastericolidae) entosymbiotic in starfish from temperate and tropical waters

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Abstract
Occurrence, nutritional physiology and symbiotic status have been studied in the pterastericolid rhab-docoels Triloborhynchus astropectinis, Pterastericola australis and P. vivipara which live, respectively, in the pyloric caeca of the starfish Astropecten irregularis (sublittoral, N. temperate), Patiriella calcar (littoral, S. temperate) and Acanthaster planci (coral reefs, tropical). Occurrence of all three flatworms is independent of host size and sex but percentage incidence (prevalence) increased in winter months in the two species (P. australis and P. vivipara) for which seasonal data was obtainable. The alimentary system is virtually identical in the three species and consists of mouth, buccal cavity, pharynx, oesophagus (partially reflexed anteriorly to form a secondary lining to the pharynx and buccal cavity) and intestine. The intestinal wall (gastrodermis) is monolayered and formed from vacuolated columnar phagocytes; smaller non-vacuolated and densely staining cells occur between these, especially posteriorly, and are believed to be immature phagocytes. There are no gastrodermal gland cells. All three flatworms are parasitic and feed on caecal tissue; digestion is predominantly intracellular and effected largely if not entirely by enzymes of host origin ingested with the food. The pattern of food storage and utilization differs markedly from that seen in other entosymbiotic flatworms; there are no significant glycogen deposits and although the gastrodermis contains much lipid this is rapidly depleted on starvation. These differences are probably a consequence of the lipid-rich diet, the caecal cells on which the flatworms feed being major lipid storage sites in each host species. All three flatworms possess a haemoglobin concentrated around the brain, pharynx and reproductive system. This is believed to be an adaptive feature allowing the flatworms to obtain oxygen preferentially from host tissues, which lack any such pigment.