The Gutless Marine Tubificid Phallodrilus planus, a Flattened Oligochaete with Symbiotic Bacteria. Results from Morphological and Ecological Studies

The anatomy of Phallodrilus planus Erseus, 1979, a relatively large interstitial tubificid from shallow subtidal sands off Bermuda, is characterized by an unequal development of the muscular layer which results in a flattening of the body shape. Like its relative P. leukodermatus, it lacks completely the digestive tract and nephridia, but is regularly associated with a thick subcuticular layer of two different gram‐negative bacteria which are involved in sulphide oxidation. This is shown by ultrastructural, histochemical and physiological data and affects the biological and ecological situation of these aberrant oligochaetes. The mutualistic nature of this bacteria/animal association is discussed.