The biology and functional morphology of Eufistulana mumia (Bivalvia: Gastrochaenacea)

Eufistulana mumia Spengler 1783 (Gastrochaenidae) is one of the rarest of all bivalves, a preserved specimen only once having been previously reported upon (Fischer, 1866). Eufistulana lies buried in subtidal sediments, entombed, except for a siphonal orifice, in an adventitious tube of its own construction. The tube comprises four layers secreted in stages by an equal number of pallial glands, housed in the reflected middle folds anteriorly, the reflected inner folds further posteriorly and the siphons. The process of tube formation is discussed. The larva probably settles on a calcareous substrate, possibly an empty shell, and then proceeds to construct a vertical tube when the initial site of settlement has been penetrated. Periodic dissolution of the original tube basally, possibly by a gland in the foot, is followed by growth downwards with a concomitant secretion of new tube material. At the end of a phase of growth a new base to the tube is secreted. The true shell of Eufistulana is modified with the anterior face very reduced, laterally expanded and, in life. covered by reflected middle folds. The posterior face is elongate and covered by reflected inner folds. The ventral margin is widely emarginate. The pedal gape is small and the foot reduced, the sole acting as a sucker attaching the animal to the tube and leaving a characteristic impression. There is a tiny, papilla-like anterior pedal projection to the foot. A septum in the tube separates tissues contained within the shell from the siphonal region, possibly restricting entry into the basal region of sediment and small, shelter-seeking, invertebrates. Similarly, the tips of the siphons possess glands that produce a secretion which probably deters would be inhabitants of the siphonal region of the tube. The major specializations of Eufistulana to its mode of life are pallial. In other respects it is a typical bivalve with little reduction of the pedal apparatus and few modifications to the organs of the mantle cavity. The labial palps are small implying little food selection in the mantle cavity. The intestine is adapted for handling large amounts of sediment. Soft deposits have been colonized in a number of ways by a number of bivalve lineages. Eufistulana is an example of another way in which this habitat has been exploited, albeit from an ancestor that was probably a borer.