THE FUNCTIONAL-MORPHOLOGY OF MACTRINULA-REEVESII (BIVALVIA, MACTROIDEA) IN HONG-KONG - ADAPTATIONS FOR A DEPOSIT-FEEDING LIFE-STYLE

The shallow subtidal mactrid bivalve Mactrinula reevesii is a deposit- feeder in the southern and south-eastern oceanic waters of Hong Kong. Buried obliquely, large quantities of fine sediment are taken into the mantle cavity and sorted on enormous labial palps. The small ctenidia probably have little value in collecting material, amounts taken in b eing too large. The mid gut is long and complexly folded inside the vi sceral mass. It is also capable of distension, although superficial vi sceral muscles maintain internal tonus. The rectum is narrow and creat es compact faecal pellets. Most interest is in the ventral mantle marg in which is, posterior to the pedal gape and the base of the inhalant siphon, united by a sheet of cuticle. There is no fourth pallial apert ure. There are, however, two pairs of flaps extending along the poster ior third of the internal ventral mantle surface. These arch over left and right mantle rejection tracts which transfer unwanted material to the base of the inhalant siphon for final expulsion. The mantle flaps prevent such material from being returned to the anterior end of the mantle cavity, for palp reprocessing, when new material arrives. They, thus, maximize sorting efficiency by separating unsorted from sorted and rejected material. Other mactrids have similar mantle flaps which they use in different ways, including the channelling of unwanted mate rial to a fourth pallial aperture for expulsion, as in Lutraria lutrar ia. The Mactridae have thus evolved a unique method of increasing the efficiency of pseudofaecal waste rejection which has thereby facilitat ed the deposit-feeding lifestyle by the diverse representatives of thi s family.