Prey preference, capture and ration in hemifusus tuba (Gmelin) (prosobranchia : Melongenidae)

Abstract

The South China Sea whelk Hemifusus tuba (Gmelin) feeds on bivalves in subtly different manners appropriate to the life style of individual prey items. Thin-shelled, deep-burrowing species with long siphons are attacked by eversion of the proboscis into the inhalant siphon (Category 1). Shallow-burrowing or epibyssate species with wide ventral marginal gapes are delicately grasped with the foot and the proboscis everted into the gape when the valves part (Category 2). A third category of bivalves comprising thickshelled, shallow-burrowing venerids with extensive mantle fusions is attacked in the same manner, but such assaults are rarely successful. As a fourth category, damaged bivalves, releasing blood, are detected, stalked, grasped and attacked at the wound. In this context there is some evidence to suggest that H. tuba can also pirate the prey of conspecific and other molluscan predators. Once the prey is penetrated, the proboscis with a delicate stenoglossan radula located at its tip cuts away the flesh, beginning with the adductor muscles so that the valves gape and release their hold on the proboscis inserted between them. The valves of thin-shelled prey species often break, such is the force of the valve adduction by the prey when attacked; this does not result from shell “chipping” by the predator. Whelks ranging in shell height from 74–148 mm (i.e., a dry tissue weight of between 1.65-18.28 g) eat between 0.3-1.9 Tapes philippinarum(Adams & Reeve) (shell lengths of 29dash34 mm) per day (i.e. a dry tissue weight of 0.2-1.4 g). This is equivalent to an average predator consumption rate of ≈4% of its own wet body weight and 6% of its dry body weight per day. Hemifusus tuba is very similar, in terms of diet and prey capture mechanisms, to the tropical Atlantic whelks Busycon spiratum and Melongena corona. Hemifusus tuba thus conforms to the general concept that tropical whelks are specialists, their temperate counterparts being more generalist.