Dynamics of aggregations of a gastropod predator/scavenger on a New Zealand harbour beach.

Observations of a New Zealand harbour beach revealed that aggregative feeding behaviour was common in the buccinid gastropod Cominella glandiformis (Reeve, 1847). The majority of such aggregations are associated with either the tellinid bivalve Macomona liliana (Iredale, 1915) or the venerid cockle Austrovenus stutchburyi (Wood, 1828). Marking experiments indicate that aggregations persist for one tidal cycle. This information, together with the density of aggregations associated with each 'prey' species allows the calculation of rate of predation by Cominella glandiformis and of the mortality of M. liliana and A. stutchburyi at different stations on the beach. Averaged over spring periods, when predation and mortality are reduced, and summer, the calculated rates suggest that between 2 to 16% of M. liliana and 3 to 9% of A. stutchburyi fall prey to Cominella each year. The majority of bivalves at the centre of aggregations appeared to be tightly shut and undamaged, suggesting that the gastropods are predators attacking live prey, although late arrivals may be attracted as scavengers by leaking body fluids. Aggregated feeding may provide the gastropod with both competitive and metabolic benefits.