LIFE CYCLE AND HABITAT SHIFT OF THE TROCHID SNAIL DILOMA SUAVIS (PHILIPPI) WITHIN INTERTIDAL MUSSEL ZONES

Abstract

The life cycle of the trochid snail Diloma suavis (Philippi, 1849), was studied on an intertidal rocky shore at Shirahama, Wakayama Prefecture, where two mytilid bivalves, Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould), formed vertically contiguous mussel beds in the upper-middle and lower zones, respectively. At low tide in April, the snail density increased with decreasing shore height and was greatest at the middle level of the H. mutabilis bed. Then, the density decreased towards the lower littoral fringe. Newly settled juveniles smaller than 2 mm in shell height appeared abundantly in late summer and autumn within algal turf on the lower shore. As snails grew larger than 2 mm, they appeared within the gaps of the H. mutabilis bed and the S. virgatus bed. They increased in size monotonically towards the next summer, but rate of growth in shell height tended to be great in autumn and small in winter. Seasonal change in the density of snails found within the gaps of the mussel beds was remarkable during a three year period, increasing from autumn to winter and then decreasing towards next summer. Reproduction occurred in summer, and adult snails disappeared by September. It is thus suggested that this species has a one year lifespan and shows a habitat shift from algal turf to the gaps of the mussel beds with growth.