ALARM RESPONSE OF THE INTERTIDALSNAIL LITTORINA LITTOREA(L.) TO PREDATION BY THE CRABCARCINUS MAENAS(L.)

Abstract

Individuals of Littorina littorea in rocky intertidal pools crawled to pool sites where they were less visible (into rock crevices; under rocks and macroalgal fronds) when either crushed conspecifics or juice from crushed conspecifics was added to these pools. A significant proportion of snails hid in 10 min or less; individual snails in one pool tested quadrupled their crawling velocities after snail juice was added. Field observations and laboratory experiments tested the hypothesis that this alarm behavior helps L. littorea avoid being eaten. Green crabs (Carcinus maenas) were observed consuming individuals of L. littorea in tide pools at night and along the shore at high tide during the day. In the laboratory, crabs required more time to locate and attack periwinkles in rock crevices than periwinkles on rock surfaces. The amount of time required to consume specimens of L. littorea depended on snail size (shell length), reflecting different methods of attack by crabs. Small snails (< 9.0 mm) were crushed, then consumed in approximately 2 min 30 sec. Crabs could not consume large snails (> 18.0 mm), but destroyed medium-sized snails (≥ 9.0, ≤18.0 mm) by cracking the shell, tearing off bits of tissue, then resuming shell cracking to expose more snail tissue. This required a mean time of 9 min 54 sec once first injury to snail tissue had occurred, which approximately equals the 10-min response time of snails exposed to crushed snail or snail juice in the field. These findings indicate that the alarm response of L. littorea serves in defense against Carcinus maenas.