Effects of trematode parasitism on the behaviour and ecology of a common marine snail (Littorina littorea (L.))

Cryptocotyle lingua (Creplin) is a digenean trematode parasite of the litto ral prosobranch gastropod Littorina littorea (L.). The literature suggests the snails become infected by grazing guano of the final host, the herring gull, Larus argentatus Pontoppidan. The parasite emerges from the snail as free-swimming cercariae. Interactions between the snail and the parasite at cellular and life-history levels are well established, but little is known of the influences the interaction has on the behaviour and the ecology of the snail. We tested the response of the snail to encounters with cercariae , examined the longevity of the guano on-shore and tested the responses of the snail to encounters with guano. Over half the L. littorea tested were a ble to detect both cercariae and a filtered homogenate of cercariae in cons pecific mucus trails, approximately one-third of animals refusing to cross the treatments. Chemoreception by the mouth or foot is considered the most likely means of detection. Guano samples (mean weight 3.22 g) naturally dep osited at approximately mid-tide level were completely washed away by one t idal inundation. We consider this period too brief to allow for ingestion o f eggs in guano by the snail. Further, snails would not cross guano placed in conspecific trails. Most snails would not cross guano diluted by 10(3) X (10 mg ml(-1)) and some snails could still detect guano diluted by 10(6) X (10 mug ml(-1)), though all were prepared to cross it. Detection of guano is again believed to be by chemoreception by the mouth or foot. These resul ts are discussed in terms of the mating and aggregating behaviour of L. lit torea. ingestion of the parasite by L. littorea is likely to take place onc e the guano has washed away as the eggs are negatively buoyant in seawater and may adhere to rock (biofilm) or algal fronds which may be grazed by the snail. (C) 2001 Elsevier Science B.V. All rights reserved.