DAO 73:63-68 (2006)  -  doi:10.3354/dao073063

ABSTRACT: Marine bivalves harbour a diversity of trematode parasites affecting population and community dynamics of their hosts. Although ecologically and economically important, factors influencing transmission between first (snail) and second (bivalve) intermediate hosts have rarely been studied in marine systems. In laboratory experiments, the effect of temperature (10, 15, 20, 25°C) was investigated on (1) emergence from snails, (2) survival outside hosts and (3) infectivity in second intermediate hosts of cercariae of the trematode Renicola roscovita (Digenea: Renicolidae), a major parasite in North Sea bivalves. Emergence of cercariae peaked at 20°C (2609 ± 478 cercariae snail^–1 120 h^–1) and was considerably lower at 10°C (80 ± 79), 15°C (747 ± 384) and 25°C (1141 ± 334). Survival time decreased with increasing temperature, resulting in 50% mortality of the cercariae after 32.8 ± 0.6 h (10°C), 26.8 ± 0.8 h (15°C), 20.2 ± 0.5 h (20°C) and 16.6 ± 0.3 h (25°C ). Infectivity of R. roscovita cercariae in cockles Cerastoderma edule increased with increasing temperature and was highest at 25°C (42.6 ± 3.9%). However, mesocosm experiments with infected snails and cockle hosts in small aquaria, integrating cercarial emergence, survival and infectivity, showed highest infection of cockles at 20°C (415 ± 115 metacercariae host^–1), indicating 20°C to be the optimum temperature for transmission of this species. A field experiment showed metacercariae of R. roscovita to appear in C. edule with rising water temperature in April; highest infection rates were in August, when the water temperature reached 20°C. Since another trematode species (Himasthla elongata; Digenea: Echinostomatidae) occurring at the experimental site showed a similar temporal pattern, trematode transmission to second intermediate bivalve hosts may peak during especially warm (≥20°C) summers in the variable climate regime of the North Sea.

KEY WORDS: Trematoda · Transmission · Cercariae · Temperature · Renicola roscovita · Himasthla elongata · Cerastoderma edule