Cockle Cerastoderma edule population mortality: Role of the digenean parasite Himasthla quissentensis

Abstract

Infection patterns of the digenean trematode Himasthla quissetensis in its second intermediate host, the edible cockle Cerastoderma edule, and the impact of this parasite on cockle populations were quantified. A 2 yr transplant experiment and a 4 yr survey were conducted in Arcachon Bay, a coastal lagoon on the Atlantic SW coast of France. Cockles free of H. quissetensis were transplanted at La Canelette, a station where cockles were usually highly infected. Infection began in May, when the water temperature reached 17°C and stopped in November (12°C). There was no new infection between November and April. Monthly surveys at 2 stations, Arguin and La Canelette, confirmed that cockle infection by H. quissetensis occurred during the warmer period of the year. Only cockles with a shell length > 8 mm became infected; therefore, cockle growth rate, which differed at the 2 stations, was also a key factor explaining infection phenology. In winter, the mean parasite abundance of H. quissetensis in cockles decreased at both stations. The mortality rate of parasites in cockles was very low, with no seasonal pattern, and did not explain this winter decrease. Consequently, the decrease of H. quissetensis was interpreted as mortality of heavily infected cockles. At Arguin, where the cockle population was characterised by moderate abundance, high growth rate and short lifespan, cockle mortality amounted to 93 % during the first year of benthic life, with no significant contribution of parasites; in the following year (Year 2), cockle mortality was 85 %, with 28 % due to H. quissetensis infection. At La Canelette, the cockle population, characterised by low abundance and low growth rate, suffered almost 100 % mortality in the first year, with no contribution by parasites, which started to infect cockles in the second year only; the remaining cockles had a long lifespan (up to 5 yr), and >46 % of their mortality was linked to H. quissetensis infection. It is concluded that both host growth rate and water temperature are important factors in the initiation of parasite infection and that the intensity of infection and its effect on host mortality closely depend on host growth and environmental factors.