Cockle emergence at the sediment surface: 'favourization' mechanism by digenean parasites?

Abstract

The aim of the present work was to assess the effect of digenean trematodes on indirect mortality of the cockle Cerastoderma edule, an infaunal bivalve. The tested hypothesis was that parasites altered the burrowing capacity of cockles and thus exposed them at the sediment surface, where they are more vulnerable to predators. If the predator is the final host, this mechanism, which drives the cockle out of the sediment, is considered as a 'favourization'. Cockle populations from 2 stations in Arcachon Bay (France) -Banc d'Arguin (oceanic situation) and La Canelette (lagoonal situation) -were sampled for 1 yr. At La Canelette, monitoring every 2 d showed that 50% of adult cockles regularly migrated to the sediment surface at a rate of 5 cockles m -2 yr -1 and disappeared in a few days. In the laboratory, 67% of these 'surface cockles' did not burrow again, suggesting that they would die in the field. Moreover, mortality measured after 7 d in the laboratory was 2 to 5 times higher than mortality of 'buried cockles', at both stations and particularly during summer. Species richness and abundance of digeneans from both stations were compared in 'buried' and 'surface' individuals to determine whether parasites played a role in cockle migration and mortality. Ten and 9 digeneans were found at Banc d'Arguin and La Canelette, respectively, with Himasthla quissetensis and Labratrema minimus being the most prevalent and abundant species at both stations. The abundance of H. quissetensis was slightly higher in surface cockles at Banc d'Arguin, but the difference fluctuated with station and cockle age (or size). L. minimus prevalence was only higher in surface cockles at La Canelette. In the latter station, we estimated that L. minimus and H. quissetensis were responsible for the emergence of 9 and 2%, respectively, of the buried cockles. Although this favourization mechanism may induce some mortality in cockles, it does not alone explain the magnitude of the observed mortalities (41 and 57% at La Canelette and Banc d'Arguin, respectively). A correspondence analysis did not show the presence of a particular parasite community in buried or surface cockles, which could explain these high surface cockle mortalities in association with the 2 dominant digeneans.

Identification and Description of Bucephalus minimus (Digenea: Bucephalidae) Life Cycle in Portugal: Morphological, Histopathological, and Molecular Data

“…The finding of B. minimus in this particular geographical site reinforces the idea that the coast of Portugal may constitute an intermediate location for parasite species found in hosts from both the Atlantic and Mediterranean coasts (here, parasitizing C. glaucum) as pointed out earlier (Maillard, 1975(Maillard, , 1976Faliex and Biagianti, 1987;Fares and Maillard, 1990;Saadfares and Combes, 1992;Faliex and Morand, 1994;ElDarsh and Whitfield, 1999;de Montaudouin et al, 2000;Sasal et al, 2000;Malek, 2001;Desclaux et al, 2002;Russell-Pinto et al, 2006).…”

“…The high number of sporocysts invading the host organs is known to induce castration of the gonadal tissue (James, 1969;Khamdan, 1998) and affect the normal function of the digestive gland (Shelley et al, 1988;Desclaux et al, 2002).…”

Distribution of parasites in their second ­intermediate host, the cockle Cerastoderma edule: community heterogeneity and spatial scale

“…Salinity remains high year-round (32 to 35), mean surface water temperature fluctuates between 9.5°C in winter and 21.1°C in summer, and sediment temperature varies from -0.2°C to 30°C. The benthic free-living fauna has previously been described (Bachelet & Dauvin 1993), as has the trematode parasite fauna of molluscs (de Montaudouin et al 2000, 2003, Desclaux et al 2002). Since 1972, Arguin (25 km 2 ) has been a national reserve with several marine bird species wintering, nesting or migrating.…”

“…Black lines in marine areas indicate tidal limits also separate flocks of birds (potential definitive hosts) by favourite foraging or repository areas (Table 1). Arguin, a group of sandy banks at the entrance of Arcachon Bay, was selected with a set of 10 stations and a 100 m scale distance ( Desclaux et al 2002, and the aim was to get a rough idea of the potential first intermediate host abundance. Unfortunately, this investigation was not performed at Stns 9 and 10 because they were added to the protocol later.…”

Parasite co-infection of two sympatric bivalves, the Manila clam (Ruditapes philippinarum) and the cockle (Cerastoderma edule) along a latitudinal gradient

“…Moreover, the fact that cockles were the bivalves most infected by digeneans may also result from a parasite strategy to optimize the likelihood of trophic transmission to complete their life cycle. Cockles are easily available to predators (final hosts of parasites) because they live in a wide tidal range and only a few millimetres under the sediment surface; they may even emerge at the surface when abundance is high, because of sediment erosion, or when they are stressed, such as by oxygen deficiency or infection (Desclaux et al 2002). Clams present several disadvantages for digenean parasites compared to cockles, such as a deeper position in the sediment and autumn recruitment, when the temperature becomes too low for the emission of cercariae (Wegeberg 1998).…”