Lack of genetic differentiation among populations of both the Patagonian Malacobdella arrokeana entocommensal nemertean and its host the geoduck, Panopea abbreviata

JOSE E. FERNANDEZ ALFAYA; BIGATTI GREGORIO; MACHORDOM ANNIE

Vitoria-Gasteiz

Congreso; 6º Congreso de las sociedades malacológicas europeas; 2011

Universidad del País Vasco Consejo Superior de Investigaciones Científicas

The nemertean Malacobdella arrokeana lives as entocommensal in the geoduck Panopea abbreviata, its exclusive host, and is the only representative of the genus in the southern hemisphere. This geoduck has been heavily fished in northern Patagonia since 1999, and different authors indicate that between 99,4 and 100% are parasited by M. arrokeana. It is interesting to note that when more than one specimen inhabits a bivalve, these nemerteans are immature. Panopea abbreviata is distributed in the north Patagonic gulfs, situated in the limit of Magallanic malacological and Argentinean provinces that comprise the San Matías, San José and Nuevo gulfs. Each of these provinces has different environmental characteristics respect to thermal gradients, thermohaline conditions, etc. Taking these particularities in mind, specimens from their main distribution area were sampled to assess their genetic structure, linking it to dispersion pattern or historical events suffered by these species. The three molecular markers showed almost no genetic variation for both species. The genetic parameters indicate population in expansion or submitted to selection. Different episodes of habitat modifications, and mainly the more recent, could explain the patterns and indices found. Such changes can provide the conditions for expansion, colonization or rapid recolonization, with the consequent loss of variation. Alternatively, a selective loss of rare nemertean genotypes by the geoduck would explain the low variability found in all their populations analysed and would indicate a strong association between the host and the entocommensal genetic structure.