Commensalism vs parasitisim : trophic relationship between M. arrokeana (Nemertea: Hoplonemertea) and P. abbreviata (Bivalvia: Hiatelidae))

JOSE E. F. ALFAYA; GALVAN D; MACHORDOM ANNIE; PENCHAZADEH PE; BIGATTI G.

Quingdao

Congreso; The 8th International Conference on Nemertean Biology; 2014

Ocean University of China

Knowledge on the biology of symbiotic nemerteans is very limited; therefore, understanding the trophic relationships with their hosts is a challenging task. Several nemertean species are considered to be true parasites, impacting negatively in the host population, while some species are considered commensal. The genus Malacobdella Blainville, 1827 includes only 6 species, all of them generally attached by the terminal sucker to the mantle cavity of marine bivalves. Malacobdella arrokeana lives inside the giant Patagonian geoduck Panopea abbreviata, both species endemic of the Southern Atlantic Ocean. Malacobdella arrokeana has never been found in other bivalve species inhabiting the north patagonic gulfs neither outside the host, while 100% of the P. abbreviata examined (n = 904) hosted at least one specimen of M. arrokeana. The aim of this study was to get further insight on the type of relationship (commensalism vs. parasitism) between the nemertean and the bivalve host. We analyzed the diet of both species by direct methods (stomach contents) and indirect methods (stable isotope analysis of C and N). A total of 30 stomach contents from P. abbreviata and 30 from M. arrokeana were analyzed and determined the δ15 N and δ13 C values of muscle tissues. Our results revealed that M. arrokeana and P. abbreviata have an omnivorous diet. Malacobdella arrokeana does not feed on host tissues, since it is suspensivorous, capturing food from the bivalve gills and incurrent water using papillae; it consumes mainly micro algae (diatoms and filament algae) and, in lower proportion, small crustaceans. P. abbreviata consumes the same items, filtering by gills. In a similar way, both species showed similar spread in their isotopic values overlaping their isotopic niches (63%). Thus, our results suggest that M. arrokeana is a commensal living in the mantle cavity of P. abbreviata