Commensalism or parasitism?: trophic relationship between Malacobdella arrokeana (Nemertea: Hoplonemertea) and Panopea abbreviata (Bivalvia: Hiatellidae).

ALFAYA J.E.F.; GALVÁN D.; MACHORDOM, A.; PENCHASZADEH P.E.; BIGATTI, G.

Qindgao

Congreso; 8th Nemertean Congress; 2014

Ocean Universtity of China

Knowledge on the biology of symbiotic nemerteans is very limited; therefore,understanding the trophic relationships with their hosts is a challenging task. Severalnemertean species are considered to be true parasites, impacting negatively in the hostpopulation, while some species are considered commensal. The genus MalacobdellaBlainville, 1827 includes only 6 species, all of them generally attached by the terminal suckerto the mantle cavity of marine bivalves. Malacobdella arrokeana lives inside the giantPatagonian geoduck Panopea abbreviata, both species endemic of the Southern AtlanticOcean. Malacobdella arrokeana has never been found in other bivalve species inhabiting thenorth 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 furtherinsight on the type of relationship (commensalism vs. parasitism) between the nemertean andthe 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 contentsfrom P. abbreviata and 30 from M. arrokeana were analyzed and determined the δ15 N andδ13 C values of muscle tissues. Our results revealed that M. arrokeana and P. abbreviata havean omnivorous diet. Malacobdella arrokeana does not feed on host tissues, since it issuspensivorous, capturing food from the bivalve gills and incurrent water using papillae; itconsumes mainly micro algae (diatoms and filament algae) and, in lower proportion, smallcrustaceans. P. abbreviata consumes the same items, filtering by gills. In a similar way, bothspecies 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