Parasite of the month

Freshwater mussels Lampsilis spp.

Mussels of the genus Lampsilis  are widespread in North American streams. While the adult has a typical filtering life-style, the first larval stage - the glochidium - is a microscopic parasite of freshwater fishes. Young mussels attach to the gills of the fish host with their shell and suck their blood until they become large enough to move to the bottom of the stream.

The fascinating part of Lampsilis life-cycle is how they can reach the fish host. Indeed, the adults are typically sessile, they evolved a strategy to attract the fish host to increase transmission rate of the young mussels: like anglers, these mussels "fish" to get close to the host.

In these shellfishes, the mantle evolved to look like a prey fish of the host. The mimicry is improved by the contractions of the mantle that imitate the tail movements of the mimicked prey. When hungry host fish get to bite on the lure, the adult mussel ejects in its mouth hundreds of glochidia that attach to the gills as showed on this video:

Actually, several mussel species have evolved comparable life-cycles, with glochidia being more or less specialists towards certain kinds of host fish. This has led scientists to hypothesize that the composition of mussel communities might to some extent be dependent on the composition of the fish community. By using data on the occurrence of several mussel and fish species in the Great Lakes, Schwald et al. (2012 Diversity and Distribution) evidenced such pattern. Their analysis revealed that host fish presence was the strongest predictor of mussel occurrence. The study notably indicates that watershed identity also explained a large proportion of mussel species composition, highlighting the importance of the history of invasion phenomenon.

sources for picture: modified from https://www.fws.gov/midwest/mussel/images/life_cycle_620.gif
and https://www.fws.gov/midwest/mussel/images/broken-rays_fishing_lure_620.jpg