Back in October 2009, we discussed the problem of Inversidens sensu lato. In Haas's (1969) conchological view, that genus was composed of 6 species from eastern Asia and Japan. However, it was subsequently discovered that two different larval morphologies were represented among those species. Whereas as shell morphology (outline, umbo position, sculpture, etc.) is highly variable, often convergent, and well known to confound a natural classification, structures of glochidium-type larvae can be downright diagnostic.
In the the particular case of Inversidens sensu lato, some species (e.g. I. brandtii) have glochidia lacking marginal hooks. That doesn't tell us much about classification since most unionid freshwater mussels have unhooked type glochidia. But, other species (like I. reinianus) have glochidia with marginal hooks. In the family Unionidae, only members of the subfamily Unioninae have hooked-type glochidia. Thus, Inversidens sensu lato was composed of representatives of (at least) two different subfamilies. To solve this problem, the genus Inversiunio in the subfamily Unioninae was created (Habe, 1991). Inversidens is currently placed in the Gonideinae.
The separation of Inversidens sensu stricto and Inversiunio based on their divergent larval morphologies is not an isolated case. An analogous situation has also occurred with certain species formerly of the genus Lamprotula (also Gonideinae) being transferred to Aculamprotula (Unioninae) because of their hoooked-glochidia. |
