There are three species currently classified in the genus Strophitus. S. undulatus, our Mussel of the Month, is widespread in the Interior Basin, Hudson Bay, Great Lakes, and the Northern Atlantic Slope drainages in eastern North America. Its genus-mates, S. connasaugaensis and S. subvexus, are both endemic to waters of the Gulf Coastal Plain. There is some interesting biogeography to discuss there, but we are going to focus on systematics and classification this month. Strophitus is emblematic of the remaining work yet to do on American mussels.
North American freshwater mussels are among the best studied in the world, and Strophitus is well known owing to its broad range. We would have expected that its classification was planted on as firm of a foundation as any other genus. It turns out that is true, but if you read the Mussel of the Month frequently, you know that many genera are pretty rickety. Strophitus is one of those.
Strophitus is a Rafinesque (1820) genus created solely for S. undulatus. The genus was mentioned throughout the 19th century in various lists, as often as not synomyzing it with the then-current concept of Anodonta. Conrad (1853) used Strophitus for a hodge-podge of species now classified in Anodontoides, Alasmidonta, Simpsonaias, Lasmigona, Pegias, as well as S. undulatus. Our current concept of Strophitus dates from Simpson (1900, 1914). He included the three species listed above, plus Anodontoides radiatus and Alasmidonta wrightiana.
Simpson (1900, 1914) recognized Strophitus as distinct based on the arrangement of the glochidia in the marsupium. Lefevre & Curtis (1912:122) described it this way:
“[Strophitus] is unique among the Unionidae in that the embryos and glochidia are embedded in gelatinous cords (called ‘placentae’ by Sterki, ‘placentulae’ by Ortmann), which lie transversely in the gills, whereas in all other cases the egg masses are placed vertically, each one occupying an entire water tube. In Strophitus, on the other hand, the cords are packed closely together, like chalk crayons in a box, a variable number being contained in a single water tube, while the blunt ends of the cords are distinctly seen through the transparent external lamella of the outer gill.”
Both Simpson (1900) and Ortmann (1912) provided descriptions of these structures, but neither were as colorful as the this! See Watters (2002) for a detailed description of those “cords” of glochidia.
These descriptions of such striking reproductive characters were based on S. undulatus, and Simpson (and everyone else) attributed these diagnostic traits to the genus. However, the other species of Strophitus were classified as such based on similarities of the hinge. A new grouping of species was established because they shared such a strikingly unique arrangement of larvae in the marsupium that Simpson coined a new word (“Diagenae”) to describe it, but it was only observed in one species.
That alone does not mean that Strophitus is not a good genus — that it is not monophyletic. After all, having a curved hinge and weak hinge teeth may in fact be shared derived homologies (i.e., synapomorphies) among S. undulatus, S. connasaugaensis, and S. subvexus. However, it turns out that they are not. At least two phylogenetic analysis to-date have included more than one species of Strophitus (Chong et al., 2008; Inoue et al., 2014), and each species appears to share a more recent common ancestor with members of other anodontine genera than with each other. However, for neither of these studies was the phylogeny of Strophitus the objective, and the sleeping dog was left to lie.
So, add Strophitus to the list of genera that would benefit from more attention by taxonomists. |
