Mussel of the Month

It is April, and occasionally we use the tradition of April Fool’s to shake things up with the Mussel of the Month. For example, Pisidium and Corbicula aren’t freshwater mussels, but they do represent other, important radiations of bivalves into fresh waters. Back in April 2021, the “Muscle” of the Month was the posterior adductor muscle of Theliderma intermedia. See: we get cute sometimes, and this post is one of those.

The Recent genus Neotrigonia is the sister-group to the Unionoida, the order of freshwater mussels. According to phylogenetic analyses based on the best available evidence, the crown-group of all freshwater mussels is a clade (Bieler et al., 2014; Combosch et al., 2017). It is monophyletic, meaning all living freshwater mussel species are descended from an ancestral species that possessed the combination of traits that distinguish the Unionoida from other bivalves— for example, living in fresh waters, having parasitic larvae that require a host fish for metamorphosis, and parental care by brooding their larvae in their ctenidia (gills) until they are sent off to seek their parasitic fortunes. All the branches of the freshwater mussel phylogeny diverge from that node on the Tree of life. If we go back towards the root of the tree one more node, we hit the branch that leads to Neotrigonia, the sole surviving genus of the order Trigonioida. The taxon composed of the Trigonioida and the Unionoida is the Palaeoheterodonta.

The thing about Neotrigonia as the closest living relative is that those bivalves are quite unlike freshwater mussels. There are only about half a dozen species (compared to more than 1000 freshwater mussels), they are all marine, they have planktonic rather than parasitic larvae, and there is no evidence of parental care. Instead, like many marine animals, these ones broadcast-spawn their gametes for external fertilization rather than sperm and egg meeting in the female’s mantle cavity and larval brooding, typical of freshwater bivalves like the freshwater mussels, Pisidium, and Corbicula. For all intents and purposes, Neotrigonia is your standard, primitive marine bivalve genus. If you know a little something about clams and mussels, you might be surprised to learn that Neotrigonia species have filibranch ctenidia like those of marine mussels such as Mytilus, as opposed to the eulamellibranch gills of more derived bivalves like freshwater mussels. If you don’t know what those ctenidia adjectives mean, don’t worry: there isn’t a vocabulary quiz, and they are esoteric enough that, as I write this, there aren’t even Wikipedia.org links I can throw you. “Filibranch” and “eulamellibranch” aren’t crucial for following this post, but they foreshadow fodder for a future discussion.

An exclusive relationship betwixt Neotrigonia and freshwater mussel has been the traditional classification for anyone alive that has ever cared. Thiele (1934) had it like that, and that’s what it said in the Treatise on Invertebrate Paleontology (Cox, 1969). As we explained almost 20 years ago (Graf & Cummings, 2006), the pre-molecular-phylogenetics era traits that diagnosed the palaeoheterodonts (i.e., Neotrigonia + freshwater mussels) boiled down to the shape of their hinge teeth, the nacreous structure of their shells, their unfused posterior mantle edges (see our discussion of Margaritifera falcata for more information about that), and maybe some similar sperm ultrastructure. A cladistic eye will recognize that the presence of nacre (mother of pearl) and posterior mantles unelaborated into distinct siphons are symplesiomorphies (shared primitive characters) — attributes that confirm what the palaeoheterodonts aren’t rather than diagnosing what they are. The main diagnostic trait that can be followed in the fossil record is their similar hinge teeth of their shells.

These three shells depict the "schizodont" hinges of Neotrigonia (left) and Unio (center) and the heterodont-type hinge of Corbicula (right).

That is pretty flimsy, and it shouldn’t have surprised anyone if that traditional hypothesis fell apart like so many other pre-cladistic bivalve classifications. But, Neotrigonia as sister to freshwater mussels has been well supported by modern molecular phylogenetic results. It is pretty nice in a world of uncertainty to have such a solid sister-group hypothesis on which to base our understanding of freshwater mussel evolution.

Our closest relative should be able to tell us something about our origins — what freshwater mussels looked like when they were first freshwater mussels. Unfortunately, Neotrigonia is so different from Recent freshwater mussels that it doesn’t tell us much. Furthermore, the evolutionary split between the ancient trigonioids and the stem lineage of freshwater mussels happened hundreds of millions of years ago. The data about what ancient trigonioids and unionoids looked like, their reproductive modes, and their parenting behaviors are going to have to come from the rocks. There is an extensive fossil record to consider, and fresh paleontological discoveries are shedding new light on the soft anatomy of the earliest freshwater mussels (Skawina & Dzik, 2011). However, for now, freshwater mussels need to be satisfied just knowing their place on the evolutionary tree: closer to Neotrigonia than anyone else.

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