Mussel of the Month

Before the cladistic era of freshwater mussel systematics, the traditional view was that Margaritifera represented the primitive condition among extant palaeoheterodonts. This hypothesis was based on margaritiferids sharing characters with Neotrigonia — the extant, marine sister lineage to freshwater mussels. These common traits are less fancy than those in the ‘higher’ freshwater mussels.

Neotrigonia anatomy is quite different from that of your typical freshwater water mussel like Ortmanniana, for example. Quite different, that is, if you are the kind of person who gets excited about the nuances of bivalve anatomy. Graf & Cummings (2006) gave a detailed description of the suite of characters that distinguish freshwater mussels from Neotrigonia, so we will only dwell on two important ones here: posterior mantle fusion and the nature of the diaphragm dividing the mantle cavity.

In almost all freshwater mussels, the posterior ends of the right and left mantle exhibit at least a little fusion. In Ortmanniana (and other Unionidae), this fusion is short and creates what is known as the supra-anal aperture. This opening is distinct from the incurrent and excurrent apertures which are separated by the fusion of the ctenidia (gills) along their entire lengths with the adjacent mantle. The mantle cavity is almost completely divided by the diaphragm manifested by the ctenidia. In other freshwater mussels, like hyriids and etherioideans, there are different patterns of posterior mantle fusion — but posterior mantle fusion nonetheless.

Neotrigonia has no mantle fusion at the posterior end, and the ctenidia are free of the mantle at their posterior ends, resulting in an incomplete diaphragm (Gould & Jones, 1974). That is quite different from the unionid condition described above, but it is very similar to what is seen in margaritiferids like our Mussel of the Month. Margaritifera species also lack posterior mantle fusion and have incomplete diaphragms (Smith, 1980). It doesn’t seem like much of a logical leap to hypothesize that the ancestral, stem group freshwater mussel — way back in the Mesozoic — had a posterior mantle like that seen in Margaritifera today.

It is a little more of a conceptual jump to regard the modern Margaritiferidae to represent the ancestor of all other freshwater mussels, but that is precisely where our heads were at the end of the twentieth century. Heard & Guckert (1970) explicitly drew it that way.

Over the last quarter century, the phylogenetic revolution has shown us that it really isn’t that simple. (It never is.) Applying cladistic theory and nucleotide characters, Margaritifera falcata and other margaritiferids are typically recovered in a clade sister to the Unionidae. That is how we have those two families classified on the MUSSEL Project Web Site: the Superfamily Unionoidea = Family Margaritiferidae + Family Unionidae.

However, the actual phylogenetic position of the Margaritiferidae is still far from settled, and it remains unclear whether the morphology of the Margaritiferidae is plesiomorphic (‘primitive’). Depending on the topology of the freshwater mussel phylogeny, it might make more sense to interpret margaritiferid anatomy as ‘reduced.’ That is, a derived loss of its posterior mantle fusion rather than retention of the ancestral condition from a common ancestor of Margaritifera and Neotrigonia.

The MUSSELp Unionoidea Cladomics page summarizes the phylogenetic studies that have included all three extant freshwater mussel superfamilies (Unionoidea, Etherioidea, and Hyrioidea). Different datasets and methods of analysis have recovered the Margaritiferidae in just about every position relative to the other freshwater mussel families.

The challenge is judging among those disparate results to settle this issue. Despite the explosion in phylogenetic studies over the last decade, the relationships among the families have received much less attention than the relationships within the families. And not even all the families: the Unionidae has been the focus, with much less attention paid to the Margaritiferidae and Hyriidae, and almost none to the etherioideans. It is fair to say for most studies that have included representatives of all three superfamilies, the goal was to root a much less inclusive ingroup — for example, the subfamilies of the Unionidae — than to discover the relationships among the families. Datasets selected to discover the relationships among freshwater mussel genera may not provide rigorous tests of deeper branches. Thus, the short internal branches separating the families tend to get easily tangled. Watch this space as more data become available!

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