The Soft-Shell Clam Mya arenaria: Biology, Fisheries, and Mariculture

Chapter 1: Systematics and Evolutionary Relationships of the Soft-Shell Clam Mya arenaria

Michael P. Lesser


The soft-shell clam Mya arenaria (Linnaeus 1758) is found worldwide in marine soft-bottom ecosystems and is native to North America. Any treatise on the biology, fisheries, and mariculture of a species should include sufficient background information on its taxonomy and evolution (i.e., systematics) to contextualize the current understanding of its ecology, historical biogeography, and commercial importance as a food source. Here, a brief description of the evolution of the Mollusca and the Bivalvia is presented as currently understood from the literature. Phylogenetically, M. arenaria is currently placed in the Protostomia clade (i.e., protostomes) of the Eumetazoa in the superphylum Lophotrochozoa, phylum Mollusca and class Bivalvia (Table 1) but a recent reorganization of the eukaryotic tree of life, based solely on molecular phylogenies, subsumes all metazoans into a new supergroup called the Opisthokonta (Keeling and Burki 2019). The readers will note many unresolved relationships in this animal group, including the uncertain placement of M. arenaria, which has not been fully resolved using paleontological, morphological, or molecular cladistics, separately or together. No detailed discussion or definitive statement is provided here on the strengths and weaknesses of one classification scheme versus another, or the relative strength of the phylogenetic signal embedded in the morphological versus the molecular characters used for those analyses.

The phylum Mollusca (Figure 1) is highly diverse, with an estimated 200,000 living species and eight recognized classes (Ponder et al. 2020). Mollusks have a long evolutionary history with an excellent fossil record due to their mineralized exoskeleton, having arisen in the Cambrian ~540 million years ago (mya) (Ponder et al. 2020). While there are several phylogenetic scenarios for the evolution of the phylum Mollusca, the phylogenetic support of a morphology-based taxonomy describes an early split of the Mollusca into two large clades: the Aculifera, containing the aplacophorans and polyplacophorans, and the Conchifera, containing all the uni-shelled groups (Wanninger and Wollesen 2018). The evolutionary time frames of the origin of the Aculifera and Conchifera suggest that the last common ancestor of the Mollusca was from the Ediacaran period ~635 mya (Wanninger and Wollesen 2018). Refinement of the phylogenetic relationships within the Mollusca has been significantly improved using molecular markers, but the lack of congruency between morphological and molecular phylogenies persists. In fact, in some phylogenies the Mollusca are not considered a monophyletic taxon and could only be considered monophyletic if the Scaphopoda and Bivalvia were considered as separate, monophyletic, taxa distinct from the rest of the Mollusca (Goloboff et al. 2009; Sigwart and Lindberg 2015). On the other hand, Kocot et al. (2011) produced a well-supported phylogenomic analysis of the Mollusca (Figure 1) based on over 300 orthologous genes, which places the bivalves and gastropods as sister taxa. The use of phylogenomic approaches (i.e., multiple genes, genomes, and transcriptomes) for unraveling taxonomic relationships has fueled the question of which approach (i.e., morphology versus molecular) has the stronger phylogenetic signal. Regardless, most recent phylogenies place the bivalves as part of a larger clade with gastropods and scaphopods while supporting the monophyly of the Mollusca (Kocot et al. 2011; Smith et al. 2011; Gosling 2015; Wanninger and Wollesen 2018).