The bivalve genus Macoma maintains wide distributions and high population densities on North American coasts, and some species in the U.S. Pacific Northwest also have circum-Arctic and Atlantic ranges. To elucidate the biogeographic history of the genus, an accurate phylogeny of the group is needed. Because no such phylogeny exists as of yet, a cladogram was inferred from Coan's (1971) monograph on the Northwest American Tellinidae, in which relationships between Macoma species are explicitly and implicitly stated but are not based on cladistic methods. For comparative purposes, I performed a preliminary morphological cladistic analysis of the twenty-one Northwest American Macoma species. Though the resulting cladogram is partially unresolved, there is some congruence between it and the inferred tree. Brooks Parsimony Analysis (BPA) carried out on the two trees gives two different results regarding the biogeographic history of the genus. The tree inferred from Coan (1971) implies an initial Arctic/Atlantic split followed by gradual southward diversification down the Pacific coast. The BPA of the character-based tree indicates an initial Arctic/Atlantic split followed by a cold-water (Aleutian and Oregonian provinces)/warm-water (Californian and Panamic provinces) split in the Pacific, with subsequent diversification taking place simultaneously in the two temperature zones. To determine which scenario is more accurate, a robust molecular phylogeny of the Northwest American Macoma is needed; morphological characters are too sparse in the genus to be truly informative. Furthermore, the high level of intraspecific and intrapopulational morphological variation in Macoma makes cladistic analysis based on shell characters difficult.

Reference: Coan, E.V. 1971. The Northwest American Tellinidae. The Veliger, vol. 14, suppl.