PHYLOGENETIC RELATIONSHIPS OF GEODUCK CLAMS PANOPEA GENEROSA AND P. GLOBOSA AND THEIR ANCIENT AND EXTANT DISTRIBUTION IN BAJA CALIFORNIA AND THE GULF OF CALIFORNIA

Panopea genus is among the largest and long-lived burrowing bivalves. These mollusks support important fisheries in the Pacific coast of Canada, the United States, and Mexico. Panopea generosa and P. globosa were sympatric in the Salton Trough region during the late Miocene to the Pleistocene Epoch.

For a long time P. globosa was considered a subspecies of P. generosa; fossils of both species have been collected in the upper Gulf of California, but the only living beds of geoduck clams that occur today in the subtropical environment of the Gulf of California correspond to P. globosa, while P. generosa is a more adapted species to temperate waters.

It is still unknown if Mexican geoducks shared a common ancestor before the formation of the Baja California peninsula and the Gulf of California. We analyzed mitochondrial and nuclear DNA sequences of P. generosa and P. globosa, as well as samples of P. abbreviata, P. zelandica, and P. japonica. Contrary to expectations, our results suggest that P. generosa is genetically more similar to P. japonica, while P. globosa probably shared a common ancestor with geoducks from the Southern Pacific (P. zelandica). Our study also corroborated that P. globosa is not an endemic species of the Gulf of California. This species can also be found in Bahia Magdalena, Baja California Sur; its fossils were collected near Santa Rosalia; San Jose del Cabo and Isla Magdalena showed that P. globosa had a wide distribution before the last glaciation period. For this reason we suggest that other subspecies of P. generosa (P. generosa var. taeniata Dall, 1918) could be really a fossil of P. globosa.

Today, P. generosa has only been found in the Pacific coast of Baja California while several ‘populations’ of P. globosa are known to exist in the upper Gulf of California (San Felipe and Puerto Peñasco); middle Gulf (Guaymas) and the Pacific coast of Baja California Sur (Bahia Magdalena).

Other unsolved questions are: (1) Are the great similarities of shell shape between P. globosa and Atlantic geoduck P. bitruncata or their fossil species P. floridiana related in geological time with the closure of the Panama istmus seaway? (2) Does P. globosa have a morphologic pattern similar to other fossil species from the Southern Pacific and Antartica such as P. parawhitfieldi, P. regularis, P. andreae, and others?