Misidentification of megalopae as a potential source of error in studies of population genetics and ecology of the blue crab Callinectes sapidus

Inaccuracy in taxonomic identification is an unknown but potentially important source of error in studies of planktonic larval ecology and evolution. We address the misidentification of blue crab (Callinectes sapidus) megalopae (post-larvae) as a source of error in investigations of genetic variation and factors influencing settlement. Callinectes spp. megalopae were sampled monthly in spring and summer from the water column at 2 locations on the Texas (USA) coast and identified by 16S mitochondrial sequences. Most of the megalopae could be assigned to C. sapidus (62%), C. similis (36%), C. rathbunae (1.5%), or C. danae (0.12%), while 5 (0.8%) were ambiguously grouped with both C. similis and C. danae. Previously used morphological characters (rostrum length, carapace length, and their ratio) were not diagnostic. Species composition differed between locations and among monthly samples. A recurring seasonal pattern in species composition was discerned, with ~95% C. similis in April shifting to ~95% C. sapidus by May/June, and variable proportions in August. This pattern strongly parallels changes in allozyme allele frequencies previously reported for blue crab megalopae at the same locations. Models selected by the Akaike information criterion indicated lunar phase, temperature, salinity, storms, and wind stress components all affecting megalopal abundance. The importance and sign of these factors differed between species. Temperature, the most important factor for each species analyzed separately, was not important when species were combined. This study demonstrates that misidentification of larvae could create the appearance of temporal genetic variation, inflate estimates of abundance, and obscure factors influencing settlement.