California Sea Grant College Program

In this dissertation I address how open ocean plankton populations can diverge genetically, and ultimately form new species. Research focuses on the oceanic calanoid copepod family Eucalanidae. A global phylogenetic study of the Eucalanidae revealed substantial cryptic diversity at the species level. Genetic data from both mitochondrial and nuclear gene loci support 13 new genetic lineages within the Eucalanidae. These new lineages range from 1.6% to 23.2% divergent from their closest relatives (16S rRNA, p-distances) suggesting that although some diverged relatively recently, other represent quite ancient speciation events. Rhincalanus nasutus was found to be a cryptic species complex, with at least 7 genetically distinct, predominantly allopatric populations worldwide. A molecular phylogeny for the family supports monophyly of the Eucalanidae, all four eucalanid genera, and the 'pileatus' and 'subtenuis' species groups.

A distinct genetic form of Eucalanus hyalinus s. I. was identified in subtropical waters worldwide. The species name Eucalanus spinifer T. Scott was removed from synonymy with E. hyalinus and applied to the smaller form, elevating the number of species in Eucalanus to a total of six. Adult females of the sister species can be distinguished morphologically by morphometric, shape, and size characters. Results suggest that species originally identified through molecular markers will also be distinguishable by morphological characters.

A global population genetic study of the sympatric sister species Eucalanus hyalinus and E. spinifer demonstrated that oceanic zooplankton species can be highly genetically structured on macrogeographic spatial scales, despite experiencing extensive gene flow within features of the large-scale ocean circulation. Habitat discontinuities at the boundaries of subtropical gyres, and continental landmasses, acted as effective barriers to gene flow for both species. The sister species differed in their global population genetic structures as well as in their oceanographic distributions. Species-specific differences in habitat were an important factor determining patterns of dispersal between populations of each species worldwide.

Two unique spliceosomal introns were discovered in the nuclear gene elongation factor 1-α in Rhincalanus and Eucalanus, and the locus was found to have limited phylogenetic utility due to difficulty in identifying orthologous, functional gene copies.