Genetics and morphology of a population of Lightiella (Cephalocarida: Hutchinsoniellidae) in Bahía Fosforescente, Puerto Rico

The class Cephalocarida is an understudied taxon of crustaceans, with a notable scarcity of distributional records, ecological data, and information about genomic architecture. Investigations of their morphology have nonetheless provided valuable insights into early crustacean evolution, while analyses of their phylogenetic position may help to resolve the sister-group relationships of Hexapoda. Cephalocarids from a newly discovered population in Bahía Fosforescente, La Parguera, Puerto Rico, were subjected to genetic and morphological analyses. A haplotype network constructed using the Median-Joining method revealed minimal genetic variation among the individuals in Bahía Fosforescente, not exceeding the degree of variation expected within populations and thereby demonstrating that the newly discovered cephalocarid assemblage belongs to a single species. Lightiella incisa Gooding, 1963 was previously reported from another site at La Parguera, and the present specimens from Bahía Fosforescente exhibit some morphological features diagnostic of that species. In corrected pairwise comparisons, however, these specimens showed a genetic divergence of ∼13.4% against a published cytochrome oxidase subunit I (COI) sequence of L. incisa. This value is high for within-species comparisons, but it was the lowest found in a search of other available conspecific and congeneric COI sequences in GenBank. Maximum Likelihood analysis based on partial COI sequences identified L. magdalenina Carcupino, Floris, Addis, Castelli & Curini-Galletti, 2006 as the closest relative to the Bahía Fosforescente population, but low bootstrap support limits our confidence in this phylogenetic arrangement. From a morphological standpoint, the present specimens resemble L. incisa, but the substantial genetic divergence between our specimens and previously published cephalocarid sequences indicates that the specimens may represent a distinct species within Lightiella Jones, 1961.