Sthenelais onca sp. nov. (Phyllodocida, Sigalionidae) from a sandy beach on the North Pacific coast of Costa Rica

Errant polychaetes of the family Sigalionidae are active, scale-covered predators inhabiting sandy marine bottoms. Knowledge of this family in tropical America is scarce, with only a few species reported from Costa Rica. In this study, seven specimens of Sthenelais were collected from the intertidal zone of Playa Naranjo, Área de Conservación Guanacaste, on the North Pacific coast of Costa Rica. Morphological examinations and Bayesian phylogenetic analyses based on DNA barcoding (COI) were conducted to determine their taxonomic identity. The specimens were recovered within the Sthenelais clade, forming a distinct and well-supported subclade that includes S. limicola from Europe and representatives from Asia, and that differs from western Pacific congeners. Morphologically, the new species Sthenelais onca sp. nov . differs from other Eastern Tropical Pacific congeners by the absence of serrations on the shafts of neurochaetae throughout the body, stylodes without papillae, and anterior elytra bearing a notch on the supra-interior margin that diminishes posteriorly. The species inhabits saturated sand in the intertidal zone and exhibits a unique elytral coloration pattern reminiscent of a jaguar’s coat. This study describes a new species of Sthenelais from sandy beaches of the Pacific coast of Costa Rica and provides an updated identification key for the genus in the region, expanding the known diversity and distribution of Sigalionidae in the Tropical Eastern Pacific.