A new species of <i>Onychocamptus</i> Daday, 1903 (Copepoda: Harpacticoida: Laophontidae) from northwestern Mexico

A new species of harpacticoid copepod, Onychocamptus fratrisaustralis (Harpacticoida: Laophontidae), is described from a coastal lagoon in northwestern Mexico (Sinaloa state). Onychocamptus fratrisaustralis appears to be related to O. krusensterni Schizas & Shirley, 1994, by the unusual formula of the P4 exopod. Onychocamptus anomalus (Ranga Reddy, 1984) shares the same formula of the P4 exopod, but differs from O. fratrisaustralis and O. krusensterni in the A2 exopod (with 4 setae in O. krusensterni and O. fratrisaustralis; with 1 seta in O. anomalus) and female P5 exopod (with four setae in O. anomalus; with three setae in O. krusensterni and O. fratrisaustralis). Onychocamptus besnardi Jakobi, 1954, also possesses a P4 exopod with two outer spines, but lacks the inner seta of the same segment. After thorough analysis of the type material of O. krusensterni (USNM259322), a number of subtle differences was found between this species and O. fratrisaustralis: armature of the antennal exopod, length/width ratio of caudal rami, dorsal ornamentation of the genital double-somite and fourth urosomite, relative length of the inner setae of second and third exopodal segments and the two innermost setae of second endopodal segments of P2-P3, relative length of the second endopodal segment of P4, general morphology of baseoendopod and relative length of the proximal setae of the endopodal lobe of P5, and relative length of the lateral outer seta of the last antennular segment