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

A new species of harpacticoid copepod, Onychocamptus fratrisaustralis (Harp acticoida: Laophontidae), is described from a coastal lagoon in northwester n Mexico (Sinaloa state). Onychocamptus fratrisaustralis appears to be rela ted to O. krusensterni Schizas & Shirley, 1994, by the unusual formula of t he P4 exopod. Onychocamptus anomalus (Ranga Reddy, 1984) shares the same fo rmula of the P4 exopod, but differs from O. fratrisaustralis and O. krusens terni in the A2 exopod (with 4 setae in O. krusensterni and O. fratrisaustr alis; 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 ou ter spines, but lacks the inner seta of the same segment. After thorough an alysis of the type material of O. krusensterni (USNM259322), a number of su btle differences was found between this species and O. fratrisaustralis: ar mature of the antennal exopod, length/width ratio of caudal rami, dorsal or namentation of the genital double-somite and fourth urosomite, relative len gth of the inner setae of second and third exopodal segments and the two in nermost setae of second endopodal segments of P2-P3, relative length of the second endopodal segment of P4, general morphology of baseoendopod and rel ative length of the proximal setae of the endopodal lobe of P5, and relativ e length of the lateral outer seta of the last antennular segment.