Two new species of the genus Mesocletodes (Copepoda, Harpacticoida, Argestidae) from the Pacific Ocean, San Diego Trough

Abstract Two new species of Mesocletodes were identified and described from the samples collected for an ecological study of benthic meiofauna in the bathyal zone of San Diego Trough (Southern California Continental Borderland, Pacific margin). Mesocletodes tetrasetosus appears to be closely related to M. langi and M. ameliae by sharing five setae in the endopod of mandibular palp. However, M. tetrasetosus differs from both congeners in the following features: (1) the second segment of antenna with seven setae, (2) the exopod of mandibular palp with two setae, (3) the terminal margin of praecoxal arthrite of maxilla with eight elements, (4) the basis of maxilla with a total of six setae, (5) the syncoxa of maxilliped with a strong or robust bipinnate seta, (6) the P1 enp-1 with an inner seta, (7) the P1 enp-2 innermost distal seta and distal spine are translocated, (6) the P3–P4 endopod with total of 4:3 elements, (7) the P5 endopod outermost seta three times as long as in M. ameliae, (8) the P5 endopod medial and innermost setae are sub equal, and (9) the caudal rami is as long as the last two urosomites. Mesocletodes nudus is closely related to M. fladensis by the presence of an outer spine along the midlength of P1 exp-1, and the P1 exp-3 distal outer element is almost as long as the exopod. However, M. nudus can be distinguished by the following character states: (1) the A1 with six segments, (2) the P4 enp-2 both distal setae are equal in length, (3) the P4 exp-3 outer spines are almost 0.5 times longer than in M. fladensis, (4) the P5 exopod with a total of three elements, (5) the P5 endopod inner distal seta is relatively slender and flexible, (6) the P5 exopod is reduced to half in its length and width, and (7) the P5 baseoendopod is represented by a transversely elongated plate, rectangular in shape. In addition, a key to the species of Mesocletodes including both new species is provided.