Kristensenia Por, 1983

Gómez, S. & Rocha, C. E. F., 2005, A new species of Kristensenia Por, 1983 and a new record and illustrated supplementary description of Halicyclops hurlberti Rocha, 1991 from Mexico, Journal of Natural History 39 (2), pp. 133-152 : 134-135

Genus Kristensenia Por, 1983

Diagnosis (amended). Darcythompsoniidae . Vermiform. Body somites of about the same length, except for cephalothorax and anal somite which are longest. Rostrum not fused to the cephalothorax; with pointed tip. Female antennule six-segmented, with bare setae only; with aesthetasc on fourth segment. Antenna with allobasis; exopod one-segmented and armed with one element. Mandibular palp two-segmented; basis with one element; endopod with four or five setae; without exopod. Maxillule without division between basis, endo- and exopod; basis with three or four setae; endopod and exopod with two setae each. Maxilla with two endites; proximal endite with one, distal endite with two setae; endopod with three elements. Maxilliped subchelate. P1–P4 exopod three-segmented. P1–P4 endopod two-segmented. Outer seta of basis of P1 and P2 spine-like, of P3–P4 seta-like. Armature formula of P1–P4 (EXP/ENP): (P1) I-0; I-0; II, 2, 0/0-0; I, I1, 0; (P2) I-0; I-1; II, 2, 0/0-0; I, 2, 0; (P3) I-0; I-1; II, 2, 1/0-0; I, 2, I; (P4) I-0; I-1; II, 2, 1/0-0; I, 2, I. Female P5 without exopod; baseoendopod armed with seven elements. Female caudal ramus with concave inner lateral contour; with seven setae. Sexual dimorphism observed in male antennule (six-segmented; subchirocer; with geniculation in third segment); inner spine of basis, P5 (with five elements), P6 (with two elements) and caudal ramus (inner lateral concave contour even in male, rather uneven in female).

The genus can be separated from Pabellonia Gómez, 2000 and Darcythompsonia T. Scott, 1906 by (1) the general shape of the maxilliped (subchelate in Kristensenia , and of a different shape in Pabellonia and Darcythompsonia ; Gómez (2002: 516) erroneously stated that the maxilliped of Pabellonia was like that in Kristensenia ); (2) shape of anal operculum (with spinular ornamentation, without sexual dimorphism and not upwardly directed in Kristensenia ; with spinular ornamentation, without sexual dimorphism, but upwardly directed in Pabellonia ; without spinular ornamentation and strongly sexually dimorphic in Darcythompsonia ); (3) male P2 endopod (sexually dimorphic in Pabellonia and Darcythompsonia ; without sexual dimorphism in Kristensenia ); (4) shape, armature and sexual dimorphism of caudal ramus ( Kristensenia possesses a female caudal ramus with straight and concave outer and inner margin, respectively, broader at its base, about 1.8 times longer than broad, with seven elements, and with the only slight sexual dimorphism being the inner lateral contour of male caudal ramus more even than that of the female; the caudal ramus of Pabellonia is about twice as long as broad, with convex and slightly concave inner and outer margin, respectively, with inner distal corner protruded into an extension upwardly directed, with five elements only, and without sexual dimorphism; the caudal ramus of Darcythompsonia is similar to that of Kristensenia , except for length/width ratio (1.8 in Kristensenia and less than 1 in Darcythompsonia ), and sexual dimorphism in the male caudal ramus of Darcythompsonia which is rather cylindrical. Pabellonia and Kristensenia can be separated from Darcythompsonia by the sexually dimorphic fan-shaped organ on the second and third male urosomites (present in the latter, absent in the two former species). On the other hand, Darcythompsonia and Kristensenia can be separated from Pabellonia by the P5 and P6 (present in both sexes in the two former species, but absent in both sexes in Pabellonia ).