SEXUAL DIMORPHISM AND A SEX-LIMITED POLYMORPHISM IN THE COPEPOD

species of the genus known to exhibit polymorphism. Among females the number of external spines on swimming legs 3 and 4 exopods is variable; a pair of legs on one animal also may express variation in external spine number. Males do not exhibit similar variation within or among animals. Sexual di-morphism in the genus Paroithona is manifested in 18 differences in number, shape, and position of various exoskeletal elements. Differences in the number of segments and/or setae on swimming legs 2-4 are unique to this group relative to three other oithonid lineages. Farran (1908) established the genus Pa-roithona from an unspecified number of fe-male specimens of Paroithona parvula found in two samples (SR 193- 54°50'N, 10°30'W; SR 197-54°57'N, ICSTW) collected be-low 1000 m off the northwest coast of Ire-land with an open, conical net (mesh siz