Postnaupliar stages of a thaumatopsyllid copepod from a reef area of the western Caribbean Sea

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Copepods of the recently established order Thaumatopsylloida are semi‐parasitic forms in which naupliar development takes place inside ophiurid echinoderms; the postnaupliar stages are free‐living. The morphology and development of the postnaupliar stages of this order remain practically unstudied. Plankton samples collected at night time following the lunar cycle in a reef area of the Mexican Caribbean coast yielded different copepodid stages of Caribeopsyllus chawayi Suárez‐Morales, one of the two thaumatopsylloid species known in the western hemisphere. Five female copepodid stages (CI-III, CV, CVI) are described and illustrated in detail. Both copepodid and adult forms are devoid of mouthparts and antennae. This species appears to moult rapidly from CIII into a pre‐adult CV by acquiring a distinctive elongation of the anal somite and a clearly different segmentation pattern of legs 1-3. Morphological and morphometric changes during postnaupliar development of several structures were analysed and compared with unpublished observations on another thaumatopsylloid (Thespesiopsyllus paradoxus Sars). Among other characters, the one‐step elongation of the anal somite in our specimens contrasts with the gradual pattern shown by T. paradoxus. It is suggested that the pre‐adult form of C. chawayi appears after the CIII stage; unlike other copepods, the number of setal elements in the CIII stage is unchanged in older copepodids. eng

The delayed formation of (1) leg 1 coxal seta, (2) the arthrodial membrane separating the proximal segments of legs 1 and 2, and (3) legs 3 segmental divisions seem to be derived characters relative to previously known patterns in the Copepoda. An expanded description of the adult is presented based on the new material. The size and general morphology of the holotype of this species corresponds to a CV; however, the presence of a genital opening at this stage suggests that females may copulate at CV. The CV and adult forms occurred exclusively during the full moon phase, whereas CI-CIII copepodids were collected during half moon only; no specimens were collected at the new moon phase. This is possibly a result of a strategy related to both the encounter of mate and host and a tide‐stream selective transport in this reef area. eng

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