The parasitic metanauplius larva of an undescribed species of Caribeopsyllus, the second thaumatopsyllid species to be reported from the Caribbean, was discovered in Ophiothrix angulata in Belize. In addition, metanauplii of Caribeopsyllus chawayi, whose free-living stages were previously known, were found to parasitize Ophiactis savignyi. Their ophiuroid hosts usually contained a single larva that caused them no discernable physical damage. Caribeopsyllus chawayi, which was initially described from Mexico, is here reported from Belize and is suggested to occur in Brazil. It might occur in the Pacific Ocean, as its host is circumtropical. Larvae of the Belizean thaumatopsyllids are exceptional among copepod nauplii for their large size, four pairs of appendages, true chelate mandibles, elaborate tripartite eyes, accessory photoreceptive structures (Gicklhorn's organ), sexual dimorphism, and endozoic parasitism. Thaumatopsyllus paradoxus and Caribeopsyllus amphiodiae, the only other thaumatopsyllids with known metanauplii, have the same suite of specializations. However, metanauplii of the latter two species crawl, whereas the Belizean metanauplii swim. Both Belizean species have an antennal arthrite used for feeding and well-developed mandibular setae used for locomotion, which are lacking in C. amphiodiae. They leave the host as non-feeding positively phototactic copepodids. Morphological features distinguishing the metanauplii of Belizean species from that of C. amphiodiae indicate that the latter should be referred to a new genus if the systematic significance of its unique morphology is corroborated. Similarities between thaumatopsyllids and siphonostomatoids in the ontogenesis of their caudal rami suggest that Thaumatopsyllidae may belong to the Siphonostomatoida, or be closely allied.
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