Although most parasitic copepods produce free-living larvae, Caribeopsyllus amphiodiae and other thaumatopsyllid copepods have a parasitic larval stage (metanauplius) that inhabits an ophiuroid stomach. Metanauplii of C. amphiodiae collected from its burrowing host, Amphiodia urtica, and reared in the laboratory gave rise to ovigerous females that released free-swimming first nauplii (NI). We provide the first full descriptions for a thaumatopsyllid of the NI, parasitic metanauplii, and free-living copepodid I to copepodid VI of the female and male based on developmental stages obtained. Extraordinary features of C. amphiodiae NI are the presence of one pair of setae on the labrum, a character unique among Copepoda, and the presence of a maxillule represented by one seta, making the larva a metanauplius by definition. In most other copepods, the one-seta or one-spine maxillule appears no earlier than NII. The mandible of the metanauplius becomes massive, and a spiniform process from the first endopodal segment forms a chelate complex with the distinctly curved, clawlike second endopodal segment. Transient vestiges of the antenna and mandible remain in the CI, but maxillule, maxilla, and maxilliped are absent during the copepodid phase. Other body structures appear earlier than reported for other copepods: pediger 5 and the bud of swimming leg 4 are present at CI, and pediger 6 and the buds of legs 5 and 6 are present at CII. During leg development setal development is accelerated, but ramal segmentation is delayed. In addition, the major body articulation of the copepodid and adult stages occurs between the third and fourth pedigers, unlike the tagmosis of most gymnoplean and podoplean copepods. Such shifts in the timing of ontogenesis, atypical naupliar morphology, and unique adult body set apart the thaumatopsyllids from other copepods.
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