METAMORPHOSIS BETWEEN NAUPLIAR AND COPEPODID PHASES IN THE HARPACTICOIDA

It has been argued that, among Crustacea, copepods show 'the most complete example of metamorphosis': from nauplius to copepodid. In groups with either specialized nauplii or copepodids of a particular shape, body form changes markedly from nauplius VI to copepodid I. During post-embryonic development, one pair of appendages is formed at each successive stage, even in cases where some of these precursors are not developed externally. The same holds for the copepodid phase in which one somite is added per moult from a proliferation zone anterior to the anal somite. Of the appendages, the antennules continue to acquire further segments, setae and spines in the course of post-embryonic development, whereas the antennae and mandibles change drastically by losing their naupliar masticatory parts and, in the case of the mandibles, by developing completely different replacement structures. The antennae lose their masticatory function and migrate pre-orally from their naupliar paroral position. Even more striking are the changes affecting the post-mandibular mouthparts. From functionless naupliar limb-buds, these achieve almost the adult state as early as C I without any precursors in previous stages, whereas precursors of swimming legs 1 and 2 are indicated at N VI in most cases but, together with legs 3-6, differentiate in each successive instar from C I onwards. The caudal rami change in shape and in the number and structure of their setae. Sexual dimorphism becomes apparent only in the copepodid phase (as early as in C I in one case). Intermoult stages are important for tracing the origin and homology of larval and juvenile structures. Although 'metamorphosis' from nauplius to copepodid is in many cases accompanied by remarkable changes, there also are examples of continuity in morphological and behavioural features.