![]() Consequently, thousands of species of parasitic copepods potentially remain to be discovered and described. Unfortunately, the majority of those researchers have focused on internal parasites (helminths) whereas externally occurring copepods, if present, have usually been overlooked or misidentified. By the end of 2022, FishBase reported 34,900 valid fish species, only a small percentage of which have been examined by parasitologists. Less specific parasites can be found on hosts representing the same genus or a family. The first is host specificity, which is usually narrow enough to allow individual fish species to have their own ”private” parasite species, some of which are copepods. Two factors indicate that this number is just the tip of the iceberg. Many of the latter group, about 2400 valid species, are parasites of fishes. Copepods are ecologically very diverse and are almost equally represented by free-living as well as symbiotic species (although this proportion may differ in most recent estimates). The class Copepoda, one of the most prominent taxa of the phylum Arthropoda, accommodates some 14,600 accepted species, and this number has been growing at a relatively fast pace. ![]() With this admittedly polemical paper, we hope to spark a discussion about this terminological problem. We conclude that, using the new integrative terminology, copepods of the family Caligidae have the following stages in their life cycles: nauplius I, nauplius II (both free-living), copepodid I (infective), copepodid II (chalimus 1), copepodid III (chalimus 2), copepodid IV (chalimus 3/preadult 1), copepodid V (chalimus 4/preadult 2), and adult (parasitic). Key concepts are illustrated in diagrams. To justify this reinterpretation, we comprehensively summarize and reinterpret the patterns of instar succession reported in previous studies on the ontogeny of caligid copepods, with special attention to the frontal filament. We see no justification for keeping “chalimus” and “preadult” even as purely practical terms. The terminology for the caligid copepod life cycle thereby becomes consistent with that for the homologous stages of other podoplean copepods. In our new understanding, both the chalimus and preadult stages should be referred to as the respective copepodid stages (II through V, in integrative terminology). Consequently, the term “chalimus” with its use currently restricted in the Caligidae to at most two instars in the life cycles of species of Lepeophtheirus, also becomes redundant. In view of recent studies, we suggest that the term “preadult” should not be used in scientific reports on Copepoda parasitic on fishes as having no explicit meaning or further justification.
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