Ultrastructural studies on the early cuticular metamorphosis of adult female Lernaeocera branchialis (L.) (Copepoda,Pennellidae)

After its final moult and fertilization an adult female of the marine fish-parasitic copepod, Lernaeocera branchialis, begins an extensive metamorphosis. This commences while the parasite is still on the flounder intermediate host and is completed once the female has established itself on the whiting final host. One early component of the metamorphosis is a considerable elongation of the parasite's abdominal region. S.e.m. and t.e.m. studies have revealed that part of the mechanism of the elongation consists of a straightening out of a highly folded abdominal cuticle. Before fertilization, the epicuticle and outer procuticular layers of this integument are thrown into a series of transverse, 4–6 µm deep pleats or folds with a density of 1–1.2 folds/µm of abdominal length. Straightening these folds can generate an approximately 6-fold length increase. The folds are already present beneath the female chalimus IV cuticle when the epidermis of this development stage starts to secrete the adult cuticle. Immediately before the final moult, the adult cuticle is super-folded with the whole cuticle displaying second-order folds, 8–10 µm deep.The capacity of Lernaeocera to engage in extensive cuticular modifications without recourse to a moult is compared with similar abilities shown by some insect species.