Development of the cartilaginous skull in solea solea: trends in pleuronectiforms

The postembryonic development of the cartilaginous cephalic skeleton in Solea solea (Linné) was observed from hatching to the juvenile stage or postmetamorphic larva. Fry were trypsin-cleared and alcian-blue-stained. At hatching, Solea solea displays no cartilaginous structure. On day 3, the fry exhibit trabecular bars, Meckel's cartilages, palatoquadrates, hyosymplectics, hyoid bars, the first two ceratobranchials, and basibranchials 1–3. On day 4, ceratobranchials 3 and 4 form. On day 12, short taeniae marginales are formed on the well-developed otic capsules and the branchial basket is nearly complete. On day 14, a first cartilage reduction begins as the posterior part of the trabecula communis separates from the otic capsules. On day 18, metamorphosis begins. The tectum posterius completes posteriorly the braincase dome. On day 20, the left eye moves to the median crest of the head and the right eye shifts downwards slightly. Consequently, displacement of the right eye induces a deformation extending to the right pterygoid process. On day 23, both eyes are located on the same side. On day 29, cartilage reduction affects the hyosymplectics; it continues with the first two ceratobranchials on day 32 and with the otic capsules on day 40. On day 50, cartilage reduction has reached the lamina precerebralis. Only a few small lumps remain at the level of the neurocranium and the splanchnocranium.