Skeletal development of the chondrocranium in the tongue sole Cynoglossus semilaevis (Pleuronectiformes: Cynoglossidae)

This study provides a comprehensive description of chondrocranial development before, during and after larval metamorphosis in the tongue sole Cynoglossus semilaevis, a commercially valuable flatfish in China. Samples were collected at regular intervals ranging from 1 to 23 days post hatching (dph). Based on observations of cleared and double-stained specimens and images from sections stained with safranin O–fast green, major morphological events during early development were described. No cartilaginous structure was visible at hatching. From 2 dph onwards, cartilaginous structures such as the trabecular bar and some elements of the mandibular, hyoid and branchial arches appeared. At this time also, cartilaginous structures of the neurocranium started to form. Hypertrophic chondrocytes could be observed in many splanchnocranium elements at 5 dph. The start of ossification was indicated by alizarin red stain visible at 14 dph. At 17 dph, most of the cartilaginous skeleton was ossified. Soon after, the right eye started to migrate and pass through a slit beneath the dorsal-fin base and above the skull. Metamorphosis was complete at 20 dph, at which time the dorsal-fin base cartilage extended onto the anterior region of the head. Meanwhile, extremities of the hyoid and branchial arch elements remained cartilaginous. At 23 dph, endochondral ossification of the splanchnocranium was nearly complete. Unlike previous observations of other Pleuronectiformes, our study indicates that endochondral ossification of C. semilaevis skull cartilage occurs before metamorphosis.