Structure and development of spines over the skin surface of the river puffer Takifugu obscurus (Tetraodontidae, Teleostei) during larval growth

The histological structure and development of spines on the skin surface of Takifugu obscurus were studied during larval development conducted artificially with an average 30‰ salinity and 18.0-20.3°C water temperature. The epidermis comprises an outermost layer, middle layer, and the stratum germinativum, and contains three types of gland cells: small spherical or flask-shaped mucous cells, larger sacciform mucous cells, and large granular cells. The dermis and subcutis follow. The spines first appear over the ventral region at 10days after hatching and consist of two parts: a central long tapering portion which projects into the epidermis and eventually outside of the body, and a short supporting basal portion that is embedded within the stratum compactum layer of the dermis. The central, long tapering portion has two very short processes on top until 25days after hatching, but these two separate spines fuse into one 30days after hatching. In contrast, the short supporting spines rooted at the base consist of three to six small spines (usually four to five spines) and are present even in the adult stage. Therefore, calcareous spines consisting of one central long spine and three to six smaller supporting spines form tetra- and septaradiate spines (mainly penta- and hexaradiate). The spines first appear over the ventral region.