Cloaca

In sharks, rays, hagfish, and lungfish, a single opening called the cloaca handles waste elimination, urine, and reproductive gamete release. Most bony fish, by contrast, have separated these functions into distinct urinogenital and anal openings. The cloaca is the ancestral vertebrate state, retained by amphibians, reptiles, and birds - lost only in mammals and most teleost fish.

Male cartilaginous fish possess paired claspers - modified pelvic fin structures adjacent to the cloaca - used for internal fertilization. Ray-finned bony fish typically have separate anus and urogenital opening. Some primitive teleosts retain cloaca-like arrangements.

Because the cloaca handles multiple functions in sharks, scientists can determine sex, maturity, and reproductive stage from a single close-up photograph.