Gill slits are individual openings to
gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of
cartilaginous fish such as
sharks, and
rays. Most of these have five pairs, but a few species have 6 or 7 pairs. Shark gill slits are not covered, but lie in a row behind the head. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow. This modified slit, called a
spiracle, lies just behind the
eye, which assists the shark with taking in water during
respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active
pelagic sharks. While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are
obligate ram ventilators and would presumably
asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species. In contrast,
bony fishes have a single outer bony gill covering called an
operculum.