what causes an orca's dorsal fin to collapse
Captive killer whales Scientists from the National Marine Fisheries Service (NMFS) have reported that "the collapsed dorsal fins commonly seen in captive killer whales do not result from a pathogenic condition, but are instead thought to most likely originate from an irreversible structural change in the fin's collagen over time. Possible explanations for this include: (1) alterations in water balance caused by the stresses of captivity dietary changes, (2) lowered blood pressure due to reduced activity patterns, or (3) overheating of the collagen brought on by greater exposure of the fin to the ambient air."[65] According to SeaWorld's website, another reason for the fin to bend may be the greater amount of time that captive whales spend at the surface, where the fin is not supported by water pressure.[66] The Whale and Dolphin Conservation Society says that dorsal fin collapse is largely explained by captive killer whales swimming in small circles due to the inadequate space in which they have to swim.[67] SeaWorld, however, claims that "Neither the shape nor the droop of a whale's dorsal fin are indicators of a killer whale's health or well-being."[66]
Gill slit Most sharks and rays have five pairs of gill slits, but a few species have 6 or 7 pairs. Shark gill slits 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. A 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 bottomâ€“dwelling sharks. Spiracles are reduced or missing in active pelagic sharks.[1] While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation"[2]. 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.[3]