The world’s most abundant vertebrate family

The bristlemouth (Gonostomatidae) is a family of deep-water marine fish found primarily in the Pacific, Atlantic, and Indian Oceans, but also residing in polar waters albeit in smaller numbers. It habitats the middle depths, generally avoiding surface waters and extreme depths. It generally has light-producing photophores aligned along the lower sides of its body. Its common name derives from its ability to open its mouth extremely wide, exposing numerous bristle-like teeth. It ranges in size from one inch to almost a foot in length. Based on worldwide survey data, it has been estimated that there are more than 20 billion (with a B) and possibly as many as a trillion individual bristlemouth fish in the oceans. The bristlemouth was first discovered during the voyage of the HMS Challenger in 1872-1876. Those oceanographers pioneered deepsea mid-ocean trawls and they recovered many new fish, including large numbers of bristlemouth. During the Cold War, US Navy sonars encountered a phenomenon they labeled the Deep Scattering Layer. Something in the ocean deeps was reflecting sonar signals so effectively that it resembled the seabed, but the depth changed somewhat between day and night. Eventually, bristlemouth (in huge numbers) and other creatures such as squid were identified as the source of the phenomenon. Unlike typical fish, the bristlemouth swimming motion is more like that of a snake; the creature undulating its entire body. Its diet consists primarily of smaller fish and crustaceans. In turn, it is pursued by a variety of larger fish and deep-diving whales. It is not targeted by commercial fishing. The bristlemouth is referred to as protandrous, meaning that it starts life as a male and then transitions to become a female. This process occurs naturally in some worms, limpets, and butterflies.