Scientists who study deep-sea fish must be both patient and persistent. Most deep-sea fish have poorly-known habits and ranges, so for any expedition there is no guarantee of finding a particular species. Since deep-sea fishing is also time-consuming and expensive, many species have only ever been observed once. Here, luck is a vital part of the scientific method.
The deepest living fish ever caught was a species of cusk eel (Abyssobrotula galatheae) in the family Ophidiidae, found in the Puerto Rico Trench in 1970. This eel, a new species at the time, was caught with a trawler net at a depth of more than 27,400 feet or 5.2 miles (8.3 kilometers). That’s almost as deep as Mount Everest, at 29,000 feet, is tall. Galathea was the name of the ship whose crew hauled up the fish, and the cusk eel’s species name galatheae commemorates the expedition (Nielsen 1977). Today, two new species of cusk eel were named (Nielsen 2015), by the very same Danish scientist who described Galathea‘s deepest catch.
Despite living in complete darkness, under high pressures, the Galathea eel is able to find its invertebrate prey, which consists mostly of crustaceans and worms (Nielson 1977). The ventral fins, used for balance in most fish, are modified into a tiny, branched sensory structure on the fish’s chin. The eyes are poorly developed but functional. By comparison, the two new species are blind. Millions of years of darkness have rendered the eyes vestigial, like the hip-bones of a whale.
Like many deep sea fish, cusk eels cannot afford to be choosy when selecting a mate — encounters are rare in the deep sea, and missing out on an opportunity to reproduce is simply not an option. Many animals can store sperm — that is, the female can mate and then wait for months or even years before using the sperm to fertilizing her eggs. In most cases, this is so the female can:
a) time the development of her young so they are born at the right time or season,
b) mate with multiple males and then later select which one’s sperm to use, and/or
c) parse out fertilization, so that not all of a female’s young are born at the same time.
Blind cusk eels do something totally different. If a male stumbles across a female who is too young to lay eggs, all is not lost. She can still mate, and simply store his sperm until she is old enough to reproduce. This way, no reproductive opportunity is squandered.
When a female cusk eel lays her eggs, she assembles them all into a gelatinous envelope, much like a frog’s egg mass. This mass then floats up into the sunlit surface waters, where the eggs develop and then hatch. Baby cusk eels feed on plankton, which is abundant at the surface, and over the course of their lives gradually make their way down to the abyss where they will live, mate, hunt, and die in total darkness.
Nielsen J.G. 2015. Revision of the aphyonid genus Aphyonus (Teleostei, Ophidiiformes) with a new genus and two new species. Zootaxa 4039 (2): 323–344.
Nielsen J.G. 1977. The deepest living fish Abyssobrotula galatheae: a new genus and species of oviparous ophidioids (Pisces, Brotulidae). Galathea Report 14: 41–48.