An anglerfish is a fish in the order Lophiiformes. This order encompasses a wide range of fish in an assortment of families, but all of them share some very distinctive traits which have made them quite famous. Unfortunately for the anglerfish, one of these traits is extreme ugliness; anglerfish are among the most bizarre looking organisms in the sea which can be seen by the naked eye, and an encounter with one is not easily forgotten.
The common name “anglerfish” comes from a unique biological adaptation exhibited by members of this order. Anglerfish have developed extremely long dorsal fins, with the part of the fin near the head being entirely separated. The section of the fin dangles in front of the face of the anglerfish, sort of like a long fishing rod. To complete the image, the fin terminates in a fleshy bulb of “bait” which is meant to intrigue other fish. In deep sea dwelling anglerfish, the bait exhibits bioluminescence, essentially creating a neon sign to attract prey.
Anglerfish are also distinguished by their extremely large mouths, which have teeth angled inwards. The angled teeth allow prey fish to swim inside, but not to escape, acting as a trap. To feed themselves, anglerfish simply cruise around the ocean, waiting for prey to swim right into their mouths.
Most anglerfish are benthic, which means they are adapted to dwell on the bottom of the ocean floor. Many are also designed to live in extremely deep water, and they have a few unique biological features to facilitate this. These fish must be able to survive in very high pressure, and they must also be adapted to handle extreme cold. As a result, many of them develop strangely compressed bodies, with organs and bones arranged in such a way that the high pressure of the ocean floor cannot hurt the fish.
Some species of anglerfish have even developed pectoral fins which act like legs, allowing them to walk along the ocean floor. Given the deep waters in which they live and their relative rarity, this particular adaptation is rarely seen in action, but it is quite remarkable when caught by scientists in submersible vehicles used for the study of the deep sea environment.
The anglerfish has one more interesting trait up its sleeve. Some species have developed a very unique form of sexual dimorphism which is designed to ensure the survival of this incredibly strange order of fishes. In these species, the male anglerfish is not designed to be self supporting, forcing him to seek out a female if he wishes to survive. The males actually latch onto the females, sharing blood supplies in a parasitic state, and over time the male atrophies away, turning into little more than a set of gonads. When the female anglerfish wishes to mate, she can trigger a hormonal state which forces the gonads to release sperm for fertilization. Some females even have multiple males to choose from!