Dagger-like Teeth in Deep-Sea Fishes , by Jason Falcone

 Adaptations for survival occur throughout the fish world. For example, the whale shark has a large mouth for engulfing masses of krill, and the sail fish has a streamlined body and a large, deeply forked tail built for speed when attacking prey. Some of the stranger adaptations can be seen the deeper one ventures underwater. At depths of over 1000 meters, strange fish forms appear. Some fish, such as the angler fish, have bioluminescent bacteria, to attract prey, while others have large expandable stomachs, such as the black swallower that can hold prey up to 10 times its own body mass. Even stranger are fish such as the fangtooth, viperfish, and dragonfish which have bizarre long teeth so large that their jaws cannot close completely. Food availability is very little at extremely low depths, so these fish have adapted fang-like teeth that can hold onto prey.

The deep-sea can be divided into four different zones; the mesopelagic zone (150-100m), bathypelagic zone (1000-3000m), abyssopelagic zone (3000-6000m), and the hadal zone (below 6000m in ocean trenches) (FAO, 2017). It is the largest habitat on earth, and water over 200 meters covers roughly 65 percent of the globe representing close to 75 percent of the global biosphere (Angel, 1982). This environment is generally characterized by low temperatures, high pressure, and little to no sunlight.

A Fangtooth, Anoplogaster cornuta, from the Bear Seamount in the Atlantic Ocean. Source: Sandra J. Raredon / Smithsonian Institution, National Museum of Natural History, Division of Fishes

Since light cannot penetrate over depths of approximately 1100 meters, there are few primary producers, and as a result, no herbivorous species. Most fish residing at such low depths become generalist feeders (Drazen, 2017). Organic material decreases exponentially with depth, making adaptations like fang-like teeth important for survival. The fangtooth fish is an example of a species that has adapted to the deep-sea environment. Relative to body size, it has the largest teeth of any marine species with teeth lengths of up to 17 centimeters (Malhotra, 2011). The viperfish is another species known for its unusual teeth size. It has curved, sharp teeth that do not fit inside its mouth, and instead, curve back close to its eyes (MESA, 2017).

Vipers Chauliuodus (Stomiidae).  Source

Since food availability is scarce in their habitat, the fangtooth, viperfish, and many other species have evolved large teeth because prey needs to efficiently captured when it is in range. When attacking, their fang-like teeth grip prey like a vice, making escape very difficult.  Evolutionary pressures have allowed fang-like teeth to persist from generation to generation and will most likely continue to be a predominant trait of these deep-sea fish in the future. 

References

Angel, M. V., Baker, A. C. 1982. Vertical distribution of the standing crop of plankton and micronekton at three stations in the northeast Atlantic. Biol. Oceanography 2:1-30.

Drazen, J. C. Sutton T. T. 2017. Dining in the deep: The feeding ecology of deep-sea fishes. Annual Review of Marine Science 9: 337-377.

Food and Agriculture Organization of the United Nations (FAO): Fisheries and Aquaculture Department. Deep-sea ecosystems. Retrieved from http://www.fao.org/fishery/topic/166310/en.

Malhotra, R. 2011. Anoplogaster cornuta, Animal Diversity Web. Retrieved from http://animaldiversity.org/accounts/Anoplogaster_cornuta/.

Marine Education Society of Australasia (MESA). Viper fish. Retrieved from http://www.mesa.edu.au/atoz/viper_fish.asp