By Tracey T. Sutton, Professor, Department of Marine and Environmental Sciences - Nova Southeastern University
The bathypelagic zone hosts a unique assortment of highly adapted fishes, most of which are extremely rare in museum and research collections. These adaptations help fishes find and eat prey, and find each other, in a permanently sunless habitat. In some cases, the adaptations have driven the radiation of entire fish families in the bathypelagic zone, where in other cases, these adaptations allow individual species of primarily shallower-living fish families (e.g., lanternfishes, hatchetfishes) to survive. Some of these adaptations, and the fishes that best exemplify these adaptations, are given below.
The constraints of darkness, high pressure, cold temperatures, and large distance from the base of the food chain (plankton, which require sunlight) limit the amount of life that the bathypelagic zone can support. Therefore, finding food, while simultaneously conserving energy, presents a major challenge. One of the most striking adaptations of predatory fishes of the deep is the astounding variety of bioluminescent “lures” that fishes use to attract prey (rather than swimming and searching, which is energetically expensive).
This adaptation largely defines the deep-sea anglerfishes, the most species-rich taxon of primarily bathypelagic fishes (167 species, 11 families). Adult female anglerfishes of most species possess a lure (termed the ‘esca’) of varying complexity, poised at the end of a “fishing rod” (termed the ‘ilicium,’ a modified dorsal fin ray;). These escae contain a single species of bioluminescent bacteria, an amazing symbiosis between vertebrates and microbes. One family of anglerfishes, the Linophrynidae, go one better – adult females also have a chin barbel whose luminescence is provided by the fish itself, not bacteria.
Representatives from a primarily mesopelagic fish family, the dragonfishes (Stomiidae) are also among the dominant predators of the midnight zone, particularly when they approach maximum size. Dragonfishes do not possess the dorsal luring apparatus of the anglerfishes, but do possess a spectacular variety of chin barbels, some of which are as long as the fish itself and terminate in a chandelier of branches and multi-colored luminescent bulbs.
Both dragonfishes and anglerfishes display another adaptation common to bathypelagic predators – large, sharp, backwards pointing teeth set in a large, terminal mouth. Presumably in an environment where prey is hard to find, once prey are lured, one does not want them to escape capture!
Bathypelagic fishes also exhibit several adaptations to prevent being food themselves. Unlike the silvery fishes of the sunlit and upper twilight zones, most bathypelagic fishes are dark brown or black, presumably to avoid reflection of bioluminescence. Some bathypelagic fishes even achieve “blackness” by being red, a strategy made possible by the absence of red light at depth. Another fish group, the “tubeshoulders” (Platytroctidae), emit a brilliant blue cloud of luminescent fluid from an internal sac, via a tube (modified scale) in the “shoulder” region (hence the common name).
The “quietness” of the midnight zone also allows fishes to detect both predators and prey by listening. The primarily bathypelagic fish families Cetomimidae (whalefishes) and Chiasmodontidae (great swallowers) have some of the most highly developed acousticolateralis systems (lateral lines and associated pores and nerves) known of any fishes. In some cases, such as the swallower genus Pseudoscopelus, the fishes likely “listen with their face”.
These are just a few of the extraordinary adaptations exhibited by fishes of the midnight zone. Our goal as ocean exploration researchers is to expand on these discoveries, as well as add much more to our knowledge of the inhabitants of this “harshest ecosystem on Earth.”