Fish are shaped by the shape of the ocean floor. That isn’t a scientific claim, but it sums up what an Ohio State team discovered when they examined almost 3,000 deep-sea fish species and charted how their bodies have evolved over time. After analyzing data on 2,882 species and publishing their findings in the journal Evolution, the researchers discovered a pattern that is cleaner than most evolutionary studies are able to produce: fish that inhabit ocean basins that have been isolated from one another for millions of years consistently arrive at the same body design.
Convergent evolution is the term for this phenomenon, which has been repeatedly observed in shallower, more accessible environments. Everybody learns about Darwin’s finches in beginning biology as an illustration of how isolated populations under comparable conditions evolve identical beak shapes.
Australia’s marsupials have developed body forms that are similar to those of placental mammals on other continents, such as the wolf and the thylacine, the sugar glider and the flying squirrel. This is because the environmental niches that these species occupy are sufficiently similar for evolution to repeatedly return to the same patterns. The Ohio State study provides evidence that the unique combination of stresses associated with living on a seafloor at great depth is causing the deep ocean to do this at an unprecedented scale and consistency.
When you consider the environment, the elongated body form that benthic deep-sea fish—those that truly live on the bottom instead of swimming in open water—keep evolving makes functional sense. A slender, tapering body has a lower profile for negotiating the topography of an abyssal plain or a seamount slope, glides effectively through sediment-heavy water close to the bottom, and lowers the metabolic cost of maintaining position in a high-pressure environment where energy budgets are limited. Even in lineages that split apart tens of millions of years ago, natural selection continues to produce similar solutions because the bottom imposes sufficiently similar limitations whether the fish is in the Atlantic, Pacific, or Indian Ocean.
One of the study’s more intriguing structural conclusions is the difference with pelagic fish, which swim in the open water column as opposed to the bottom. Pelagic deep-sea fish have a significantly greater variety of body shapes, including stubby profiles and eel-like ribbons of muscle and cartilage, as well as rounder, compressed forms and incredibly extended ones. Compared to the seafloor, the open water column at depth provides greater dimensional freedom. The various foraging techniques available in open water, such as ambush predation, active pursuit, filter feeding, and scavenging, promote greater morphological variation because there is no substrate placing a constant physical constraint on body form. In contrast, the spectrum of effective body layouts is reduced by the seafloor, and this reduction is stable enough across basins that convergent evolution consistently reasserts it.
Evolutionary scientists will likely look closely at the depth-evolution rate association that the study finds. Not simply increased convergence, but faster rates of general morphological evolution were linked to deeper ocean depths. Extreme conditions obviously limit some outcomes while accelerating change within that constricted range, which goes against a basic perception that says what’s conceivable is limited. The deep ocean’s energy scarcity may put a particularly strong selection pressure on body efficiency, favoring forms that even marginally lower metabolic costs and eradicating forms that are unable to survive in the resource-constrained abyssal environment. However, this study does not fully explain the mechanism linking depth to evolutionary pace; rather, it is a discovery that suggests more research rather than a definitive answer.
A comparative morphological examination of 2,882 species is an exceptionally big dataset for deep-sea fish research, and it’s important to note that the collection of museum specimens used in this analysis was assembled over the course of more than a century of ocean exploration. A large portion of our knowledge regarding the morphology of deep-sea fish originates from specimens gathered in trawl surveys that were never intended for evolutionary study; these creatures wound up in the collections of natural history museums as byproducts of larger sampling initiatives. The Ohio State study, which was arranged and examined using phylogenetic techniques that were unavailable when the majority of those specimens were gathered, is partially the result of that accumulated institutional record.
It contributes to our understanding of deep-sea evolution by providing a large-scale confirmation of something that scientists had suspected but had not shown at this level of detail: fish have been independently reaching the same conclusions about body shape in every ocean basin where the appropriate pressures are present.
