For decades, the idea that the deep-sea may contain a huge diversity of yet to be discovered species has sometimes been stated as a fact. The “stability hypothesis” suggests that it is environmental stability, as occurs in the tropics, allows species to diversify. Numerous graphs of species richness with depth, for different taxa at different locations, have show conflicting gradients, sometimes peaking in mid-depths. However, there are three reasons this theory is doubtful:
(1) Indeed, the deep-sea environment is stable on ecological timescales, at least compared to shallow seas. However, whether the deep-sea has been stable on evolutionary timescales is unclear; for example, perhaps it suffered de-oxygenation during the last world mass extinction.
(2) The fridge like conditions in the deep-sea at less than 4 oC below 500 m depth, and with low productivity due to the absence of light, would limit organisms growth, delay reproduction, and lengthen generation times by comparison with shallow tropical seas. This would slow potential speciation.
(3) Because the deep-sea is the largest habitat on Earth, with a similar environment over large areas, then deep-sea species have large geographic ranges, including large depth ranges. Species with large geographic ranges are discovered sooner than more endemic species (Costello et al. 2015). Thus it is likely that deep-sea species diversity is better known than expected by its limited exploration and inaccessibility.
As part of her PhD, Chhaya Chaudhary plotted the richness with depth for a dataset of over 50,000 species in 50,000 km2 equal-area hexagons. She used four measures of species richness and all declined rapidly with depth. The total number of recorded species rapidly declines in the first few hundred metres, but is highly influenced by sampling effort. Similarly, the average number of species (shown with error bars) also declines, but is influenced by sampling effort. However, ES50, the number of species in 50 random samples repeated so as to calculate a standard error and thus accounts for sampling effort, also clearly declines with depth.
Costello MJ, Lane M, Wilson S, Houlding B. 2015. Factors influencing when species are first named and estimating global species richness. Global Ecology and Conservation 4, 243-254. http://dx.doi.org/10.1016/j.gecco.2015.07.001.
Costello MJ, Chaudhary C. 2017. Marine biodiversity, biogeography, deep-sea gradients, and conservation. Current Biology 27, R511–R527. DOI: https://doi.org/10.1016/j.cub.2017.04.060.
Chaudhary C, Costello MJ. 2017. Species richness decreases with depth in the ocean. Deep-Sea Life No. 10, November 2017, 11-12.
The data for the above graphs is here Depth_gradient_500m_intervals