While coral reefs are known to harbour an astonishing and extensive diversity of species, few studies have been performed on the role of the pelagic environment - or, in other words, the deep blue sea - in species diversification. However, a recent study by the University of Liège has shown that this pelagic environment and other associated environments can lead to a greater diversity of shapes in some groups of fishes, compared with reef environments.

Speciation is a complex and fascinating phenomenon, and a breeding ground for many scientific publications. Its complexity can be explained by the fact that numerous factors, both intrinsic (genetic diversity, morphology, etc.) and extrinsic (new ecological niche, competition, predation, etc.), may be responsible for diversification processes. 

In the marine realm, it has been proved that tropical coral reefs were (and are) a driver for diversification among several fish clades. This complex and highly productive habitat offers many possibilities for diversification, both from a morphological point of view (i.e. disparity) and in terms of species (speciation). For many marine fish families, different studies have thus identified greater diversity in reef environments than in non-reef environments. 

Life outside the reefs

Bruno Frédérich, from the Laboratory of Oceanology at the University of Liège, and his colleagues from the universities of Turin and Pisa, decided to go against these findings: what about the role of non-reef environments, such as the pelagic environment (in the open sea, more poetically known as the 'deep blue sea'), or sandy areas close to the coast, in the evolution of species?

"The idea behind this paper (1) is to demonstrate the opposite of what is generally accepted concerning reef environments", explains Bruno Frédérich, the main author of the study."We started out from the hypothesis that it is actually the pelagic marine environment, i.e. the 'deep blue sea', or even shallower environments albeit associated with sandy areas, that can be conducive to diversification".

And to test the hypothesis? A fish clade. One that must have a sufficiently large number of species, including enough fossils, in order to be able to follow the evolution of their diversification, and the possibility of finding species associated with reef and non-reef species. The winner? The group of carangoid fishes, i.e. teleost fishes such as jacks or remoras with suckers, known for their phoretic association with sharks and other large fish. 

Databases made available by scientists worldwide 

It was essential to obtain morphological and molecular data for the study in order to apply a timescale to this clade's diversification. This data was collected from public databases and museums.

"The current trend in the scientific world is to make data accessible to everyone, especially through websites such as Morphobank, or sites offering genetic data, in particular PubMed." 

Example of images that were used for the morphological study of Carangoid fishes. Trachinotus blochii top left, Carangoides chrysophrys top right and Exheneis sp. below.

(1) Non-reef environments impact the diversification of extant jacks, remoras and allies (Carangoidei, Percomorpha), Frédérich Bruno et al. Proceedings of the Royal Society B : Biological Sciences, nov. 2016.