Morphological evolution in the deep blue sea

FIG2 Arbre Carangidae

Summary of the ecological (diet and habitat) histories on the consensus timetree of carangoid fishes using stochastic mapping. We can see here that it isn't possible to fully ascertain whether the common ancestor lived in a reef or non-reef habitat (50/50). 

"In our analyses, we divided the type of biodiversity into two different blocks: the number of species and disparity, which corresponds to morphological diversity. Hence, there are two types of diversity that are explored differently. With time-calibrated phylogeny, the length of the branches express the time lapse between the divergence of two lineages. With the information regarding time, we can calculate the rate of speciation. And in our case, we didn't observe any differences in the rate of speciation, regardless of whether the species is a reef-dweller or not."

FIG3 Disparite habitat

...but an effect on disparity

However, as regards disparity, analyses of morphospaces reveal something completely different, as Bruno Frédérich explains: "There is greater diversity in shape in the current non-reef environment than several million years ago. Therefore, the calculations regarding shape disparity levels  coincide with the calculations concerning rate of morphological evolution: this shows that 50 million years ago, there was clearly less morphological diversity in the non-reef environment compared with today."

> Level of disparity according to the periods and habitats, through two methods of calculation, (a) and (b). The two graphs show that disparity is currently greater in the non-reef environment than during the Eocene.

"This shows that the non-reef environment clearly had an impact on the diversification of these fishes. If we use phylogeny to do the calculation, there is strong support for a model with two rates of morphological diversification:  the rate of morphological evolution among non-reef dwellers was nearly twice as much compared with reef-dwelling species."

Ecological niches left vacant

It is necessary to be able to swim fast, over long distances and be a master of camouflage in pelagic environment, which offer very little physical shelter. Elongated bodies and scales reflecting the light are therefore major advantages in terms of survival. "Several types of shape appeared, which are a compromise between travelling over long distances, at high speed, and optimising camouflage."

Hence the study's conclusions on the factors that may have led to this diversification: "Our hypothesis is that since many species have disappeared from the pelagic environments, the Carangoid fishes were able to 'sweep' into the available space and occupy the niches that had become available. But as to why it was these fish that explored this environment, that's another question to which I don't yet have an answer. However, we do show that the pelagic environment was a factor of diversification for this group of fish."

An important discovery because up until now, very few studies have been able to prove that the pelagic environment could also lead to diversification.

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