Pisces: Scales, bony plate or bare skin?

Scales, bony plaques or bare skin? Researchers at the University of Geneva (UNIGE) have gone back to the evolutionary history of the largest line of fish (ray-fed fish) to better understand these properties. This allowed them to discover a link between the presence of scales and fish habitat.

Study on scale loss and bone plate gain from trunk in ray-finned fish, published in the journal Development letter, Goes back to studying the common ancestor of ray-finned fish (wings with thorns or rays) 420 million years ago. According to this study, “Ray-winged fish (Actinopterigrins) With over 33,000 described species, they represent the most diverse lineage of vertebrates in the world. The majority of these fish have scales as a protective shield. However, many lines show body armor made of bony plates on the trunk or lacking any protective structures ”.

Scales, among others a protective structure

During development some fish belonging to this great dynasty lost their scales. It may disappear before the presence of bony plaques in some cases. Bone plates are made up of full or partial pectoral bone as opposed to scales. Researchers observed that only fish that had previously lost their scales, for example, were likely to develop bony plaques such as the hippocampus.

Other fish will remain “naked”, that is to say, without scales or bony plates. In fact, very few cases of re-acquisition of scales have been seen in fish that have lost them, so it would be difficult to reverse the occurrence of loss of scales.

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Housing and Scale Disadvantages: A Surprising Relationship

Hence researchers Alexander Lemopoulos and Juan I. Montoya-Burgos tried to understand the difference between these fish. It appears that fish without scales have a greater tendency to live in bentic environments (under oceans or lakes), in contrast to those bearing scales. In addition, those who are devoid of it increase the respiratory capacity of the skin, which is necessary in a quadratic environment where oxygen content is often reduced and water speed is limited. This is a better immune response for these fish, which seems beneficial in a sea full of germs. In fact, it has been shown that skin areas without scales, for example, show relatively high expression of immune genes after parasitic infection.

Thus, the researchers found that the absence of scales would be associated with habitat. However, it is difficult to determine whether this development is a cause or a result of the living environment. In fact, it is not yet known with certainty whether fish without scales moved to live in bentic environments because they are more adapted to it, or if fish living in bentic environments suffered further scale losses. They (by genetic modification) had better approaches to the living conditions of this ecosystem that remained there. Nevertheless, the presence of fish families without scales in open water and not only in the aquatic floor has been observed, but the study confirms the hypothesis of “pre-adaptation”. First, fish lose their scales while living in open water, and secondly, they will colonize the bentic environment because the absence of scales will give them a selective advantage. Thus, researchers have concluded that the loss of the scales would have allowed effective colonization of the ecological niche of the sea and lake beds, which would partially explain the vast current biodiversity of ray-finish fish.

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