Le site de vulgarisation scientifique de l’Université de Liège. ULg, Université de Liège

The farmer fish
1/6/15

Damselfish morphoAnother feeding habit

The researchers underline that the loss of this ligament is not linked to a change of life with regard to the proximity of the reef area. Some species which do not have the ligament do not wander far from the reef, like most damselfishes. “And even so, when we are talking about species that live in the water column, they remain quite close to the reefs, do not wander more than a few meters from it and only develop in shallow water. At night, they return to sleep among the coral reefs or rocky anfractuosities. The morphological evolution is therefore not linked to a change of habitat but to a dietary specialization”, continues Damien Olivier. Usually, damselfishes are quite opportunistic when it comes to eating as the observations of the young researcher have shown in the context of his thesis. According to the species, they have developed three different types of diet. The first consists of algae attached to the substrate, a second, consists of zooplankton in the water column, and a third which groups together a series of intermediate species. “What we noticed, however, was that all the species lacking the ligament are exclusively zooplanktivorous. Among the species with the ligament, the three types of diet were observed. During the course of evolution, some lineages went from being zooplanktivorous to algivorous and vice versa. Once they lost the ligament however, they no longer ate algae. It seems that there was no going back after the loss of the ligament”.

So how did the ligament disappear during evolution? The question remains without a definitive answer even though the logic of the mechanics of evolution makes it possible to put forward plausible theories. “Just because a fish began to feed itself exclusively with zooplankton does not cause the loss of the ligament during the evolution”, Damien Olivier interjects. “There is no causality of this type. On the other hand, what is possible is that at a given time and due to genetic mutation, a fish was born without this ligament and survived by continuing to catch zooplankton, and that it transferred its genes during reproduction thus introducing a new phenotype without the ligament. By adopting a new morphology during the course of evolution, this made them better hunters of zooplankton, the species without the ligament would have been able to perpetuate their line and diversify. This is one theory which we are attempting to verify by means of a new study” Bruno Frédérich continues, “Or there may have been a sexual selection made linked to sound. Perhaps certain females were more attracted by another acoustic sound linked to the absence of the ligament that the species reproduced more often and this phenotype was able to perpetuate”. 

A more rapid morphological evolution without the cerato-mandibular ligament

The discovery of the ligament and its role in the feeding habits of damselfish makes it possible to identify its presence as a veritable functional adaptation. It was a key innovation which enabled damselfishes to forge a place in the coral reefs by adopting this ‘farming’ behavior. Within fifty million years, the damselfishes successfully radiated in the modern coral reef ecosystems. This success can be partly explained by this morphological trait.

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