Abstract 

Most coral reef fish have a biphasic life cycle, in which an oceanic larva transforms into a reef juvenile/adult. This transition is called metamorphosis. Metamorphosis is triggered by a peak in thyroid hormones that activates or represses target genes responsible for many morphological, physiological, and behavioural changes that occur during metamorphosis. The thyroid hormones that trigger and regulate this transition process, are sensitive to many biotic factors (presence of predators, intra- or interspecific competition) or abiotic factors (temperature, pH, salinity, or pollution). 

The modification of these factors in the environment is defined as environmental variations. Whether anthropogenic or natural, environmental variations can alter metamorphosis by interfering with the thyroid hormone signalling pathway. During this thesis, I studied the effects of an endocrine-disruptor insecticide on the anemonefish, Amphiprion ocellaris and demonstrated in laboratory, the alteration of metamorphosis at the molecular, hormonal, and morphological level via thyroid hormones disruption. I then attempted to understand how the environment could modulate this crucial transition for coral reef fish by analysing the morphological variations induced by different types of habitats during recruitment in the convict surgeon fish, Acanthurus triostegus. Differences in habitat (mangrove, beach, rocky area) during settlement have led to variations in growth rate, shape trajectory and morphological disparity. In addition, using treatments that decrease or increase the thyroid signal in anemonefish, I was able to demonstrate the involvement of thyroid hormones in shape variation processes during metamorphosis. This work thus highlighted the important role of metamorphosis and the environment in the production of phenotypic diversity. In the context of increasing climatic variability, this thesis highlights the importance of the environment in life history transition, such as metamorphosis.