Last but not least, I wish to express my deepest gratitude for my family: my mom, my dad, who took care my little daughter during three years of my research and study. They sacrificed their happiness for my happiness, their days for my days. I would like to thank for my sisters and my brother for your effort to look after my daughter during my absence. Above all, I would express my special thanks to my husband Cao D.V and my daughters, Lan Chi and Bao An, for your endless love, understanding and support. You are the most important persons in my life and have been my greatest motivation.

In this study, we investigated the host choice of naïve Amphiprion ocellaris, a specialist, at two different stages of development (newly settling juveniles and post‐settlement juveniles). The fish were exposed to their natural and unnatural host species in the laboratory and their fitness was assessed in terms of activity and growth rate. Newly settling juveniles exhibited little host preference, while post‐settlement juveniles immediately associated with their most common host in the wild. The analysis of fish activity confirmed that A. ocellaris is diurnal; they are most active in the morning, less at midday and barely move at night. The average travelling distance of juveniles was shorter in the groups living with their natural host, increasing in the groups living with an unnatural host and was highest in groups that did not become associated with any other unnatural host species. Post‐settlement juveniles living with the natural host species grew better than those living with unnatural hosts or without anemone contact. These results suggest that the welfare of A. ocellaris in captivity will be optimized by keeping them with their natural anemone host species, although more generalist Amphiprion species may survive in association with other hosts.

Anemonefish are iconic examples of marine fishes living in mutualistic symbiosis with sea anemones. In a given sea anemone, the anemonefishes have a stereotyped social organization with a dominant female, a semi-dominant male, and several juveniles. A strict size-based hierarchy governs the social interactions within these colonies, with each individual differing from the previous or next fish in the order by +/- 20% size. This social organization is conserved across the Indo-Pacific in all 28 species of anemonefish found on any of ten giant sea anemone species. We report the existence of huge "megacolonies" of up to 100 fish living in large carpets of sea anemones. This alternative organization was observed for different fish and anemone species in different coral reef locations (French Polynesia, Japan, Taiwan, and Vietnam). In these colonies, the strict size-based hierarchy is no longer recognizable, and the level of aggressivity of the different members appears lower than in "normal" colonies. These megacolonies may correspond to a previously overlooked type of social organization that could be linked to host availability and offer a unique opportunity to understand anemonefish's behavioral, social, and hormonal plasticity.