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.

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Invasive species are of increasing concern to the local biodiversity and ecology as the magnitude of biological invasions is increasing globally. The genetic structures of newly established invasive populations may reveal insights towards the invasion processes, making population genetics an important tool for understanding current invasions. Here we study newly established populations (<10-20 years before first sampling) of the cosmopolitan alien ascidian Botryllus schlosseri in four Puget Sound (Washington, USA) marinas, using eight polymorphic microsatellites. Up to seven sampling sessions over a period of 19 years revealed populations with fluctuating allelic richness (AR=2.693-4.417) and gene diversity (He=0.362-0.589). The populations were well differentiated on spatial and temporal scales and subjected to moderate genetic drift (Fs'=0.027-0.071). The obtained significant heterozygote deficiencies, positive inbreeding coefficients (Fis) and population structure measures (Fst) revealed that no population was under the Hardy-Weinberg equilibrium. Comparing these parameters with those from two Californian sites (Moss Landing and Santa Cruz, 1200 km southerly; invaded by Botryllus during 1940's) revealed a connection between Moss Landing and Puget Sound, while Santa Cruz remained isolated. On the US west coast scale, this study revealed no major difference in invasive population dynamics between recently and decade long established populations, except for fewer alleles and lower He. When comparing ten worldwide sites, only few microsatellite loci displayed strong regional differences. With globally the lowest numbers of alleles and lowest genetic indices, the Puget Sound Botryllus populations exhibit genetic characteristics of recently established populations, further emerging as one of the youngest B. schlosseri populations, worldwide.