Anemonefish facilitate bleaching recovery in a host sea anemone

Ocean warming is causing the symbioses between cnidarians and their algal symbionts to breakdown more frequently, resulting in bleaching. For sea anemones, nutritional benefits derived from hosting anemonefishes increase their algal symbiont density. The sea anemone-anemonefish relationship could, therefore, facilitate bleaching recovery. To test this, bleached and unbleached sea anemones, both with and without anemonefish, were monitored in the laboratory. At the start of our experiment, algal symbiont density and colour score were lower in the bleached than unbleached sea anemones, whereas total chlorophyll remained similar. After 106 days, bleached sea anemones with anemonefish had an algal symbiont density and colour score equal to the controls (unbleached sea anemones and without anemonefish), indicating recovery had occurred. Furthermore, total chlorophyll was 66% higher in the bleached sea anemones with anemonefish than the controls. In contrast, recovery did not occur for the bleached sea anemones without anemonefish as they had 78% fewer algal symbionts than the controls, and colour score remained lower. Unbleached sea anemones with anemonefish also showed positive changes in algal symbiont density and total chlorophyll, which increased by 103% and 264%, respectively. Consequently, anemonefishes give their host sea anemones a distinct ecological advantage by enhancing resilience to bleaching, highlighting the benefits of symbioses in a changing climate.

Range extensions in anemonefishes and host sea anemones in eastern Australia: potential constraints to tropicalisation

Species distributions at range edges show complex shifts with climate change. The present study examined anemonefish and host sea anemone abundance at their southern distribution limits on the eastern coast of Australia, to identify factors influencing the geographic responses of the symbiosis. Roaming surveys (30min) were conducted at 21 sites (~30.01–30.95°S) on rocky reefs in a tropical–temperate transition zone. Two species of host anemones (Entacmaea quadricolor and Heteractis crispa) and anemonefishes (Amphiprion akindynos and A. latezonatus) were found, along with Dascyllus trimaculatus. Nearly all anemones and anemonefishes (>99.5%) were associated with vegetated islands and rocky islets, and abundance was influenced by seawater temperature. Greater numbers of H. crispa and A. akindynos than E. quadricolor and A. latezonatus were found; however, most A. akindynos were juveniles. H. crispa provided nursery habitat for both fishes, whereas adult fishes utilised only E. quadricolor. A southern range extension was found for E. quadricolor, and overwintering of A. latezonatus had extended poleward since the mid-1990s. The paucity of islands and rocky islets south of our surveys, and host-usage patterns, could constrain future range extensions. These findings showed climate-driven latitudinal shifts are complex and are likely to be variably constrained for different species.

Cohabitation promotes high diversity of clownfishes in the Coral Triangle

Global marine biodiversity peaks within the Coral Triangle, and understanding how such high diversity is maintained is a central question in marine ecology. We investigated broad-scale patterns in the diversity of clownfishes and their host sea anemones by conducting 981 belt-transects at 20 locations throughout the Indo-Pacific. Of the 1508 clownfishes encountered, 377 fish occurred in interspecific cohabiting groups and cohabitation was almost entirely restricted to the Coral Triangle. Neither the diversity nor density of host anemone or clownfish species alone influenced rates of interspecific cohabitation. Rather cohabitation occurred in areas where the number of clownfish species exceeds the number of host anemone species. In the Coral Triangle, cohabiting individuals were observed to finely partition their host anemone, with the subordinate species inhabiting the periphery. Furthermore, aggression did not increase in interspecific cohabiting groups, instead dominant species were accepting of subordinate species. Various combinations of clownfish species were observed cohabiting (independent of body size, phylogenetic relatedness, evolutionary age, dentition, level of specialization) in a range of anemone species, thereby ensuring that each clownfish species had dominant reproductive individuals in some cohabiting groups. Clownfishes are obligate commensals, thus cohabitation is an important process in maintaining biodiversity in high diversity systems because it supports the persistence of many species when host availability is limiting. Cohabitation is a likely explanation for high species richness in other obligate commensals within the Coral Triangle, and highlights the importance of protecting these habitats in order to conserve unique marine biodiversity.