Cohabitation promotes high diversity of clownfishes in the Coral Triangle

Abstract

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.

Keywords: amphiprion; anemonefish; biodiversity; coral reefs; mutualism; symbiosis.

Figure 1.

The relationship (r = 0.68, n = 20, p < 0.01) between the proportion of anemones that contained interspecific cohabiting clownfishes and clownfish diversity (S–W Index). Surveys were conducted between 2005 and 2014. The distance of each site from Hoga is illustrated by the shading of each point (see Legend). 95% confidence interval is shown (grey dashed lines).

Figure 2.

The proportion of cohabitation in clownfishes throughout the Indo-Pacific. The proportion of cohabitating clownfish at a location was determined by dividing the number of anemones per location hosting more than one species of clownfish by the total number of anemones hosting clownfish at the location. Surveys were conducted between 2005 and 2014 and published data from Elliott & Mariscal [27] and Riccardi et al. [26] were included. Grey shading indicates the proportion of anemones that were cohabited by different species of clownfish. The black line delineates the Coral Triangle as designated by Allen & Werner [16]. The dashed circle indicates Hoga Island.

Figure 3.

Aggression of (a) Amphiprion clarkii and (b) A. perideraion in inter- and intraspecific social groups on reefs around Hoga Island, South East Sulawesi, Indonesia. Aggression was determined by observing the dominant (largest) clownfish of each social group for a total of 10 min. In interspecific social groups, aggression was defined as intraspecific or interspecific, i.e. within or between species.

Figure 4.

Spatial patterns in usage of anemones by clownfish in intra- and interspecific social groups for Hoga Island, South East Sulawesi, Indonesia. The percentage of time clownfish spent in each quadrant was determined and colour coded accordingly. Territory analysis was determined for intra- and interspecific social groups, and for the dominant (largest) and subordinate (smallest) individuals.