Host specialist clownfishes are environmental niche generalists

Abstract

Why generalist and specialist species coexist in nature is a question that has interested evolutionary biologists for a long time. While the coexistence of specialists and generalists exploiting resources on a single ecological dimension has been theoretically and empirically explored, biological systems with multiple resource dimensions (e.g. trophic, ecological) are less well understood. Yet, such systems may provide an alternative to the classical theory of stable evolutionary coexistence of generalist and specialist species on a single resource dimension. We explore such systems and the potential trade-offs between different resource dimensions in clownfishes. All species of this iconic clade are obligate mutualists with sea anemones yet show interspecific variation in anemone host specificity. Moreover, clownfishes developed variable environmental specialization across their distribution. In this study, we test for the existence of a relationship between host-specificity (number of anemones associated with a clownfish species) and environmental-specificity (expressed as the size of the ecological niche breadth across climatic gradients). We find a negative correlation between host range and environmental specificities in temperature, salinity and pH, probably indicating a trade-off between both types of specialization forcing species to specialize only in a single direction. Trade-offs in a multi-dimensional resource space could be a novel way of explaining the coexistence of generalist and specialists.

Keywords: anemone; anemonefish; intraspecific variation; mutualism; trade-off.

Figure 1.

Result of aggregating 100 stochastic character maps (the 10 used in the analyses and 90 others). The degree of shading gives the posterior probability (relative frequency across stochastic maps) of host specialization. Darker branches indicates more generalist mutualistic interactions with sea anemones. While the figure shows a specialist–generalist continuum, the original character maps are binary. Panel (a) shows results of the grouping C1 where specialists are interacting with a single host. Panels (b) and (c) show the results of the groupings C2 and C3, respectively. The number of potential hosts is shown left of the species names. ‘P.’ stands for Premnas and ‘A.’, Amphiprion.

Figure 2.

Posterior distributions of niche breadth optima (θ) for the second PC axis. The environmental niche breadth of host generalists species is shown in black and host specialists are in white. Vertical lines show the modes of the distributions and are coloured accordingly. The 95% confidence intervals (CI) are shown below the curves. Results of the grouping C1 are shown in panel (a), C2 in (b) and C3 in (c).