Coral reefs are incredibly diverse marine ecosystems, supporting hundreds of thousands of species worldwide (1). In their shallow sunlit waters, coral reef fishes have evolved for over 50 million years (2), diversifying into myriad colors, shapes, and sizes.

Today, coral reef fish are found in tropical waters around the world, but where did they first emerge? We don’t know for sure, but evidence strongly suggests that the area of reefs bounded by Indonesia and the Philippines is their evolutionary crucible (3).

This region falls in the Coral Triangle (the area of reefs bounded by Indonesia, The Philippines, and Papua New Guinea). It is the epicenter of marine biodiversity today, containing half of the planet’s reefs, with the highest diversity of reef fishes and corals known (4) . It’s likely that all modern reef fish species are derived from this center of origin and have radiated out over time.

Why here? The answer is, in part, likely related to the geography of the Indonesian and Philippine Region. Many small islands are also scattered throughout the area, which could have served as isolated, distinct habitats. Over time, fish living on adjacent, but isolated reefs would have met unique challenges, and evolved into many distinct species to meet those challenges (3).

In recent evolutionary history, sea level has also changed frequently throughout the Coral Triangle. As sea level changed, some fish populations would have been split by geographic barriers– perhaps isolated by very deep, or by very shallow channels. A history of frequent isolation and reintroduction through sea level change could also contribute to the rapid diversification of reef fishes (3).

For more on reef fish evolution and larval dispersal see this excellent video lecture by California Academy of Sciences research biologist Luiz Rocha.

What’s it like to study coral reef fish? Amy McDermott and Erin Eastwood write about life in marine research, while studying coral reef fish evolution and conservation in Fiji.

Dig deeper references:

1. M.L. Reaka-Kudla, in Biodiversity II, M.L. Reaka-Kudla, D.E. Wilson, E.O. Wilson, Eds. (Joseph Henry Press, Washington, DC, 1997), pp. 83-108

2. Bellwood, David R., Lynne van Herwerden, and Nicolai Konow. "Evolution and biogeography of marine angelfishes (Pisces: Pomacanthidae)." Molecular phylogenetics and evolution 33.1 (2004): 140-155.

3. Mora, Camilo, et al. "Patterns and processes in reef fish diversity." Nature 421.6926 (2003): 933-936.

4. Bellwood, David R., and Christopher P. Meyer. "Searching for heat in a marine biodiversity hotspot." Journal of Biogeography 36.4 (2009): 569-576.

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