Ice Ages and Butterflyfish

When most people think of the end of the last ice age, images of woolly mammoths and sabre-toothed cats combing the permafrost for their next meal are conjured up. Paleolithic man (Homo sapiens), armed with custom stone spears and donning functional furs, might actually have been trailing not that far behind. While this might reflect the state of affairs on land, the evolution of fish in our oceans were probably influence by the repeated glaciations during the Pleistocene Epoch (~2,588,000 to 11,700 years ago) through sea level changes. Indeed, for many of our world’s tropical marine fish species inhabiting insular seas, such as the Red Sea and Arabian Gulf, a low sea level means that fish here might be cut-off from populations in less isolated waters, or extirpated altogether.

In our study we focused on the evolution of the unique and brilliantly coloured butterflyfish species (family Chaetodontidae) from the coastal waters of the Arabian Peninsula and their close relatives. Using advanced genomic tools to isolate ultraconserved element (UCE) loci, long stretches of DNA often conserved across vertebrates, we sequenced the more variable flanking regions of these loci to build our fish tree of life, albeit a greatly pruned one.

We tested three hypotheses using this butterflyfish tree as a template. First, we were interested in whether endemic butterflyfishes that evolved in this peripheral part of the tropics intermittently seeded the broader Indo-West Pacific during the Pleistocene. Think a cradle (i.e. source) versus a museum (i.e. sink) of biodiversity. Second, we tested the extent to which butterflyfish maintained a continuous presence in the Red Sea during the major environmental fluctuations of the Pleistocene. An entire camp of geologists believe that the Red Sea dried out every few 100,000 years or so leaving fishes no option but to flee or die. Alas there is no relation in this scenario, however, to the stories of Moses parting the Red Sea for the fleeing Israelites. Third, we tested whether the endemic butterflyfish might possess particular biological traits making them better adapted to the unique environmental conditions in these harsh seas.

In our study we found that despite generating significant biodiversity in the form of endemic butterflyfish species in the coastal waters of the Arabian Peninsula over the last two million years, few led on to increased species richness in adjacent seas (museum/sink = 1, cradle/source = 0). We also found that even though deep reef environments were drastically reduced during extreme low sea level stands of glacial ages, shallow reefs persisted, and as such there was no evidence supporting mass extirpation of butterflyfish in this region. These fish are survivors of much harsher times, but it is still not clear where and how they hunkered down for tens of thousands of years. We also found that the unique environmental conditions in this region probably contributed to the formation of endemic butterflyfishes via specialisation to their shallow reef habitat. This makes perfect sense because when the sea level drops, the only reefs that remain are shallow; slow growing corals rarely make sudden movements.

It is important to note that none of the dispersal-related traits in these butterflyfishes were associated with endemism, suggesting that factors other than those related to the distance its larvae can travel prior to choosing its final coral reef home to settle on may be important. These tiny larvae can see, smell, hear, feed, and swim, and are therefore capable of traversing large oceanic divides. There must be reasons why some butterflyfish species settle here and not there. As always with science though, our answers only generated more questions to investigate.

Joseph DiBattista – Scientific Officer