Birdnest Coral Shows Us How Reefs Might Recover From Massive Bleaching

With all the news that the Great Barrier Reef is dying, and has suffered very substantial damage and countless corals lost, it can be quite depressing to be a coral or reef lover. But if there’s one thing the media is good at is focusing on the negative and alarming, because “if it bleeds, it leads”.

For sure there’s some serious degradation going on with wild coral reefs but there’s also some researchers that are focusing on how reefs will change and recover over time. There’s a very interesting and inspiring school of thought called the Deep Reef Refuge Hypothesis, investigating how deep living corals might repopulate the shallows (provided conditions are suitable for coral growth again).

A shallow water union of Seriatopora caliendrum on the left, and S. hystrix on the right.

The Deep Reef Refuge Hypothesis only pertains, naturally, to those species of corals that are flexible enough to grow in shallow and rather deep water, so shallow water specific-corals like most Acropora species can’t really benefit from this deepwater reserve. But corals like Seriatopora, Stylophora and some species of Montipora can have a rather wide depth distribution for a single species.

One recent study looked at Seriatopora birdnest corals which can be found growing in shallow waters in as little as 15 feet of water, all the way down to 120 feet in clear water. The purpose of the study was to investigate how the shallow and deepwater colonies differ in their breeding behavior, and yielded a ton of insight, at least from an aquaristic point of view.

Brooded larvae of Seriatopora can be clearly seen in fluorescent green inside the corallites of this branch fragment. On the right is a free swimming coral larvae called a planula

Due to living at depth, with less access to light and energy, deepwater birdnest corals had a reduced reproductive period, and their larvae were smaller and weaker than the shallow water counterparts. This somewhat hinders their ability to be robust recolonizers of shallow water reefs, but this is a physiological trait, not a genetic one.

With subsequent generations of deep living corals migrating a little bit more shallow, it is possible for these mesophotic corals to repopulate shallow water reefs, but on a pretty long timeline. Barring any kind of severe mixing or water flow disturbance, it won’t happen any time soon, but if we can restore and preserve the coral reef environment, the Deep Reef Refuge Hypothesis does hold hope that the ocean has a natural repository for coral reefs of the future. [Mesophotic]

You can clearly see the difference between the moderate depth Seriatopora above, and the more compact and tighter branching colony below from shallow water.

Jake Adams

Jake Adams has been an avid marine aquarist since the mid 90s and has worked in the retail side of the marine aquarium trade for more than ten years. He has a bachelor’s degree in Marine Science and has been the managing editor of ReefBuilders.com since 2008. Jake is interested in every facet of the marine aquarium hobby from the concepts to the technology, rare fish to exotic corals, and his interests are well documented through a very prolific career of speaking to reef clubs and marine aquarium events, and writing articles for aquarium publications across the globe. His primary interest is in corals which Jake pursues in the aquarium hobby as well as diving the coral reefs of the world.