Exploring Deep Reefs

Anjani Ganase, PhD candidate studying coral reef ecosystems at the University of Queensland in Australia, presents her colleague’s study of differences between coral reefs in shallow and deeper water. The findings in Dr Pim Bongaerts’ research may have implications for coral reefs around the islands off the north of Tobago.


Latest scientific findings reveal that shallow water coral reefs that are more prone to disturbances such as wave action, storm damage and bleaching events, are unlikely to be reseeded by corals living in deeper, more protected waters.  Even though coral species may occur across a large depth range, they can evolve into different strains or  “breeds” adapted to either the shallow or deep water environments, limiting the connectivity between these sections of the reef. These were the finding of Dr. Pim Bongaerts, a coral reef scientist at the University of Queensland in Australia. The story of corals and coral reefs gets even more complicated.

Pim’s interest in coral reefs began in the Western Caribbean. While taking a break from exploring his first love rainforest ecosystems, he learned to dive in Bocas del Toro, an island off the coast of Panama. Pim never returned to the forests, instead he focused his career on understanding the complexities of corals. I met Pim six years ago when he invited me to work with him at the Heron Island Research Station in the Southern Great Barrier Reef. As the head of the Deep Reef Scientific Team (affectionately known as the “Deep Cats”) for the XL Catlin Seaview Survey, he was busy planning his biggest expedition to the Great Barrier Reef where he would explore the largely unknown deep, mesophotic or low-light sections of coral reefs. Researching deeper coral reefs, from 40 m to 150 m, requires considerable planning and training in technical diving to ensure safety while surveying these reefs.

Dr Pim Bongaerts surveying mesophotic (low-light) coral reefs using a dive propulsion vehicle. Photo by The Ocean Agency, XL Catlin Seaview Survey.

The Deep Refuge Hypothesis suggests that while shallow reefs are naturally more prone to physical disturbance such as wave action, or coastal activities, reefs located at greater depth may be more protected from the surface disturbances. Should a shallow reef be damaged or destroyed, these deeper reefs ought to be a potential source of coral re-growth during the recovery stages of the shallow water reef.

Pim wanted to test this theory by looking at the genetic differences between coral species living at different depths. Strong genetic differences between corals from shallow and deep reefs may imply that the coral species have adapted to these distinct reef environments, and that the deep coral may not necessarily be able to survive the shallow water environment. Conversely, a lack of genetic differences means that the corals form a single, interbreeding population and that deep reefs may provide a source of larvae for the shallow. However, proving this genetic variation was not as straightforward as one might think. Pim needed to explore and apply novel genome sequencing techniques for identifying such genetic variations within the same coral species.

Pim in submersible transporting coral samples at depth. Photo by Substation Curaçao.

For this research, Pim went to the remote island of Bermuda in the Western Atlantic. Multiple sites around the island were surveyed at two depths -12 and 40 m. At each location, two coral species with two distinct modes of sexual reproduction (brooding and broadcast spawning) were sampled. Reproduction by broadcast spawning refers to the simultaneous release of coral eggs and sperm into the water column and their fusion in the water column results in coral larvae. The larvae then look for a suitable location to settle and grow. Conversely, brooding corals carry out fertilization internally within the colony and then larvae is released into the water column. As the eggs and sperm from the broadcast coral spend a longer time in the water column, their larvae are more likely to drift much farther distances. These reproductive modes might have influenced the connectivity of coral species between shallow (12 m) and deep (40 m) sites, since the brooding coral species showed greater genetic difference between depth and location compared to the broadcasting coral. Therefore, even though shallow and deep coral reefs may have overlapping coral species, strains of coral species may prefer one depth to the other.

Dr Pim Bongaerts using a remote operated vehicle (ROV) to survey mesophotic coral reefs. Photo by Global Change Institute, University of Queensland.

What does this mean in terms of the survival of our reefs in the broader Caribbean region and Tobago? Pim’s study forces us to consider the diversity of our own coral reefs at different locations and at different depths. There may be greater variations at depth than we can imagine; and therefore different sensitivities to disturbances. Consider the reefs that surround the islands off Speyside in the Atlantic Ocean. Do we even know how much variation exists? The reefs in the sheltered bay of Goat Island are likely to be very different from those on the exposed side of Little Tobago; and different again when we explore deeper. To conserve these coral reef habitats, it is crucial first to protect them – the Marine Protected Area proposed by Environment Tobago is a start – then we can study to fully understand the depths, diversity and uniqueness. This is the nature of eco and educational tourism that we might be exploring.

Link to full publication:
http://advances.sciencemag.org/content/3/2/e1602373.full