Exploring the twilight zone in the British Indian Ocean Territory

For the last week, scientists on the Bertarelli Foundation’s expedition to the Chagos Archipelago in the British Indian Ocean Territory have been carrying out research in a wide range of ecosystems. Dominic Andradi-Brown, from the University of Oxford, is taking part in the expedition and has provided the following update about the work he has been conducting in the ‘twighlight’ or mesophotic zone:

For the past week Catherine Head and I from Oxford’s Ocean Research and Conservation (ORC) group have been lucky enough to take part in the Bertarelli Foundation funded expedition. We’re involved in studying the health of the reefs, particularly in the face of widespread coral bleaching which is currently occurring across the globe. In addition to this reef health monitoring, a major focus for my work is to conduct some of the first exploration of the twilight zone reefs of the Chagos Archipelago.

The twilight zone, known scientifically as mesophotic coral ecosystems, includes coral reefs from 30m to 150m depth. These reefs are characterised by light dependent ecosystems, but are adapted to very low levels of light. Due to the remote nature of the archipelago, diver surveys have only been listed to a maximum depth of 25m, meaning that most mesophotic reefs have never been scientifically surveyed.

So why are we interested in the twilight zone?  Many of the impacts that cause most damage on shallow reefs in Chagos, for example processes such as coral bleaching and direct storm damage, are believed to decline in severity at greater depths. This means that twilight zone reefs may act as a refuge for shallow reef life.

We’re using a remote operated vehicle (ROV) to survey the upper twilight zone around the Chagos Archipelago in the 30-60m depth range. Already we’ve had many exciting findings! For example, the charismatic Chagos Clownfish (Amphiprion chagosensis), found only in Chagos, had previously been found down to 25m, we’ve extended that known depth range down to 37m after documenting several individuals in an anemone off Peros Banhos in the north of the archipelago earlier in the expedition.

The structure of the reef changes a lot in the twilight zone. One of the most common corals found on the shallow reefs belong to the genus Porites. On shallow reefs these corals have distinctive rounded boulder shapes. At twilight depths we’ve documented very flattened plate-like Porites colonies. We think this change in shape is an adaptation to the lower light levels on these deeper reefs, as this pattern has been observed on twilight reefs elsewhere in the world. However, researchers are still trying to understand the advantages to corals of becoming flatter, particularly at the fine scale (something Jack Laverick in the ORC group is actively working on).

As well as the seabed reef-specific twilight zone surveys, when deploying the ROV we’ve often found lots of sharks at twilight depths. Mostly these have been grey reef sharks (Carcharhinus amblyrhynchos) that have been interested in the ROV unit, circling in closer to look. On a couple of occasions, during ROV surveys in one of the Chagos atoll lagoons we found black-tip reef sharks (Carcharhinus melanopterus). What is clear from the ROV surveys is that sharks in Chagos are regularly visiting twilight reefs, further reinforcing the importance of these deeper reef habitats to larger mobile predatory species in the marine reserve.