Exploring the twilight zone in the Chagos Archipelago

For the past couple of weeks Catherine Head and I from Oxford’s Ocean Research and Conservation (ORC) group have been lucky enough to take part in the Berteralli Foundation Chagos Archipelago expedition. The Chagos Archipelago, officially known as the British Indian Ocean Territory, is the largest continuous no-take (i.e. no fishing) marine protected areas in the world, covering 397,667 sq miles. On the expedition we’re involved in studying the health of the reefs, particularly in the face of widespread coral bleaching currently occurring (see Catherine’s recent blog post). 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 Chagos.

Trevallys (left) and large, fragile sea fans (right) on twilight zone reefs at 58m in Chagos.

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 adapted to very low levels of light. Due to the remote nature of the archipelago, in recent times diver surveys have been limited to a maximum depth of 25m, so most twilight zone reefs in Chagos 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.

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Dom setting up the ROV unit.

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 Chagos earlier in the expedition.

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The endemic Chagos Clownfish (Amphiprion chagosensis) adjacent to an anemone at 37m.

The structure of the reef changes a lot in the twilight zone. One of the most common corals found on the shallow reefs of Chagos 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).

Shallow reef Porties (left) is much more rounded, whereas Porties found in the twilight zone forms flattened plates (right).

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.

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Grey reef sharks at 30m on a ROV twilight reef survey.

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