UNSEEN Expeditions - Cleaning Session in the Twilight Zone

Mesophotic Coral Ecosystems are coral habitats found between 30 and 150 or even 200 meters in tropical and sub-tropical regions. Due to the difficulty for divers to reach such depths, these ecosystems are poorly studied worldwide and a particularly important knowledge gap exists in Indonesia and other South-East Asian countries. These ecosystems can play a critical role in coral reef resilience, potentially providing refuge for some species at threat from human pressure in shallower waters. They also host their own unique range of endemic species. Therefore, like the better-known shallower coral reefs, they should also be considered when it comes to ocean conservation.

In the Bali region Mesophotic Coral Ecosystems offer cleaning opportunities to a wide range of species, notably to the emblematic Bumphead sunfish, Mola alexandrini. M. alexandrini is the heaviest species of bony fish roaming our Oceans, but curiously nothing much is known about its behaviour. In the eastern part of Bali, Indonesia, Bumphead sunfish seem to gather on the shallow coral reefs in specific regions, seeking interactions with cleaner fish for removal of skin parasites. But how about the deep cleaning stations ? Are they effective alternative to the shallow, heavily dived reefs ? What are the differences between the two ?

In an effort to contribute in answering these questions, Blancpain supported an international team of scientific deep divers working in close collaboration with local scientists from Udayana University. Three groups were working together: an oceanography team collecting oceanographic data such as temperature, salinity, etc., a shallow dive team surveying the depth range from the surface to 30 metres, and a deep team surveying anything below 40 metres and down to depths of 80 to 110 metres depending on the site. The two dive teams were recording any encounter with the fish species involved in M. alexandrini cleaning events, as well as all information related to the surrounding environment and any special sighting such as dolphins and sharks.

Data still needs to be analysed in detail, but preliminary analysis seems to indicate that the Nusa Penida area presents very sudden changes in water parameters over short periods, even on the deeper part of the reef. This, together with the daily strong currents, influences local biodiversity. Regarding the cleaning events, the main difference between shallow and deep reefs appears to be in the abundance and diversity of the cleaner fish species. Deeper cleaning areas (below 60 metres) seem to only host the Longfin bannerfish, Heniochus acuminatus, when shallower ones display up to five different cleaner fish species. Finally, some potential new species unknown to science so far as they are probably endemic to great depths, have been photographed.