The Remarkable Cleaning Behaviour of Reef Manta Rays
July 2023
A new study uncovers the remarkable cleaning behaviour of reef manta rays from underwater video footage.
Manta rays are in a global state of decline and are vulnerable to extinction. The Maldives is home to the largest known population of reef manta rays and although protected, they still face threats from human impacts. It is important to identify key aggregation sites, such as cleaning stations, to provide effective conservation efforts.
Cleaning stations are among some of the most crucial sites for manta rays. They provide grounds for a behaviour commonly known as “cleaning”. Cleaning is a process where small cleaner fish remove bacteria and detritus from manta rays, helping them to maintain their physical health. Injured manta rays use cleaner fish to help keep their wound free from infections, allowing quicker recovery and scar tissue regrowth. Cleaning stations are also areas of “sociability” for reef manta rays and are important grounds for mating rituals known as “courtship trains”.
A reef manta ray passes in front of a remote underwater camera in Raa atoll, Maldives © Jessica Haines
In 2020, the Manta Trust team based on InterContinental Maldives Maamunagau Resort began deploying underwater video cameras at three cleaning stations frequently used by reef manta rays inside Maamunagau Lagoon, situated on the southwestern corner. In 2022, Annabel Kemp, masters student at the University of Exeter, set out to delve deeper into the world of cleaning manta rays and use the remote underwater full video footage to answer some of our unknown questions.
Over three years, 470 sightings of 98 individual manta rays were recorded on these remote video surveys. Of these sightings, more than 83% were juvenile manta rays. These observations suggest juvenile reef manta rays have a stronger affinity for cleaning stations. This research provides more exciting evidence of a potential nursery ground and suggests that reef manta rays strongly rely on these important sites.
A Manta Trust researcher attaches a remote camera to a manta ray cleaning station © Jasmine Corbett
Before this study, no one had used RUVS to assess the cleaning activity of reef manta rays beyond capturing IDs. But with the help of a photographic identification database, Annabel investigated this remarkable behaviour by analysing fragments of over 500 hours of video footage. This is the first study that quantifies the length of time adult and juvenile manta rays spend cleaning at these grounds.
Interestingly, adults cleaned for much longer than juveniles. Even more so, adult females cleaned for an incredible average of 24 minutes longer than adult males. The most likely explanation for this is that size matters. Adults, particularly adult females, are much larger than juveniles. Therefore, they may take longer to be cleaned. Alternatively, it could be that cleaner fish prefer larger manta rays over small-bodied individuals. Either or both reasons could explain why adults spend longer at cleaning stations.
A reef manta ray passes in front of a remote underwater camera in the Maldives © Simon Hilbourne
As well as providing exciting knowledge about how long mantas spend cleaning, Annabel investigated how manta cleaning activities are affected by different environmental factors, including the moon and tidal phase. Both of which are known environmental cues for manta feeding activities. Interestingly, during conditions where feeding activities increased, cleaning activities also increased. For example, during a full and new moon, strong forces create dense patches of zooplankton, a source of food for manta rays. In this study, the full moon was linked to significant peaks in cleaning activities. The link between feeding and cleaning activities is recognised. However, it is unclear as to why this may be. It could be that after optimal feeding conditions, mantas require more thorough cleaning. Or they could visit cleaning stations to physiologically prepare or recover from feeding activities.
This research provides important information that can be used to inform important management decisions in the Maldives to help support the world’s largest known population of Reef Manta Ray. Although there are still unanswered questions, this study marks an exciting first step in learning more about the charismatic Reef Manta Ray.
ANNABEL KEMP
University of Exeter MSc student