Encountering a manta ray is a magical experience, and not uncommon in the Bazaruto Archipelago seascape. Both the reef manta ray (Mobula alfredi) and giant manta ray (Mobula birostris) are found in these waters. With manta season around the corner, we are already seeing aggregations of reef mantas feeding on the surface on trapped plankton fronts during expeditions, while further south from the archipelago there are various cleaning stations, where both species of manta rays regularly get cleaned by fish. Despite being a favourite species among divers, manta rays are threatened, with reef manta rays currently listed as ‘Vulnerable’ and giant manta rays as ‘Endangered’ by the IUCN Red List. By recording sightings of mantas and other animals, and correlating them with simultaneous oceanographic and environmental sensor parameters, we can learn more about these gentle giants.
Giant manta rays & reef manta rays
Manta rays have sparked the interest of scientists globally for decades, for one because the species have the biggest brain-to-size ratio of any fish, indicating high intelligence. Research has proven that manta rays can recognise themselves in the mirror, which is considered a high cognitive function. Though, only in 2008 scientists discovered that there are two species of manta ray, not one.
The two species look very similar, however, reef manta rays are typically 3 to 3.5 meter in disc width (the largest reef manta rays have been found in Mozambique, sizing up to 5m in disc width), while giant oceanic manta rays can impressively reach up to 9 meters in size. Other differences between the manta ray species are the fact that there are no spots in between the gills of giant manta rays and the dark edges at the end of the bottom of their fins. Reef manta rays are found globally in (sub)tropical waters, whereas oceanic manta rays are found in both (sub)tropical and temperature ocean waters. Being pelagic, giant manta rays travel further than reef manta rays, as they embark on ocean crossings, covering the Pacific, Atlantic and Indian Ocean, where the reef manta ray is only found in the Indian ocean and part of the Pacific Ocean and usually keeps to smaller, coastal areas throughout their lives, for example the Bazaruto Archipelago.
Reef manta rays (Mobula alfredi) sighting in the Bazaruto Archipelago, July 2022. (Photo by Daniel Escayola)
Distribution in the Bazaruto Archipelago
Manta ray season in the Mozambique Channel starts around February, when seawater temperatures drop from the peaks of the summer, and large amounts of plankton are trapped in fronts and eddies as well as after upwelling events. Strong and sudden Mozambique Channel wind reversals trigger upwelling of colder water (water being pushed up from the deeper ocean to shallower water columns) resulting in higher density of plankton: the primary food source of both reef manta rays (M. alfredi) and oceanic manta rays (M. birostris). The colder water contains high quantities of nutrients, which combined with sunlight warms up and may trigger phytoplankton blooms. After the arrival of phytoplankton, zooplankton typically follows as zooplankton feeds on phytoplankton. It is during this period that we see more manta rays in the Bazaruto Archipelago and surroundings. Our surface-sighting records show that manta rays predominantly occur in two areas: south of the Bazaruto Archipelago on the east side of Magarruque Island and near the Canyon (see below), as well as off the north side of Bazaruto Island. Reef manta rays are encountered regularly during scuba dives in other parts of the archipelago (e.g. 3-Trees, 9-mile, and 2-mile reefs), but giant manta rays are rare, as they spend most of their time in deeper waters. We have sighted both species at various cleaning stations in the region during dives. In February 2021 we saw a massive aggregation of giant mantas deep in the Canyon feeding on a plankton front, where more than 30 individual were seen in a single day.
Distribution of M. alfredi and M. birostris in the Bazaruto Archipelago, based on BCSS’s sightings.
Distribution in the Bazaruto Archipelago
We often encounter the manta rays surface feeding on plankton or, while scuba diving, at cleaning stations. Large mats of plankton often gather in the upper water columns, on which the manta rays feed for hours, gulping massive volumes of water and filtering the plankton through their gills. From the surface, it is easy to sight this feeding behaviour as the manta rays usually expose their backs above the water, occasionally slapping their pectoral fins as they change direction, creating paths of turbulent water. This behaviour is sometimes seen in groups, called cooperative feeding, also known as aggregation behaviour. They form a ‘chain’ of individuals, each manta ray hovering slightly above the one in front of them.
Another spectacular encounter is to see manta rays at cleaning stations. Both giant manta rays and reef manta rays pay frequent visits to cleaning stations in Mozambique, where their skin is being cleaned by cleaner fish species including butterfly fish (Chaetodontidae) and wrasses (Labridae). The fish gather around coral-rich areas on the reef and eat parasites, bacteria and dead skin off of the body and the inside of the mouth of the manta rays. This process is called a symbiotic relationship as both the manta rays and the cleaner fish benefit from the interaction. The exchange is incredibly important for the manta rays, since the fish eat the infected skin tissue and remove any ectoparasites living on the skin. Simultaneously, the fish gain a reliable food source.
A reef manta ray (M. alfredi) feeding on trapped plankton fronts (yellow/green layer on surface behind the animal). (Photo by Dr. Mario Lebrato)
The importance of manta rays
Manta rays’ feeding behaviour makes them one of the key species for the reef health. As they travel regularly between the deep ocean and coral reefs, the waste they accumulate while feeding on zooplankton acts as a fertiliser on the reef. The faeces contain nutrients not found in coral reefs surroundings, which help corals grow. The species also control plankton abundance and diversity and therefore act as an important connection between the deep sea and shallower water columns of the ocean.
Unfortunately, the commercial fishing industry poses a big threat to manta rays, as they are not only often targeted for their gills and dorsal fins to be sold in international trade, but also caught as by-catch of big fishing vessels. Discarded marine gear also opposes a threat as the gear gets tangled around their cephalic lobes and fins. Their slow reproduction rate (averagely one pup every 4-5 years) means it is difficult for the species to recover from the ‘Vulnerable’ and ‘Endangered’ IUCN statuses, and with their habitats and food sources steeply declining due to climate change, the need for better protection has never been more needed.
For questions about this article, please contact:
Iris Uijttewaal, Bazaruto Center for Scientific Studies
Bazaruto Center for Scientific Studies
Host of the first permanent Ocean Observatory focused on multi-ecosystem time series research in Africa, the Bazaruto Center for Scientific Studies (BCSS) was established in 2017 as in independent, non-profit organisation with a mission to protect and support the fragile ecosystems of the Bazaruto Archipelago, Mozambique. The research station is located on Benguerra Island, off the coast of Mozambique.