Reef manta rays (Manta alfredi) are familiar to divers and snorkelers of tropical waters in the Indian Ocean. They normally are sighted in shallow coastal reefs where they gather to feed on tiny plankton or to reproduce. But these captivating animals face significant threats from commercial fishing where they now often categorised as targeted by-catch. Populations in many countries have rapidly declined and the IUCN has listed the species as “vulnerable to extinction”.
Despite these threats, we still know very little about this enigmatic creature; for instance, nobody knows how deep they can dive or how far they travel. With this in mind, a group of researchers tagged nine manta rays off the coast of Al Lith, Saudi Arabia in the south-central Red Sea. Unlike previous projects, this study used satellite telemetry which enabled the researches to precisely document the diving depths of the manta rays for the very first time.
The data revealed that the mantas tended to remain in the upper 50 m of the water column every day. However, there was a clear difference between their night-time and daytime activities. During daylight hours, the manta rays stayed close to the surface of the water, but once dusk fell they descended to an average depth of 35 m. This ties in with previous studies which have speculated that manta rays filter-feed on plankton as it rises from the deep at night.
But the data also revealed another intriguing side to these animals: very occasionally they dived to much greater depths. One individual was even recorded as reaching 432 m, the greatest depth ever recorded for this species. This finding completely changes our view of this animal, which had previously been regarded as a predominantly reef-orientated species.
These infrequent deep dives all occurred in the same way, with slow descents, relatively short bottom times, and rapid ascents. This led the authors to believe that these dives were used as a means of travelling a particular distance and not for consuming plankton. Mantas are negatively buoyant and will sink slowly downwards if they stop swimming. This means that these deep powerless glide dives followed by active ascents are actually a far more efficient means of travelling from one place to another than continuous swimming would be.
The reef manta ray now joins a small group of species, including the Japanese flounder and whale sharks, which are known to use this type of gliding behaviour. In this way, they may act as an unexpected link between surface waters and the deep sea.
Reference: Braun CD, Skomal GB, Thorrold SR, & Berumen ML (2014). Diving behavior of the reef manta ray links coral reefs with adjacent deep pelagic habitats. PLoS ONE, 9 (2) PMID: 24516605
Image: Marcio Lisa