South of the great barrier reef, there is an island where manta rays (Manta alfredi) group together. A recent study on these rays has revealed the secret that drives their feeding behaviour, and may indicate why so many gather there. Mantas did not gather at a particular ‘lunchtime’ and it wasn’t based on how big or the type of available prey. The factor that determines their foraging is a power that influences oceans across the globe: the tide.

Previously, it seemed as though manta rays were stuck in a tricky situation. They feed by filtering plankton from the warm tropical and subtropical waters in which they live, and while this may seem pleasant enough, tropical seas have low levels of nutrients. In comparison to temperate and polar seas, there aren’t quite so many plankton there. The bountiful plankton aggregations in cooler waters are out of reach for the manta ray, whose thermal boundaries won’t let them venture too far from the tropics. It is just too cold for them.

At certain times of the year, there are predictable groupings of tropical plankton that many filter feeding marine organisms take advantage of, but these seasonal bounties are not enough to sustain the manta ray throughout the year. The strategies that the rays employ to forage successfully under normal conditions remained a mystery – until now.

Manta alfredi feeding Wikimedia Commons

Manta alfredi feeding
Wikimedia Commons

A recent study has reported that manta ray feeding is  observed predominantly just before the low tide. The reason for the rays’ affinity for this particular tidal phase is likely a result of the density of plankton in the water.

Once the researchers had made this connection, they looked back over around 5 years of manta ray sighting data, and were able to confirm that rays were most often observed feeding just before low tide.

Zooplankton biomass at the study site off Lady Elliot Island, Australia, was seen to change dramatically throughout the tidal cycle. The highest densities of plankton here were found approaching the low tide. The number of plankton is not changing during the short-term tidal cycle however, but the low tide results in up to 40 times more plankton concentrated into a smaller body of water.

As a result, the amount of plankton in each cubic meter increases, and filter feeding becomes a whole lot more efficient.  If the rays opted to feed at times and locations with a reduced density of plankton, it is likely that there would be a net loss of energy. So the rays will only reach foraging threshold when the plankton concentration is high enough.

Whilst this study provides evidence for the tide influencing manta ray foraging, it does not fully explain the oceanographic processes that cause plankton to accumulate in certain areas. And it doesn’t fully reveal why the manta rays gather here.

Manta rays have been observed to engage in courtship and cleaning behaviour in the waters off Lady Elliot Island, so there may be more to their grouping than just food. At the least, this study has highlighted the importance of aggregation sites to this species. Since plankton abundance is likely to be intimately related to climate change, the presence of these aggregations of manta rays is a useful indicator of the health of the plankton base of the tropical food chain.

Manta rays are classified as Vulnerable to extinction by the IUCN. If we can fill in the gaps in our knowledge of manta ray distribution and the reasons behind their movements, we can stand a better chance at implementing conservation methods. It is likely that this research will be used to identify other likely aggregation sites for manta rays, allowing us to further understand the biology and ecology of this graceful warm-water species.

References

Armstrong, A. O., Armstrong, A. J., Jaine, F. R., Couturier, L. I., Fiora, K., Uribe-Palomino, J., Weeks, S. J., Townsend, K. A., Bennett, M. B. & Richardson, A. J. (2016). Prey Density Threshold and Tidal Influence on Reef Manta Ray Foraging at an Aggregation Site on the Great Barrier Reef. PloS one, 11(5), e0153393.

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