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Feeding Frenzy

Mass feeding aggregation of reef manta rays at Hanifaru Bay MPA in the Maldives.

Despite being one of the largest creatures in our oceans, manta rays feed almost exclusively on some of the tiniest animals in the marine world. In order to survive on these miniature planktonic snacks, mantas have evolved into perfect plankton feeding machines; roaming the oceans in search of their tiny prey which they filter from the water with giant mouths and sieve-like gill plates. This page talks about the manta rays and their microscopic prey; how they find it, and what morphological and behavioural adaptations they have evolved to fully exploit this planktonic food source…

What is Plankton?

To understand how and why mantas feed in the ways they do we first need to understand the food they are chasing.  The word plankton is derived from the Greek language and translates to errant, which can be taken to mean “drifter” or “wanderer”.

A sample of manta food zooplankton magnified.

The term is applied to a group of organisms which are defined by their ecological niche rather than their taxonomic classification. Plankton are defined as weakly swimming or drifting micro-organisms that inhabit the pelagic zone of the oceans, seas and bodies of fresh waters and which are unable to resist ocean currents. They encapsulate a huge variety of different species, many of which are planktonic only for a portion of their lifecycle; for example, most coral reef fish larvae drift around as part of the planktonic soup until they mature into adults. Plankton are divided into two main functional groups; the first being phytoplankton, which includes the plants, such as algae, which are able to produce their own food through photosynthesis, while the second group contains all the animals which are referred to as zooplankton.  While still tiny, the zooplankton are generally much larger than phytoplankton, and so it is this food source which the manta rays, and other large marine filer feeders such as the whale shark and basking shark, feed upon.

One of the most diverse and abundant zooplankton groups found in the oceans are the copepods, and it is these tiny animals (along with arrow worms, mysid shrimps, and a host of other tiny critters) which the manta rays prefer to dine upon. Each of these tiny water flea-like crustaceans is rich in energy, but in order to sustain themselves manta rays must extract vast quantities of these animals from the water column.

Although plankton could be considered to be one of our oceans most abundant food sources, the nature of plankton means it is not evenly dispersed throughout the marine realm; concentrating in certain ‘hotspots’, which are often ephemeral, forming with the ebb and flow of the tides or the shifting of the seasons.  Mantas have become experts in predicting and seeking out these productive areas of food and taking advantage of them when they occur.

Feeding Machines

Two specialized cephalic "head" fins funnel plankton rich water through five huge pairs of gill plates.

So how are mantas adapted to make the most of this quite unusual food source? Manta rays are known as filter feeders and this term most accurately describes the way in which mantas are able to consume their microscopic food. Ingeniously designed manta rays are  efficient and expert foragers, roaming the seas and oceans in search of food. Their sleek and hydrodynamic bodies appear almost two-dimensional at first glance, driving them through the ocean in search of food. Horn-like projections in front of each eye border the edges of a wide smile that only hints at the depths of a cavernous mouth which lies within.

Upon finding an abundant food patch mantas transform into feeding machines, unfurling their horn-like cephalic fins from either side of their giant mouths to act as a funnel, the once flattened body and mouth becomes a giant black hole with wings, powering the manta through the water as countless litres of water flow in through the mouth and stream out over the five pairs of gill slits that line the throat of the animal. Any plankton flowing over these gills that is larger than a grain of sand is sieved out of the water and trapped by feathered gill plates. Once the plates have netted a mouthful of planktonic food, the manta closes its mouth and coughs, back flushing the trapped plankton from the gills into the back of its throat before swallowing its mouthful of highly nutritional prey.

Feeding Strategies

To help manta rays maximise their planktonic rewards for the energy they expend while feeding, these adaptive animals have evolved a wide variety of feeding strategies to maximise the food they consume. While the overall feeding technique is simple; swim through the water with massive mouth agape, while ‘paddle-like’ cephalic fins funnel plankton rich water through specially modified gills; it is the specific variations in the mantas swimming positions, and the cooperative techniques which the mantas choose to employ, which are key to their feeding successes.

Almost breaking the surface, this manta completes back flip after back flip as it barrel rolls through the plankton rich water.

Some mantas feed singularly, barrel rolling themselves backwards, like a dog chasing its own tail, as they feast on densely concentrated patches of plankton. This technique is often favoured by particular individuals who sometimes spend hours repeatedly looping around and around in a dizzying display of underwater ballet.

A manta ray feeding on the seabed holds out its cephalic fins to funnel more plankton into its mouth.

Others bottom feed by scooping up plankton along the seabed, their cephalic fins and bellies passing millimetres above the sea floor as they scoop up the plankton which has sunk down to the seabed to try and avoid predation. Some of these individuals also change the feeding position of their cephalic fins, which are splayed outwards away from the mouth, enabling the mantas to inch even closer to the seabed, while at the same time funnelling more plankton towards their mouth with outstretched cephalic fins. This feeding technique often results in red abrasions and scratch marks to the edges of the mantas cephalic fins and upper gill slits, but the pay-off must be worth the minor wounds as some individuals spend hours feeding in this way.

Chain feeding mantas line up head to tail.

Another feeding strategy is to surface feed, the mantas hug the water’s surface instead of the seabed, exposing their backs to the tropical sun as powerful thrashes of their wing-like pectoral fins propel them through the water, their heads tilted slightly upwards in order to reach the denser patches of plankton which seek refuge just beneath the water’s limit.

As the plankton becomes thicker mantas may begin to feed in groups, lining up head-to-tail the mantas form feeding chains of several dozen individuals, working cooperatively together to improve their planktonic food rewards. The smaller male mantas also piggy-back the larger females; swimming directly on top of their backs they match the beats of the female’s pectoral fins to take advantage of the extra food in the same fashion as chain feeding.

A mass aggregation of cyclone feeding manta rays at Hanifaru Bay Marine Protected Area in the Maldives.

At one location in the world, when conditions are perfect and the plankton levels become extremely dense, the chain feeding mantas loop around to form a spiralling column of as many as 150 individuals. These ‘cyclone feeding’ events are spectacular and only occur about a dozen times each year at Hanifaru Bay in the Maldives. The column of mantas literally acts like a cyclone, spinning around and around in tight formation the mantas create their own current as a vortex forms in the eye of the cyclone and the filtered water moves up towards the surface. Plankton (and indeed divers) positioned on the outside of the cyclone get pulled inexorably closer as the mantas rush past in their feeding frenzy, forming the wall of the cyclone with the sheer mass of their spiralling bodies they literally suck the planktonic feast towards their gaping mouths.

 

How Much Food is Enough?

A huge mouth open wide to engulf dense patches of zooplankton at the water's surface.

Just about all that we know to answer this question comes courtesy of mantas held in captivity. Captive manta rays held in aquariums are feed roughly 12% of their body weight each week, that’s 27 kg (60 pounds) of microscopic plankton, fish larvae, copepods, and other zooplankton in a single day for an average sized, 3 metre wide, manta which weighs over 1.5 tonnes. This is a large amount of food and, as a result, these captive animals have shown extremely fast growth rates. However, it is highly unlikely that under natural conditions wild manta rays are able to find this quantity of food to feed themselves on a daily basis. Certainly the growth rates observed in long term studies of juvenile wild manta rays (by re-sighting the same individuals year after year), appear to be much slower than those few specimens held captive in aquariums.

Feeding Giants

The majority of manta ray research has to date focused mainly on the resident reef mantas which are generally much more frequently observed by scientists and divers, and therefore easier to study. But every now and then encounters with feeding oceanic manta rays have given scientists brief glimpses into the feeding strategies of these elusive giants. The giant oceanic mantas appear to be more solitary than their reef cousins, with few documented feeding aggregations of these animals. Although it is simply possible that because they spend so much more time out in the open oceans, where humans seldom venture, we just have not observed any large feeding aggregations yet. Indeed, anecdotal observations by sailors far out at sea have reported massive feeding aggregations of giant manta rays at the surface in the Pacific Ocean.

© 2017 Manta Trust