In one of my Tonga posts last year I included a photo of a Manta Ray. In the light of recent publicity about a manta dubbed “Freckles” on Ningaloo reef, I thought it might be a good time to show some more. One disclaimer; I didn’t take any of these manta photos. Vili Takau took them for me using my camera, for reasons I’ll explain later.
Vili’s first photo sets the scene.
You can see that the ray is in shallow water clouded with sediments. The cloudiness is caused by an incoming tide covering previously exposed sections of reef; the shallow water creates strong currents that stir up the organic matter clouding the water. This in turn attracts small fish, turning the sea into a diluted soup for creatures like mantas that are equipped for filter-feeding.
The mantas know this, and exploit it very effectively by swimming over the reef and against the current. This means that they are moving much faster through the water than they are over the ground – similar to an aircraft in a head wind – and so pushing a high volume of water through their food-filtering mechanism without travelling very far. To help this along, they curl their cephalic lobes (the “horns” at the front) to funnel water towards their mouth. When they reach the end of the reef, they turn around and swim back to where they started, and begin another feeding run parallel to the previous one.
The black manta above is starting one of these feeding runs. The current has stirred up a lot of sediment, giving a haze I’ve only partially removed. I’ve cropped the next image to give a clearer view of the mouth open between the curled “horns”.
As the manta draws abreast of Vili, it looks as though the pilot fish is “slipstreaming” rather than hanging on. The side view also gives a better view of the gill slits, which are open to allow the easy flow-through of water from the mouth.
As the manta goes past, the perspective shifts to its rear quarter:
In all these shots you can see the sinuous beauty of the manta’s “wings” on their up and down strokes. The bubbles show how much power the manta needs to even progress slowly in the current.
As the manta goes past, the two pilot fish come into view more clearly.
Until this final view:
Another manta has white undersides and markings, not unlike Freckles the Ningaloo manta.
As it goes past Vili, it descends, giving a clear view of its small dorsal fin.
The reason I didn’t take these photos is that burdened with a camera, I was too slow a swimmer for those currents. Only one person on board was able to cope comfortably - Vili, the local whale guide, who with his metre-long fins could seemingly swim as far and as fast as a Long Lance torpedo (that’s him in the photo below, pointing out a whale in deep water). He offered to take my camera in if I set it up for him, an offer for which I am truly grateful.
The mantas we saw were almost certainly Reef Manta Rays (Mobula alfredi). They can have a span of 5 metres or more, though these ones were more like 3 metres. Their very similar cousins, the Oceanic Manta Rays (Mobula birostris), grow larger - up to 7 metres. Mantas are part of the Elasmobranch subclass of fish, which includes sharks, rays and skates. They differ from the Teleosts or bony fishes in having cartilaginous skeletons and up to 7 pairs of uncovered gill slits. Mantas have 5 pairs of gill slits, each supported by a cartilaginous gill arch. These gill arches also hold pallets of spongy tissue – rakers - that trap the food particles as the water flows through them, giving them their filter-feeding abilities. Unfortunately, this also makes mantas vulnerable to floating bits of plastic.
Like some other elasmobranchs, mantas are ovoviviparous. The females hold their fertilised eggs in their oviducts while the pups hatch. The pup then remains in the oviduct for a while, feeding on a milky secretion produced by the mother. When it eventually emerges, a year or so after conception, it weighs around 9 kilos and has a span of 1.4m.
For me, the real eye-opener was how intelligent mantas are. Having long ago dissected dogfish (Scyliorhinus canicula) and seen how their cerebrum is dwarfed by their olfactory scent lobes, I assumed that all elasmobranchs must be equally dim-witted because they were so distant from us. A dangerous assumption, given the intelligence of some birds (whose ancestral line split from ours over 300 million years ago), and octopods (over 500 million years ago - they don’t even have a backbone). Unlike their distant dogfish cousins, manta rays have large brains, proportionately and absolutely, with well-developed areas normally associated with learning and problem-solving. Recent experiments with mirrors suggest that they possess self-awareness, in the way that chimps and dolphins do.
Like many other large fish, mantas use cleaning stations. These are spots, usually on a reef, staffed by specialist cleaner shrimp and species of wrasse and goby. The larger fish approaches, holding open its mouth to reveal it needs cleaning. It then stops, allowing the cleaners to safely remove and eat any parasites. The cleaners even swim inside the manta’s mouth and gills to do the job, to the mutual benefit of both parties.
That led me to think about Freckles, the Ningaloo manta. Given manta intelligence and familiarity with cleaning stations, it’s perhaps not surprising that Freckles approached her human acquaintance to remove what was effectively just another parasite embedded in her skin. It puts us humans in our place – we’re just large cleaning shrimps!
My whale-swimming trip is described here, a favourite whale encounter here, and some Tongan land animals (mostly birds) here.