Mantas look a bit like flying carpets, and in fact “manta” is Spanish for blanket. They are one of the holy grails for divers. Mantas are calm, elegant giants. It is like meditation to watch them slowly flying in the water. So let’s try to find a story around mantas!
The typical introduction of mantas in a dive briefing would include the fact that they are harmless, huge, and beautiful. You would typically visit them in their cleaning stations, that’s often huge rocks, where so called cleaner fishes live. The mantas know, and visit these places to get rid of parasites and rubbish on their body. For the cleaner fish the parasites are food. Fun fact: At least two species of cleaner wrasses (the genus Labroides has five species) seem to clean themselves using their customer’s rough skin to get rid of their own parasites! They really make the best of their work day. But I sense some tension in the cleaner staff, since they seem not to clean each other.
it is a funny coincidence that both, mantas and cleaner wrasses, are considered to be able to pass the mirror test. This test might or might not prove self-awareness in animals. If you see someone with a sticker on the forehead, are you able to recognise when it is you in a mirror? Only a handful of animals can do that, and the maybe only two fishes that passed live in a close symbiosis, they are mirror test buddies! I would say we have to learn a lot about the intelligence of animals.
Popular species for divers are also popular for science, and the attention they get can be used to draw attention to environmental issues. Read on asianbestiary.org how Mantas are killed for TCM, and how much ignorance harms our fauna and ourselves. They mention that Manta gills, though they contain heavy metals, are recommended for breastfeeding mothers to increase their milk production… There’s much more unproven marketing nonsense.
Bach to the holy grail. It is no secret that reef mantas, M. alfredi, are the smaller and more commonly seen sibling of the oceanic manta, M. birostris. The differenciation between those two species is not that easy, but the size is maybe the most obvious feature. The other 9 species of the genus Mobula are often called devil rays, and are much smaller than the before mentioned. Just to be clear, we’re in the class Chondrichthyes, subclass Elasmobranchii, order Myliobatiformes.
Until 2017, the two largest Mobula species were separated in the genus Manta. That linked paper records 5.5m max. size for alfredi and 7m for birostris. Interestingly they also mention that these two species are closer related than most of the other Mobula species. Wikipedia, and this website explain the differences between M. alfredi and birostris. In photos I can’t see the caudal sting at the tail on M. birostris, and we for sure won’t see the larger teeth. So the missing spots on the underside of the wings are one key. I find the white shoulder markings have more contrast in M. birostris, and they say it forms a T shape rather than the Y shape in M.alfredi. The inactive caudal sting and the T pattern are the features mentioned in the key to the genus Mobula! I find it hard to see, and overall hard to distinguish both. I’m not alone with that, until 2009 both species were put together under Manta birostris!
When it comes to the reproduction behaviour of M. alfredi, I found there is not too much knowledge around recreational divers, and luckily it is pretty interesting. One reason why mantas are considered a vulnerable species is that they reproduce once every two years, and produce only one baby manta then. It starts with an egg or occasionally two. The baby hatches in the mama’s oviduct, and gets out later as fully developed, tiny manta. Tiny in that case means more than one meter. But how do they mate? Seems not many people have witnessed it.. Read this stunning paper from Project Manta Trust.
It sounds familiar that males chase willing females in so-called trains. The female would perform some tricks, like backwards rolling, and the males would copy those. Obviously the female judges the males performance, and if there’s someone good enough for her, he might hover above her. In order to get the private parts together, the male has to grab one fin with his teeth. Usually this will be the left fin, and the bite will leave some markings afterwards. So a manta with gentle bite marks, is a female. In this lefty tighty position mating happens.
On YouTube is this video of a manta-mating, with only eleven thousand clicks in eleven years! Be one of the few humans to have seen Mantas mating!
This mating ritual is held at cleaner stations, and so far we can only guess how much the divers influence the mating behaviour. It should be a matter of common sense to keep a great distance while watching mantas, so that we do not disturb them.
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