The word ‘Manta’ comes from the Spanish for cloak or shawl and in the Maldivian language mantas are known as En Madi, which in English means ‘small fish eating ray’. Both of these terms are very descriptive of the manta rays, but what do we really know and just as importantly what do we still have to learn about these giant fish?
How many kinds of manta are there?
Manta rays belong to the taxonomic family Mobulidae. This family contains eleven species of plankton eating rays. Within this family there are two genera, Mobula and Manta and more specifically within the genus manta there are two species, Manta birostris (the giant oceanic manta) and Manta alfredi (the resident reef manta). These two species have much in common, but a few important differences in life history exist between the two. These are important to note as they have very different implications for the management and conservation of these species. Whilst the main focus of the work of the Manta Trust is the two known manta species we are also trying to improve knowledge and understanding of the nine species of mobula.
What kind of animal are they?
Manta rays are cartilaginous elasmobranch fishes. This means they are a close relative of all sharks and rays. The fact that they are fish means they don’t breathe air, but instead they use their gills to respire underwater. Mantas must keep moving in order to keep water flowing over their gills, which means they can never stop to sleep or rest on the seabed!
How large do they grow?
Oceanic mantas (M.birostris) are the bigger of the two manta species; reaching a wing span (that’s wing tip to wing tip) of up to 7 metres (23ft)! A large oceanic manta might weigh in at up to 2 tonnes (4,440 lbs), making them a real ocean giant! Although smaller than the oceanic mantas, the reef mantas (M.alfredi) are still pretty big fishes, growing to an average wing span of 3-3.5 metres (9-11.5ft) and a possible maximum of 4.5 metres (15ft), reaching weights of up to 1.4 tonnes (3,100 lbs). In both species the wing span is roughly 2.2 times the length of the body.
How long do they live?
Research would indicate that mantas probably live to at least 50 and possibly up to 100 years. However, until scientists have been studying these animals for another 20 years we are unlikely be able to answer this question more accurately. Ping-Pong (M12), is a Maldivian reef manta who was first photographed by divers in 1989 when she was already fully grown and sexually mature. Ping-Pong is still seen now at the same site almost every year! It’s thought that mantas are 15-20 years old when they reach sexual maturity, which would make Ping-Pong at least 40 years old!
Where are they found?
Mantas are found all around the world; however some important differences in range and habitat exist between the oceanic mantas and their smaller reef manta counterparts. Both species live pelagic lives in the open ocean, visiting reefs to feed and be cleaned. However, oceanic mantas have the wider geographic range of the two species occurring in tropical, sub-tropical and temperate waters. This species is more migratory in its nature, commonly sighted along productive coastlines with regular upwellings, oceanic island groups and offshore pinnacles and seamounts. Reports of this species range from to 31ºN (South Carolina, USA) and 36ºS (North Island, New Zealand). The resident reef mantas are more commonly sighted inshore around coral reefs, tropical island groups, atolls and bays, as well as along productive coastlines. As the name suggests, this species is more resident to tropical waters with smaller home ranges.
What do they eat?
Despite their status as an ocean giant, manta rays feed on some of the smallest organisms in the sea! They are planktivores, feeding especially on zooplankton; tiny animals such copepods, mysid shrimps and arrow worms. Mantas are known to make seasonal migrations in order to take advantage of particularly abundant areas of food.
How do they reproduce?
It is believed that mantas are sexually mature at around 10-15 years of age. Courtship in mantas can last days or even weeks and is an intricate process led by the female. The males compete to mate with the female, following her around the reef in what is known as a mating train; we’ve counted as many as 30 males following a single female. Watching a mating train of mantas is an enthralling experience, with the female leading the males as she races, twists, turns and dives around the reef with each male following her every move. In doing this the female tests the fitness and persistence of the males as they try to keep up with her and gain the first place in the train. At the end of this process the female selects a single male and the two mate as a pair, belly to belly in the mid water, the male biting down on the female’s left wing to secure them together.
Mantas are ovoviviparous, meaning that once they have mated the fertilised egg grows within the female’s uterus until it has developed into a fully functioning manta pup. In aquaria the time between mating and pupping has been observed to take around one year. Mantas usually give birth to a single pup, but occasionally two developing embryos have been recorded inside a pregnant female during autopsies. At birth mantas measure around 1.5m from wing tip to wing tip. It’s thought that mantas are probably born in shallow water at night, but to date no wild births have ever been recorded.
Currently it is not know how often females give birth, but the latest research suggest birth rates average around one pup for each mature female every 2-5 years. This extremely slow rate of reproduction, coupled with the long gestation period, late maturity age and small litter size, makes manta rays very vulnerable to exploitation.
Are there any threats to mantas?
Just like all wild animals mantas, have a number of natural predators. Certain species of sharks, as well as orcas and false killer whales have all been recorded predating upon manta rays. Shark bites are seen within every population of manta rays that are studied. Often these bites are taken from the trailing edges of the manta’s wings and in many cases these heal up quite well and leave the manta with no lasting damage.
In addition to natural predation, mantas face significant threats from humans and our activities. Fishing line can cause severe entanglement resulting in deep, and often life threatening lacerations if mantas become entangled in it. Gill nets and other fishing nets also cause mantas to become entangled, usually resulting in death. Unfortunately, mantas cannot swim backwards and so attempts to disentangle themselves often result in greater entrapment. Mantas which become entangled in nets quickly die because they are unable to actively pump oxygen rich water over their gills in order to respire.
Most recently mantas rays have become a desirable catch in a number of countries around the world. The reason for this increase in demand is the manta ray’s gill plates which have become highly desirable in the Chinese medicine trade. The manta’s gill plates are cartilaginous finger-like projections that branch off the gill arch and are used by the mantas to filter their planktonic prey from the water. The trade in these plates has increased significantly in recent years, posing a worrying threat to manta and mobula populations around the world.
Understanding mantas, their life history, threats, reproduction and movements are all crucial to understanding what kind of conservation measures are required for these animals. Both species of manta ray have recently been reassessed for the IUCN Red List which looks at different species against a range of criteria to see what issues are of concern to the species survival. Both species of manta are considered to be ‘Vulnerable’ on this listing. Giant mantas have also recently been listed on Appendix I and II under the Convention on Migratory Species (CMS).
What do we still need to find out?
In the last ten years scientists have made a good start at uncovering the secret lives of mantas rays, however so far we have just uncovered the tip of the iceberg in-terms of really understanding these animals. Much still remains to be found out about even the basics of their biology, and with the largest brain of all the world’s fishes, there is much still to understand about their intelligence, curiosity and social interactions. Learning more about these species is vital to being able to provide them with effective protection; ensuring their persistence in years to come.