Author Bee Smith
Mobula birostris & Mobula alfredi
These ocean giants are a giant mystery!
Key Features & Appearance
As recently as 2009, it was believed that there was only one species of manta ray, but we now believe there are two: the oceanic manta (Mobula birostris) and the reef manta (Mobula alfredi). In terms of genetics, these species are indistinguishable based on mitochondrial DNA, but there are differences at nuclear markers, suggesting that their speciation was a recent event and there has been post-divergence gene flow. In terms of appearance, these species do look very similar but can be distinguished by a few traits:
Size: The oceanic manta is also known as the giant manta, and for good reason as it can reach 7-9m/23-30ft, whilst the reef manta is smaller (though not exactly small) at 3-5m/10-16ft.
Dorsal surface: The oceanic manta has a defined T-shaped white patch on its back, whereas the reef manta has a V- or Y-shape that is more of a gradient.
Ventral surface: The oceanic manta has a clear black border on its underside, whilst the reef manta’s border is more gradual. Additionally, the oceanic manta has a blister at the base of the tail (a remnant of its ancestor’s stinging spine) which the reef manta does not have.
Spot patterns: Both species have spot patterns on their underside, which are unique like human’s fingerprints. In the oceanic manta, these spots are clustered around the lower region, whilst in the reef manta they can also be all over the underside.
Oceanic Manta Ray Credit: Andy Murch sharksandrays.com
Habitat & Distribution
These two species don’t just look different; they also lead different lives. As their names suggest, the oceanic manta is a more pelagic species, whilst the reef manta is more coastal in nature. The oceanic manta is generally thought of as a migratory species, though it is known to show some residency, whilst the reef is thought of as more resident, though is also known to also undertake long distance movement. The oceanic manta is widely distributed around the world in tropical, subtropical, and temperate waters, whilst the reef manta is found in the tropical and subtropical indo-pacific (so they live side by side in some areas!). Both species generally stay in shallow waters, but they can perform dives far into deep sea with the reef manta’s record being around 700m and the oceanic manta’s being over 1,000m!
In addition to these two known species, ongoing research is investigating the possibility of a third species of manta ray in the Western-Central Atlantic Ocean. This potential species occupies a similar niche to the reef manta, living in the reef habitats of the Caribbean Sea and Gulf of Mexico.
Reef Manta Ray Credit: Andy Murch sharksandrays.com
Diet
Both species of manta rays are filter feeders. They use their large cephalic fins to channel water into their mouths and then filter out tiny planktonic organisms, such as copepods, shrimp, and larvae, using their gills rakers. They have also been known to eat small fish! Mantas perform some amazing behaviours to maximise their food intake from dense plankton patches, such as barrel rolling, where they backflip with their mouths open to continue feeding in the same spot, and cyclone feeding, where many mantas circle a large patch of zooplankton, forming a swirling cyclone.
Reproduction
Mantas are generally solitary, but feeding is one of the reasons they come together, and another reason they come together is to mate. Like all sharks and rays, manta rays reproduce sexually through internal fertilisation, where the male and female have to lie stomach-to-stomach. In order for females to select their mates, they are known to lead a group of males in a chase to assess their fitness.
Sharks and rays tend to have life history traits that mean they have a slow rate of reproduction, and manta rays have one of the slowest rates. Their lifespans can be as long as 50 years, they may not reach sexual maturity until around age 10, and they only have a few pups over their lifetime.
The pups grow inside the mum in eggs, which they hatch from and then are given birth to later as live young that look like miniature versions of the adult - this reproductive strategy is known as ovoviviparity. These tiny pups, measuring 1-1.5m/2-ft, are born with their wings folded over, like a burrito, which they then unfold to swim away, as their mother no longer cares for them after birth.
Threats
Manta rays’ conservative life history makes their populations very vulnerable, as they struggle to recover from increased mortality above natural levels.
One of the biggest threats facing manta rays is bycatch, when they are accidentally caught by fisheries targeting other species. Bycatch of mantas is common and pervasive in both commercial and artisanal fisheries, particularly in purse seine and gillnet fisheries. Since they are not the target species, they may then be released, but this can be tricky due to their large size and powerful wings, which make them difficult to handle and potentially dangerous to crew members. Sadly, even if manta rays are released, they may not survive due the stress of the event or sublethal injuries.
Manta rays are also deliberately targeted by fisheries in some regions of the world, for their meat and their gill plates. Their gill plates are used for traditional medicine, and this is a growing trade.
This fishing pressure has led to rapid population declines, such as a 99% decline in reef manta sightings and 92.5% decline in oceanic manta ray sightings in Mozambique. Fortunately, manta rays are now protected in Mozambique, as well as in other former fishing hotspots like Indonesia and Taiwan. However, in some places where they are protected, illegal fishing still occurs.
Status
Both the oceanic and reef manta rays are classified as Vulnerable by the International Union for Conservation of Nature (IUCN) due to their slow reproductive rates and the high levels of human exploitation they face. They are listed under Appendix II of the Convention on International Trade in Endangered Species (CITES), which regulates international trade in endangered species, and also included in the Convention on the Conservation of Migratory Species (CMS).
Fun Fact
Here are some more facts about manta rays that highlight just how mysterious they are:
The oceanic manta ray has the largest brain-to-body size ratio of any fish and shows signs of intelligence such as passing the mirror self-recognition test.
Manta rays appear to be able to rapidly change colour in response to stimuli, such as social interactions or feeding.
Manta rays can jump completely out of the water, but we don’t know why they do this. it may be for mating or communication purposes or they could simply be removing parasites.
Works Cited
• ‘Manta rays change colour’ (2014) Nature, 512(7512), pp. 9–9. doi:10.1038/512009a.
• Ari, C. (2014) ‘Rapid coloration changes of manta rays (Mobulidae)’, Biological Journal of the Linnean Society, 113(1), pp. 180–193. doi:10.1111/bij.12321.
• Booth, H. et al. (2020) ‘An integrated approach to tackling wildlife crime: Impact and lessons learned from the world’s largest targeted Manta Ray Fishery’, Conservation Science and Practice, 3(2). doi:10.1111/csp2.314.
• Braun, C.D. et al. (2014) ‘Diving behavior of the reef manta ray links coral reefs with adjacent deep pelagic habitats’, PLoS ONE, 9(2). doi:10.1371/journal.pone.0088170.
• Bucair, N. et al. (2021) ‘Underestimated threats to manta rays in Brazil: Primacies to support conservation strategies’, Global Ecology and Conservation, 30. doi:10.1016/j.gecco.2021.e01753.
• Fisheries, N. (2024) Giant Manta Ray, NOAA. Available at: https://www.fisheries.noaa.gov/species/giant-manta-ray (Accessed: 28 November 2024).
• Kashiwagi, T. et al. (2011a) ‘Habitat segregation and mosaic sympatry of the two species of manta ray in the Indian and Pacific Oceans: Manta Alfredi and M. Birostris’, Marine Biodiversity Records, 4. doi:10.1017/s1755267211000479.
• Lassauce, H. et al. (2020) ‘Diving behavior of the reef manta ray (Mobula Alfredi) in New Caledonia: More frequent and deeper night-time diving to 672 meters’, PLOS ONE, 15(3). doi:10.1371/journal.pone.0228815.
• Marshall, A.D. et al. (2023) ‘Southward range extension and transboundary movements of reef manta rays Mobula Alfredi along the East African coastline’, Journal of Fish Biology, 102(3), pp. 628–634. doi:10.1111/jfb.15290.
• MARSHALL, A.D., COMPAGNO, L.J. and BENNETT, M.B. (2009) ‘Redescription of the genus manta with resurrection of Manta Alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae)’, Zootaxa, 2301(1). doi:10.11646/zootaxa.2301.1.1.
• Nicholson-Jack, A.E. et al. (2021) ‘A hitchhiker guide to manta rays: Patterns of association between Mobula Alfredi, M. Birostris, their symbionts, and other fishes in the Maldives’, PLOS ONE, 16(7). doi:10.1371/journal.pone.0253704.
• Samantha Andrzejaczek et al. , (2022) Diving into the vertical dimension of elasmobranch movement ecology.Sci. Adv.8,eabo1754 .DOI:10.1126/sciadv.abo1754
• Stevens, G.M., Hawkins, J.P. and Roberts, C.M. (2018) ‘Courtship and mating behaviour of manta rays Mobula Alfredi and M. birostris in the Maldives’, Journal of Fish Biology, 93(2), pp. 344–359. doi:10.1111/jfb.13768.
• Stewart, J.D. et al. (2016) ‘Spatial ecology and conservation of Manta Birostris in the indo-pacific’, Biological Conservation, 200, pp. 178–183. doi:10.1016/j.biocon.2016.05.016.
• Stewart, J.D. et al. (2024) ‘Get them off the deck: Straightforward interventions increase post-release survival rates of manta and devil rays in tuna purse seine fisheries’, Biological Conservation, 299, p. 110794. doi:10.1016/j.biocon.2024.110794.
• Venables, S.K. et al. (2024) ‘Persistent declines in sightings of manta and devil rays (mobulidae) at a global hotspot in southern Mozambique’, Environmental Biology of Fishes [Preprint]. doi:10.1007/s10641-024-01576-5.
• Zhou, X. et al. (2024) ‘Consumer characteristics and preferences for Mobulid Gill plates in China’, Conservation Biology, 38(4). doi:10.1111/cobi.14244.
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