Catching Some Rays in Mexico
For many years it was believed that there was a single species of manta ray distributed globally in the oceans, Manta birostris, the Giant manta ray. This was despite the fact that there is a significant range of manta size across different locations, and that some mantas appear to be migratory pelagic animals, while others are more coastal and resident. In 2009, genetic studies showed that the smaller coastal animals were a separate species. Designated Manta alfredi, the Reef manta ray, this species had been proposed nearly 150 years ago, but not formally adopted. The identification of manta rays has remained complicated, however, as the morphology of both recognized species is variable and hybrids between M. birostris and M. alfredi have been identified. There was also evidence for a third species of manta in the Caribbean Sea, but these animals raised even more questions as they had morphological characteristics of both M. birostris and M. alfredi. Work just published in the journal PeerJ by Hinojosa-Alvarez and colleagues now provides morphological and genetic evidence to show that a third manta species does in fact exist in the Caribbean Sea - named the Yucatan manta ray for its range in the coastal waters of Mexico’s Yucatan peninsula. The authors surveyed Caribbean waters near Isla Holbox off the northern Yucatan peninsula, finding manta rays present from May through August. Known to be the period of time when spawning tuna bring many filter feeding pelagics to the area, the mantas are likely there to feed on massive amounts of fish eggs. The use of photo identification to catalog mantas near Isla Holbox allowed for a detailed characterization of Yucatan manta rays with respect to morphological features - such as the color of the mouth and the dorsal surface, the spotting of the ventral surface, and the presence or absence of dorsal spines - and to compare these with previously reported M. birostris and M. alfredi. The authors found that Isla Holbox manta rays exhibit the morphological characteristics of both M. birostris and M. alfredi.
Skin biopsies were also taken from 12 mantas for genetic analysis, which showed that animals appearing morphologically to be M. birostris or M. alfredi actually carried the unique genetic pattern of the Yucatan manta ray. Analysis of mitochondrial DNA found that some of the Holbox rays carried haplotypes that were shared with shared with M. birostris/M. alfredi, but four animals had novel haplotypes not seen in either of the previously known manta species. These complicated genetic patterns indicate that the Yucatan manta ray is genetically distinct from M. birostris and M. alfredi, but there is also evidence of their interbreeding with M. birostris. This could indicate that 1) the Yucatan manta is a relatively new species, recently diverged from M. birostris, 2) that it is a subspecies of M. birostris, or 3) that some animals may hybridize by breeding back to M. birostris. While M. alfredi is estimated to have diverged from M. birostris between 275,000 and 1 million years ago, the Yucatan manta is a more recent offshoot of the parental line, between 28,000 and 56,000 years before present. This timing corresponds to a period of dramatically higher seas that opened a greater channel between the Atlantic Ocean and the Gulf of Mexico. M. birostris animals that migrated into the Gulf may have remained there as sea levels fell, their greater isolation leading to eventual speciation.
The species determination of manta rays just became quite a bit more complicated, with a need for detailed studies that may define non-genetic characters to more easily identify these animals.
The open access article is: Hinojosa-Alvarez, S, Walter, RP, Diaz-Jaimes, P, Galván-Magaña, F and Paig-Tran, EM. (2016) A potential third Manta Ray species near the Yucatán Peninsula? Evidence for a recently diverged and novel genetic Manta group from the Gulf of Mexico. PeerJ 4:e2586. It can be found at: DOI 10.7717/peerj.2586