The Manta Ray (Manta birostris)
in Puerto Vallarta

By Griffin Page
Naturalist ~ Eco-guide

For many divers and snorkelers, Manta rays (Manta birostris), also commonly called devilfish, giant mantas and in Spanish manta rayas, are a most sought out species. Their angel like flight under water is a most spectacular sight. So graceful and agile are these creatures that they often offer us a show resembling a classical ballet performance almost equal to those I’ve seen on big production stages. Their leaps out of the water are also quite impressive….up to 7 feet! Why they do this? Many hypotheses have been brought forth but very little is actually know about why they jump clear out of the water the way they do. Some say it’s perhaps to clean themselves of parasites and shed some dead skin, or to play, perhaps some kind of fishing technique and I even heard someone say they do it to help push a newborn out of their womb. Wow! I think I’ll wait for a more complete scientific study to form my own opinion.

Until recently, manta rays were divided into different species (Manta alfredi, M. ehrenbergi, M. hamiltoni, M. birostris) based on coloration differences, size variation, and geographical location. However, investigations by scientists such as Tim Clark from the University of Hawaii show that manta rays are all one species, Manta birostris. Tim plans to publish a comparison of genetic samples from several worldwide populations to establish this hypothesis.

Manta rays have a generally triangular body plan. The pectoral fins have evolved into wide triangular wings with which the head has fused and giving the manta a broad blanket-like body, thus earning them the name 'manta', the Spanish name for cloak. This wonderful species is related to the shark family. Sharks, unlike fish (which have a bony skeleton), have a body structure made completely of cartilage, allowing them more flexibility of motion. Their coloring may vary from black, blue grey or dark brown dorsally and generally have a white underside, sometimes with varying darker patterns.

Each Manta ray has a unique color pattern. Both the dorsal and ventral color characteristics are used to identify individuals. They may attain a disk width of 29 feet (9 meters) maximum but will usually average more like 22 feet (6.7 meters) at full growth. They can attain a weight of 3000 pounds (1350 kilograms) and their life expectancy is estimated at around 20 years although not yet confirmed. They can also dive to depths over 720 ft, sail several yards out of the water, and reach speeds of over 14 miles/hour.

These underwater giants inhabit temperate and sub-tropical waters around the globe (between latitudes 35° N and 35° S). They tend to travel a lot, mainly in the search of plankton rich areas. They are filter eaters, as are most Mobulids (which means it belongs to the family of Mobulidae). They feed on microscopic organisms such as copepods, mysids (minuscule shrimp-like animals) along with larvae of fish, lobster, and octopus that float and swim in the open water. Some have been seen looping vertically for extended periods while feeding. They will eat about 2% of their body weight daily. This represents 56 pounds for an adult weighing 2800 lbs!

There is little sexual dimorphism between males and females. Males are distinguished from females by the presence of two claspers (male sex organs) located adjacent to the inside edge of each pelvic fin. Females will reach sexual maturity when their wingspan has reached 11 feet and it is estimated that it will take them approximately 5 years to reach it. Reproduction will usually take place near rocky reefs and many males can be seen courting one female. In order to mate, the male will bite the pectoral fin of the female before placing himself in a belly to belly position. He will then insert one clasper into the cloaca (cavity where intestinal, urinal, and reproductive canals come together) of the female. Gestation is believed to take 13 months Manta with Remoras. Photo by: Steve Jones but the exact time is unknown. Mantas reproduce via aplacental (without a placenta) viviparity (give live birth). A pup hatches from its thin-shelled egg inside the mother. After hatching the pups are fed by uterine milk until they are ready to be expelled. Females give live birth to one or perhaps up to two pups that are about 3-4 feet (1-1.5 meters) wide and weigh approximately 20 pounds. The pectoral fins are curled around the pup in an S-shape when inside the mother. When the pups are expelled they unfold their wings and start to swim. Young mantas grow very rapidly and are thought to double their size in the first year!

Manta rays were once commercially harvested from Australian and Californian waters for their liver oil and their skin which was used as an abrasive and durable leather goods such as wallets. Although it is now rarely hunted, Manta meat is still a delicacy in the Philippines and I have seen it served here in our bay on a few occasions, mostly, at beach side restaurants of very small towns. It is still actually fished in certain areas of the world including La Paz in Baja Califirnia Sur. As a matter of fact, the IUCN classifies this species as DD (Data Deficient) and their report is as quoted here: “This common and widespread large coastal plankton-feeding ray is very widely distributed in tropical shelf waters and around oceanic islands. The species is highly Manta ray fishery. Photo by Jonathan Roldan vulnerable due to its life history, giving birth to just one very large pup every two or three years. There are no target fisheries in most parts of its range, and by-catch is rare under the present fishing methods in use world-wide. Unfished subpopulations are not considered threatened. However, wherever there are fisheries the species quickly becomes vulnerable, population declines have been observed where target fishing has taken place, and there is a commercial market for them in various places of the world. Reportedly, they are now very scarce in the Gulf of California. Populations will rapidly decline unless the fisheries are carefully managed. More data on population status is required to assess the species' conservation status.”

Dive tourism has benefited greatly from the manta in locations where they are reliably encountered and sometimes approach divers. In these areas, where divers often touch and interact with mantas, the rays can develop skin lesions in response to the removal of the thin protective mucous layer. Let’s dive consciously!

“Nature could be such a wonderful teacher if only we saw it for what it really is” ~ Monachí