The Spotted Eagle Ray and Its Relatives

Looking out towards the edge of the reef, I am so glad I turned my head just in time to see the distinctive white spots on top of the dark disk-shaped body gracefully glide past me.  It’s always a treat to see a beautiful spotted eagle ray swim past while diving or snorkeling.  The spotted eagle ray (Aetobatus narinari) is a type of elasmobranch (Class Elasmobranchii), characterized by its cartilaginous skeleton.  This class includes sharks, skates, and rays, all of whom have a skeleton made of cartilage not bone.  Other common names for the species include eagle spotted ray and white spotted eagle ray.  Many ocean lovers do not know much about spotted eagle rays or their relatives, including stingrays, other species of eagle rays, and manta rays.  Throughout the article, I hope to give readers a better understanding of these majestic animals by sharing spotted eagle ray facts along with information on their relationships with other rays and facts about these other rays.  You can also read more about manta rays here.

Spotted Eagle Ray (Aetobatus narinari)

Spotted Eagle Ray (Aetobatus narinari)

Spotted Eagle Ray Identification

The spotted eagle ray is best identified by its flat, disk-shaped body, with a dark dorsal surface, or back, covered in white spots, a white ventral surface, or underbelly, and a long thin tail.  In addition, these rays have a distinctive head that protrudes into a snout.  Individual spotted eagle rays have been recorded with disk widths up to 3 meters (9.8 feet), however, it is much more common to see individuals with disk widths around 2 meters (6.5 feet) or less.  The pectoral fins, often described as the animal’s wings, appear triangular in shape, coming to a point at the ends.  The spotted eagle ray swims via undulation of its pectoral fins, which give the appearance that the animal is flying or gliding through the water.  Its mouth is located on the underbelly, positioned to aid in the consumption of bottom dwelling animals.  When thinking about the prominent traits of stingrays, including eagle rays, many people immediately think of the barbs on the tail.  The spotted eagle ray will have between one and six short barbs, or spines, containing venom.  The barbs are found near the base of its tail and are used for defense.

Spotted Eagle Ray Feeding Habits

Spotted eagle rays feed on both benthic invertebrates, such as clams, shrimp, oysters, and urchins, and small benthic-dwelling fish.  They have a flat tooth plate on each jaw, which they use to crush shells before digesting the shellfish meat.  It is believed  that spotted eagle rays will use their unique snout shape to dig in the mud to find food.

Spotted Eagle Ray Range

A. narinari Range

Spotted Eagle Ray (A. narinari) Range

Spotted eagle rays are found worldwide in tropical and warm temperate coastal waters.  In the Western Atlantic they range from North Carolina down to Brazil, including the waters of the Gulf of Mexico and the Caribbean Sea.  They can also be found in the Eastern Atlantic off the coast of West Africa, from Mauritania in the north to Angola in the south.  Spotted eagle rays are widespread in the Indo-Pacific, ranging from South Africa in the west to Hawaii in the east, and from Japan in the north to Australia in the south, and the species can also be found in the Red Sea.  Finally, A. narinari has populations in the Eastern Pacific off the West Coast of the Americas, ranging from the Gulf of California to Peru.  Spotted eagle ray populations show high site fidelity, with individuals consistently found in the same location, however, it is unknown whether the animals remain in these locations all the time or whether they will leave and continue to return to the same site.  Spotted eagle rays swim in open water near coasts, bays, and coral reefs, and can be found at depths up to 80 meters (262 feet), although it is believed that they spend most of their time near the surface.

Spotted Eagle Rays: One Species or Several?

Some experts believe that A. narinari, or the spotted eagle ray, is actually a species-complex, meaning that what we currently define as the spotted eagle ray actually encompasses at least four different species.  That being said, at present, A. narinari continues to be recognized as originally described.  Researchers point to differences in external morphology, coloration, and distribution as evidence there may in fact be several species represented in different populations of spotted eagle ray.  Reports from the Philippines have shown distinct narrow disk and wide disk morphs, and researchers suggest that these might not just be morphs of the same species but be distinguishing characteristics of two different, presently undescribed species.  It has also been suggested that Indo-Pacific populations differ morphologically from Atlantic populations.  Researchers are currently working on molecular studies to shed more light on the possibility that the currently accepted species A. narinari is actually a species complex.

Spotted Eagle Ray Life History

The life history of a species describes changes that an individual goes through during its life, often with a strong focus on reproductive strategies.  There are still a lot of unknowns about the life history of the spotted eagle ray, including their longevity, natural mortality rate, and reproductive frequency.  Both males and females reach sexual maturity been four and six years of age.  Spotted eagle rays, along with the other elasmobranchs, are ovoviviparous.  This means that eggs develop and then hatch within the female where the pups continue to develop before live birth.  Unlike the majority of species with internal development whose young are nourished by a placental connection with the mother, the pups developing in an ovoviviparous animal are nourished by their yolk sac.  Spotted eagle rays are estimated to have a gestational period of around one year.  At birth, female pups had a measured disk width of 26 centimeters (10.2 inches) and male pups had disk widths ranging from 16 to 36 centimeters (6 to 14 inches).  Keep in mind that relatively few pups have been measured at birth but current measurements give an idea of approximate pup size.  Each female can carry a litter of one to four pups, although it is uncommon for a mother to have four pups.  Spotted eagle rays have a low reproductive output, especially as many scientists believe that individuals may not reproduce annually, which is a factor contributing to population decline of A. narinari.

Spotted Eagle Ray Individual Identification

A spotted eagle ray’s spot pattern is unique to the individual and allows for identification of individuals, much like a human fingerprint..  Photo identifications are an important tool for spotted eagle ray researchers, as it allows them to track the movements of individual rays.  Researchers just need to collect identification photos with information about the location where the individual was seen, and can then compare locations and other information about the individual over time as photos are added to the database.  This type of identification is also used in studies of whale sharks and manta rays.

Threats To Spotted Eagle Rays

The spotted eagle ray (A. narinari) is given near threatened status by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species.  The species was assessed and the listing published in 2006.  Prior to 2006 the spotted eagle ray was considered data deficient by the IUCN Red List.  A species is listed as near threatened if it is close to qualifying for a threatened status or is likely to qualify in the future.  The combination of the species’ low fecundity or low reproductive output, its decreasing population, and its exposure to inshore fishing gear led experts to classify it as near threatened.  Species with low fecundity are naturally vulnerable to overfishing, and, as previously mentioned, spotted eagle rays inhabit inshore, coastal, and coral reef areas worldwide, all of which are subject to heavy fishing pressure.  Although spotted eagle rays tend not to be targeted fisheries species, with an exception for aquarium trade, the mere presence of inshore fishing gear leads to many unintended captures.  Just a few of the fishing methods threatening A. narinari are longline fishing, seine nets, gillnets, and trawling.  Fisheries in much of the animal’s range are unregulated, especially in Southeast Asia where is extremely high, usually unregulated, fishing pressure in inshore water, prompting the IUCN Red List to give A. nariarni vulnerable status in Southeast Asia.  Species that are listed as vulnerable to extinction are considered to have a high risk of becoming extinct in the wild.  Fishing pressure is slated to increase substantially worldwide, which, unfortunately, spells bad news for a huge number of marine organisms.

Spotted Eagle Ray Conservation

The spotted eagle ray not only plays a valuable role in the marine ecosystem, but it also has a high tourism value.  Divers and snorkelers cherish spotted eagle ray sightings and spotted eagle ray focused eco-tourism can be a boom to the economies where these animals are found.  The spotted eagle ray is afforded protection by a few governments worldwide.  The species is protected in all Florida state waters, where it is illegal to harvest, possess, land, purchase, or sell spotted eagle ray.  In Australia, the A. narinari is protected within the Great Barrier Reef Marine Park along the country’s east coast, and along the country’s northern coastline where the species carries spiritual significance by some indigenous communities and where Turtle Exclusion Devices (TEDs) are required to be used by the prawn trawl fisheries.  TEDs decrease catches of megafauna including turtles, rays, and sharks by allowing these larger animals to escape when accidentally caught in nets.  The island nation of the Maldives have created marine protected areas around popular dive sites to protect all sharks and rays, as leaders recognize that these animals are worth much more alive and used the tourist industry than they are when they are fished out.  Furthermore, in 1995 the Maldives banned the export of rays, and in 1996 the nation banned the export of ray skins, to further protect the rays that are valuable to their burgeoning tourist industry.  A. narinari populations in South Africa have also been afforded protection by limits placed on recreational line fishing that enacted a one bag limit per species per person per day of unspecified organisms within the class Chondrichthyes, which includes the spotted eagle ray along with the other cartilaginous fish.  In addition, the removal of shark nets from several of South Africa’s beaches have protected populations of A. narinari from accidental entanglement.  In order to protect this majestic animal, more research is needed to understand more about the species’ life history and range, and about natural and fisheries induced mortality rates.  Most importantly, initiating fisheries management and enforcement will help to protect the spotted eagle ray along with many other marine organisms.

Eagle Rays

The spotted eagle ray is one of the most well-recognized eagle rays, but there are many other species of eagle ray.  All of the eagle rays share similar life histories, and are characterized by their long tails and their well-defined diamond-shaped bodies, which are created by the triangular pectoral fins, or wings.  Eagle rays tend to swim in open water, rather than resting on sandy substrate like their stingray relatives.  A few other common eagle ray species include the common eagle ray (Myliobatis aquila), the bat ray (Myliobatis californica), and the bullnose ray (Myliobatis freminvillii).

Stingrays

There are many types of stingrays found throughout the world’s oceans and, even though stingrays and eagle rays share many characteristics, including their flattened bodies, circumglobal distribution, diet, ventral mouth, and presence of a stinging barb on the tail, there are several distinguishing features between the two.  While eagle rays spend most of their time swimming in the water column, stingrays tend to be less active, burying themselves in the sand as a form of camouflage from predators.  Stingrays tend to have more rounded pectoral fins and, therefore, more oval body shapes, while eagle rays tend to have triangular shaped pectoral fins and diamond-shaped bodies.

A Note on Classification

As scientific researchers continue to study the many organisms living in our oceans, aquatic environments, and on land, our understanding of the relationships between different animals continuously evolves.  This often leads to changes and disputes over the taxonomic classifications of many organisms as more information becomes available.  Many taxonomists place eagle rays in the Family Myliobatidae.  Within this system, the Family Myliobatidae is split into three Subfamilies that include eagle rays, cownose rays, mobula rays, and manta rays, with 42 individual species split between the three Subfamilies.  However, not all taxonomists are in agreement and some experts, including the IUCN Red List, place eagle rays in the Family Myliobatidae and manta rays in the Family Mobulidae.  Within this system both Families, along with skates, guitarfish, and several stingray families, are part of the Order Rajiformes.

Manta Rays

(Mantas in Kona, Hawaii seen during a night excursion. Photo courtesy of: http://www.jacksdivinglocker.com/diving/boat-diving/manta-night-dive/)

Reef Mantas (Manta alfredi)

Despite the aforementioned debate over the exact evolutionary relationship between eagle rays and manta rays, taxonomists agree that they are closely related species, regardless of whether they belong to the same Family or the same Order.  Manta rays are the largest rays in the seas, and these gentle giants are revered by divers and ocean lovers.  There are currently two described species of manta ray, however, efforts are underway to formally describe a third, distinct species of manta.  The currently described species are the Giant manta (Manta birostris) and the Reef manta (Manta alfredi).

Mantas Ray vs. Stingrays

Manta rays are more similar to eagle rays than stingrays, so I will begin with a comparison between mantas and stingrays and then compare mantas and eagle rays.  Mantas are the largest known rays, as well as the only filter feeding rays (a feeding strategy adopted by many large marine animals, including whale sharks).  As filter feeders, mantas have a terminal mouth, or a mouth located at the front of their head, which is an adaptation that allows them to filter feed on suspended plankton in the water column in front of them.  On the other hand, stingrays have their mouths on their underside so that they can eat invertebrates and small fish living on the substrate below.  As mantas evolved, they lost their functional stingers or spines, whereas all stingrays have some type of stinging barb.  The giant manta (Manta birostris) possesses a non-functional caudal spine that is found within a calcified mass on its tail.  It is possible to see the caudal spine on giant mantas if you look closely at its tail.  A more prominent feature of mantas that is absent in stingrays is the horn-like cephalic fins extending from the manta’s face.  All of the aforementioned differences between manta rays and stingrays can be extended to comparisons of manta rays and eagle rays.  However, eagle rays are often larger than stingrays, putting them closer in size (but still smaller!) than mantas.  In addition, eagle rays and mantas both have triangular shaped pectoral fins while stingrays have much more rounded or circular pectoral fins.  Although the outlined differences between manta rays, eagle rays, and stingrays help to differentiate between the animals, these are generalized traits.  It is good practice to look up animals that you have seen in a reef life reference book to be sure about exactly what you saw.

Manta Ray Feeding Habits

manta_mouth_bottom_feeding_hanifaru_a

Bottom Feeding Manta

As mentioned previously, mantas are filter feeders, eating zooplankton, which are small organisms that cannot swim against the current.  Mantas will also occasionally eat small fish swimming through the water column.  Like most filter feeders, mantas rely upon the RAM feeding method: swimming forward with their mouth open to consume their small, non-moving prey.  Mantas feed in zooplankton rich waters and rely upon their gill rakers to grab the tiny animals from the water column and separate them from the water, allowing the food to be eaten while the water passes over the gills.  When in the water with mantas, it appears as though the mouths of these giant animals could swallow up a diver but, fear not, the animal’s throat is about the size of a human fist!  It is most common to see mantas feeding mid-water where plankton tend to be evenly distributed throughout the water column.  However, if plankton is floating together at the surface, mantas will move up in the water column to swim through plankton-rich surface waters.  When mantas are surface feeding, observers will often be able to see the animal’s large dark shadows just below the surface while their pectoral fins can be seen rising up out of the water as the mantas swim.  Another observed feeding behavior of mantas is barrel rolling, when the animals will swim in large vertical loops mid-water.  This behavior allows the mantas to feed when plankton that is concentrated in one area, as the motion allows the animal to stay in that one place.  Mantas have also been observed bottom feeding, which allows them to access plankton that has sunk down from the water column; however, this is rarely seen.  Individuals will swim low along a sandy bottom, sucking in the bottom-dwelling plankton, in a vacuum-like manner, and some mantas have been seen with scrape marks on their undersides after bottom feeding.

Manta Species Identification

In 2009 prominent manta researcher Andrea Marshall, often referred to as the Queen of Mantas, formally described M. alfredi as a separate species.  Prior to 2009, it was believed that all mantas belonged in a single species, M. birostris.  Marshall’s research team used external characteristics and measurements of form, such as coloration, dentition, size at maturity, spine morphology, maximum disk width, and denticle morphology, to differentiate M. birostris and M. alfredi.  One of the easiest ways for divers or snorkelers to differentiate between giant mantas (M. birostris) and reef mantas (M. alfredi) is based on their dorsal and ventral coloration.  Beginning on the dorsal side, or the back, both species of manta have white shoulder patches that vary enough to aid in species identification.  Giant mantas have very large, conspicuous white shoulder patches that create a T-shaped black marking running parallel to the mouth and down between the two shoulder patches.  The reef manta’s shoulder patches are gray or white in color giving an appearance of soft shading, with a less pronounced, y-shaped area of black shading above the mouth and down between the shoulders.  Moving on to the ventral side, or underside,  all mantas have white bellies with dark patches.  The pattern on the underside of each manta is unique, much like a human fingerprint, and can used by researchers to identify individuals.  Both giant and reef mantas have two rows of five gill slits on their ventral side.  There are several differences in ventral coloration that can be used to identify the species of manta.  Giant mantas (M. birostris) have dark triangles beneath their fifth gill on each side, and they have markings only on the belly, not between the gill slits.  On the other hand, reef mantas (M. alfredi) will frequently have markings all over their underside, including between the gill slits.  They will not have the dark triangles below the fifth gill slit.  In addition, giant mantas tend to have dark shading along the back edge of their pectoral fins while reef mantas will have light shading or no shading in this area.  The two species also differ in range, size, behavior, and several morphological characteristics, however, for casual observers, these traits are nearly impossible to use for species differentiation.  As you might expect from the name, giant mantas grow to larger sizes than reef mantas.  Giant mantas have been reported to reach a maximum disk width of approximately 7 meters (23 feet) while reef mantas are reported to reach a maximum disk width of approximately 5 meters (16 feet).  Although this may seem a large difference, individuals will vary in size within each species and all mantas will appear huge when seen underwater.

Distinguishing features of the Giant Manta (M. birostris) on the dorsal (top photo) and ventral (bottom photo) sides. Note the t-shaped shoulder marking, black triangles beneath the 5th gill slit, and lack of marking between the gills).

Distinguishing features of the Giant Manta (M. birostris) on the dorsal (top photo) and ventral (bottom photo) sides. Note the t-shaped shoulder marking, black triangles beneath the 5th gill slit, and lack of marking between the gills).

Distinguishing features of the Reef Manta (M. alfredi) on the dorsal (top photo) and ventral (bottom photo) sides. Note the y-shaped shoulder marking, lack of black triangles beneath the 5th gill slit, and markings between the gills).

Distinguishing features of the Reef Manta (M. alfredi) on the dorsal (top photo) and ventral (bottom photo) sides. Note the y-shaped shoulder marking, lack of black triangles beneath the 5th gill slit, and markings between the gills).

Manta Ray Distribution

Manta rays have a global distribution and have been seen in every ocean except the Arctic Ocean although they are most common in tropical waters.  Even though both species have circumglobal ranges, they tend to be found in different locations and do not appear to share habitat.  Both species of manta are migratory, however, giant mantas are often found swimming in offshore waters and researchers believe that they are more migratory than reef mantas, who tend to remain in coastal waters.  Reef mantas are usually found further inshore, close to reefs, atolls, island groups, and coastlines with regular up-welling of nutrient-rich water.  This means that most divers spot reef mantas.  Despite the species’ large range, Individual populations of both giant and reef mantas are fragmented due to their specific habitat use and resource needs.

Giant Manta (Manta birostris) Range

Giant Manta (Manta birostris) Range

Reef Manta (Manta alfredi) Range

Reef Manta (Manta alfredi) Range

Manta Color Morphs

Both species of manta have been observed with two different color morphs (black and white), in addition to their standard coloration.  A common misconception is that black mantas and white mantas are separate species, however, both color morphs have been observed in giant and reef mantas.  Because both

Melanistic (Black) Color Morph

Melanistic (Black) Color Morph

morphs have been observed in both manta species, researchers believe that the morphs represent genetic mutations that occurred before mantas evolved into two separate species.  The more common of the two is the melanistic morph, or black morph, which causes the entire ray to appear black with few areas of white coloration.  The more uncommon leucistic, or white, morph causes individuals to appear nearly or completely white.  Leucistic mantas are not albinos as they display some darker pigment on their bodies, including in their eyes.

Manta Ray Life History

Like their relatives, including the spotted eagle ray, mantas have a low reproductive output and are ovoviviparous.  Reef and giant mantas are estimated to live for about 40 years with females reaching sexual maturity around 8 – 10 years old.  Mantas will have one, occasionally, two pups after a gestational period of one year.  It is believed that they do not reproduce every year, with a reproductive periodicity ranging from two years to five years.  Pups of both species of manta are born with disk widths of approximately 1.5 meters (5 feet).  Both species of manta have decreasing population sizes, as their life history leaves them extremely susceptible to overfishing and population decline.

Manta Ray Threats and Conservation

drying_mobula_manta_gill_rakers_c

Manta and Mobula Gill Rakers Drying Before Being Sold for Use in Chinese Medicine

Both species of manta ray are classified as vulnerable to extinction by the IUCN Red List of Threatened Species.  Mantas populations are being decimated by heavy fishing pressure, from both direct fisheries and accidental bycatch.  Mantas are targeted as part of the gill raker trade, with manta gill rakers sold for high prices as a Chinese medicinal product.  Gill raker fisheries are unsustainable and completely unmanaged, but continue as poor fishermen are able to sell manta parts for high prices.  Mantas are also fished for their meat, however, this is a much smaller trade than the gill raker trade, usually confined to small artisanal fisheries where manta meat supplements fish-based diets.  In addition to being victims of direct fisheries, mantas are often caught as bycatch of other, large-scale fisheries that use gill nets, drift nets, and seine nets to catch their target fish species.  Many countries are beginning to recognize the high value of living mantas for tourism, with global manta-based ecotourism estimated to generate about $50 million annually, worth much more than a one-time payment of about $250 to $500 for a dead manta in a market.  Clearly, mantas can be a huge boon to ecotourism worldwide, however, tourist interactions with these beautiful and inquisitive creates needs to be regulated, to avoid negative effects on manta populations.  Several countries worldwide have implemented legislature aimed at manta conservation.  The following places protect at least one, if not both, species of manta: the Republic of Maldives, Guam, Hawaii, Mexico, the Philippines, Yap, Ecuadaor, Brazil, Western Australia, New Zealand, Honduras, Florida, the Yaeyama Islands of Japan, and Indonesia.  Although national protection is a positive step in manta conservation, international management is vital due to the animal’s migratory patterns.  In addition to be listing as vulnerable to extinction by the IUCN Red List, mantas have been listed on Appendix II of the Convention of International Trade in Endangered Species (CITES) and the giant manta ray (M. birostris) is protected under the Convention on Migratory Species (CMS).  CITES is an agreement undertaken between nations worldwide to manage the international trade of wild animals and plants.  A listing on Appendix II means that trade in the species must be controlled closely or restricted.  The CMS is the only convention working to protect migratory species and has over 100 signatory nations.  M. birostris is listed on both Appendix I and Appendix II of the CMS, which means that all signatory nations must enact strict protection of giant mantas and their habitat.  Although the increasing protection afforded to manta rays is a step in the right direction, stricter enforcement and even more widespread management is needed to conserve both giant and reef mantas and end the devastating gill raker trade.

Citizen Science Through Manta Matcher

Manta researchers with the Marine Megafauna Foundation rely upon photo identification of individual mantas for many of their studies.  Because mantas have a unique pattern on their underside, individuals can be identified from photos of their ventral surfaces.  These unique identifications are used to study migratory patterns, population size and composition, habitat usage, and population trends over time.  The Marine Megafuana Foundation, in conjunction with ECOCEAN USA, launched Manta Matcher to engage researchers and citizen scientists to create a global database of manta rays.  Anyone able to take photos of mantas underwater can contribute to the Marine Megafauna Foundation’s ongoing research on manta rays.  All you need to do is upload your ID photos with information on the date, time, and location of the encounter to Manta Matcher.  Manta Matcher can be accessed online at mantamatcher.org.

Note on Measurements

Most science is conducted using the metric system, which is less understood by American readers.  Therefore, I converted any metric measurements into the imperial system of feet and inches for ease of understanding, with all conversions showing approximations not exact figures.

Sources

Allen, G. R., Steene, R. C., Humann, P., & DeLoach, N. (2002). Reef fish identification: Tropical Pacific. Jacksonville, Fla: New World Publications.

Encyclopedia of Life

Florida Museum of Natural History, Biological Profiles

IUCN Red List of Threatened Species

Manta Ray of Hope

Marine Megafauna Foundation

Marshall A. D., Compagno, L. J. V. & Bennett, M. B. (2009) Redescription of the genus Manta with resurrection of Manta alfredi (Krefft,1868) (Chondrichthyes; Myliobatoidei; Mobulidae). Zootaxa 2301: 1–28.

National Geographic, Stingrays

The Manta Trust