Creatures of the Coral Reef – Manta Rays, Silent Sentinels of the Sea.

Silent Sentinels of the Sea

Manta Ray Trio, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)

Manta Ray conservation is important to protect this marvelous species, what can you do to help?

These magnificent creatures are the epitome of elegance and grace.  Long ago feared by seamen around the world due to the similarity of their projecting cephalic fins to “devils horns” and their triangular shape resembling a cape.  However, these plankton feeding giants are anything but dangerous.  They are instead the lead players of a graceful underwater ballet, flying gracefully through the water performing amazing pirouettes, back flips and even leaps from the water into the air.

This article is another in our series on “What are Coral Reefs” with a focus on the creatures of the coral reef.  I thought how appropriate to begin reviewing “Creatures of the Reef” than to look at this under appreciated and magnificent creature.

Manta Rays were first described by Dondorff in 1798 and named Manta birostris. Other synonyms include Cephalopterus vampyrus Mithchell 1824, Cepahalopterus manta Bancroft 1829, Manta americana Bancroft 1829, Ceratoptera johnii Müller & Henle 1841, Ceratoptera alfredi Krefft 1868, Brachioptilon hamiltoni Hamilton & Newman 1849, Raja manatia Bloch & Schneider 1801, Manta hamiltoni Hamilton & Newman 1849, and Manta alfredi Krefft 1868.

The Manta Ray has also been known by many names around the world including, Manta ray, Atlantic manta, Australian Manta Ray on Reef, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)devilray, blanketfish, devil ray, devilfish, devil-ray, eagle ray, giant devil ray, giant manta, giant Atlantic manta, great devilfish, manta, manta ray, Pacific manta, prince alfreds ray, sea devil, and skeete are common names in English language. Other names include diable de mer (French), duivelsrog (Dutch), jamanta (Portuguese), manta (Italian), manta atlantica (Spanish), oni-itomaki-ei (Japanese), raya (Spanish), teufelsrochen (German), and urjamanta (Portuguese).  Source: Florida Museum of Natural History

Manta Rays are thought to be highly intelligent and based upon some theories the Manta Ray, which has the largest brain among fishes,  possesses the most developed cognitive abilities of any fishes and may be equivalent to the black-furred gibbon (Sympsyndactylus). Source: The Open Anatomy Journal.

Manta Ray facts and Anatomy

Manta Ray, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)The Manta Ray is one of the largest fishes, and has been know to reach 9 m (29.5 ft) and weigh as much as 1,350 k (3,000 lb).  The Manta Ray’s lifespan is thought to be about 20 years.  They are close relatives of sharks, which are also one of their main predators along with certain types of whales.  They are a close relative to the stingray, but they do not have a stinging tail.  The body of the Manta Ray is covered with a protective mucus membrane.  The mucus serves two basic functions: it acts as a barrier against infection and it reduces friction so the Manta Ray can move more easily through the water.  This mucus membrane can be damaged through human contact, so if you get close to a Manta Ray please refrain from touching it.

The Manta Ray has a highly developed pectoral fin to help it glide swim through the water.  The up-and-down movements of the huge pectoral fins resemble the wings of a bird in flight. The Manta Ray also has two Cephalic fins or lobes on its head just in front of its mouth that can be “rolled up” when swimming and unfurled when feeding.  The mouth of the Manta Ray is in the terminal position, that is it is on the front of the head.  Other types of rays, such as the sting ray have mouths that are in the inferior position, that is underside of the head.   The eyes are located on the side of the head, just behind the Cephalic lobe.    The Manta Ray has five gill slits on the underside of its body.  They also have large openings behind their eyes known as spirals.

Manta Rays are filter feeders.   The filtering mechanism consists of plates of pinkish-brown sponge-like tissue located Manta Ray and Diver, Manta birostris,  (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)between the successive gill rakes that support the gills.  The Manta Ray “fly’s” through the water with its mouth open and the Cephalic fins unfurled directing plankton-rich water towards the mouth.  The unfurled Cephalic fins funnel water more directly into the mouth of the Manta Ray.  They tend to aggregate in areas that offer large concentrations of zooplankton, and have been seen in groups of up to 50 individuals.  It is fascinating to watch them swimming in slow vertical loops  feeding on the zooplankton.

The oceanic islands and submarine ridges provide precious few sites containing nutrient-rich waters and an abundance of zooplankton, in the otherwise nutrient poor tropical regions.  Scuba diving operators have adopted the use of underwater night lights to attract plankton which in turn attracts Manta Rays, creating a “symbiotic” relationship where the Manta Rays can feed more easily and the dive operators can offer unique underwater encounters with this marvelous creature.

The Manta Ray does have a tail, but their tail does not have a stinger.

Habitat and Behavior

Manta Ray HabitatThe Manta Ray inhabits temperate, tropical, and subtropical waters worldwide ranging between 35° N and 35° S latitudes. In the western Atlantic Ocean, this includes South Carolina (US) south to Brazil and Bermuda. Occasionally this ray is observed as far north as New Jersey and San Diego. Other locations include the east coast of Africa, in the Gulf of Aden, Red Sea, Arabian Sea, the Bay of Bengal, as well as the Indo-Pacific.

Habitat of the Manta Ray ranges from near shore to pelagic waters, occurring over the continental shelf near reef habitats and offshore islands.

Manta Rays sometimes swim in loose aggregations and have Manta Ray Breaching, Manta birostris, (Walbaum, 1792), Maui Hawaii (Steven Smeltzer)been observed in groups of up to 50 individuals  The Manta Ray spends considerable time near the surface of water but can also dive to depths of up to 300 meters (1,000 ft). Manta Rays have been observed breaching, jumping clear of the water and returning with a splash. Three types of jumps have been observed, forward jumps landing head first, forward jumps landing tail first, and somersaulting. Groups of these animals have been seen participating in this behavior, breaching one after the other. While it is not understood why this behavior is exhibited, some speculate it may play a role in attracting mates or is a form of play. Source Florida Museum of Natural History

Reproduction

Manta rays are ovoviviparous which means that they produce eggs but the egg “hatches” inside the mother where the young mantas mature and the mother gives birth to live young;  ovum (egg), vivius (alive), and papere (to produce).  The egg has a thin membranous egg case and the baby manta rays hatch in the female’s oviduct and remain inside of the mother feeding on the mother’s uterine milk until fully developed. Babies are born with their pectoral fins folded over themselves. When they stretch out their “wings,” they are known to be approximately 3′ across.

Observation of Manta Ray births are extremely rare.  The only reported birth was for a resident manta female in captivity at the Okinawa Expo Aquarium following what was determined to be a 12 -13 month gestation period. The newborn was abandoned immediately after birth and the mother was seen mating again within hours.  Pregnancy rates for resident mantas have been estimated using long term sighting records of females observed pregnant every 2-3 years on average, Manta Ray Trio, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)with some females becoming pregnant in consecutive years . Although giving birth to a single pup at a time appears to be the norm for both species based on dissections of pregnant females, two pups may be conceived on occasion. Source: Captive Records of Manta Rays in Okinawa Churaumi Aquarium.

The sex of a manta ray can be determined by the presence of claspers in males, and their absence in females.  Among males, calcification of the Manta Ray Female, No clasper, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)claspers occurs rapidly over a relatively narrow range of growth and the majority of calcification occurs once the claspers have extended beyond the length of the pelvic fins. Since the onset of clasper calcification in many shark species coincides with a rapid rate of clasper growth and gonadal maturation claspers extended beyond the pelvic fins were used as a reliable indicator of sexual maturity in male manta rays.

Based upon observation of mating behavior, mating events appear to be restricted to certain times of the year that vary by region.  Male suitors line up in a “train” behind the female trying to bite the pectoral fin.  When a male succeeds in biting the pectoral fin he positions his ventral side against hers and inserts his clasper into the female’s cloaca for somewhere between 60 to 90 seconds.  Following this he removes his clasper and continues to hold her pectoral fin in his mouth for several minutes.  Following this the male releases the female and both swim apart.  Off Ogasawara Island, Japan, most mating trains were observed between March and October with occasional chases observed outside the summer season. Mating behavior in Yaeyama Island was reported during the spring and autumn and lasting for about a one month duration. The wet season in this area begins in May and lasts through June, and the typhoon season (May – November), typically peaks in September. In the Maldives, the majority of mating trains were observed between October and November, towards the end of their wet season (April to October), with occasional trains observed outside this season . In Mozambique, fresh mating scars were observed primarily during the summer months. In Hawaii, the mating season appears to be during the winter months. Although mating trains can be reliably seen, observation of acts of copulation in free ranging mantas are rare.  [5,6,7]

History of the Manta Ray Encounters in Kona Hawaii

Manta Ray Duet, Manta birostris,  (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)Before the Kona Manta Ray Night Dive became so popular, few people went scuba diving at night with the manta rays. For years, hotels and restaurants along the coast would shine bright lights into the water to offer romantic views to guests. This light attracted plankton, the plankton attracted the mantas. The Kona Surf Hotel that opened in 1971 was one of these places. In the early 1990’s, dive operators began leading divers in the waters off the Kona Surf with repeated success of manta sightings. During the early days, some divers would reach up and pet the passing mantas. As the number of divers increased, petting increased. In 1992, the first year that the dive gained popularity, we began noticing pink patches on the undersides of the mantas. Our search for manta information led us to the Waikiki Aquarium where one of their marine scientists informed us that many sea creatures have a thin coating of protective mucous on their skin and this mucous can be rubbed off.

Because the diver’s underwater light attracted a dense cloud of food, the mantas would come very close to individual divers to optimize their feeding opportunity. Back then, most mantas would endure some contact by the divers to get at the concentrated food source. Each manta seemed to have its own tolerance level for the amount of human touching they would endure. Some mantas left after the first touch. Most mantas tolerated some touching and then would leave. One named Lefty endured petting more than most.

Lefty was the first manta ray we ever got to know. She was easy to identify from the others because of her broken left cephalic fin. The fin hangs unfurled and limp and often times it blocks half of her mouth forcing the flow of water and food away. Consequently, we consider her ‘feeding challenged.’ Because of this, we believe Lefty stayed and fed in the abundance of food provided by the divers even though she was rubbed repeatedly. Sensing a need to protect Lefty and the other mantas, respected people from the dive community came together and produced a set of conservation guidelines for the Manta Ray Port Side, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)Kona Manta Ray Night Dive.
Originally drafted by The Ocean Recreation Council of Hawaii and PADI’s Project AWARE (Aquatic World Awareness, Responsibility, and Education), the following updated version of these guidelines are set forth for all divers on the Kona Manta Ray Night Dive.

1. OBSERVE ONLY: No touching. Resist the urge to “pet” the mantas. This will rub off their protective mucus coating. Do not chase, grab, or try to take a ride on the mantas. This doesn’t benefit the animal in anyway.

2. DIVER POSITION: Divers please stay on or near the sand, rubble or boulder bottom. An open water column is necessary for the mantas to maneuver. Avoid contact with coral, sea urchins, or other marine life. Form a semi-circle with your group.

3. SNORKELER POSITION: Snorkelers please stay on the surface. Do not dive down into the water column where the mantas are feeding.

4. LIGHTS: Divers please shine lights up into the water column to attract plankton. Snorkelers please shine lights down.

5. BUBBLES: Divers please avoid exhaling bubbles directly into the manta’s face.

6. TAKING PHOTOS or VIDEO: Photographers and videographers please be considerate of others. Adhere to these guidelines and let the mantas come to you.

Dive operators enforced these guidelines with their dive groups immediately and soon Lefty’s pink patches healed. Over a decade later, she is as healthy as ever and one of the most consistent dinner guests on the Kona Manta Ray Night Dive.  This approach to interacting with marine species demonstrates the value that can be gained from a proactive and responsible manner of approaching marine life.  In this case the Manta Rays are able to access a rich feeding zone and divers and snorkelers can interact closely with the Manta Rays in a “once in a life-time” encounter. Source: Manta Pacific Research Foundation

Manta Ray Seasonal CyclesThere is also some interesting research that was performed by, Kara Osada as part of her Masters Thesis at the University of Hawaii.  The study was to review the relationship of zooplankton abundance to the number of Manta Rays that visited a particular “Manta Ray Dive Site” such as Garden Eel Cove off the Kona coast in Hawaii.  The study tracks the relationship of zooplankton to lunar phases and to the presence of Manta Rays.  It is also interesting to note the date of the study which appears to be 2008 from the dates on the included Manta Ray Garden Eel Cove,  Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)charts.  It is also interesting to note the number of individual Manta Rays that were visiting the site at that time maximum monthly average of 5 individuals a night over a 12 month period.  It is interesting to note that when I dove the site in April we were just past the midpoint of between the full moon and the new moon phase and we had 37 Manta Rays on the site.  Talking to one of the long-time videographers on the site they had been having record sightings over the prior two week period and it will be interesting to see if the trend continues.  Zooplankton abundance

You can also check out my post from my Manta Ray Dive in Hawaii in April of this year…great dive.

Some well known resident manta ray aggregation areas worldwide that have become popular tourist destination areas include Komodo Marine Park, Indonesia, Yap, Micronesia, Palau, Yaeyama, Okinawa, Japan, Kona , Hawaii, Bora Bora, French Polynesia, Mozambique, the Republic of Maldives, Ningaloo Western Australia and in the Republic of Maldives.

 

Conservation

Manta Ray Gill Slits, Manta birostris,  (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)Large sharks and killer whales (Orcinus orca) have been reported to prey on manta rays. It is unknown how many attacks result in fatalities and weather or not the shark will consume the entire manta ray. In Mazambique, about 1 in 8 manta rays shows injuries likely caused by a shark attack, compared with only 1 in 4 in the Maui, Hawaii population.

The status of most manta ray populations worldwide is poorly understood. They are classified by the IUCN Red List for Threatened Animals as “near-threatened”,  meaning that manta rays could be threatened with extinction in the near future if conservation efforts are not implemented.

Currently this species has a high value in international trade and directed fisheries exist that target this species in what is certain to be unsustainable numbers. Artisanal fisheries also exist that target this species for food and medicine. Individuals are also taken as bycatch in everything from large-scale fisheries to shark control programs/bather protection nets.

The rate of population reduction appears to be extremely high in several regions, as much as 80% over the last three Manta Ray Encounter, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)generations (approximately 75 years), and globally a decline of >30% is strongly suspected. Sustained pressure from fishing (both directed and bycatch) has been isolated as the main cause of these declines. Certain monitored sub-populations appear to have been depleted, such as in the Philippines, Indonesia, and parts of Mexico and are believed to be decreasing in other areas such as India and Sri Lanka as a result of sustained pressure from fishing. Of particular concern is the targeting of this species at critical habitats or well-known aggregation sites where numerous individuals can be targeted with relatively low catch-per-unit-effort. Source: IUCN Red List

The explosive growth of trade in the gill rakers of Manta Rays has driven up the amount of direct fishing to an unsustainable level.  The main reason is demand from China, where their gill rakers (filaments that filter the animals’ food from the water) are marketed as a supposed cure for a variety of ailments.

The southern Chinese city of Guangzhou is the hub of the trade in the dried parts, which retail for as much as $500 a kilogram (roughly $225 a pound), according to the research team’s findings.  The researchers note that the gills had not previously been prescribed in traditional Chinese medicine, and many of its practitioners conceded in interviews for the study that gill rakers were not effective in treating illness and that many alternatives were available. The rising popularity of the ingredient seems to be the result of traders’ efforts to create a market.

Because of the extreme vulnerability of the manta and mobula rays, the race to save them is critical.  It took years of campaigning before shark sanctuaries and bans on shark fin possession began to materialize.  With manta and mobula rays, there may not be time to go through years of raising public awareness before action is taken. Source: New York Times

Actions

Manta Ray Feeding on Plankton, Manta birostris, (Walbaum, 1792), Kona Hawaii (Steven W Smeltzer)Scuba Diving tourism involving the Manta Ray is a growing industry and it has clearly demonstrated that sustainable tourism significantly enhances the economic value of such species in comparison to short-term returns from fishing. However, tourism related industries can also negatively impact individual behavior, entire populations and critical habitat for this species. Thoughtful development of “Manta Ray Diving” can be a critical component of sustaining this marvelous creature. Governments and Divers worldwide are encouraged to work together to help create both sustainable industry and local economies balanced with the need to protect our fragile oceans. Otherwise more and more Manta Ray habitats will be depopulated such as those Indonesia and the story of “Diving with the Mantas” may be only a story that we can tell to our grandchildren.

1) Ban trade in gill rakers

2) Provide support to develop “Eco-tourism” opportunities for local economies dependent upon fishing for Manta Rays and Mobula Rays

3) Support long-term redevelopment of reef systems and reintroduction of Manta populations in areas where the populations have been decimated

Series:

  1. What are Coral Reefs
  2. Introduction to the Underwater World
  3. Creatures of the Coral Reef
  4. Manta Rays, Silent Sentinels of the Sea

 

Other Reading and Resources

Hawaii Institute of Marine Biology

Manta Pacific Research Foundation

Florida Museum of Natural History

Open Anatomy Journal.

1. Kara Osada, Relationship of zooplankton emergence, manta ray abundance and SCUBA diver usage Kona Hawaii

2. Uchida, S., M. Toda, and Y. Matsumoto, Captive Records of Manta Rays in Okinawa Churaumi Aquarium in 2008 Joint Meeting of Ichthyologists and Herpetologists, M.A. Donnelly, Editor. 2008: Montreal, Canada. p. 520.

3. Marshall, A. and M. Bennett, Reproductive ecology of the reef manta ray Manta alfredi in southern Mozambique. Journal of Fish Biology, 2010. 77(1): p. 169-190.

4. Mark H. Deakos, The reproductive ecology of resident manta rays (Manta alfredi) off Maui, Hawaii, with an emphasison body size, Springer Science+Business Media, 2011

5. Homma, K., et al. Biology of the manta ray, Manta birostris Walbaum, in the Indo-Pacific. in Indo-Pacific fish biology: proceedings of the fifth international conference on Indo-Pacific fishes, Noumea, 1997. 1999. France: Ichthyological Society of France.

6. Stevens, G. and R.D. Rubin. Reproductive Behaviour, Mating and Male Competition in Manta Rays (Manta birostris) in the Indian Ocean  in 2008 Joint Meeting of Ichthyologists and Herpetologists. 2008. Montreal, Canada.

7. Yano, K., F. Sato, and T. Takahashi, Observations of mating behavior of the manta ray, Manta birostris, at the Ogasawara Islands, Japan. Ichthyological Research, 1999. 46(3): p. 289-296.