History

Distribution

While manta rays have a circumglobal distribution (meaning that they are found across the globe) and are found in all oceans but the Arctic, they are mostly commonly found in tropical waters. The different species of manta rays have very different distributions, however, and are not commonly found residing together or using similar habitat.
Manta birostris appears to be more commonly found off shore and may be a more migratory species. This species has been documented to occur as far north as southern California and Rhode Island on the United States west and east coasts, Mutsu Bay, Aomori, Japan, the Sinai Peninsula, Egypt and the Azores Islands in the Northern Hemisphere and as far south as Peru, Uruguay, South Africa and New Zealand in the Southern Hemisphere.
Manta alfredi, is commonly sighted inshore, within a few kilometres of land. Found around coral and rocky reefs as well as along productive coastlines with consistent upwelling, tropical island groups, atolls and bays. This species is widespread in the Indian Ocean, with images and sightings of M. alfredi from the Red Sea in the north to Durban, South Africa in the south, and from mainland Thailand in the north to waters off Perth, Australia in the south. In the eastern and south Pacific, M. alfredi occurs from the Yaeyama islands, Japan in the north to the Solitary Islands, Australia in the south and is sighted as far east as French Polynesia south of the equator and the Hawaiian islands north of the equator. Two reports and photographs of M. alfredi from the north Atlantic off the Canary Islands and the Cape Verde Islands and historical reports and photos of M. alfredi off the coast of Senegal in north west Africa (Cadenat 1958) are the only evidence of populations of M. alfredi in Atlantic waters.
The two Manta species are sympatric in a few locations around the globe including Mozambique, Hawaii, and Indonesia but at most of these locations, geographical separation, fine-scale habitat use, or seasonal movement patterns typically keep them from coming in contact with one another. (See map)

Biology, Ecology& Behaviour

Manta rays have very distinct body colouration including natural spot patterns on their ventral surface (stomach) that can be used to differentiate individual rays. Amazingly, aside from the natural colouration pattern most commonly seen in manta rays, two additional ‘colour morphs’ occur. Firstly there is a black or melanistic morph, where the entire ray is black aside from a variably sized blaze of white on its ventral surface. No other species of shark or ray has been observed to have a melanistic or black colour morph. While ‘black mantas’ are common in some regions, such as the indo pacific and along the west coast of the Americas, in some regions specimens are entirely absent, such as along the eastern coast of Africa where no black manta rays have been registered. Although far more rare than the melanistic morphs, a white or leucistic morph also occurs, where there is a notable absence of pigment on the body of the manta ray creating an overall whitish effect. These individuals are not true albinos as they still retain some pigment (particularly in their eyes) but they can be extremely devoid of pigment in some cases. It is not known when the genetic mutations that caused these rare colour morphs first occurred or why they have persisted over time, but it is certainly interesting that these colour morphs occur in both of the recognized species of Manta, suggesting that these genetic mutations occurred in an ancestral form (i.e. before the species split).

How old to manta rays live? At the moment, common techniques used to age other sharks and rays are not working for manta rays and it is unclear how long they live or at what age they reach sexual maturity. However, we expect that like many of the other large pelagic species of elasmobranchs, manta rays are long-lived and reach maturity at a late age. Researchers are busy trying to modify existing techniques and explore new ones so that we may finally figure out the longevity of both Manta species.

We do know that manta rays appear to spend all of their time in the water column; there is currently no evidence to suggest that they rest on the sea floor like other rays. They do appear to partition their time up doing different things. During the daylight hours, manta rays appear to most commonly spend their time inshore and are often seen at cleaning stations or feeding along the coast. During the evening, they appear to commonly move offshore or into deeper waters and it has been proposed that they may spend the majority of the nighttime hours feeding.

Manta rays, along with the devil rays, have the largest brain of any fish (which includes the other sharks and rays) relative to body size. While this might at first seem odd, it is becoming increasingly clear that these magnificent animals are highly social, a common trigger for rapid growth in brain size. Aside from the highly social and complex behaviours they exhibit, many of which we still do not fully understand, manta rays may also use sound to communicate to one another (intra-specific communication). There is evidence to suggest that the noise generated by breaching (jumping clear out of the water) may serve to signal to other rays in the vicinity.
It is often noted that manta rays look very different from other rays. The reason for their dramatic differences have a lot to do with their diet and their lifestyle, which ultimately required them to have a different body plan than their demersal relatives. As such, manta rays have undergone dramatic modification and as a result have many new and unusual adaptations. One of the most obvious differences lies in the diamond shape of their body; their massive pectoral fins are capable of propelling them through the water in the same fashion as a bird in the sky. These massive wings which have a mix of both white and red muscle (used for both quick bursts of speed and long sustained swimming) and provide manta rays with agility, maneuverability and speed and most importantly, enable them to undertake massive migrations. Manta rays are known to travel substantial distances sometimes over 200 kilometers a week and over 1,000 kilometers two months in a single direction. Manta rays also have what is known as lateral eye placement, meaning that instead of having eyes located on the top of their head (like demersal rays) they are located on the side of their head. This placement allows them greater range of vision in the pelagic environment where they spend their time. Despite this clever eye placement, manta rays do have distinct blind spots, which is evidenced by ‘rear approach’ predation injuries and line entanglement scars between their cephalic fins. Their cephalic fins themselves are a unique adaptation, only occurring in the mobulid rays. These lobes attached to the end of their head on either side of their mouth appear to aid in the capture of zooplankton, presumably making them more efficient feeders. Unlike other planktivorous elasmobranches like whale sharks and basking sharks, which are only capable of opening their mouths, manta rays are capable of angling these fins in different ways to direct water flow into their mouths. When not feeding, manta rays will roll these fins up to make themselves more hydrodynamically efficient whilst traveling. Early sailors and explorers thought that they resembled horns and thus named the group the ‘devil fish’, which they are still often referred to today. This often leads to negative feelings about these rays so we encourage using the name manta, which derives from the Spanish word for blanket or cloak.

Like many other large marine megafauna, manta rays are planktivores, feeding on small marine invertebrates called zooplankton and occasionally small fish. Manta rays employ many different types of feeding modes when trying to capture this tiny food source. Commonly, when plankton is distributed evenly in the water column they will simply feed at the surface or mid-water with their enormous mouths open sieving the tiny particulate animals from the water with specialized and highly modified gill rakers. These gill rakers prevent the food particles from passing over their gills (where oxygen is extracted from the water and ultimately exits via their ten gill slits). After being trapped, plankton particles are transported to the back of their mouth where they are swallowed. The opening of their esophagus or their ‘throat’ is approximately the size of an average human fist. It is for this reason that they cannot eat large items and pose no threat to humans. While the above-mentioned foraging method is the most commonly observed feeding mode, manta rays will alter their behaviour to suit conditions. If plankton is densely concentrated in one location manta rays are often observed barrel rolling in the water column (which facilitates them staying in one location or in a specific area). While making dramatic vertical loops, individuals are able to feed more efficiently on the concentrated plankton. Similarly, if plankton accumulates or is trapped just above the sea floor, manta rays are capable of feeding off the ground. Like a vacuum, the rays will almost scrape their huge terminal mouths along the seabed. They will also often use their cephalic fins in a unique manner, like a funnel, to direct plankton into their gaping mouths.

Despite swallowing their food whole, manta rays still possess teeth. Measuring only about 1-2mm in size, individual manta rays can have up to 4,000 of these minute teeth, which are embedded in a tooth band that stretches almost the entire length of the lower jaw. These teeth are not used for feeding but rather are used in their reproductive behaviour. Abrasive, like Velcro, male manta rays will grasp onto the pectoral fins of the female ray when mating using these teeth to help them maintain their grip. The teeth are not large or sharp enough to cut the female’s fin, but they do leave scratch marks that appear red when fresh (i.e when the female has been recently mated) and turn white to grey over time.

Research Efforts

Population Estimation & Demography

Long-term monitoring of manta rays at study sites in Mozambique has allowed us to ascertain population estimates along this stretch of coastline and track population composition seasonally. Using photo identification techniques manta rays are ‘visually tagged’ and ‘re-sighted’ over time, allowing us to examine the proportion of re-sighted individuals to new individuals that are continually being identified. These values can be used to estimate both the annual population size at our monitored aggregation sites and also the super population size (number of rays that may occur in this region).

Our current study is the first of its kind in Mozambique or even Africa, with our population estimates serving as the world’s first data on an identified manta population. To date, the population estimates for this immediate stretch of coastline suggest that we have approximately 800 reef manta rays (Manta alfredi) and approximately 600 giant mantas (Manta birostris) although giant manta rays are both observed less commonly and identified individuals re-sighted less often. To date, this is one of the largest populations of manta rays identified anywhere in the world. Combined with the fact that manta rays use this coastline year-round, southern Mozambique may offer divers and researchers alike the best opportunity to interact with these gentle giants.

While it is an extremely important first step to estimate the population sizes of manta rays, it is equally important to assess whether their numbers are stable, increasing or decreasing. This year we will be looking into a notable decline in the ‘sightability’ of rays over the last seven years along our famous ‘manta ray coastline’ and the factors that may be contributing to their gradual decline in these waters.

Reproductive Ecology

Telemetry Studies

Despite their curious and tolerant nature, manta rays are elusive animals that are typically only encountered in surface waters where they are cleaning or feeding. As such we still know very little about these threatened species which continues to hamper our efforts to manage populations effectively, assess their threats and assign them an appropriate conservation status.

While most of our work is built around our non-intrusive photo identification program, various tagging programs have been started to establish how manta rays use near-shore environments. These programs also aim to identify critical habitats such as feeding areas, cleaning stations, and areas used for mating and birthing.

The first of our programs makes use of coded acoustic tags, which are placed on a random selection of individual rays. These tags relay information to listening stations (acoustic receivers) that we have strategically placed down on reefs or other important aggregation areas. Tagged animals will continually register (several times a minute) on these listening stations if they are using the area within 500 metres of one of the stations. Even if an individual only passes by, the stations will still record their presence. Over time, this acoustic-based system gives us an uninterrupted view of how these individuals use various habitats along the coastline, giving us an idea of which areas are heavily trafficked during which time periods, which area may serve as critical habitats that must be carefully monitored or protected, and which areas area used infrequently or seasonally. The more receivers used the better information we get back and it is our intention in 2010 to have over 20 listening stations deployed at aggregation sites along a 200 km stretch of coastline in southern Mozambique. This passive acoustic array will continue to gather valuable information on manta rays for years to come, with the batteries on tags lasting up to five years. Our local array will also link in with the Wildlife Conservation Society’s Indian Ocean ‘Meganet’ which has similar arrays operating in many locations in the western Indian Ocean, including Madagascar, Tanzania, Kenya and South Africa.

Over the long term this study will provide some of the data needed to support the design of a marine protected area in the region for manta rays and whale sharks. From a conservation angle, we will be able to target protection measures where they are most needed thus increasing the effectiveness of management efforts. By sharing movement data with other regional nodes of the meganet, we can look at the linkages between countries and start planning for more successful international management measures.

Genetic Studies

For the past half decade, our team has been collecting tissue samples of identified individuals in the Mozambican population. We are also collecting tissue samples from manta rays across the globe. To extract these samples we use highly efficient Hawaiian slings with biopsy tip that remove small plugs of tissue from the pectoral fins of individual rays. These samples allow for many types of studies. To date we have used our collection to examine the differences between the two populations of Manta alfredi and Manta birostris at our study site in Mozambique. We are also continuing our efforts to compare populations around the globe to better understand their movement patterns and relatedness. In the future we also hope to look in more detail at the relatedness of individuals at the population level here in Mozambique and overlay this information with the detailed ecological information that has been collected in the field.

Feeding Ecology

Our feeding ecology team, team lead by Chris Rohner, will be trying to determine the dietary preferences and seasonal shifts in the diet of manta rays along this coastline. The ocean off Tofo is incredibly productive. As opposed to tropical seas, which are usually very clear, the waters along the Inhambane coastline are often full of plankton. Thanks to this increase productivity, we have a high abundance of both whale sharks and manta rays along the coastline. While manta rays and whale shark often feed together in the same area, observations have revealed that each of the animals have their own specialised strategies to make the most of the microscopic feast. Additionally, while preliminary observations suggest that both animals feed on the same or similar plankton species, it is still unclear if each of the species have dietary preferences.
Scientists from the Foundation began in 2009 to look into the plankton dynamics in our local waters. Even though this is work is still in progress, a nice success story of a little planktonic animal called Diacavolinia longirostris is emerging. This tiny gastropod is often seen by divers on their safety stop in the area and our researchers have long suspected that the mantas might particularly seek out and feed on this zooplankton species. Lately, our team has been building more and more evidence to support is theory. Diacavolinia is often seen when large feeding aggregations of manta rays occur and we have observed individual manta rays ingesting them. Plankton tows in the immediate vicinity of barell-rolling mantas have further supported this theory as has our analysis of manta waste (yes…we even collect this in the field! Is there no end to our dedication?).

International Collaborations and Comparative Studies

As human pressures increase worldwide manta ray populations have declined in many areas where they were once common. There is a notable contrast between the few reasonably protected populations, such as in Yap, the Maldives and the Hawaiian Islands, where economically valuable ecotourism and dive operations exist, and populations in areas where fisheries target species, such as along the coast of Africa, India, South East Asia, and Central America. Sadly, due to many of their life history traits mantas are highly vulnerable to over-fishing, and there looms a genuine threat of localised extinction of certain populations.

Currently there are no comprehensive management programs for manta rays anywhere in the world, yet they are listed by the IUCN as ‘near threatened’ to extinction. This conservation assessment has not been updated since the discovery of two distinct species. The recent reclassification of the genus has major implications for the conservation assessment of the two species. Each species faces different and specific threats in various regions of the world, and the worldwide IUCN status of the genus requires urgent revaluation in light of this revision. Our team is currently leading efforts to make this reassessment.

Acquiring accurate information on population dynamics, lifespan, reproductive parameters, growth rates and natural mortality rates is crucial to understanding the conservation requirements of a species. Additional information on the life history of manta rays is sorely needed to supplement the paucity of existing data, particularly for giant mantas (Manta birostris) for which little biological information is known. Site fidelity, movement patterns and habitat usage are also essential pieces of information needed to properly manage populations, once the biological parameters of a species are known. This type of information can highlight an animal’s potential susceptibility to fishing pressures and help determine critical habitats and seasonal migration routes. Elucidating these types of large behavioral trends in elusive pelagic animals can prove challenging but with the help of modern technology we are better equipped to try than at any other point in history.

At the end of the day, management itself is the true challenge and real effort needs to be made to protect populations or critical habitats once they have been identified. In doing this, the animals themselves need to be protected from intense anthropogenic pressure of any kind. While directed fishing pressure is the most obvious threat, management must also extend to recreational boating and fishing, as well as, activities involving human interactions (e.g. scuba diving). All such activities must be regulated in order to ensure that even cryptic threats are monitored and minimized, the health and behaviour of animals is not dramatically altered and the sustainability of eco-tourism is maintained.

Eco-tourism has proven to be an excellent way of generating a positive, sustainable balance between protecting populations of animals and creating an economically viable form of tourism. In 2002, the Big Island of Hawaii was reported to have generated over $2.5 million US dollars in combined revenues from manta related activities in their coastal waters. Surveys in the Maldives have reported that the local economy generates approximately $7.8 million US dollars in combined revenue from manta ray related activities in their islands. These figure are astonishing when compared to the $250-500 US that a dead manta can yield in the fish markets across the world. Some of the directed fisheries that supply this market can harvest as many as 1,500 individuals per annum. Manta rays are predominantly used for their skin, cartilage, and gill rakers, which are used in bogus Chinese medicinal products and for ornamental goods. In some areas, like along the eastern coast of Africa, manta rays are also still taken in subsistence fisheries.

Manta ray populations are frequently small in size and they are one of the least fecund species of sharks or rays (low number of offspring). Intense fishing pressure can threaten the stability of local populations and their numbers can be decimated in the span of a few years. Our conservation efforts in Mozambique aim to raise the profile of manta rays in the region and ultimately formally protect both species of manta rays in Mozambican waters. Long-term we hope to assist with the development of marine protected areas for manta rays and whale sharks in critical habitats such as the Tofo coastline south of Inhambane. Globally, foundation scientists will continue to contribute through our involvement with the IUCN redlist assessment for manta rays. We also will continue to collaborate with international manta ray programs to uncover population trends, assess threats, and develop management plans.

What can you do to assist with conservation efforts?

To get more involved we suggest the following: