While mantas might seem like nomadic travellers, voyaging the oceans in constant search of food rarely interacting with other species, this is not the case. As well as interacting with other members of their own species during feeding events, at cleaning stations, during courtship and of course when mating; mantas also lend a helping hand to some other species, playing host to a variety of hitchhiking parasites and remoras. Furthermore, social interactions between the manta rays themselves may be much more complex than we currently believe…
Ocean Oasis & Hitch Hikers
Manta rays are like floating islands, oasis sanctuaries for a whole host of animals which seek the shelter, protection and sustenance these giants can provide. Tiny parasites attach themselves to every surface of their bodies; inside their pharynx and gills, around their mouths and in the mucus on the manta’s backs. Juvenile Golden Trevalleys (Gnathanodon speciosus) ride the pressure waves in front of the giant’s head, like bow riding dolphins, these tiny companions appear to lead the mantas, piloting them on their voyage across the open seas. As soon as they are large enough to survive by themselves, the golden trevalleys leave the shelter of the giants to seek protection together in shoals.
Another group of fishes have taken their association with, and dependency on, their larger hosts to a whole new level. These fish are called remoras or suckerfish which, as their names suggest, actually suck onto the bodies of their hosts using modified dorsal fins which act like a suction cup. This enables these fish to cling onto the bodies of the mantas, saving energy; they even venture deep inside their host’s mouths to rest, find food or escape predation. One species, the White Suckerfish (Remorina albescens), actually spends its whole adult life inside the mouth of manta rays, which must certainly be a little discomforting for the host.
The benefits to the remoras and suckerfish for living in such close association with their larger hosts are clear, but there is debate about whether the manta rays receive a significant return in benefits from these relationships. Certainly there are several negative impacts; reduced swimming efficiency through extra drag created by having to carry their hitch hikers, skin abrasions and sores created by the constant suction attachments of the remoras, and the overall discomfort caused by having to put up with as many as two dozen of these fish scurrying around on, and inside, their bodies.
In most coastal reef manta ray populations there are several species of remoras which can be found living on the bodies of the resident mantas. By far the most common is the Slender Suckerfish (Echeneis naucrates); sometimes as many as a dozen of these 50cm long remoras can swarm around a single manta, although 1-2 individuals per manta are most common. Another species often encountered is the Short Remora (Remora remora), although this large, compact and brown remora is much more commonly seen on the oceanic mantas, appearing to prefer life in the open ocean.
The world’s oceans can be an itchy place to live for its inhabitants, especially the larger ones, whose bodies present a much bigger target for water bourn parasites to launch their attack upon. Tiny ectoparasites live and feed on their host’s bodies, hiding themselves inside mouths, spiracles, gills and anywhere else they can secure a firm grip out of harm’s way. And for filter feeding animals like the manta rays, which spend much of their time swimming around with their mouths wide open, particles of food and detritus become lodged in their gills, creating an unwanted build-up of dental detritus.
On land larger animals, such as birds and mammals, often cope with this problem by grooming and cleaning themselves; using teeth, hands, beaks and claws to rid themselves of these irritating parasites. However, fishes like manta rays do not have hands, or any other necessary tools with which to clean themselves, so instead, they have found a group animals that can do the job for them. These animals are called cleaners, and the vast majority are small reef fishes, although many shrimp species have evolved to become specialised cleaners as well.
The cleaners are small out of necessity; their size enabling them to survive on, and pick off, the individual parasites which cover their clients, even venturing right inside the bodies of their clients to reach those areas which would be inaccessible to a larger cleaner. However, this size makes the cleaners less mobile, so instead of travelling around in search of clients to clean, their clients come to them. The cleaners set-up shop at specific locations, usually a prominent reef outcrop or coral bommie, and a whole host of client species make the trip each day to visit these ‘cleaning stations’.
This relationship between ‘cleaner’ and ‘client’ has long fascinated marine ecologists and is widely showcased as the perfect example of mutually beneficial symbiosis, with both the cleaner and client receiving an overall benefit from this cooperative behaviour. For the cleaner the benefit comes from a ready supply of food which is brought to them on a plate, which in this case is the body of their clients. They remove irritating ectoparasites, algal growth, they clean wounds by picking off dead skin and infectious bacteria and fungus, and they also perform dentistry by swimming right inside the mouths of their clients to remove food particles which have become lodged inside. For the clients the benefits appear to be obvious; having a local dentist, doctor and body salon all rolled into one package, with limited waiting times and no reservations required!
Both species of manta rays visit cleaning stations and indeed manta ray cleaning stations are often the best locations in the world to dive or snorkel if you want reliable encounters with these animals. In the Maldives for example there are hundreds of known manta ray cleaning stations, but some are much more commonly used by the mantas than others. The most active sites are often those closest to the mantas favoured feeding sites, although this may fluctuate over time as seasonal abundances of food, or other environmental factors, cause the manta rays to shift their preferences to other locations. It is likely that human disturbance, through sound pollution or diver pressure is also responsible for abandonment of some cleaning stations by the resident manta ray populations.
When a manta ray visits a cleaning station they glide within inches of the reef; slowing their forward movement to the point at which they begin to stall and the rear end of their body begin to sink. If there is a strong current the mantas are able to remain stationary above the cleaning stations, positioning themselves head first into the current to gain uplift, they hover with effortless ease. In slower currents the mantas move in slow circles above the station, lining up one behind the other in a graceful slow motion dance, each manta taking its turn on top of the cleaning station before peeling off to join the back of the queue. Upon reaching the cleaning station the manta rays unfurl their cephalic fins and open their mouths wide, inviting the cleaners to search every nook and cranny of their body for the irritating parasites.
The most prevalent cleaner fish species are all members of the wrasse family; a highly diverse and successful family of marine fish who owe their success to the extendable tube-shape design of their mouths which have evolved to allow them to exploit a wide variety of food sources. Two of these wrasses, the Blue-streaked Cleaner Wrasse (Labroides dimidiatus) and the Bi-colour Cleaner Wrasse (Labroides bicolour) are specialised cleaners who spend their entire lives searching the bodies of a multitude of coral reef fishes in search of ectoparasites. Another two fish species which regularly act as manta ray cleaners are the Moon Wrasse (Thallasoma lunare) and the Two-tone Wrasse (Thallasoma amblycephalum), both are gregarious by nature and often gather in huge numbers, rising up like a swarm of bees to meet the approaching manta ray, which can provide the mother of all feasts for these tiny cleaners. It’s not uncommon for a single manta to be covered with over a hundred of these industrious little cleaners as they scour the manta for ectoparasites.
Manta rays are infested by a variety of parasite species, each adapted to inhabit specific areas of the mantas body. Some ectoparasites live inside the thick mucus layer which coats the dorsal surface of the mantas skin. These little lice are about the size of a grain of rice and can actually been seen scurrying around on the backs of the mantas as they hover above the cleaning stations. Scientists inspecting the bodies of dead manta rays have also discovered a host of other parasites which infest the gills and mouths of the manta rays, so it is no surprise that the cleaner wrasses also concentrate their search for food in these areas as well. The different cleaner species also divide the focus of their search; with the Two-tone Wrasses targeting the mantas backs and bellies, while the other three species prefer to enter right into the mantas mouths and gill openings in search of trapped food and internal parasites.
Sometimes the tranquillity of the cleaning is broken when a cleaner bites a particularly sensitive part of a mantas body, sending it flying away from the cleaning station as great speed, trailing a stream of fleeing cleaners which pour from the mantas body as it goes. Occasionally these accelerations are directed upwards towards the water’s surface, as the manta leaps clear out of the water before gravity bring its immense bulk crashing back into the sea.
Manta rays in the Maldives regularly spend many hours each day visiting favoured cleaning stations, cruising along the reef or hovering above the cleaning station to allow the cleaners access to their bodies. These sites are obviously extremely important to the population as a whole; serving as focal points for mating activity as well as the obvious function of cleaning. However, these explanations, while they are undoubtedly important contributing factors, they do not fully explain the reasons why some sections of the population spend so much more time at cleaning stations than others. While male and juvenile manta rays both regularly visit cleaning stations, the frequency of their visits, and the length of time they spend cleaning, are significantly less than the large females in the population.
Over five years researchers have identified over 400 individual manta rays visiting one particular cleaning station in the Maldives, amassing thousands of visitation records for the population as a whole which is split roughly evenly between males and females. However, during the study period a core group of 60 large females accounted for over 50% of all recorded sightings, indeed of this site’s top 100 visiting manta rays, 73 are large females. If the motivation for manta rays to visit cleaning stations was simply a mechanism for having parasites removed from their bodies, we would not expect to see such variations in visitations within the population, as parasite loads are unlikely to vary significantly enough between the sexes to warrant such polarized trends.
So why do the females spend so much more time at these cleaning sites than the males? The simple answer is that we just don’t know, however a likely possibility is that male manta rays spend more time roaming from location to location in search of mates, while the females remain faithful to a specific home range, only venturing further afield if the drive for food dictates. In the Maldives female manta rays show very strong site fidelity, returning to the same feeding and cleaning stations on a regular basis throughout their adult lives, which span many decades. Each area appears to have its own core population of individuals which have learned how best to maximise their planktonic food requirements from their home range, which often occurs in very patchy bursts of abundance with the regular ebb and flow of the tides.
As a result, it appears these individuals have plenty of time on their hands (or rather fins) to play with, and many of them make a bee line for their favourite cleaning station during these periods of lull in feeding activity. Once at the cleaning station, the females may engage in mating activity at specific times of the year, get cleaned, or simply enjoy the sensation of being groomed by the cleaner fishes. The first few hours after sunrise are the peak periods for cleaning activity, which is not surprising as cleaner fish are active only by day, so each morning the mantas are itching to be cleaned.
It is also possible that these core groups of females are linked by more than simply habitat utilization, with social associations and possibly genetic relatedness binding the group together, as younger females learn the habits of the older and more knowledgeable individuals. The extent to which these clearly defined groupings at specific cleaning stations link these females together on a larger scale, both geographically and temporally, is only just beginning to be pieced together. And to what degree social interactions, or merely functions of habitat utilization, are the ultimate driving forces behind these associations still remains to be seen.