Sea flap flaps, sea pancakes, Haray Potter, these are just a taster of names that can be applied to the beautiful and majestic animals that I’m fortunate enough to study as part of my post-graduate research program. I say “fortunate” because as a student of bioscience I’ve come to realise through experience that despite a love for everything that moves, there are certain species that you can grow tired of observing after weeks of staring at them. Stingrays however, haven’t stopped providing me with new interests to investigate, behaviours to observe and science to ponder on…
I’m fairly sure that most people have an image pop into their mind when they hear of a stingray. Perhaps it’s the singing ray of Finding Nemo or maybe it’s the unfortunate thought of Steve Irwin’s passing. But all in all we know them as moderate to large flat bodied animals that seem largely comprised of wing like structures, with a long tail that should be signed “do not touch” due to the stinging barb hidden somewhere along its length. Rays are fascinating to look at simply for these characteristics, they seem alien like and certainly unique amongst most fish species. In fact it is not even hard to find somewhere to gawp at these animals as they are often showcased in aquariums around the world, almost to the point of being an expected feature of the day out. This said, beyond description our common knowledge of the stingray is often somewhat restricted.
Things to know about Rays
Cartilaginous skeleton – All ray species have a distinctive biological characteristic. Instead of a typical bone skeletal structure, theirs is made from cartilage, the same material that allows our ears to bend. Cartilage enables faster, more flexible movements when hunting prey as it’s far lighter than bone and more resistant to breaking.
Flattened sharks – A little fact that many people aren’t aware of is that rays are actually related to sharks. Sharks, skates, rays and chimeras are all cartilaginous fish who make up the class Chondrichthyes. Whilst chimeras go into their own subclass (Holocephali), sharks, skate and rays stay closely related with in the subclass Elasmobranchii. Not only do they share a cartilaginous skeleton with sharks but they also have the same highly developed electro-receptive sense. Sharks and rays have pores filled with a gel like substance called ampullae of Lorenzini around their rostrums and mouths that detect the electric pulses given off by muscular movements and body functions of prey. For a ray this helps them home in on prey items buried under the sediments of the sea floor.
Super duper old – Rays dates back to around 420 million years ago to the late silurian period. They, as well as other Elasmobranchs, were among the first jawed vertebrates on Earth providing more efficient prey handling and consumption. In fact, they date back so far that they were present on Earth before trees!
Diverse and distributed – Rays have diversified into over 600 species that are evolved to occupy all marine and aquatic bio-regions across the world. Whilst often considered a species of the tropics, rays are also present in temperate waters such as the UK where there are actually 18 species of ray and skates. You may have noticed how I’m referring to them as rays and not the typical stingray. This is because I’m a little science poo and a true stingray is actually referring to only one family of ray, the Dasyatidae… I’m a nerd, deal with it.
No aggro – The infamous barb of the stingray leads to many people fearing their presence. During my research in the Bahamas it was a common occurrence to meet locals who simply would not step into the sea through fear of stepping on a ray. It is such a shame because these animals are really not aggressive. It is actually pretty costly for a stingray to use its barb so will only do so if in fatal danger. The barb is actually brittle, shattering within the wound of the struck opponent. Although this does more damage to the unfortunate recipient, it does leave the stingray defenceless and in need of using precious energy resources to slowly grow another. So basically as long as you are not grappling, trapping or stomping on a ray, be calm and enjoy your encounter with such a beautiful animal.
Ecosystem maintenance – The presence and abundance of rays in an environment can be incredibly beneficial. Rays are known as mesopredators, this means they fill a predatory role that sits around the middle of the food chain/trophic system. Having this role makes them extremely important as they are an essential link between the lower primary consumers such as crustaceans and worms, and the apex predators e.g. sharks species. In some environments there is the potential that the removal of such a role could cause a trophic cascade effecting the populations of species both above and below in the food chain. Through their predation of benthic (sea floor dwelling) species they cause bioturbation. Bioturbation is the natural disturbance of sediments and benthic material caused by the behaviour of an organism and is performed by rays as they dig through the sediment to access prey items hidden below. Rays notoriously bury themselves in sediment to avoid detection by predators which also contributes to the process. This bioturbating behaviour can help maintain a healthy ecosystem biologically, chemically, and physically meaning their presence in vulnerable but productive habitats such as reefs and mangrove creeks is vital.
Help needed! – A huge focus of conservation is on sharks at the moment and rightly so, their populations are under crazy extrinsic pressure, but there is a little devil called data deficiency which really threatens many ray species. Data deficiency (DD) is a status produced by the International Union for Conservation of Nature (IUCN). The status is applied to species populations just like Near Threatened (NT), Endangered (EN) etc, however its meaning is almost more worrying. A DD species status means that there is not enough ecological understanding of said species to be able to determine the scale that anthropogenic threats such as habitat loss are having on their populations. In effect it is a big “I DONT KNOW”, meaning there is potential for a DD classified species to be going extinct right under our noses without any form of conversational effort. Nearly half (46%) of all Chondrichthyians are DD of which a large proportion is made up of ray species. This indicates that research needs to be done on those species to better our ability to conserve populations.
The Masters by Research program (MRes) I’m studying as a post-graduate degree is focused on the ecology of two data deficient ray species found in coastal waters around islands of the Bahamas. In no way is my research alone going to change the status of both these species but it may contribute to a better understanding which could develop into conservation frameworks. I’ll dive into the science behind the research in my next post but for now let me introduce you to these beauties.
Southern stingray (Hypanus americana)
Southern stingrays are commonly seen within coastal waters/creeks and around reefs further out to sea. Their colouration is a dark brown to black on top and clear white on their under side (ventral surface). The body surface is incredibly smooth apart from the spinal ridge down the back of an individual which is lined with small dorsal spines to the base of the tail. The tail itself is long and slender with 1-2 barbs present around 4 inches down from the base. These barbs can be fairly long, I’d estimate the largest I saw to be 15-20 cm or so in length. On the underside of the tail is a ventral skin fold which is a significant identifying feature of the species. A pointed rostrum gives an almost diamond like shape to the body leading to the two powerful undulating wings of the ray. The eyes of a southern stingray are beautiful. The pupil is this amazing peanut like shape surrounded by blue and yellow hues, constantly appearing to be staring you in the eye with the stern look of a disappointed teacher.
This species is often seen in aquarium tanks particularly in the Americas where their populations are distributed. They are also the subjects of a tourist spot at the Cayman Islands called Stingray City, where paying visitors have an opportunity to be surrounded by wild southern stingrays. I’m personally not a huge fan of the place although I have never been. Like I said they are still wild animals so by no means forced into the interactions, but some evidence has pointed towards health issues in individuals such as disease and competitive injury caused by the large amounts of rays concentrated in a small location attracted by hand fed squid (not part of their natural diet!) from the tourist companies. Due to this, I personally do not approve of the attraction but there is plenty of literature out there on the yays and nays of it all so feel free to investigate for your own opinions. From what I’ve seen in the Bahamas the southern stingray population appears to be fairly healthy however it’s very hard to tell these things with observations alone. Southerns are a species that are commonly caught up in coastal fisheries and there are even instances of fishermen mutilating tails of live individuals to prevent harm to themselves when removing them from their catch. Whilst they are not specifically targeted, a multitude of factors could be causing an unseen decline in population structure due to their data deficiency.
Caribbean whiptail ray/Chupare ray (Styracura schmardae)
Now these puppies are a mysterious bunch. Scientifically they are extremely unknown with extremely limited literature on their ecology. Most research that has been done on the species is about the parasites that have been found inhabiting its body tissues. In fact such a small amount is known of them that only just this year have they been officially recognised to be an inhabitant of the Bahamas despite by physically observed there since the early 1900s. The lack of understanding for this species life cycle and ecology overall really worries me because it is just being totally overlooked at the moment. Researchers at the Cape Eleuthera Institute (CEI), where my MRes fieldwork was based, worked hard to get its official recognition in the Bahamas and have been leading efforts to study the animals. I have to say that during my time working with both the southern stingray and chupare rays, a favouritism did develop towards the chupes. Everything about them makes them incredibly interesting. Its likes finding a rare item every time you come across one, I just always felt so lucky to have the chupe as a study species giving me opportunities to be alongside them in their environment, interacting with individuals and sampling their biology. Their calm mentality makes them likeable, it is almost as if they are incredibly high, just swimming along seemingly not minding interactions with dorky scientists as we caught, sampled and released them.
Both study species can grow to become extremely large animals, but from what I’ve been told by researchers at CEI some chupes have been observed to be significantly larger than the southerns. During my time there we recorded one individual whose total body length was just under 2 meters and body width a meter, certainly an intimidating sight. This said it is only intimidating if you actually see them because they are cryptobenthic, meaning that they naturally blend into the seafloor environment making them far more elusive than its southern stingray counter parts. Unlike the southern stingray they do not have the dorsal spines down their back. Instead their entire dorsal/upper surface is covered in dermal denticles, tiny scale like spines that give their surface a sand paper texture. These denticles team up with the protective layer of mucus to trap sediment from the sea floor helping camouflage the rays to great effect, when looking out for them you are basically looking for moving sand. The body shape is far more rounded than that of southern stingrays, with an extremely thick and muscular tail extending to great length behind it with no ventral skin fold. Again they have 1-2 barbs that are considerably thicker and longer than those of a southern stingray.
In my next post in this section, I’ll dive into exactly what it is that Molly and I are researching into and the science behind it all. We really believe that research such as ours is crucial for knowing more about these amazing animals and helping to develop conservation frameworks not only for them as species but for their ecosystems as a whole. So please keep your eyes peeled for the next post if you are interested in learning more about it.