There are so many wonderful species of sharks and rays in our oceans, so I want to share with you some of our favourites. Once a month we will shine the spotlight on an elasmobranch we love and throw some fun fishy facts at ya.
The smooth stingray
This month I am introducing to you my absolute favourite elasmobranch of them all - the smooth stingray. I'll admit I am a bit biased though, as these beauties have been my study species for the last 3 years. I am a PhD candidate in The Behaviour, Ecology and Evolution of Fishes Lab at Macquarie University, and I study the behaviour and ecology of smooth stingrays in Jervis Bay, NSW. For an insight into what I do check out this blog I wrote about last years' fieldwork.
What's in a name?
Smooth stingrays are also known at short-tail stingrays. The former refers to their smooth skin, which is due to their lack of dermal denticles. Dermal denticles are what give sharks and many other species of ray their sandpaper-like skin. Dermal denticles are kind of like fish scales but are actually made up of dentine (the stuff teeth are made of). Instead smooth rays have a thick layer of epidermal mucus which protects them from parasites. The latter refers to the fact that their tails are relatively short in comparison to the length of their disc.
The scientific name for smooth stingrays is Bathytoshia brevicaudata. Bathytoshia is derived from the Ancient Greek bathy meaning "deep" or "ocean", and brevicaudata is derived from the Latin brevis, meaning "short" and cauda meaning "tail".
Scientific classification of smooth stingrays
Species: B. brevicaudata
Smooth stingrays are a species of stingray (Suborder Myliobatoidei). This means they have (yep - you guessed it) stingers! More specifically, they are a whiptail stingray, which puts them in the family of Dasyatidae. All species of ray in the Dasyatidae family, with the exception of the porcupine ray, have venemous barbs on their tail. When a stingray is threatened or stepped on, they can flick up their tails and lodge this barb into the predator in defence. By nature, stingrays are not aggressive. When someone is 'stung' (because stabbed sounds far worse) it is usually because the ray is startled, be it by someone stepping on it or a shadow creeping across it. The best way to avoid startling a stingray while you're our enjoying the ocean, is to do something called 'the stingray shuffle'. When walking into the water, simply shuffle your feet through the sand as opposed to stepping. If the vibrations don't alert the ray to your presence, then you still won't step on the ray accidentally. A soft nudge to the ray will be enough to send them swimming off.
Steve and the stingray
On the 4th of September 2006, Australia and the World heard the news that Steve Irwin had been killed by the barb of a stingray to the chest while filming a documentary in Queensland. Due to inclement weather, Steve and his cameraman, Justin Lyons, decided to head to Batt Reef near Port Douglas to see what else they could film. While snorkelling in waist deep water, they came across a “massive, eight-foot-wide (2.4m) stingray” said Justin. Wanting to get footage of it swimming away, Steve swam up behind the stingray at which point his shadow likely startled the animal and it started stabbing "hundreds" of times before swimming away, says Justin. Not realising that the ray had hit Steve, it wasn't until Justin panned back that he saw Steve "standing in a huge pool of blood". Many may remember the media saying that Steve or the crew had tried to remove the barb after the incident which inevitably killed him, but this wasn't the case. During one of the strikes, the barb had hit Steve's heart; "he had a two-inch injury over his heart with blood coming out of it.”
It's widely assumed that Steve was killed by a smooth stingray. While Batt Reef is a bit far north for them, few marine rays reach sizes anywhere near the described 2.4m.
Big, bold and benthic
Smooth stingrays are the largest species of marine stingray in the world, growing up to 2.5 metres in width (4m in total length) and up to 350kg! To put that into perspective, that's wider than a car and about 1/4 as heavy! There has even been unconfirmed reports of sooth rays reaching 3m in width.
They're a common species and found in the southern waters of Australia, New Zealand and South Africa, found cruising shallow coastal bays, estuaries, inlets, coastal rocky reefs, offshore islands, the open sea floor and near the surface over the outer continental shelf. Off Australia, they have been found to 150 m depth, to 200-250 m off New Zealand, and 180 to 480 m off South Africa.
Being coastal, these rays often have encounters with humans and human activity. Here in Australia, they are really common at boat ramps where fishermen discard their fish scraps, capitalising on a free feed. In South Africa, they are really common where people go cage-diving with great white sharks because they are attracted by the baits used to attract the sharks. In fact, operators usually see more smooth stingrays than sharks!
Even though they're common, we actually know very little about them and much of what we do know is speculation.
Behaviour and ecology
It has been suggested that smooth stingrays have small home ranges (<25km). They forage on a myriad of benthic foods such as shellfish, crustaceans, and fishes an they can detect the magnetic fields and vibrations of their prey using their Ampullae of Lorenzini which are electroreceptors covering the underneath of their snout and around their mouths and their lateral line system - this is very handy when your eyes are on the back of you head!
Due to their size, smooth stingrays have few natural predators, but they do fall prey to copper sharks (Carcharhinus brachyurus), smooth hammerheads (Sphyrna zygaena), great white sharks (Carcharodon carcharias), and killer whales (Orcinus orca).
During summer, smooth stingrays aggregate in large number around Poor Knights Islands in New Zealand and it is considered that this area may be a mating and nursery ground for this species. It is the only such site described for this species despite their broad distribution. It is unknown at what size or age smooth stingrays reach sexual maturity, however when mating, a single receptive female may be followed by several males, each trying to bite onto the wing of the female. The successful male, when he has a good hold, will flip upside down to mate with the female, belly to belly! Female smooth stingrays in captivity have actually been seen mating with three different males rays in a row. Here is a neat video of mating marble rays off Cocos Island by Undersea Hunter.
Smooth stingrays are ovoviviparous (or aplancental viviparous). This means, the young develop inside eggs within the mothers uterus until they are ready to hatch. They then hatch inside the mothers body and are born live and free-swimming. Smooth give birth to 6-10 pups ranging in size from 32-36 cm disc width. Then they are born, the rays are curled up like burritos after which they unfurl and begin their lives. It is unknown how long gestation is, but from my observations in the field we think it is once annually, and mating/birthing occurs throughout the year. In other words, there does not seem to be a breeding season. Pupping and nursery areas are unknown but they may use specific nurseries. This is something we hope to uncover with our research.
It may surprise you to hear that as a group, rays are more threatened than sharks. According to Dulvy et al. 2014, "Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays".
Accorinng to the IUCN Red List of Threatened Species, smooth stingrays are threatened indirectly by fishing, being caught as by-catch in trawl, Danish seine, longline and purse seine fisheries. Most often they are discarded but they are sometimes sold in Australia as 'flake'. They are also commonly taken by recreational fishers and sometimes speared for sport. In South Africa, smooth stingrays are occasionally caught in shark nets.
Often, smooth stingrays are encountered with missing or partially amputated tails. When caught recreationally or commercially, on purpose or by accident, fishers will often cut ofd tails before releasing them to reduce the risk of injury or entanglement in nets and fishing gear. A silver lining here though is that because so many are seen with missing tails, their survivability of such an act is high and they survive fine without tails with most being fully healed over.
Thankfully though, despite the threats they fact, smooth stingrays are listed as Least Concern on the IUCN Red List of Threatened Species, meaning they are "widespread and abundant" and not currently threatened with extinction.