The undulating elasmobranch…
The sea floor shudders, and suddenly a two-metre disc of sand takes flight. The camouflage tumbles off the departing like a flag in a gale. This is not my first encounter with a stingray, but I am no less bewitched than I was when playing in the shallows as a child. I remember one feeding out of my hand, and visits so regular we named the individuals—there was Ray Charles, Sugar Ray and Two O’Clock Tony, named for his particular timeliness. They all ghosted through the shallows of the sandy beach where we spent our summer holidays, sometimes with the curling edge of their pectoral fins cutting the surface. We were taught to splash when entering the water, and if we didn’t bother them “they wouldn’t bother us”. And so it proved to be true, but their tantalising aura of mystery never left me, and it has infected others—a mystery still being unravelled by marine biologists to this day.
When Steve Irwin was barbed through the heart on Australia’s Great Barrier Reef, the media seized on the story of a “stingray attack”—Irwin, who had survived crocodile attacks and snake bites, was finally topped off by a ray, and the genera surged up the popular rankings of man-killers. Suddenly, the relatively harmless ray found its kind lumped in, both by taxonomy and reputation, with its trouble-making cousins of the same elasmobranch group—sharks.
Needless to say, it was a grisly end for Irwin. At the base of a stingray’s tail are one or two barbs, serrated like a steak knife. Proteinous and highly virulent venom is stored in a solid state in grooves and, when thrust into flesh, it dissolves in the wound, instantly being absorbed into the bloodstream. The pain is excruciating, often leads to infection, and in extremely rare cases, the barb will sever arteries or penetrate organs, culminating in death.
Yet the only stingray fatality in New Zealand was a Thames swimmer in 1938, and one occurrence in local history hardly makes for a killer-species. Indeed, ACC statistics warn that you are 60 times more likely to be killed tripping over a floor rug; so unless we are about to elevate mats to the status of apex predators, it seems we are being misled by sensationalism. But we all knew that. Irwin’s death was what it was; an astonishingly rare accident that reflected all our worst fears and killed a much-celebrated Australian.
Given their size and relative abundance on our coast, you could be forgiven for thinking we know a fair bit about their habits, habitats and persuasions. We know that they are viviparous; meaning the embryo develops in the female’s uterus and the live young (pups) are born fully formed after a long gestation period, which may last up to 12 months. We know that stingrays feed on benthic invertebrates, fish and zoo-plankton in a similar fashion to eagle rays and other batoids by repeatedly inhaling sediments and water through the mouth and venting this out through the gill slits. We even know that during the mating season, the teeth of mature male stingrays change shape to a sharp cuspidate form—this is known as sexually dimorphic dentition and is thought to aid males in holding females during mating activity.
This is true of the three species that are most common in New Zealand waters; the long-tailed stingray Dasyatis thetidis, the short-tailed stingray D. brevicaudata and the eagle ray, Myliobatis tenuicaudatus. And we know that during some summers short-tailed and long-tailed stingrays congregate in large numbers around the underwater arches and pinnacles of the Poor Knights Islands off Northland’s east coast. Scores of stingrays, with wingspans up to two metres, “fly” like a squadron of bombers throughout the water column—it’s a dramatic sight, and one of the few places in the world where this behaviour can be witnessed.
But come late April, most of the stingrays leave the Poor Knights, and we’re left with nothing but clear blue sea. The trail goes cold and the scientific record dwindles into hypotheses and best guesses. We do not know where they go. We do not know why. It’s not even clear whether the seasonal migration is in response to biological factors such as reproduction, feeding or predator avoidance, or the result of seasonal physical changes in sea temperature, photoperiod or currents. Or a combination of both factors.
Marine biologists have suggested they head for the deeper sea, because that’s where some other fish go, and it’s also where we are tempted to put things when we don’t know where they are. Perhaps they go to deeper water for a more stable environment. Or perhaps not. And despite the relative abundance of the genus, there have been very few observations of mating activity. And, come to think of it, there are few recorded observations of stingray pups. We can’t even suggest with any conviction where they are born—in estuaries, like many shark species seeking better protection for their young, or in deep water. However, almost miraculously, they turn up as juveniles at the Poor Knights the next spring, and much to the bewilderment of marine biologists, the riddle begins all over again. Sometimes our knowledge of life on earth seems embarrassingly thin.
Enter Agnès Le Port, a PhD student at the University of Auckland’s Leigh Marine Laboratory. For the last two and a half years she has been diving among the stingrays at the Poor Knights Islands; counting them, latching them with biopsy prods and tagging them with all manner of high-tech paraphernalia. Every month she dived down to 25 m on scuba and swam transects (defined areas for survey purposes) for a half hour at a time, suspended in water as cold as 12ºC, counting stingrays as they filed past or were resting on the bottom. In winter there were few, sometimes none, but by late February she sometimes counted more than 30 on a dive. “I was amazed at the sheer numbers of large stingrays cruising along the walls,” Le Port says, recounting her first impressions. “I immediately wanted to know more about where they all came from, why they came to the Poor Knights Islands and where they went in winter when divers stopped seeing them.”
Her detailed underwater counts confirmed what was already known; that short-tailed and long-tailed stingrays follow a repeatable seasonal pattern of abundance, their numbers at the Poor Knights Islands swelling five-fold from spring to autumn and declining in winter, which correlates with changes in sea temperature. But then they disappear to Poseidon only knows where. What has emerged is that the aggregations are mainly of large females, with recent wounds across their back and pectoral fins, supporting a theory that the aggregations are in fact part of a larger event that included mating.
If that’s why stingrays gather at the Poor Knights, why do they leave again? And where do they go? For answers to questions like that, Le Port turned to GPS. The global positioning system was first developed for military purposes, but as is often the case, defence budgets have served the cause of science, and now we find a system traditionally employed to guide a warhead down a ventilation shaft used to plot the sex-life of a stingray with the same level of accuracy. Thermobaric bombs to elasmobranches in a single step—it’s heartening somehow. The technology has revolutionised the study of marine fauna, particularly the knowledge of migratory species.
For marine species, pop-up satellite archival tags (PSAT) are often used. These tags record temperature, depth and light intensity from which sunrise and sunset, and therefore also location, can be deduced. At a pre-programmed time, the PSAT releases from the animal and floats to the surface where it transmits data to orbiting Argos satellites, which relay the data back to the researcher’s computer.
Data returned from the short-tailed stingray tags suggests that small females, around 1 m in body width, spend most of their time at greater depths (100-200 m) in winter and do not stray very far; only 15-20 km from their summer locations. “We are now getting a clearer picture of where short-tailed stingrays at the Poor Knights Islands may be going during the winter when they disappear,” says Le Port.
But are there other aggregation points in New Zealand? And do stingrays ‘commute’ between them? Or, as the PSAT tags suggest, are stingrays in fact quite parochial, never straying more than 30 kilometres from home?
If you were beginning to believe that this feature poses more questions than it answers, you’d be right, because that accurately reflects the depth of the stingray knowledgebase. The irony is that due to our measures to conserve the commercially valuable and endangered, we sometimes know more about the rare species in New Zealand than we know about the most abundant. We have no definitive data for the total New Zealand population of long-tailed or short-tailed stingrays, no yardstick by which to gauge the environmental and anthropogenic pressure on the species, and no basis for management, protection or further study. All of which makes work like that of Agnès Le Port crucial.
To try and determine the relationship between the Poor Knights population and others around the country, Le Port turned from satellites high above to another line of enquiry leading deep within; probing the building blocks of life at a sub-cellular level. Analysis of mitochondrial DNA allows scientists to map genetic distances between discreet populations, even of the same species. A large genetic distance implies either a long period of diversion, or a rapidly evolving species, or both. Just such analysis of genetic distance between indigenous people finally verified what was assumed of the migration of humans around the earth. And Le Port hoped genetic distance would also shed light on mating behaviour and migratory patterns of New Zealand’s stingrays.
Whereas human DNA can be sampled with a delicate cheek swab, a suitable stingray biopsy requires a more liberal use of force. Beneath twenty-odd metres of water Le Port hovered with what looked like a two-metre-long cattle prod. And as the leviathans cruised past she gave them a taste of their own medicine, extracting a 14mm-deep disc of tissue on the sharpened tip of the lance. Each biopsy takes just an instant, but collecting 270 short-tailed and long-tailed stingray samples from the Poor Knights Islands and several locations around the coast of North and South Islands took years.
These biopsies have demonstrated little genetic variation so far, which may not be so surprising. We know that stingrays, like sharks, are ancient in design and evolve slowly, so the results don’t necessarily imply that the stingrays in New Zealand move around much. In fact, the DNA reveals little more variation between the New Zealand and Australian short-tailed stingray populations, groups that have been isolated for several million years. Are stingrays migratory or highly regional? Those results, says Le Port, are still inconclusive. “I haven’t answered all the questions that sparked my interest originally, but I’ve learned a lot about stingrays and gained some insights into their behaviour.”
After all that was revealed with satellite-tracking and submarine biopsies-on-the-fly, her research has come full-circle; back to observations, hypotheses and peer review. Le Port uses phrases like ‘most likely’, ‘suggests’, ‘perhaps’ and ‘possibly means’ to describe stingray behaviour. She’s far from completing the puzzle, but some of the pieces have been found.
As a family, stingrays have survived on earth since the Upper Cretaceous more than 65 million years ago, so perhaps as enthusiastic newcomers to earth we can be excused for our impatience to know and understand every detail. Theirs is a world of silence where nothing needs to explain where it’s going or what it intends to do when it gets there. The shadowy form of our contrary ray may be the best metaphor we have for its perplexing habits; behaviour that has so far eluded American aerospace technology and the latest genetic science. “If they could speak it would save us a lot of time,” complains Le Port, but they don’t, and she loves them for it.