Early research leading to discoveries, answers and many more questions about these majestic, unique and native sea creatures

Of all the fascinating creatures that live beneath Bermuda’s waters, one of the most spectacular is the spotted eagle ray. These Bermuda natives have wingspans that can reach up to six-and-a-half feet wide; they are known to dive down to depths of 70 metres; their markings are individually unique with black, dark brown or dark grey backs, covered by white spots and rings. And these creatures are incredibly powerful, both in the way they move and how they eat.

“They swim, seemingly effortlessly in strong currents, and often glide in a nearly motionless fashion as they head towards the bottom, where they can wreak havoc on clams, conch and their hard-shelled relatives,” explains Dr Matt Ajemian, who has been studying Bermuda’s spotted eagle rays, on and off, for seventeen years. “These animals have incredible, powerful jaws and fused teeth that allow them to bust through rock-like shells.”

Dr. Ajemian is an associate research professor for fisheries ecology and conservation at Florida Atlantic University’s Harbor Branch Oceanographic Institute and it is only by chance that he happened upon our eagle rays. His interest was first piqued around 2007 by an article about the impact overfishing of large sharks was having on mesopredators such as rays. A mesopredator sits in the middle of the food chain and is both predator and prey. In North Carolina, for example, there had been an explosion of cownose rays which led to a collapse in the bay scallop industry. Was this happening elsewhere?

Dr. Matt Ajemian

“Bermuda was interesting because large sharks had been overfished there for many years,” continues Dr Ajemian. So he contacted his Bermudian graduate school colleague, environmental consultant Thad Murdoch, who confirmed that not only were there a number of spotted eagle rays in Bermuda, but that very little was known about them. And so began his research. “We were expecting to see transitions and migrations of animals, but Bermuda’s also isolated. The animals don’t have anywhere to go. So the thought was, maybe these significant predatory effects are also happening in Bermuda?”

Since then, he has discovered that, while spotted eagle rays can be seen all around the island, they historically like Harrington Sound and Flatts inlet where the females in particular stay for long periods of time. Males are more transitory, frequently moving to the North Lagoon and the reefs. Their location habits, however, seem to be evolving and Dr Ajemian believes it’s because their available food sources may be changing. During his first years of research, there was an abundance of calico clams in Harrington Sound, which are a popular meal choice for the spotted eagle ray. However, more recently, these have plummeted in number. “This brings about new questions regarding what the rays are feeding on, and how they are spending their time,” he says. “We seem to be noticing less time spent in Harrington Sound than in previous years, potentially because the available prey has changed there. The West Indian topshell population on the other hand, seems to have taken off in Bermuda and there are more of these animals around than I’ve ever seen before.”

While there are many aspects of the spotted eagle ray Dr Ajemian would love to research, his first priority is to get a better understanding of their foraging ecology and behaviour. His current project, which brings him back to Bermuda this summer, involves listening to the rays eat shellfish to understand the acoustics of shell fracture.
“We are doing this so that we can develop detectors in certain environments in order to track where and when the rays are eating,” he explains. “We can do this because their method of eating is somewhat unique compared to other shellfish predators.”
The spotted eagle ray’s mouth is on the underside of its head, and they have hardened dental plates in the upper and lower jaw for grinding and crushing mollusc shells. In order to feed, they submerse their heads into the sand and excavate buried prey. They can also stick their heads out of the water to forage for shells along the rocks.

Monitoring these creatures is no mean feat. The ray first has to be captured, then fitted with a multi-sensor biologging tag, which is attached using suction cups on the smooth, flat area of the head where remoras are known to attach themselves. Remoras are small fish which attach themselves to larger fish using a sucker. The ray, therefore, may already be used to having something on its head. “The great thing about it is, we would collect the animals, put the tag on them, and 15 minutes later they had no problem feeding.”
The tags record the rays swimming and feeding behaviours with a hydrophone, which is an underwater microphone, and a forward-looking video camera. “These tags have provided us with some amazing perspectives on how these animals experience the Bermuda environment and have captured feeding in habitats that we have not explored. We are using the rays to tell us where their prey now lies.”

With one solution, however, came another discovery. Before coming to Bermuda, the tags had been tested on rays in captivity. When they fitted them to Bermuda’s rays, they discovered that their heads had “divots”: “We got to Bermuda with this tag and it didn’t really fit properly,” explains Dr Ajemian. “What we noticed when we did this on the first animal, there’s a little bit of a divot in the head. We had to move this thing much further back than when testing it on animals in Florida. We’ve seen it on a few other ones too. It’s like a channel between both sides of the cranium.”

Does this make Bermuda’s spotted eagle rays unique?

“We’re not sure if it means there’s something different morphologically about those animals in Bermuda, or whether we just encountered a few that were undernourished at the time. We have a study ongoing that will compare their genetics to other populations around the Atlantic. I suspect that analysis will demonstrate the Bermuda population is isolated and unique from the others; however, I do not have that evidence just yet.”

What they do suspect, though, is that Bermuda’s rays have been resident here for centuries and over that time, the population and its genetics may have diverged. “We’ve done some tagging work, none of which suggested that the animals ever left,” he explains.
“We also take photographs of their dorsum and that pattern is individually unique. We use that to identify, after several years, the same individuals. All of this is leading us to converge on the idea that it’s likely a resident population. Also, it’s a long swim to get anywhere else!”

Each discovery leads to more questions, and more research that Dr Ajemian wants to do: “Their formation in groups of individuals is also interesting to watch as there are seemingly behaviours that are communicated between them that we simply don’t yet understand. It’s a species that has so much to it biologically.

“They’re really important not just to the culture of Bermuda, but the ecology of these systems. They connect the sound to the reef through their movements. They transport nutrients back and forth. There’s so much to learn and so much that needs to be done.”