Stingray blue
In the lead-up to our science communication event on stingrays later in September, we discovered that some exciting new research on stingrays has been led from Hong Kong.
Dr Mason Dean and his team at the City University of Hong Kong published articles on structural colour on stingrays in Advanced Optical Materials and Frontiers in Cell & Developmental Biology earlier this year. Croucher News caught up with Dean to find out more about his work.
Dean’s interest in the colour blue in the animal kingdom was piqued by a talk he attended on the subject. On the back of that, he started to investigate the blue spots on the blue-spotted Ribbontail stingray. It turned out that there was essentially no research on how colour gets created in rays or sharks (which are a separate lineage from rays but closely related). What Dean and his team found was that a previously undescribed way of producing structural colour was at work.
“There's a neat collaboration of a pigment-like cell in their skin and then a structural colour cell, an iridophore, you could call it, which is a cell with a bunch of little crystals inside of it. And the crystals are arranged in such a way that then the combination of the iridophore and the pigment cell makes it so that blue gets reflected, but all the extraneous longer wavelengths get absorbed. So you end up with this very robust electric blue colour coming off the skin.”
It's not known what role this vivid blue colouration plays for stingrays. At this point, it’s only possible to speculate. “Like other bright colours, it could act as a near-field warning signal,” Dean said. “But at present we don’t know.” You can find out more fascinating details and a short video about this special way of creating blue here.
A lot of Dean’s other research into stingrays has been about their ultrastructure—the fine-scale construction only visible with high-resolution tools but critical to how tissues work. “One of the distinctive features of sharks and rays is that they have skeletons made of cartilage, not bone. It seems like a really terrible building tool for a skeleton, especially if you're doing active stuff. And many sharks and rays are really active,” said Dean.
“Compared to our cartilage, that of sharks and rays has a really odd design, almost a chainmail armour on the outside of the cartilage. The armour is cobbled together from thousands of little mineralised blocks like the tiles you'd see on a bathroom floor, but covering all the unmineralised cartilage underneath it. It's what engineers would call a biocomposite.”
“Bone and cartilage are used really differently in human skeletons, in terms of where you find them, like the cartilage in our noses or joints. But in the stingray, the structural reinforcement of the cartilage by this interesting outer layer allows the tissue to allow the tissue to behave differently, depending on the forces it experiences,” Dean explained.
He went on: “Imagine a bunch of building blocks in series; they're connected by soft material. If you squish them together, you end up having the blocks knock against each other, and you get stiff behaviour. But if you put tension on them, they behave softly. So then you put a whole array of that over an entire 3D object, and then you can end up getting prescribed flexibility and stiffness in different orientations, which makes for a super cool kind of smart biomaterial in terms of dictating when to be flexible and when to be soft.”
The tiling of a surface is known as tessellation. “As soon as you start doing this research, you start seeing tessellations everywhere, including in man-made building materials. It turns out nature just makes a ton of them in lots of different forms.” The diverse biological tilings used for everything from armour to optics could be of huge interest to material scientists, says Dean.
Dean first became interested in stingrays as a child during visits to an aquarium. “The stingrays were always the ones that I would drag my parents to let me go see for a while.” When he became a scientist, Dean came across more fascinating examples of rays, learning not all species have stingers. “There’s a species called Narcine bancroftii. It's a little electric ray, one of the species that produces its own electricity. There'd been an anecdotal observation that it protrudes its jaw to an extreme degree. And as a Master’s student, I was finally able to get some high-speed video of it. And it extends its jaw about 100% of its head length in about 100 milliseconds to catch its prey.”
There are many avenues of research that Dean would love to explore with his team. “I'm interested in biomedicine regarding cartilage behaviour. One thing that we've found is that sharks and rays have blood vessels in their cartilage, even as adult animals, which is extremely bizarre from a mammal standpoint. Our blood vessels eventually retreat from our cartilage during development, and that's partly why our cartilage doesn't repair itself very well. And you have these animals with skeletons entirely made of cartilage. And they, for some reason, have persistent blood vessels in their cartilage that likely are helping to keep the cartilage vital. Some of these animals have lifespans that are much longer than ours. So the natural variation we see in animal tissues, evolved with diverse and exceptional qualities, then offers a perfect playground for discovery, for novel engineered materials or biomedicine.”
To carry out this work he feels so passionate about, Dean feels fortunate to be working a great team in his lab to keep the momentum of his research rolling forward. “Discovery is organic and fun when you have nice people to work with who are curious, motivated, and collaborative,” he said.