Stingray
I recently visited Sea World at Awa-Kamogawa in Japan. I noticed that all the fish had broadly similar shapes except for the rays, which have huge delta-shaped wings. What is the particular advantage of this shape, and do these fish follow different behaviour patterns to the fish with the more common shapes?
鈥� Rays are one of the few groups of fish that spend the majority of their time on the ocean floor. Others include species such as plaice and skate.
The flat configuration of rays allows them to swim just over the top of the substrate in search of food. This profile means their height above the seabed is minimal and they are less likely to be spotted by predators. You may also have noticed that the dorsal (upper) side of the rays is dark or mottled and that the ventral (lower) side is lighter in colour; this acts as camouflage.
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Most species of ray flap their wings in the way that birds do. However, unlike birds, which also have to work against gravity, their primary goal is forward motion. A good example of a flying ray is the manta ray. These are frighteningly large, but if you meet one in the water it will just glide by, filtering seawater in search of krill and other marine invertebrates.
David Townsend
Ipswich, Suffolk, UK
鈥� Flatfish and rays live on the seabed, where they find food. One might deduce that rays that are not seabed dwellers have evolved from those that were, taking off again to follow their food into the higher layers of the ocean.
Chris Avey
Norwich, Norfolk, UK
We were not sure whether free-swimming rays evolved from ones that lived on the bottom so we consulted Aidan Martin, director of the ReefQuest Centre for Shark Research () and a research associate of the zoology department of the University of British Columbia 鈥� Ed
鈥� A dorsoventrally (top-to-bottom) flattened, or 鈥渄epressed鈥�, body form makes helps make fish inconspicuous when lying flat on the seabed, or partially buried in it. But there are other kinds of flatfish, too. In fish of the order Pleuronectiformes, the body is flattened laterally (from side-to-side), in what is known as a 鈥渃ompressed鈥� body form. This develops during larval stage, when one eye migrates across the head. The side that ends up with both eyes becomes pigmented, while the other often loses it. The transformed fish lies on its unpigmented side with eyes and pigmented side uppermost. That two unrelated groups of creatures independently evolved a similar body form and lifestyle that has persisted for millions of years suggests that it must have significant advantages.
Skates and rays can extend their jaws to a surprising degree, allowing them to grasp prey from the seafloor with forceps-like dexterity. Skates have pointed, grasping teeth and feed on shrimps, amphipod crustaceans, polychaete worms and teleost, or ray-finned, fish. Stingrays have flattened, cobblestone-like teeth that are suited to crushing shells of crustaceans and molluscs. Eagle rays possess remarkable molar-like plates that can crush the hardest-shelled molluscs, such as oysters and snails. They often excavate their prey from its shell by a blowing a jet of water from the mouth.
Manta rays have tiny teeth that do not function in feeding because these exquisitely graceful fish are plankton feeders that use filtering plates that bridge the spaces between successive gill bars to strain minute crustaceans from the water. Electric rays of the order Torpediniformes have simple grasping teeth, but are able to stun prey or discourage would-be predators, using stacks of modified gill muscles that are all enervated on the same side so that they can generate as much as 220 volts of electricity.
The secondarily pelagic (or higher ocean-dwelling) rays, such as eagle rays, cownose rays and manta rays, have evolved more recently than bottom-dwelling skates and stingrays. The earliest of the pelagic rays dates back to the lower Eocene, about 55 million years ago. In the actively swimming eagle, cownose and manta rays, the pectoral wings are longer, with concave rear margins, and they move as a result of a wing-like flapping. In the bottom-dwelling forms, locomotion primarily comes from undulations of the edges of the pectoral disc. An interesting addition to this basic pattern is found in the so-called 鈥渓eg skates鈥�, which use limb-like extensions of the pelvic fins to punt along the bottom.
All skates are oviparous, depositing yolky eggs in rectangular leathery egg cases with a long process at each corner to anchor it. After a gestation fuelled entirely by its pre-packaged yolk, the baby skate emerges from the egg case. Empty skate egg cases, known as mermaid鈥檚 purses, are often found washed up on beaches after a storm.
Electric rays and stingrays are viviparous, with young staying inside their mother throughout their development. Their yolk sacs are depleted about halfway through, and must be augmented with a protein-rich bath called histotroph, or uterine milk. In some eagle rays, which are also viviparous, extensions of the uterine lining enter the spiracles (respiratory openings) of each developing ray to secrete histotroph directly into the cavity behind the mouth.
Skates and rays are both classified in the group known as elasmobranchs, which also includes the sharks, which date back to the lower Jurassic period, about 200 million years ago. Skates and rays, collectively termed batoids, evolved more recently.
No one knows why elasmobranchs evolved. One possibility is that adopting their lifestyle and flattened body form enabled these weird and wonderful creatures to feed on the seafloor, where they could hide from predators. Perhaps most intriguingly of all, some rays 鈥� notably eagle and manta rays 鈥� have proportionately the largest brains of any elasmobranch, and can exhibit surprisingly complex social and courtship behaviour.
Aidan Martin
University of British Columbia, Canada
This week鈥檚 questions
Inside out
What would happen to the world as we know it if the Earth were entirely hollow beneath the crust, assuming that it did not collapse inwards?
Terrence Douglas
Halifax, Nova Scotia, Canada
Spice attack
Why does turmeric stain everything an indelible yellow, including surfaces that appear impermeable to other substances? Other powdered spices such as cinnamon, paprika and chilli do not leave the same legacy. And what is the best way to remove turmeric stains?
Hefin Loxton
Huddersfield, West Yorkshire, UK