LIKE AN EAGLE

For eagle rays, the ocean is their sky. Indeed, these elegant sea creatures have been seen leaping several metres into the air. Naturalist Georgina Jones reveals more.

MANY MARINE ANIMALS ARE named for land animals with varying degrees of accuracy: catsharks, dogfish, seahorses, cow sharks, butterfly fish, bull sharks and tiger sharks, to mention just a few. Since when does a cow shark resemble a cow? And it’s true that eagle rays don’t very much resemble eagles, except in their movement. They look just like they are flying through the water, and sometimes even leap several metres up into the air.

Eagle rays have flattened bodies with triangular fins that they flap up and down to move through the water, technically known as oscillatory pectoral fin locomotion. That is, as opposed to other rays that use a rippling, waving motion of their fins, again, technically known as undulatory pectoral fin locomotion. Their large duckbill-shaped snouts are rounded and they have a small dorsal fin at the base of the body as well as a long slender tail with several defensive spines that may be more than twice their body length.

Like other members of the stingray family, eagle rays use their spines to protect them from predators from above. The spines are sharp and barbed, and use a complex chemical system to deliver their venom involving venomous tissues that mix with the mucous membrane on the surface of the spine when triggered, producing a cocktail of toxins.

This is an effective deterrent to their predators, usually sharks. As well as the injury from the cut of the spine itself, their venom produces swelling and muscle cramps and can be very painful to human beings, who sometimes step on them. Since the barbs typically break off inside the wound, they may require surgical removal. However, the wounds are only life-threatening if the spine stabs a vital area.

It’s worth noting that these rays are usually very skittish around people and, unless stepped on when buried in sandy shallows, will typically retreat if approached.

They have cartilaginous skeletons like their relations the sharks. In common with sharks, they also hunt their prey using electromagnetic organs, the ampullae of Lorenzini. These can be seen as small sensory pits dotted around the snout and function to detect the life signals of crustaceans, worms or molluscs hiding in or on the seafloor. Though they will also eat small fish if they can catch them, eagle rays don’t have sharp teeth but flattened hexagonal plate teeth that they use to crush exoskeletons and grind up shells.

While they have been found at depths of 800m, they usually occur in water shallower than 50m and over sandy bottoms. This is their preferred environment, as they can burrow in the sand to detect their prey as well as hide from their predators by burying themselves under it. Since they have flattened bodies with their eyes on top, they rely on their sense organs to detect their prey in and on the sand. While hunting or hiding, they use spiracles, openings just behind their eyes, to take in water to breathe. This is one of their two methods of breathing. While swimming, they use the more typical system of taking in water through their mouths. This is a more efficient method, because a larger volume of water can be taken in to flow over their gills for extracting oxygen, but while lying in wait for prey or hiding in the sand, the rays can’t use their mouths for breathing, and luckily don’t need as much oxygen as when swimming.

A school of eagle rays.

Eagle rays are ovoviviparous. After an extended mating period, the embryos develop from eggs in the mother’s uterus, first absorbing the nutrients in the yolk sacs and, once these are depleted, then being fed by a kind of milk supplied in the uterus by the mother. The pups are born alive and independent after about eight months of gestation; usually in litters of three to seven.

There are at least three species known from South African waters, one rather perplexingly known as a bull ray, ranging in size from 1.5m to 2.5m and weighing between 59 and 98kg. The common eagle ray is brown with irregular black spots, bull rays are also brown but with blue to grey crossbars, and the spotted eagle ray, the biggest of the three species, is usually dark with many white spots. They all have white bellies that help with countershading: looking up from below, a white belly tends to blend into the light sky and gives some protection from predators.

They can live for up to 20 years and, due to their preference for sandy bottoms, can sometimes be seen in large aggregations in shallow lagoons and brackish estuaries as well as flying, rather like eagles, through our coastal oceans.