Frequently Asked Questions

A skate onboard a ship.

A skate onboard a ship.

Have you ever wondered how big or small skates and rays really get? Whether skates vomit and excrete? Whether skates and rays get cancer? Here are the answers to some weird and wonderful questions about skates and rays. Learn more about these fascinating creatures by clicking on the questions below.

How big (or small) do skates and rays get?

Answer:

Skates and rays range widely in size, from just a few centimeters in width to over 9 meters wide! The smallest rays in the world are the shortnosed electric rays (Family Narkidae), which are approximately the size of a pancake, measuring 10 cm across weighing only 0.5 kg. The biggest ray is the manta ray (Manta birostris), which can reach up to 900 cm in width and may weigh many tons (thousands of pounds)! The smallest skate in the world is the starry skate (Raja stellata) which only reaches a maximum total length of 76 cm. The common skate (Dipturus batis) is the largest skate in the world, reaching up to 250 cm in length.

How long have rays and skates existed?

Answer:

Skates and rays are among the oldest surviving group of jawed vertebrates, first appearing in the fossil record during the Lower Jurassic (about 150 million years ago). They are exceptionally resilient, having survived a number of major global extinctions. All of the major taxa are known by the Upper Cretaceous (approximately 100 million years ago) to the Paleocene (approximately 50 million years ago). However, the exact fossil record of skates and rays is difficult to determine given that all cartilaginous fishes leave little behind in the fossil record, with the exception of occasional teeth and scales.

Are electric rays really electric?

Answer:

Yes they are! All electric rays have a pair of specialized organs capable of producing an electric discharge, varying from 8 to 220 volts depending on the species, which is used to stun or kill prey. Each organ is composed of stacks of 500 to over 1,000 striated muscle plaques all enervated on the same side, so that when the muscles of the organ are contracted, a summed electrical shock is produced. For example, due in part to its large size, the electric organs of the Atlantic torpedo (Torpedo nobiliana) are capable of producing 220 volts of electricity! And interestingly, skates are also "electric", producing weak electric pulses with organs in their tails. The sensory system of skates - in turn - appears to be "tuned" to the appropriate stimulus. For example, the mean electrical pulse frequency produced by the electric organ of adult clearnose skates (Raja eglanteria) is 2.5 Hz, which is closely aligned with their peak electrical frequency sensitivity (2 - 3 Hz). This match may represent facilitate communication during social interaction and mating.

Do skates and rays have "enemies"?

Answer:

Although we generally tend to think of cartilaginous fishes mainly as predators (largely due to books and movies such as Jaws), many species of elasmobranch are important components in the diets of other species. For example, skate egg are eaten by animals ranging from carnivorous gastropods to sperm whales, and are likely a very important food source in temperate marine ecosystems around the world. Young and old skates and rays are also eaten by many predators, such as sharks, other skates and rays, and seals. However, since elasmobranchs lack bone and otoliths, hard remains of these fish are rarely found in stomach contents or feces of their predators, making accurate predictions of predation rates difficult to obtain. Occasionally though, dermal scutes of skates and spines of rays have been recorded in stomach contents and seal feces. Small rays have also been found whole in the stomachs of a number of dolphins and small toothed whales. In other areas of the world, predators have been observed actively preying upon ray species. For example, in tropical environments hammerhead sharks have been observed attacking and eating stingrays, and in New Zealand, killer whales have been observed preying upon both eagle rays and stingrays.

And ultimately, the greatest "enemy" of skates and rays is humans. Many species of skates and rays have declined significantly in abundance in the last few decades due in part to over-exploitation by direct and indirect fisheries. For more information on human uses and abuses of skates and rays, go to the Skate Conservation section of this website.

How do skates and rays avoid predators?

Answer:

Skates and rays have many ways to avoiding predators, with some examples described below:

  1. The most common strategies employed by skates and rays are probably deterrence, defense, and flight. Skates have thorny discs for deterring predators, and many ray species have venomous spines capable of inflicting serious wounds. They are also capable of moving quickly in short bursts.
  2. Another very common predator avoidance strategy used by skates and rays is hiding (or crypsis). Skates and rays frequently bury themselves in the substrate to avoid detection, and many species have dorsal surfaces that are the colour of the substrate, providing camouflage.
  3. Skates and rays can also change their habitat use to avoid predators. For example, cowtail rays (Pastinachus sephen) in Shark Bay, Western Australia have been observed resting in extremely shallow waters with sand bottoms that are free of predatory hammerhead and tiger sharks.
  4. Skates and rays can vary their activity levels and patterns to coincide with times when predators are fewer/more numerous. For example, Hawaiian stingrays (Dasyatis lata) exhibit lower activity rates during the day when predators are more likely to see them.
  5. Lastly, group formation might be another way that skates and rays avoid predators; however, it has not been observed commonly in the wild. One possible example of a species that practices group formation is the cowtail ray (Pastinachus sephen) in Shark Bay, Western Australia. This species forms groups when predator risk is high (i.e. when light levels are low and the water is more turbid), presumably to increase vigilance.
Do skates vomit, excrete, and urinate?

Answer:

Yes they do! Vomiting is common among elasmobranchs, but it is rarely observed in the wild. Under laboratory conditions, a vomiting reflex has been induced in rays, skates and dogfish. In other shark species, vomiting is achieved through a complete eversion of the stomach. Fishermen that catch sharks with line and/or net have reported stomach eversions for species ranging from the blue shark (Prionace glauca) to the tiger shark (Galeocerdo cuvier), and anecdotal evidence suggests that stomach eversion and subsequent re-swallowing occurs naturally in almost all carcharhinid shark species. This type of regurgitation and eversion, however, has not been observed in any batoid species to date.

Excretion and urination are essential elements in the digestive process of skates and rays. In other words, a portion of the food that is consumed by skates and rays is not absorbed by the digestive tract, and is egested as feces or excreted as nitrogenous waste in urine. Few studies have been carried out on the digestive efficiency elasmobranchs; however studies on some species of elasmobranchs (such as the lemon shark Negaprion brevirostris) have reported that food remains in the digestive tract of elasmobranchs for long periods of time (i.e. up to 18 days) relative to teleosts. A study on digestion in the little skate (Leucoraja erinacea) in the laboratory found that evacuation rates varied between prey types, depending on the composition of the prey surface integument, the fat content of the tissue, and the internal skeletal structure of the prey species. Krill and clam foot/tissue were digested faster than polychaetes and sand lance.

Do skates and rays live only in the ocean? Or are there freshwater skates and rays?

Answer:

Elasmobranchs are primarily marine fishes. However a few species live in brackish bays and estuaries, and about 3% of the species (approx. 30 species) are permanently restricted to freshwater. For example, South American stingrays of the Family Potamotrygonidae are the quintessential freshwater elasmobranchs, residing entirely in freshwater. Many species of elasmobanch that reside in freshwater are in decline due to pollution and destruction of their restricted habitats. For example, in 2004 the Australian government added the Maugean skate (Raja sp. L) - the only non-marine skate known of the 400 or so skates throughout the world - to its threatened species list. This species was recently discovered in Tasmania in 1988, and is confined to the upper reaches of two biologically unique estuarine systems characterized by low nutrient and low salinity waters.

How have humans used skates and rays in the past?

Answer:

Skates and rays have been used in many different ways by maritime societies around the world, due in part to their historical prevalence in the marine environment. Along the northwest coast of North America, for example, a variety of skates and sharks were caught for food, but also played an important role in myths and legends. Preserved vertebral centra, teeth, spines, and dermal denticles have been found in many Pacific coast archaeological sites. Unfortunately, estimating the actual significance of skates and rays in prehistoric economies from archeological sites is often difficult because the remains of cartilaginous fish preserve relatively poorly compared to those of bony fishes.

The ancient Greeks and Romans also used batoids. In particular, physicians prescribed electric discharge from electric rays as a treatment for many illnesses. Scribonius Largus, a Roman physician, was the first to record the use of torpedo fish "shock therapy" for treatment of headache and gout. Electric rays were also used by the ancient Greeks during operations and childbirth as a kind of anesthetic, with the electricity supposedly acting to numb the pain. In fact, the word 'narcotic' stems from narke which is the Greek word for these types of rays.

Do skates and rays get cancer?

Answer:

In recent years, crude shark cartilage extract had been promoted as an alternative cure for cancer. This belief is based on the misconception that sharks do not develop cancer. In actual fact, sharks, skates and rays do develop cancerous tumors. A study conducted in 2004 examined the occurrence of tumors among members of the class Chondrichthyes. Although actual cancer rates were not determined, forty-two cases of malignant or benign tumors were found in the literature, including cases in 16 skates or rays, including the thorny skate (Amblyraja radiata) in the North Atlantic. Other batoid species that were found to have cancerous cells were the gray skate (Dipturus batis) - which had cutaneous fibrosarcoma - and the thornback skate (Raja radiata), a number of which were found with melanomas. Rays with tumors were from the Family Dasyatidae (whiptail stingrays). Unfortunately, the alleged lack of cancer in sharks has contributed to at least two significant negative outcomes: 1) a dramatic decline in shark populations worldwide, and 2) the diversion of patients away from effective cancer treatments.

What is the "stingray shuffle"?

Answer:

The "stingray shuffle" is a way in which people can avoid being stung by stingrays when wading in shallow water close to the beach. To do the "stingray shuffle", slowly slide (or shuffle) your feet in the sand. This gives any stingrays in the vicinity warning of your approach so that they are able to swim out of your way. Also, polarized glasses are the proper attire for the "stingray shuffle", as they will assist you in spotting and avoiding stepping on stingrays in shallow waters.