Marbled electric rays, scientifically known as Torpedo Marmorata, accompany us on many of our local dives, but this week we have come across a considerable number of them! They are thought to be the most electro-sensitive animals, even more so than sharks, and an unforgettable species to dive with.
Although commonly perceived as a tropical species, the marbled rays are found as far North as the North Sea, the Eastern Atlantic, throughout the Mediterranean and down to Southern Africa.
Each fish has two electric organs; one on either side of the head, stacked with columns made up of jelly filled electroplates that act like a battery. This facilitates a current up to 30amps to travel from the lower to upper surface of the ray, producing shocks of 70-80volts (a similar effect to submerging a mainspowered hairdryer into a bathtub).
When a ray comes in contact with prey or predators its initial shock will be the strongest, with the intensity declining as it becomes fatigued. Interestingly, below temperatures of 15degrees Celcius, the nerves innervating the electric organs stop functioning and therefore rays may go entire winters (particularly in the North) without the ability to produce a current.
Typically, like many stingray species, the marbled electric rays are sedentary during the day, concealing themselves in the sand and most active at night. They are well-adapted to waters with minimal oxygen, such as deoxygenated bottom water and small pools during falling tide. They have a very low oxygen carrying capacity and heart rate, and stop breathing all together when the partial pressure of oxygen drops to 10-15 torr. Amazingly, though, are able to comfortably survive for five hours in this state without breathing.
In addition to their primary defence mechanism, electric rays use their shocks for hunting. Two capture behaviours have been specifically identified: “jumping” and “creeping”. With prey swimming close to the rays head, it will “jump” and produce an initial shock causing tectonic contraction of the prey, breaking the vertebral column. Whereas, with stationary or slow-moving prey, the marbled electric rays will use their disc and tail movements to draw prey closer in increments. When close enough, it will suck the prey in, at times producing electric shocks throughout ingestion. Bethnic bony fishes, such as hake, sea bass, jack mackerel and goatfish compose 90% of electric rays diet (by weight) and are consumed whole.
Interestingly, the gestation period for Marbled Electric Rays is longer than humans, between 9-12 months and producing a litter range of 3-32. There are striking differences between males and females: males sexually maturing when 5, while females maturing only at 12-13 years. Life expectancy also varies between sexes; males typically 12 years while females up to 20. As a result of the lengthy gestation period, females are only able to mate biennially while the males can every year.
For all intents and purposes the flesh of the Marbled Electric Rays is inedible, with only a gelatinous texture to offer. Consequently, they are redundant to fishing vessels and often thrown overboard dead. Historically however, their economic value has been significantly greater. The Greeks and Romans used the electric shocks from the rays for chronic headaches and gout, while eating the flesh was recommended for epileptics. In the 1800s the oil was used as fish oil and a source of fuel for lamps and lights. The oil was also believed to have medicinal properties – claims that have since been entirely disregarded by the scientific community.
Nowadays the Electric Rays are most valuable in medical research, as model organisms, because the electric organs are rich in a particular acetylcholine receptor which plays an important role in the human nervous system.