Latin name

Torpedo marmorata

Other names

Torpedo marmorata

Identification

Their bodies are soft and flaccid. On either side of the head, electrically paired kidney-shaped organs peek through the skin. Behind the small eyes are large oval spatulae, the edges of which are covered with finger-like projections, almost converging in the middle. Behind the spiracles are 5-7 protruding mucous pores on the "back of the head". Between the nostrils is a rectangular flap of skin, much wider than long, almost reaching the small curved mouth. The small teeth end in a single point and form a kind of barbed "rasp" on both jaws. On the underside of the disc are five pairs of small gill slits.

Features of fish fins

The pectoral fins of these stingrays form a nearly circular disk that is about 59-67% of the total length. The two dorsal fins with rounded tips are close together. The base of each is about 2/3 of its height. The tip of the first dorsal fin is level with the tip of the pelvic fin bases. The first dorsal fin is larger than the second. The tail is short and thick, with leathery folds running along the sides. It ends in a triangular caudal fin with rounded corners. 

Fish colouring

These stingrays are easily distinguished by their characteristic coloration and the protruding edges of the spiracles. The dorsal surface of the body is dark brown and covered with numerous spots. Some individuals have a uniform coloration with no spots. The ventral surface is white with darker disc edges. 

Distribution

Occupies the eastern Atlantic from Scotland and the southern North Sea to the Cape of Good Hope, South Africa, including the Mediterranean.

Habitat

They live at depths of up to 370 meters. Marbled electric rays prefer water temperatures that do not exceed 20°C. They are usually found at depths of 10-30m off the coast of Britain, 20-100m in Italy and deeper than 200m in Tunisian waters. As bottom dwellers, these rays are found near rocky reefs and in algae thickets on sandy or muddy bottoms. 

Size

Maximum recorded length is 100 cm, with males and females rarely exceeding 36-38 cm and 55-61 cm, respectively. There is probably a geographical correlation in size. Maximum recorded weight is 3 kg.

Behavior

During the warm summer months, pregnant females migrate to Arcachon Bay, France, in shallow waters near oyster beds. In addition, stingrays of this species may migrate northward into the waters of the British Isles during the summer and fall.

Lonely and slow, these fish can remain motionless for several days. During the day, they lie on the bottom under a layer of sediment, with only their eyes and spouts visible. 

Survival ability

Their slow heart rate (10-15 beats per minute) and low blood oxygen saturation allow them to consume less oxygen than sharks and rays of comparable size. They are resistant to hypoxia and can survive in oxygen-deficient bottom waters and tide pools. When the partial pressure drops below 10-15mm of mercury, stingrays stop breathing altogether and can live in this state for up to 5 hours. They are able to cope with these extreme conditions thanks to anaerobic glycolysis and an additional alternative energy pathway in the mitochondria that slows the accumulation of potentially harmful lactate in the cells. 

Ability to generate electricity

Like other members of their squadron, they are capable of generating electricity. Each of their paired electrical organs consists of 400-600 vertical columns, which in turn represent a bale of about 400 jelly-filled "electroplates" that act like a battery. The electrical discharge produced by these stingrays has a voltage of 70-80 volts, with an estimated maximum potential of 200 volts. The stingray emits a series of discharges, gradually discharging the "battery" and lowering the voltage. Experiments in artificial conditions have shown that at temperatures below 15°C, the nerves of the electrical organs cease to function effectively. In nature, when the water temperature naturally drops in winter, these fish may stop using these organs. Alternatively, stingrays may have an unknown physiological mechanism that adapts these organs to the cold. 

Food and feeding habits

They hunt from ambush and stun their prey with an electric shock. Vision does not play an important role in the hunt; when stingrays are on the bottom, their eyes are often hidden under a layer of sediment. Instead of visual cues, they respond to signals from lateral line mechanoreceptors, so they will only attack a moving object. In addition, Lorenzini's ampullae help them detect prey.

The diet consists of 90% benthic bony fishes such as hake, goby, sea bass, mullet, mackerel, sea bream, mullet, damselfish,  clownfish, wrasse, sea eel and flounder. Secondary food sources include cephalopods such as European squid and Sepia elegans. Stingrays swallow their prey whole.

Defense

Because of their large size and ability to generate electricity, they are prey for other animals such as sharks. When defending themselves, stingrays act differently depending on which part (tail or disk) the predator is trying to grab. 

If the disk is touched, the fish quickly turns to face the threat and electrocutes itself, after which it flees in a straight line and may burrow back into the sediment. 

If the tail is touched, the stingray turns its belly outward and shrinks into a ball; having completed this maneuver, it does not swim away, but remains in this position, displaying its electrical organs as high as possible toward the threat. These movements are accompanied by strong electrical discharges. 

This species of ray uses electricity to defend its tail more than its disk.

Reproduction

They reproduce by oviparity, with the developing embryos feeding first on egg yolk and then on histotrophs produced by the mother's body. Adult females have two functional ovaries and two uteri; the inner surface of the uterus is covered with a series of longitudinal folds. The reproductive cycle in females is probably 2 years, while males can reproduce annually. Mating occurs in November-December, and pups are born the following year at 9-12 months. There are 3-32 newborns in a litter, and the number of litters is directly related to the size of the female.

Organs begin to form in embryos 1.9-2.3 cm long, at which time they already have eyes, pectoral and ventral fins, and external gills. As the embryos grow to 2.0-2.7 cm, their dorsal gill slits close, leaving only ventral gills, as in all rays. At the same time, 4 blocks of primary cells that form the electric organs unite. In embryos 2.8-3.7 cm long, the pectoral fins enlarge and fuse with the snout, forming a rounded disk typical of stingrays. At 3.5-5.5 cm, the external gills disappear and pigmentation appears. Embryos 6.6-7.3 cm long are capable of producing an electrical discharge. During gestation, the strength of the discharge increases by a value of 105, and in embryos 8.6-13 cm long, it reaches 47-55 volts, which is comparable to the discharge strength of adult stingrays.

Newborns are born at approximately 10-14 cm in length and are capable of displaying characteristic hunting and defensive behaviors from birth. Males and females reach sexual maturity at 21-29 cm in length at about 5 years of age and 31-39 cm and 12 years of age, respectively. The maximum life span of females is estimated to be 20 years.

Fishing

Of no interest to commercial fisheries. May be caught as bycatch in commercial bottom fisheries. 

Relationship with a person

Capable of delivering a painful but non-lethal electric shock to humans. The ability of Marbled electric ray to generate electricity is used in medicine.