| Marbled electric ray | |
|---|---|
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Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Torpediniformes |
| Family: | Torpedinidae |
| Genus: | Torpedo |
| Species: | T. marmorata |
| Binomial name | |
| Torpedo marmorata A. Risso, 1810 | |
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| Range of the marbled electric ray[1] | |
| Synonyms | |
Torpedo diversicolorDavy, 1834 | |
Themarbled electric ray (Torpedo marmorata) is aspecies ofelectric ray in thefamilyTorpedinidae found in the coastal waters of the easternAtlantic Ocean from theNorth Sea toSouth Africa. Thisbenthic fish inhabits rockyreefs,seagrass beds, and sandy and muddy flats in shallow to moderately deep waters. It can survive in environments with very littledissolved oxygen, such astidal pools. The marbled electric ray has a nearly circularpectoral fin disc and a muscular tail that bears twodorsal fins of nearly equal size and a largecaudal fin. It can be identified by the long, finger-like projections on the rims of itsspiracles, as well as by its dark brown mottled color pattern, though some individuals are plain-colored. Males and females typically reach 36–38 cm (14–15 in) and 55–61 cm (22–24 in) long respectively.
Nocturnal and solitary, the marbled electric ray can often be found lying thesea floor buried except for its eyes and spiracles. This slow-movingpredator feeds almost exclusively on smallbony fishes, which itambushes from the bottom and subdues with strongelectric bursts. It defends itself by turning towards the threat, swimming in a loop, or curling up with its underside facing outward, while emitting electric shocks to drive off the prospective predator. Its pairedelectric organs are capable of producing 70–80volts of electricity. This species isaplacental viviparous, with the developingembryos sustained byyolk andhistotroph ("uterine milk") produced by the mother.Mating takes place from November to January, and females bear litters of 3–32 pups every other year after agestation period of 9–12 months. The newborn ray is immediately capable of using electricity to hunt.
Theelectric shock delivered by a marbled electric ray can be severe but is not directly life-threatening. Its electrogenic properties have been known sinceclassical antiquity, when live rays were used to treat conditions such aschronic headaches. This and other electric ray species are used asmodel organisms in biomedical research. Various coastaldemersalfisheries take the marbled electric ray asbycatch; captured rays are usually discarded as they have little commercial value. The impact of fishing on its population is uncertain, and thusInternational Union for Conservation of Nature (IUCN) has listed this ray as Vulnerable. In theMediterranean Sea, it remains the most common electric ray and in some areas may be increasing in number.
FrenchnaturalistAntoine Risso described the marbled electric ray asTorpedo marmorata in his 1810Ichtyologie de Nice, ou histoire naturelle des poissons du département des Alpes maritimes (Ichthyology ofNice, or natural history of fishes in theAlpes-Maritimes). Thespecific epithetmarmorata means "marbled" inLatin, and refers to the ray's color pattern.[2] Because notype specimens are known, in 1999 Ronald Fricke designated Risso's original illustration as the specieslectotype.[3]
Within the genusTorpedo, the marbled electric ray belongs to thesubgenusTorpedo, which differs from the other subgenusTetronarce in having fringed margins on their spiracles and generally ornate dorsal coloration.[4] Othercommon names for this species include common crampfish, marbled torpedo, numbfish, and spotted torpedo.[5]
Widely distributed in the easternAtlantic Ocean, the marbled electric ray is found fromScotland and the southernNorth Sea southward to theCape of Good Hope, and north at least as far as Durban, likely further.South Africa. In addition it is present in theMediterranean Sea, with higher densities in western areas.[6] It prefers temperatures cooler than 20 °C (68 °F).[1][7] This species is typically found at depths of 10–30 m (33–98 ft) offBritain andIreland,[8] 20–100 m (66–328 ft) offItaly, and down to 200 m (660 ft) offTunisia.[9] It has been recorded from as deep as 370 m (1,210 ft).[5] The marbled electric ray tends to be found deeper than thecommon torpedo (T. torpedo), which shares the southern portion of its range.[9]
Bottom-dwelling in nature, the marbled electric ray inhabits rockyreefs andseagrass beds, as well as nearby areas with sandy or muddy bottoms.[7] During warm summer months, pregnant females are known tomigrate intoArcachon Bay in northwesternFrance, where they are commonly found in very shallow, muddy pools nearoyster beds.[10][11] This species may conduct a northward migration in summer and autumn, into the waters of theBritish Isles.[8]
The body of the marbled electric ray is soft and flabby, and entirely lacksdermal denticles. The thickpectoral fin disc is nearly circular and comprises about 59–67% of the total length; the pairedkidney-shapedelectric organs are visible beneath the skin, outside of the small eyes. Immediately posterior to each eye is a large, oval spiracle, which bears 6–8 long, finger-like projections on the rim that almost meet at the center. On the "nape" behind the spiracles, there are 5–7 prominentmucous pores. Between the nostrils, there is a quadrangular curtain of skin much broader than long, that almost reaches the small, arched mouth. The teeth are small with a single pointed cusp, and are arranged with aquincunx pattern into a pavement-like band in either jaw. The five pairs ofgill slits are small and located beneath the disc.[5][12][13]
The twodorsal fins have rounded apexes and are placed close together; the base of each fin measures about two-thirds its height. The rear of the first dorsal fin base is located behind the rear of thepelvic fin bases. The second dorsal fin is only slightly smaller than the first.[5][12] The short, robust tail has skin folds running along either side, and terminates in a largecaudal fin shaped like a triangle with blunt corners.[7][13] The upper surface has a dark mottled pattern on a light to dark brown background; some individuals are uniformly brown.[12] The underside is plain off-white with darker fin margins.[14] This species can grow up to 1 m (3.3 ft) long,[5] though few exceed 36–38 cm (14–15 in) long for males and 55–61 cm (22–24 in) long for females. The much larger sizes attained by females can be attributed to the resource investment needed for reproduction. There seems to be little geographic variation in maximum size.[15][16] The maximum weight on record is 3 kg (6.6 lb).[5]
Solitary and slow-moving,[14] the marbled electric ray may remain motionless for several days at a time.[10] It ismore active at night and spends much of the day buried insediment with only the eyes and spiracles showing.[1] Consistent with its sluggish nature, the marbled electric ray has a low blood oxygen carrying capacity andheart rate (10–15 beats/min), and consumes lessoxygen than other sharks and rays of similar size.[10] It is highly tolerant of being deprived of oxygen (hypoxia), allowing it to cope with deoxygenated bottom waters or being stranded in small pools by the fallingtide. The ray stops breathing entirely when the oxygenpartial pressure in the water drops below 10–15Torr, and can survive such a state for at least five hours. It deals with extreme hypoxia by couplinganaerobicglycolysis to additional energy-producing pathways in itsmitochondria, which serves to slow down the accumulation of potentially harmfullactate within itscells.[17]
Like other members of its family, the marbled electric ray can produce a strongelectric shock for attack and defense, produced by a pair of electric organs derived frommuscletissue. Each electric organ consists of 400–600 vertical columns, with each column composed of a stack of roughly 400 jelly-filled "electroplates" that essentially act like abattery.[11] This ray has been measured producing up to 70–80volts, and the maximum potential of the electric discharge has been estimated to be as high as 200 volts. The strength of the electric shock declines progressively as the ray becomes fatigued.[13] Experimentsin vitro have found that thenerves innervating the electric organ essentially stop functioning at temperatures below 15 °C (59 °F). As the water temperature in the wild regularly drops below this threshold in winter, it is possible that the ray does not use its electric organ for part of the year. Alternately, the ray may have a yet-unknownphysiological mechanism to adapt electric organ function to the cold.[18]
Known parasites of the marbled electric ray include thetapewormsAnthocephalum gracile[19] andCalyptrobothrium riggii,[20] theleechesPontobdella muricata andTrachelobdella lubrica,[21] themonogeneansAmphibdella torpedinis,[22]Amphibdelloides kechemiraen,[23]A. maccallumi,[22]A. vallei,[23]Empruthotrema raiae,E. torpedinis,[24] andSqualonchocotyle torpedinis,[25] and thenematodesAscaris torpedinis andMawsonascaris pastinacae.[7]
The marbled electric ray is anambush predator that employs electricity to capture prey. Vision is of little importance in hunting, as the ray's eyes are often obscured as it lies buried on the bottom. Instead, it likely relies on themechanoreceptors of itslateral line, as it only attacks moving prey. Theelectroreceptiveampullae of Lorenzini may also contribute to prey detection.[26]
Small,benthicbony fishes constitute over 90% of the marbled electric ray's diet by weight;[27] these includegobies,hake,sea bass,mullets,jack mackerel,sea breams,goatfish,damselfish,wrasses,conger eels, andflatfish.[5][9][12]Cephalopods such asEuropean squid (Loligo vulgaris) andelegant cuttlefish (Sepia elegans) are a minor secondary food source. There is a single record of an individual that had swallowed apenaeid prawn,Penaeus kerathurus,[27] and a study of captive rays found that they reject liveMacropodiacrabs.[28] Off southernFrance, by far the most important prey species is theleaping mullet (Liza saliens).[27] Food items are swallowed whole; there is a record of a ray 41 cm (16 in) long that had consumed athree-bearded rockling (Gaidropsarus vulgaris) 34 cm (13 in) long.[12]
Two distinct types of prey capture behavior have been observed in the marbled electric ray. The first is "jumping", used by the ray to attackprey fish that swim close to its head, typically no farther than 4 cm (1.6 in). In the "jump", the ray pulls back its head and then thrusts its disc upwards, reaching about two or three times as high as the prey fish is from the bottom. Simultaneously, it makes a single tail stroke and produces a high-frequency (230–430Hz, increasing with temperature) burst of electricity. The initial electric burst is very short, containing only 10–64 pulses, but is still strong enough to causetetanic contraction in the body of the prey fish, often breaking itsvertebral column. As the ray glides forward, the motion of the jump sweeps the now-paralyzed prey beneath it, whereupon it is enveloped by the disc and maneuvered to the mouth. Electric bursts continues to be produced during this process; the total number of electric pulses over a single jump increases with size, ranging from 66 in a newborn 12 cm (4.7 in) long to 340 in an adult 45 cm (18 in) long. The jump lasts no more than two seconds.[26][28]
The second type of prey capture behavior is "creeping", used by the ray for stationary or slow-moving prey; this includes stunned prey that may have drifted out of reach from a jumping attack. In creeping, the ray makes small up and down motions of its disc coupled with small beats of its tail. The raising of the disc draws water beneath it and pulls the prey towards the ray, while the lowering of the disc and the tail beats move the ray towards the prey in small increments. When it reaches the prey, the ray opens its mouth to suck it in. Short electric bursts are produced as necessary, depending on the movement of the prey, and continue through ingestion.[28]
Because of its size and electrical defenses, the marbled electric ray does not often fall prey to other animals such assharks.[7] This species exhibits different defensive behaviors depending on whether a prospective predator grasps it by the disc or the tail. A ray touched on the disc will quickly turn toward the threat while producing electric shocks; this is followed by it fleeing in a straight line, after which it may re-bury itself. A ray touched on the tail will propel itself upward into a loop; if it has not escaped after the maneuver, the ray will curl into a ring with the belly facing outward, so as to present the area of its body with the highestelectric field gradient (the underside of the electric organs) towards the threat; these behaviors are accompanied by short, strong electric shocks. The ray tends to produce more electric bursts when protecting its tail than when protecting its disc.[28]
The marbled electric ray exhibitsaplacental viviparity, in which the developingembryos are nourished initially byyolk, which is later supplemented bynutrient-richhistotroph ("uterine milk") produced by the mother. Adult females have two functionalovaries anduteruses; the inner lining of the uterus bears a series of parallel lengthwise folds.[29] The reproductive cycle for females is probably biennial, while males are capable of mating every year. Mating occurs from November to January, and the young are born the following year after agestation period of 9–12 months.[15][16] The litter size ranges from 3 to 32, increasing with the size of the female.[12][16]
The electric organs first appear when the embryo is 1.9–2.3 cm (0.75–0.91 in) long, at which time it has distinct eyes, pectoral and pelvic fins, andexternal gills. At an embryonic length of 2.0–2.7 mm (0.079–0.106 in), thegill clefts close dorsally, leaving the gill slits beneath the disc as in all rays. At the same time, the four blocks of primordial cells that make up each electric organ rapidly coalesce together. The embryo's pectoral fins enlarge and fuse with the snout at a length of 2.8–3.7 cm (1.1–1.5 in), giving it the typical circular electric ray shape. When the embryo is 3.5–5.5 cm (1.4–2.2 in) long, the external gills are resorbed andpigmentation develops. The embryo can produce electric discharges by a length of 6.6–7.3 cm (2.6–2.9 in). The strength of the discharge increases by a magnitude of 105 over the course of gestation, reaching 47–55 volts by an embryonic length of 8.6–13 cm (3.4–5.1 in), close to that of an adult.[11]
Newborns measure approximately 10–14 cm (3.9–5.5 in) long,[1] and are immediately capable of performing characteristic predatory and defensive behaviors.[11] Malesmature sexually at approximately 21–29 cm (8.3–11.4 in) long and five years of age, while females mature significantly larger and older at 31–39 cm (12–15 in) long and twelve years of age. Themaximum lifespan is 12–13 years for males and around 20 years for females.[1]
The shock delivered by the marbled electric ray can be painful but is seldom life-threatening, although there is a danger of a shocked diver becoming disoriented underwater.[7] Its electrogenic properties have been known sinceclassical antiquity, leading it and otherelectric fishes to be used inmedicine. Theancient Greeks andRomans applied live rays to those afflicted with conditions such aschronic headaches andgout, and recommended that its meat be eaten byepileptics.[14][30]
The marbled electric ray iscaught incidentally inbottom trawls,trammel nets, andbottom longlines; it has little economic value and is mostly discarded at sea when captured. TheInternational Union for Conservation of Nature (IUCN) assesses it as Vulnerable. At least in the northern Mediterranean, surveys have found that it remains the most common electric ray, and is perhaps becoming more abundant in Italian waters.[1] This and other electric ray species are used asmodel organisms in biomedical research because their electric organs are rich inacetylcholine receptors, which play an important role in the humannervous system.[31]
