The zebra shark (Stegostoma tigrinum) is a species of carpet shark and the sole member of the family Stegostomatidae. It is found throughout the tropical Indo-Pacific, frequenting coral reefs and sandy flats to a depth of 62 m (203 ft). Adult zebra sharks are distinctive in appearance, with five longitudinal ridges on a cylindrical body, a low caudal fin comprising nearly half the total length, and usually a pattern of dark spots on a pale background. Young zebra sharks under 50–90 cm (20–35 in) long have a completely different pattern, consisting of light vertical stripes on a brown background, and lack the ridges. This species attains a length of 2.5 m (8.2 ft).
Zebra sharks are nocturnal and spend most of the day resting motionless on the sea floor. At night, they actively hunt for molluscs, crustaceans, small bony fishes, and possibly sea snakes inside holes and crevices in the reef. Though solitary for most of the year, they form large seasonal aggregations. The zebra shark is oviparous: females produce several dozen large egg capsules, which they anchor to underwater structures via adhesive tendrils. Innocuous to humans and hardy in captivity, zebra sharks are popular subjects of ecotourism dives and public aquaria. The World Conservation Union has assessed this species as Endangered worldwide, as it is taken by commercial fisheries across most of its range (except off Australia) for meat, fins, and liver oil. There is evidence that its numbers are dwindling.
Female zebra shark at the Children’s Aquarium at Fair Park
Taxonomy[edit]
Early taxonomists thought that juvenile zebra sharks were a different species because of their different appearance from adults.
The zebra shark was first described as Squalus varius by Seba in 1758 (Seba died years earlier; the publication was posthumous). No type specimen was designated, though Seba included a comprehensive description in Latin and an accurate illustration of a juvenile. Müller and Henle placed this species in the genus Stegostoma in 1837, using the specific epithet fasciatus (or the neuter form fasciatum, as Stegostoma is neuter while Squalus is masculine) from an 1801 work by Bloch and Schneider. In 1984, Compagno rejected the name “varius/m” in favor of “fasciatus/m” for the zebra shark, because Seba did not consistently use binomial nomenclature in his species descriptions (though Squalus varius is one that can be construed as a binomial name). In Compagno’s view, the first proper usage of “varius/m” was by Garman in 1913, making it a junior synonym.[2][3] Both S. fasciatum and S. varium are currently in usage for this species;[2] until the early 1990s most authorities used the latter name, but since then most have followed Compagno and used the former name.[4] A taxonomic review in 2019 instead argued that S. tigrinum is its valid name. This name was omitted in Compagno’s review in 1984, possibly due to confusion over its year of description (in a publication in 1941, Fowler mistakenly listed it as being described in 1795). Squalus tigrinus was described by Forster in 1781, two years before Squalus fasciatus was described by Hermann. Consequently, the former and older is the valid name (as Stegostoma tigrinum), while the latter and younger is its junior synonym. As the name proposed by Forster in 1781 has been used in tens of publications since 1899, it is not a nomen oblitum.[4]
The genus name is derived from the Greek stego meaning “covered”, and stoma meaning “mouth”.[5] The specific epithet fasciatum means “banded”, referring to the striped pattern of the juvenile.[6] The juvenile coloration is also the origin of the common name “zebra shark”. The name “leopard shark” is sometimes applied to the spotted adult, but that name usually refers to the houndshark Triakis semifasciata, and is also sometimes used for the tiger shark (Galeocerdo cuvier).[2] Due to their different color patterns and body proportions, both juveniles and subadults have historically been described as separate species (Squalus tigrinus and S. longicaudatus respectively).[3]
Phylogeny[edit]
There is robust morphological support for the placement of the zebra shark, the whale shark (Rhincodon typus), and the nurse sharks (Ginglymostoma cirratum, Nebrius ferrugineus, and Pseudoginglymostoma brevicaudatum) in a single clade. However, the interrelationships between these taxa are disputed by various authors.[7] Dingerkus (1986) suggested that the whale shark is the closest relative of the zebra shark, and proposed a single family encompassing all five species in the clade.[8] Compagno (1988) suggested affinity between this species and either Pseudoginglymostoma or a clade containing Rhincodon, Ginglymostoma, and Nebrius.[3] Goto (2001) placed the zebra shark as the sister group to a clade containing Rhincodon and Ginglymostoma.[7]
Description[edit]
Close-up of zebra shark
The zebra shark has a cylindrical body with a large, slightly flattened head and a short, blunt snout. The eyes are small and placed on the sides of the head; the spiracles are located behind them and are as large or larger. The last 3 of the 5 short gill slits are situated over the pectoral fin bases, and the fourth and fifth slits are much closer together than the others. Each nostril has a short barbel and a groove running from it to the mouth.[9] The mouth is nearly straight, with three lobes on the lower lip and furrows at the corners. There are 28–33 tooth rows in the upper jaw and 22–32 tooth rows in the lower jaw; each tooth has a large central cusp flanked by two smaller ones.[3]
There are five distinctive ridges running along the body in adults, one along the dorsal midline and two on the sides. The dorsal midline ridge merges into the first dorsal fin, placed about halfway along the body and twice the size of the second dorsal fin. The pectoral fins are large and broad; the pelvic and anal fins are much smaller but larger than the second dorsal fin. The caudal fin is almost as long as the rest of the body, with a barely developed lower lobe and a strong ventral notch near the tip of the upper lobe. The zebra shark attains a length of 2.5 m (8.2 ft), with an unsubstantiated record of 3.5 m (11 ft).[3] Males and females are not dimorphic in size.[10]
The color pattern in young sharks is dark brown above and light yellow below, with vertical yellow stripes and spots. As the shark grows to 50–90 cm (20–35 in) long, the dark areas begin to break up, changing the general pattern from light-on-dark stripes to dark-on-light spots.[3] There is substantial variation in pattern amongst adults, which can be used to identify particular individuals.[10] A rare morph, informally called the sandy zebra shark, is overall sandy–brown in color with inconspicuous dark brown freckles on its upperside, lacking the distinct dark-spotted and banded pattern typical of the species. The appearance of juveniles of this morph is unknown, but subadults that are transitioning into adult sandy zebra sharks have a brown-netted pattern. Faint remnants of this pattern can often be seen in adult sandy zebra sharks. This morph, which is genetically inseparable from the normal morph, is only known from the vicinity of Malindi in Kenya, although seemingly similar individuals have been reported from Japan and northwestern Australia.[4]
In 1964, a partially albino zebra shark was discovered in the Indian Ocean. It was overall white and completely lacked spots, but its eyes were blackish-brown as typical of the species and unlike full albinos. The shark, a 1.9 m (6.2 ft) long mature female, was unusual in that albino animals rarely survive long in the wild due to their lack of crypsis.[2]
Distribution and habitat[edit]
Zebra sharks are often seen resting on sand near coral.
The zebra shark occurs in the tropical waters of the Indo-Pacific region, from South Africa to the Red Sea and the Persian Gulf (including Madagascar and the Maldives), to India and Southeast Asia (including Indonesia, the Philippines, and Palau), northward to Taiwan and Japan, eastward to New Caledonia and Tonga, and southward to northern Australia.[3][5]
Bottom-dwelling in nature, the zebra shark is found from the intertidal zone to a depth of 62 m (203 ft) over the continental and insular shelves. Adults and large juveniles frequent coral reefs, rubble, and sandy areas.[3] There are unsubstantiated reports of this species from fresh water in the Philippines.[5] Zebra sharks sometimes cross oceanic waters to reach isolated seamounts.[10] Movements of up to 140 km (87 mi) have been recorded for individual sharks.[10] However, genetic data indicates that there is little exchange between populations of zebra sharks, even if their ranges are contiguous.[11]