Coral Reefs: Diversity on Display
At first glance, the distinctive and beautifully-marked Zebra Shark (Stegostoma fasciatum) seems poorly named. The term ‘zebra’ hardly seems descriptive of this amber to cream-colored shark with leopard-like spots.
Just the Facts:
Habitat: Intertidal, Sandy Plains, Rocky Reefs, Kelp Forests, Coral Reefs
Depth: intertidal - 215 ft (65 m)
Distribution: Southern African, Madagascaran, Arabian, Indian, South East Asian, Western Australian, Northern Australian, Japanese
The Zebra Shark’s equine moniker refers only to juveniles of the species, which — up to a length of about 20 inches (50 centimetres) — are dark brown or purple with white or pale yellow vertical bars and spots. By the time this species reaches a length of about 35 inches (90 centimetres), the dark saddles break up into small brown spots on a yellowish background. As this species grows, its pattern of dark spots becomes less linear and more uniformly distributed over the body, eventually forming an attractive pattern of leopard-like spots. The name ‘Zebra Shark’ is retained, however, to avoid confusion with other species known colloquially as the “leopard shark”, including the Tiger Shark (Galeocerdo cuvier)
Why does the Zebra Shark change its stripes for leopard spots? Such radical changes in pigmentation pattern between juvenile and adult are nearly universal among carpet sharks of the order Orectolobiformes and coral reef fishes in general. In reef-dwelling teleosts, it is believed that juveniles of many species wear a distinct ‘uniform’ that advertises their low social status, indicating that they are not a threat to — and thereby avoid being attacked by — territorial adults. But there is no conclusive evidence of territoriality in any shark species.
The functional significance of the bold stripes of juvenile Zebra Sharks is hindered by the fact that, although they are occasionally encountered and a few have been collected as museum specimens, no one seems to know the habitat where the juveniles typically live. One source suggests they may live in water deeper than 165 feet (50 metres), while another attests that — at least off India — they are very common in shallow, coastal waters while adults of this species are typically found farther offshore. At present, the evidence seems to favor the latter contention. If this is true, then juvenile Zebra Sharks may form loose aggregations as a protective measure, relying on their strongly-marked bodies writhing and wriggling near one another to visually confuse a would-be predator. Perhaps observations of juvenile Zebra Sharks in the wild or more detailed capture records will eventually shed some light on this mystery.
Adolescent and adult Zebra Sharks are most often encountered during daylight hours, lying quietly on sandy bottoms between coral heads. This species often lies in passes or channels through the reef crest, places where tidal flow squeezes and accelerates oxygen-bearing seawater. In these passes, Zebra Sharks are typically observed facing open-mouthed into the current, propped up on their pectoral fins. By elevating their mouth and gills above the substrate, where water flow is not impeded by friction with the bottom, these sharks may facilitate breathing while at rest. Zebra Sharks are probably nocturnal, actively hunting for prey at night.
In India, this species is known as the “monkey-mouth shark”. This wonderfully evocative name refers to the ‘face’ formed on the underside of its head, which features a blunt snout with prominent nares (possibly suggestive of eyes?) and nasal barbels (cheek ‘whiskers’?) immediately in front of the short, transverse mouth. The small mouth and thick throat and gill muscles of this shark enable it to forcefully suck up prey animals, including snails, bivalves, crabs, shrimps, and small teleosts. These are grasped between multiple rows of trident-shaped teeth which are stout enough to crush even the sturdiest of tough-shelled prey.
The long, flexible body and tail of the Zebra Shark enable it to enter reef crevices and caves to capture concealed prey. Nearly half the total length of this shark consists of its resplendent, pennant-like caudal fin. As in many other orectoloboid sharks, the elongate caudal fin of the Zebra Shark is almost parallel with the body axis and has virtually no lower lobe. Such a tail has a low thrust angle and is relatively inefficient for high-speed or long-distance swimming. But, coordinated with the other fins, it is wonderful for fine control of bodily yaw, pitch, and roll. Zebra Sharks typically swim with an exaggerated, eel-like wriggle. In areas with a gentle but persistent through-current, it is not uncommon to see one of these sharks ‘surfing’, delicately adjusting its fins to remain more-or-less stationary in the water column with its tail waving sinuously, like a silk ribbon caught in a draft.
Each summer and autumn in the tropical and warm temperate waters of Australia, Zebra Sharks migrate into shallow coastal areas. There, they lie on sandy bottoms or sand patches between coral reefs, forming loose aggregations. These aggregations may be associated with Zebra Shark breeding. Zebra Shark mating behavior has never been observed, but must be incredibly sinuous and graceful to behold. Courtship in this species probably involves the successful male grasping a receptive female’s pectoral fin in his mouth. Copulation probably occurs on the bottom, the male pushing the female onto her back, then arching his body to bring his belly along hers before inserting a single clasper into her vent. But all this is mere speculation on my part.
Shortly after mating, female Zebra Sharks lay four or more egg cases. These egg cases are dark brown or purplish-black, measuring 7 inches (17 centimetres) long by 3 inches (8 centimetres) wide and about 2 inches (5 centimetres) thick. In addition, each egg case has along both sides thick tufts of fine, hair-like fibers that probably serve to anchor it to the substrate. After an undetermined gestation period, those egg cases lucky enough to avoid being eaten will hatch. One end of each egg case splits open. Then a tiny, zebra-striped shark wriggles forth, ready to find food and evade predators. If it is successful at meeting these twin challenges, the young Zebra Shark will grow, eventually exchanging its birthday suit for the leopard spots that adorned its parents.