Similan Islands -Nurse Sharks

As taken from the files these Similan Diving Experts

The Nurse Shark is rare, but is found among the boulders of the Similan Islands. If you would like to learn more about Sharks in the Similan Islands please check out our Shark Page. If you have any questions about how to dive with sharks – check here


Nebrius ferrugineus

The Tawny Nurse Shark’s common name is derived from its ability to suck up prey using a powerful sucking motion with its throat, just like a baby being nursed. This talent is used to vacuum out octopus, fish and crabs from the reef or it can be reversed to fire a jet of water at any captor.

A large, tropical inshore shark of the continental and insular shelves of the Indo-Pacific,often in the intertidal in water scarcely able to cover it and from the surf line down to a few meters depth, commonly at 5 to 30 m and ranging down to at least 70 m on coral reefs. It occurs on or near the bottom in lagoons, in channels, or along the outer edges of coral and rocky reefs, in areas with seagrass and sand on reefs, sandy areas near reefs and off sandy beaches. It prefers sheltered areas in crevices and caves on reefs but often occurs in more exposed areas in depressions or crevices. Young prefer crevices in shallow lagoons but adults are more wide-ranging.

The tawny nurse shark is primarily nocturnal, resting in the daytime in shelters but prowling slowly about around reefs at night, although some individuals may be active in the day. In Madagascar it is described as day and night-active, and in captivity they get quite active and vigorous when food is presented to them during the day. They are social, gregarious sharks when at rest and form resting aggregations of two to a half-dozen or more in shelters, and are often seen piled inertly across or on top of one another. When resting, they are extremely sluggish. This shark has a limited home range, and individuals often return to the same area every day after foraging.

Reproduction ovoviviparous (aplacental viviparous) with cannibal vivipary or uterine cannibalism in the form of oophagy Inside the eggs, they eat each other!!!). This shark has been described as an oviparous or post-oviparous shark that retains the egg-cases until they hatch and the young are born, but recent evidence indicates that this is incorrect. Pregnant females collected from Okinawa have had one or two foetuses per uterus, 297 to 595 mm (the latter near term), with the yolk sac reabsorbed and a greatly expanded stomach filled with yolky material in foetuses 338 and 595 mm long, and also had cased eggs in the uterus. Apparently this species practices oophagy on relatively large, cased nutritive eggs (unlike many lamnoids which have very small nutritive eggs), and is the first orectoloboid known to have uterine cannibalism. It is not known whether foetuses of this species eat other foetuses (adelphophagy) as with the sand tiger (Carcharias taurus). The presence of two foetuses 338 and 297mmin the same uterus, with the smaller one slender and the larger bloated with yolk, suggests that competition between siblings for the relatively big eggs is likely and could even eliminate the less successful sibling. Adelphophagy is less likely although early stages need to be examined to eliminate it. Number of young per litter uncertain; at least four young per uterus has been suggested from cased eggs, but the size and enormous girth of the near-term Okinawan foetus and the two smaller foetuses in a litter suggests that litters are smaller, possibly one or two per uterus or even one per female, and that numbers of cased eggs in the uteri cannot be used to extrapolate litter sizes in this species. In captivity adult females lay cased eggs on the bottom, but these do not develop, and could be nutritive and unfertilized. Such free eggs may have been the basis of the suggestion that this species is oviparous. The tawny nurse shark breeds in July and August off Madagascar. Food of this shark includes corals, crabs, lobsters and other crustaceans, Octopus, squid and probably other cephalopods, sea urchins, and reef fish including surgeonfish (Acanthuridae), queenfish (Carangidae) and rabbitfish (Siganidae), and occasionally sea snakes. While foraging the tawny shark moves along the bottom and explores depressions, holes and crevices in reefs. When it detects prey it places its small mouth very close to the victim, and uses its large pharynx as a powerful suction pump to rapidly suck in reef organisms that may be out of reach of its teeth. A few large individuals dissected had quantities of small, active reef fishes in their stomachs, presumably sucked in by the sharks as the prey fishes lay inert in shelters or on the bottom at night. Individuals caught by fishermen may reverse this sucking action, and blast streams of water out of their mouths and into the faces of their captors; they are said to make a grunting sound between blasts. It is not known if spitting water is deliberate and defensive or if the sharks are actually aiming the water at the anglers. They also tend to spin when hooked on a line, making them difficult to handle and subdue.

The body form of the tawny nurse shark (littoral morphotype) is more fusiform and streamlined than other nurse sharks, with narrow-based, falcate, plesodic pectorals, pointed dorsal and anal fins with the anal-fin apex raked posterior to the free rear tip, a short ventral caudal-fin lobe, lateral eyes and gills, a narrow head, flat wedge-shaped snout, and compressed, semi-blade-like, imbricate teeth in discrete series. The tawny nurse shark superficially resembles certain other large, partly or mostly sympatric, active reef sharks including the sand tiger shark (Carcharias taurus), sicklefin lemon shark (Negaprion acutidens), and reef whitetip shark (Triaenodon obesus). Whether it is behaviourally divergent from other nurse sharks awaits a detailed comparative study of nurse shark behaviour. Its status as a game fish in Australia, unlike the nurse shark in the western Atlantic, suggests that it may be a more active swimmer when not resting on the bottom.

Size

Size: Maximum about 314 to 320 cm, though most individuals are smaller; size at birth has been reported as about 40 cm (Fourmanoir and Laboute, 1976) but subsequent data from a pregnant female captured off Okinawa suggest that it may reach 60 cm or more at birth (Teshima et al. 1995), while a 79 cm female from Navotas market in Manila, Philippines had a somewhat bloated stomach full of yolk, suggesting that it was newborn or a term foetus; males are mature at about 250 cm and reach at least 301 cm; adult females are 230 to at least 290 cm.

Interest to fisheries…

Common or formerly common in areas where it occurs, and caught inshore by fishermen in Pakistan, India, Thailand, and Philippines, and probably widely captured elsewhere.
It is utilized fresh and dried-salted for human food, its liver is rendered for oil and vitamins, its fins are used in the oriental sharkfin trade, and offal is processed into fishmeal. Its thick, armour-like hide is potentially valuable for leather.Off Queensland, Australia, it has been fished as a big-game shark, and large individuals are prized as powerful fighters by sports anglers. Apart from anglers who target this fish, it is apparently primarily caught only as an untargeted bycatch of fisheries in inshore waters in nets, on line gear, and in fish traps.

Conservation Status : The conservation status of this shark is uncertain and urgently needs investigation despite its wide range. In some areas, including the Gulf of Thailand where it was commonly caught in the 1960s, it may have been depleted due to increasing fisheries activity and habitat degradation. Also, reef habitats have been extensively damaged or destroyed by dynamiting and poisoning in parts of its range, including Indonesia and Philippines, which probably have had an adverse effect on this species both directly and through decimation or elimination of its prey. Its docility and inshore habitat makes it particularly susceptible to a wide variety of fishing gear, to harassment and injuries by divers, and to reef destruction and pollution.

Threat to humans: This has been described as a much more docile species than its close relative, Ginglymostoma cirratum, and apparently tolerates close proximity of divers and usually allows humans to touch and play with it without biting. However, there are a few records of these sharks biting their tormentors, and clamping tightly onto them. Because of its size, strength, powerful jaws and small but sharp cutting teeth, the tawny shark should be treated with the respect due it.

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