| Epaulette shark | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Orectolobiformes |
| Family: | Hemiscylliidae |
| Genus: | Hemiscyllium |
| Species: | H. ocellatum |
| Binomial name | |
| Hemiscyllium ocellatum (Bonnaterre, 1788) | |
 | |
| Range of the epaulette shark | |
| Synonyms | |
Squalus ocellatus Bonnaterre, 1788 | |
Theepaulette shark (/ɛpəlɛtʃɑːrk/, Hemiscyllium ocellatum) is aspecies of longtailed carpet shark of thefamilyHemiscylliidae, found in shallow, tropical waters offAustralia andNew Guinea (and possibly elsewhere). Thecommon name of this shark comes from the very large, white-margined black spot behind eachpectoral fin, which are reminiscent ofmilitary epaulettes. A small species (usually under 1 m (3.3 ft) long), the epaulette shark has a slender body with a short head and broad, paddle-shaped paired fins. Thecaudal peduncle (to which thetail fin is attached) comprises over half the shark's length. Adults are light brown above, with scattered darker spots and indistinct saddles.
Epaulette sharks havenocturnal habits and frequent shallow water oncoral reefs or intidal pools. This shark has evolved to cope with the severe night timeoxygen depletion (hypoxia) in isolated tidal pools by increasing the blood supply to its brain and selectively shutting down non-essential neural functions. It is capable of surviving completeanoxia for two hours without ill effects, and at a much higher temperature than most other hypoxia-tolerant animals. Rather than swim, epaulette sharks often "walk" by wriggling their bodies and pushing with their paired fins. This species feeds on a wide range of smallbenthicinvertebrates andbony fishes. Epaulette sharks areoviparous, with females depositing pairs of egg capsules around every 14 days from August to December. Due to their hardiness and small size, epaulette sharks are popular with bothpublic andhome aquaria. TheInternational Union for Conservation of Nature (IUCN) has assessed this species as ofLeast Concern, as outside of the small aquarium trade it is of little interest tofisheries.
The epaulette shark was originally described asSqualus ocellatus by the FrenchnaturalistPierre Joseph Bonnaterre in the 1788Tableau encyclopédique et méthodique des trois règnes de la nature. The name was later changed to the currently validHemiscyllium ocellatum. Thetype specimen was a 35 cm (14 in) long immature male caught nearCooktown,Queensland,Australia.[2] Other common names for this species are the itar shark and the blind shark (also used forBrachaelurus waddi).[3] Goto's 2002morphological analysis of thecarpet sharks showed the genusHemiscyllium as apolytomy, meaning that thephylogenetic relationships between the epaulette shark and its sister species could not be resolved.[4]
The range of the epaulette shark extends from the southern coast ofNew Guinea to the northern coast ofAustralia, as far south asSydney.[5] The Capricorn-Bunker Group of theGreat Barrier Reef contains a particularly large population, with thousands estimated to inhabit the reefs aroundHeron Island alone.[1] The estimated population density of these reefs is approximately 0.3 to 1.2 individuals per 100 square meters.[6] Additionally, there are unsubstantiated reports of this species fromMalaysia,Sumatra, and theSolomon Islands.[2] Epaulette sharks are found in shallow water to a maximum depth of 50 m (160 ft), and are often seen in water barely deep enough to cover their bodies. They prefertidal pools,coral flats, and stands ofstaghorn coral.[2][3]
The epaulette shark has an elongated body, over half of which is comprised by the slender caudal peduncle. The snout is short and rounded, with thenares placed almost at the tip along with a pair of tinybarbels; there are grooves running from the nares to the mouth. There are 26–35 tooth rows in the upper jaw and 21–32 tooth rows in the lower jaw. The teeth are small, with broad bases and triangular cusps. The eyes are oval in shape and elevated, with a largespiracle below each. The five pairs ofgill slits are small, with the fourth and fifth very close together.[2]
The pectoral andpelvic fins are broad and rounded and are thickly muscled. The two similarly sizeddorsal fins are placed well back on the body. Theanal fin is low and positioned just in front of thecaudal fin. The caudal fin has only an upper lobe, which contains a prominent ventral notch near the tip and is angled almost horizontally relative to the body. Adult epaulette sharks are beige to brownish above with many widely spaced brown spots and subtle darker bands. There is a very large black spot ringed in white located behind each pectoral fin; this pair of spots are the origin of this shark's common name. Juveniles have alternating light and dark bands over their body and fins, which break up as they mature.[5][7] Epaulette sharks are typically 70–90 cm (27.5–35.5 in) long; the maximum reported length is 107 cm (42 in).[3][7]
As anadaptation for navigating its complex reef environment, the epaulette shark moves by seemingly walking, bending its body from side-to-side and pushing off of the substrate with its paddle-shaped pectoral and pelvic fins. Its gait is similar to that ofsalamanders, an example ofconvergent evolution. The shark is capable of swimming, but often prefers to walk along the sandy or coral bottom even when the water is deep enough to allow it to swim freely. Thecartilaginous supports of the epaulette shark's paired fins are reduced and separated when compared to other sharks, allowing them to be rotated for use as limbs.[8] This mode of locomotion even enables the shark to crawl out of the water to access isolated tidal pools.[9] The gait of the epaulette shark isconvergently similar to those oftetrapods such assalamanders, suggesting that the movements needed for walking on land may predate, and perhaps contributed to the evolution of, the first terrestrialvertebrates.[10] Epaulette sharks are largelynocturnal and are most active in low water. They often hide inside or below coral heads, though it is enough for the head to be covered even if the rest of the body is exposed.[2] Sometimes they perch in the open on sandy flats or atop reefs facing into the current, a form of orientation known as rheotaxis that may improverespiration or predator awareness.[11]
Epaulette sharks are preyed upon by larger fishes such as other sharks.[5] Its coloration provides protectivecamouflage,[7] while its epaulette is speculated to be aneyespot for distracting or deterring predators.[2] Epaulette sharks are almost allparasitized by the praniza (parasitic)larval stage ofgnathiidisopods. The larvae feed onblood and mostly attach to the skin around thecloaca and theclaspers, though they are also found inside the mouth and on thegills. These parasites cause little damage and are not believed to adversely affect the health of the shark.[12] Other parasites of this species include a species ofmyxosporean in the genusKudoa, which infests theskeletal muscles,[13] the hemogregarineprotozoanHaemogregarina hemiscyllii, which infects the blood,[14] theostracodSheina orri, which attaches to the gills,[15] and thenematodeProleptus australis, which infests the stomach.[16]
At night, the shallow reef platforms inhabited by the epaulette shark often become isolated from the ocean by the receding tide. The amount ofdissolved oxygen in the pool can drop 80% or more overnight from the combined respiration of all the organisms within the pool. The epaulette shark has evolved to cope with these hypoxic conditions, being able to survive for over three hours in 5% of the atmospheric O2 level without losing behavioral responsiveness. In the laboratory, epaulette sharks have survived for an hour without any oxygen at 30 °C (86 °F), which is also unusual in that most animals capable of tolerating oxygen deprivation do so at low temperatures.[17]
The physiological responses of the epaulette shark to low oxygen are mediated by thenucleosideadenosine. In hypoxic conditions, the heart and ventilation rates drop sharply.[18] The shark'sblood pressure falls by half as theblood vessels dilate to deliver more blood to the brain and heart. Unlike in bony fishes and tetrapods, the blood flow rate remains constant and there is no elevation of bloodglucose levels.[19] The brains of sharks only consume a third as muchATP as those ofteleosts.[17] The epaulette shark is able to lower this energy demand further by reducing themetabolism of certain areas of its brain, e.g. keeping the sensory nuclei functional while deactivating the motor nuclei. This allows the shark to supply enough ATP to preventneuron death, while still remaining alert to its environment.[20]
Epaulette shark development rate and fitness are strongly affected by temperature during development.[21] Temperature rises due toclimate change are sufficient to weaken the sharks, which risks damage to ecosystems.[21]
The epaulette shark is an opportunistic predator ofbenthiccrustaceans, worms, and smallbony fish. OffHeron Island, over 90% of the epaulette shark's diet consists ofpolychaete worms andcrabs, with juveniles taking mostly the former and adults mostly the latter. It hunts most actively at dawn and dusk, though feeding can occur at any time during the day.[10] This shark relies mainly on itsolfactory andelectroreceptive senses to locate hidden prey.[2] It is capable of sucking prey into its mouth by expanding its muscularbuccal cavity.[10] While searching for food, the epaulette shark sometimes turns over debris with its snout or thrusts its head into the sand, swallowing food items while expelling the sand grains through its gill slits.[2] Unlike most sharks, the epaulette shark maychew its food for up to 5–10 minutes.[10] Its teeth can be depressed to form a flat surface for crushing hard-shelled prey.[5]
Mating in the epaulette shark takes place from July to December, though in captivity reproduction occurs continuously.[1][22] Courtship may be initiated by the female following and biting the male. The male then holds onto the pectoral fin of the female with his mouth and lies alongside her, while inserting one of hisclaspers into hercloaca.Copulation lasts about one and a half minutes.[23] Epaulette sharks readily breed in aquariums, even in tanks as small as 135 gallons,[24] but larger tanks are recommended to allow room for the hatched offspring. This species isoviparous, with females depositing eggs from August to December. The female drops the egg capsules two (rarely four) at a time every 14 days, producing a total of 20–50 eggs per year. Each egg case measures 10 cm (3.9 in) long and 4 cm (1.6 in) wide. The young emerge after 120–130 days, at a length of 14–16 cm (5.5–6.5 in). The growth rate of the young is slow at first but increases to about 5 cm (2.0 in) per year after three months. Both males and femalesmature sexually at a length of 54–64 cm (21.5–25 in), corresponding to an age of at least seven years.[1][10]
Epaulette sharks are harmless to humans, though if handled they may nip their captors. They are easily observed and handled by beachgoers as they move slowly whilst out of water, and show little fear of humans. This species adapts readily to captivity and is displayed by manypublic aquariums in the United States, Canada, and Australia.[2] In an article forAquarium Fish Magazine, Scott W. Michael referred to the epaulette shark as "the best shark for the home aquarium." They will breed in captivity, even in tanks as small as 510 L (135 gal), though full-grown sharks are best housed in tanks of 680 L (180 gal) or more. They are not compatible with community tanks as they will eat other fish.[25]
TheInternational Union for Conservation of Nature (IUCN) has assessed the epaulette shark as ofLeast Concern. This species is common in Australian waters, where it is not captured by eithercommercial orrecreational fisheries, and is minimally affected by collection for the aquarium trade and fisherybycatch. Much of its Australian habitat is encompassed by protected marine parks. Off New Guinea, the epaulette shark has been assessed asNear Threatened, as there it faceshabitat degradation,destructive fishing practices, andover-exploitation byartisan fishers.[1]
In 2015, the behaviour of an epaulette shark was filmed in detail by theBBC for the first episode of a new documentary series namedShark, released around the 40th anniversary ofJaws.[26] The documentary shows an epaulette shark's ability to walk over land, and its hunting behaviours in the ebbing tide.
In normal temperatures, the sharks emerged from the egg cases after 125 days. But in 31C waters, they emerged after 100 days. The researchers also measured the fitness of the baby sharks, and found that it peaked at 29C but then fell sharply at 31C... Weaker sharks were less efficient hunters ... which could then have a knock-on effect across the coral reefs where they live, upsetting the balance of the ecosystem.
