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Shark of the week: Winghead shark — 15. January 2018

Shark of the week: Winghead shark

Did you know that hammerhead sharks did not gradually develop such a large hammer, as it had been believed for decades? Genetic tests showed the opposite. Sharks with giant cephalofoil (hammer) like the Winghead shark, where it is as wide as up to 50% of the shark’s total length, are the primary species, from with later evolved species with a smaller hammer. The benefits of the hammer have to exceed the hindrance of its deformity by far.

Eusphyra blochii X-ray.jpg
X-ray of Eusphyra blochii by Sandra Raredon/Smithsonian Institution – http://eol.org/collections/14770, Public Domain, Link

The Winghead shark (Eusphyra blochii) forms its own genus within the family hammerhead sharks and has been named after the German naturalist Marcus Elieser Bloch, who described it for the first time in 1785. It is a slender shark (hence the additional name Slender hammerhead shark) and has got not only particularly protruding hammer wings (which are at birth folded back along the body and unfold only later on) but extremely long nostrils, too. After a gestation period of 8 to 11 months, where approximately 11 embryos have got each their own compartment inside the uterus and are nourished by yolk and later by a placental connection (ovoviviparous), they are born at a length of 13 to 19 in – 32 to 47 cm. They are slow growing and reach up to 6.2 ft – 1.9 m – at length and 21 years of age.

The Winghead shark lives in coastal tropical waters of the Indio-West Pacific. Due to the high fishing pressure for its fins and meat and the degradation of its habitat in these areas, the IUCN has been classified it as endangered since 2016 (prior to that it was only considered near threatened, but it is absent from several areas by now).

Sources: here and here

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Shark of the week: Scalloped hammerhead — 8. January 2018

Shark of the week: Scalloped hammerhead

Sharks do have special healing abilities. They often get wounds (due to mating rituals, combat, collisions with reef substrate or fishing gear) which, however, heal remarkably quickly. A part in it is the fact that dermal-denticles are teeth and are continually replaced like real teeth. Thus wounds are quickly covered with temporary, larger denticles, and the resulting scar become invisible by normal shaped denticles within 6 month.

An example of this was found in Scalloped hammerhead sharks (Sphyrna lewini). This species of Hammerhead sharks (also known as bronze, kidney-headed, or southern hammerhead sharks) lives all over the  world offshore in warm and tropical waters near the surface, partly in large schools. This and the fact that there are several distinct sub-populations makes this species highly susceptible to fishing pressure. They are caught  commercially for their meat and skin, but manly their fins, often illegally (IUU), as bycatch or as big game fish by recreational fishermen. That’s why they are worldwide considered endangered by the IUCN.

Hammerhead shark, Cocos Island, Costa Rica.jpg
Scalloped hammerhead by Barry PetersFlickr, CC BY 2.0, Link

Scalloped hammerheads are up to 14 ft – 4.3 m – long (but on average only up to 8.2 ft – 2.5 m – as female and 5.9 ft – 1.8 m – as male). They are ovoviviparous (once the yolk sack is depleted it converts into a placental connection) and give birth after 9 to 12 month in specific, shallow nursery areas to up to 40 living young. The pups grow slowly and often fall prey to larger sharks, what explains the relative large litter size.

Sources: here, here and here

Shark of the week: Portuguese shark — 18. December 2017

Shark of the week: Portuguese shark

Sharks of the family sleeper sharks are bad ass: they like it cold (like the Greenland shark) as well as hot (like the Pacific sleeper shark). But one of them also likes it deep: the Portuguese Shark (Centroscymnus coelolepis) is the record holder for greatest depth attained by any shark. It usually inhabits depths below 1,300 ft -400 m-, but has been caught as deep as 12,057 ft -3,675 m. It is special in other aspects, too. It has very large dermal denticles shaped like scales of bony fish, and its eyes are well adapted to deep sea conditions: large and green due to a reflective layer named tapetum lucidum (like cat eyes), its cells are optimized for detecting movement and bioluminescence.

A small, dark brown, heavy-bodied shark with large green eyes and small fins, lying on the ground next to a meterstick
Centroscymnus coelolepis by Jon Moore – Marine Fisheries Review 65(4), Public Domain, Link

The Portuguese shark can be found worldwide, but patchily, in several distinct population in the Atlantic and Mediterranean Sea, Pacific and Indian Ocean. It is on average 3.3 ft – 1.0 m – long and ovoviviparous with an average number of 12 pups after one year of gestation (due to the vast extend of the deep sea, it seems that copulation trigger ovulation).

There is depth segregation by size and sex; since pregnant females are found in shallower water of the deep sea, they are more at risk by fishing pressure. It is targeted for the squalene in its liver (22 to 49% by weight) and its meat, but mostly taken as bycatch by bottom trawlers. Thus the IUCN considers the Portuguese shark as Near Threatened and even Endangered in European waters, despite a zero Total Allowable Catch since 2010, because even if discarded, as deep sea species the survival rate is likely to be low. It seems that a separate sub-population in the Mediterranean Sea is secure because it occurs at depths that are outside of the scope of existing fisheries, additionally there has been a ban on deepwater fisheries operations below depths of 3,300 ft – 1,000 m – since 2005.

Sources: here, here and here

Shark of the week: Prickly shark — 11. December 2017

Shark of the week: Prickly shark

The Prickly shark (Echinorhinus cookei), looking like the big brother of the Ninja lanternshark, is thankfully not as irritable as its name suggests. On the contrary, despite its size of up to  13.1 ft -4 m- it is rather shy around humans.

Echinorhinus cookei head2
head of Echinorhinus cookei by D Ross Robertson [Public domain], via Wikimedia Commons
The Prickly shark, as well as its brother Echinorhinus brucus from the family Bramble sharks or Echinorhinidae, is known for its thorn-like denticles. Thus its scientific name Echinorhinus, which is greek for echinos = hedgehog or sea urchin and rhinos = nose. It inhabits deeper coastal waters of the Pacific ocean from California and Hawaii to Chile and China to New Zealand and stays as a homebody in a rather small home range of 0.85 sq mi  -2.2 km2. It likes it rather cool and stays by day in depths of up to 4900 ft -1500 m-, but migrates at night upwards near the surface.

Prickly sharks are not targeted directly (its meat isn’t tasty), but are often victims of bycatch of deepwater trawl fisheries to sate our increasing need for fish meal (used for aquaculture and pig, chicken and even cow husbandry). Therefore they are considered as near threatened by the IUCN, even though a female has been found with up to 114 embryos (they are ovoviviparous).

Sources: here, here and here

Shark of the week: Shorttail nurse shark — 4. December 2017

Shark of the week: Shorttail nurse shark

The Shorttail nurse shark (Pseudoginglymostoma brevicaudatum) has got a very long scientific name. This is due to the fact, that, since 1986, it hasn’t been sorted into the genus Ginglymostoma like Pacific nurse shark and Nurse shark but has been named as false (pseudo-) nurse shark.

As the name implies, this nurse shark has got an extreme short tail. It is also the smallest shark of its family with up to 2.46 ft – 75 cm – length. Unlike the Tawny nurse shark, who is in fact ovoviparous, but whose egg cases pile up in captivity and are unfertilized ejected, the Shorttail nurse shark lays indeed eggs (oviparous), out of which after around 5 month young hatch, proven by the successful breeding in Amsterdam’s zoo since 2006.

egg-case Pseudoginglymostoma brevicaudatum
Egg capsule of Pseudoginglymostoma brevicaudatum, © Chris Avila, Toronto Bentic Sharks, Canadian Marine Aquaculture, Toronto

Living on coral reefs off east-Africa and Madagascar in distinct sub-populations, the Shorttail nurse shark is considered “vulnerable” due to high fishing pressure (it is taken as bycatch and for its fins, meat and skin) and habitat destruction. It has been caught for aquaria, too, which could be supplied now as well from breeding programmes.

Sources:hereherehere and here

Shark of the week: Tawny nurse shark — 27. November 2017

Shark of the week: Tawny nurse shark

The Tawny nurse shark (Nebrius ferrugineus) is a docile shark of the family nurse sharks and lives in the Indian Ocean from the Red Sea to South Africa and India and in the western Pacific from Australia and Japan to South Sea islands. It inhabits shallow coastal waters in depths up to 230 ft – 70 m – on coral reefs. It is nocturnal and feeds from small fishes, corals, sea urchins, sea snakes, crustaceans and especially octopuses, which it sucks into its small mouth.

The female Tawny nurse shark is ovoviviparous. The embryos feed (after depleting their yolk sack) from big eggs (oophagy), they compete for food and maybe prey on weaker siblings (cannibalism) – in any case mostly only one out of 4 embryos per uterus survives. The pups are born at 16 to 31 in -40 to 80 cm- and grow up to 10 ft – 3.20 m- length. They mature at 8 ft 2 in – 250 cm – (males) and 7 ft 7 in to 9 ft 6 in – 230 to 290 cm – (females) and loose their spots on the white belly. Their color is yellowish brown (since their name).

Even if Tawny nurse sharks are mostly gentle and can be trained (it is said that on some South Sea islands children even ride them), they get wild and spit, grunt and bite when provoked or captured. That’s why they are targeted by recreational fishermen as big game fish. Except in Australia they are fished commercially for their meat, fins, skin and liver oil (partly using explosives or poisons) and caught for aquariums. Additionally they are harassed by divers and their habitat gets often destroyed. Therefore they are considered vulnerable worldwide by the IUCN.

Sources: here, here and here

Shark of the week: Smalltail shark — 30. October 2017

Shark of the week: Smalltail shark

Another example to prove the Plate tectonics, anyone? Well, the Smalltail shark (Carcharhinus porosus – named after really big pores behind its eyes) was considered, until 2011, to live in coastal waters on both sides of the Isthmus of Panama: in the western Atlantic Ocean from the northern Gulf of Mexico to southern Brazil as well as in the eastern Pacific from from the Gulf of California south to Peru. A momentous mistake that seems to continue until now (see IUCN Red list and homepage of the Florida Museum). In fact, its sister species Pacific smalltail shark (Carcharhinus cerdale) can be found in the eastern Pacific, and the Smalltail shark lives only in the western Atlantic part. Both had been separated around 2.8 million years ago and developed into different species (just like the Whitenose shark and the Blacknose shark a proof of evolution, as well).

Carcharhinus porosus SI.jpg
Carcharhinus porosus by D Ross Robertsonhttp://biogeodb.stri.si.edu/caribbean/en/gallery/specie/100, Public Domain, Link

This small (generally only up to 3.6 ft -1.1 m- long) requiem shark is slow growing and viviparous with two to nine young every two years after an approximately 12-month gestation period. It can be found near the bottom of coastal waters and estuaries and forms large aggregations segregated by sex.

Along the northern Brazilian coast, where some of its nursery areas are, the Smalltail shark is the most common shark, and in Trinidad the most economically important shark (under the name puppy shark). Fishing pressure as bycatch as well as targeted (for its meat, fins, cartilage and liver oil) caused a significant decline in numbers, resulting in an ICUN-classification as vulnerable in Brazil, where mostly juveniles have been caught, but only data deficient overall. Since its habitat is much more narrow as previously assumed (see above), the IUCN should revise that urgently (in my opinion).

Sources: here and here

shark of the week: Tiger shark — 23. October 2017

shark of the week: Tiger shark

Like last weeks Blacktip reef shark, many sharks are targeted by recreational fisherman. After the film “Jaws” it became cool and manly to catch “man-eating” sharks as big-game fish, either alone or in tournaments. Rarely they are of use after (unlike in artisanal fisheries) – it is only for the thrill. And the kill, even if some tournaments try a catch-and-release-approach (if the shark survives the injuries and stress, and even the pull-up can be dangerous for example for deep sea sharks or highly pregnant females). Since the Tiger shark (Galeocerdo cuvier) has the reputation of a man-eating killer machine, it is a main target. As pointed out here, this results in many killings. In California alone, tiger sharks caught recreationally outnumbered the commercial side 6-to-1. Commercially, they are taken as bycatch or targeted for their fins, meat (the mercury content of this shark’s flesh is lower than in other large requiem shark species due to their diet), skin, cartilage and liver oil.

Yes, sharks can bite. And due to its size (females up to 16 ft 5 in – over 5 m- and males up to 13 ft 1 in – 4 m- length), power and dentition, an adult Tiger shark can cause significant harm -its serrated teeth are able to slice through turtle shells, after all. According to the ISAF, it is second only to the White shark in number of reported attacks on humans, many of them fatal. But if you watch this video, they seems rather docile and sluggish. They are big, curious and potentially dangerous, but no evil killing machines.

Tiger sharks live in tropical and subtropical waters all around the world and undergo a long-distance seasonal migration. As opportunistic feeders they prey mostly on fish, cephalopods, crabs, sea snakes, sea birds and carrion, but also with age more and more on seals, turtles, dugongs, dolphins and injured whales. As Neil Burnie said: “Tiger sharks like the dead, the dying and the dumb” (I would add dump, since they sometimes eat garbage, too), because they can’t be picky. To grow to such a size in relatively nutrient-poor warm waters is no mean feat (White sharks inhabit mostly cooler waters).

Tiger sharks are unique among their family. Firstly, they have got spiracles and a capacious stomach with a muscular wall that is proportionately three times thicker than that of any other requiem shark. Also unlike any other requiem shark, the female Tiger shark is ovoviviparous and not viviparous. That means the pups hatch in the uterus, but are not feed by a placental connection after the yolk supply is depleted. During the gestations period of up to 16 months the mother does provide unfertilized eggs (oophagy) and the one or other sibling to nibble on (in-utero cannibalism). But, other than in the Sandtiger shark, many survive, resulting in big litter sizes of 10 to 80 pups (unlike almost all other requiem sharks).

The 20 to 30 in -51 to 76 cm- long newborn Tiger sharks stay in their shallow nursery areas and grow fast. That is necessary, since their slender and delicately built bodies with over-sized fins and unique tail structure (an elongate upper caudal lobe) condemns them to an eel-like wriggle, an inefficient swimming style that renders them extremely susceptible to predators like bigger sharks (even other Tiger sharks). They do not outgrow their ‘awkward stage’ until they reach a length of about 8 feet -2.4 metres- at about four years of age. After that, their powerful build and fins make them successful predators with startling speed bursts.

Like the Blacktip reef shark, the Tiger shark has got large eyes with a reflective tapetum lucidum like a cat for nightly hunting. Normally they enter shallow, coastal waters at night and spend the day offshore in deeper water. But if their prey, like the Hawaiian Monk Seals, remains near shore during daylight hours and forages in deeper waters under cover of darkness, the Tiger Sharks adapt and revers their usual day-night pattern, remaining inshore during the day and moving offshore at night.

Tiger sharks are considered “near threatened” by the IUCN due to fishing pressure and questionable shark control programmes. But their widespread distribution and high growth and reproductive rates give reason to hope. It has been discovered that juvenile survivorship increases where adult Tiger Shark populations have been depleted by fisheries and hence predation of young is lessened.

Sources: here, here and here

Shark of the week: Blacktip reef shark — 16. October 2017

Shark of the week: Blacktip reef shark

The Blacktip reef shark (Carcharhinus melanopterus – derived from melas = black and pteron = wing or fin (see Pterosaurs)) is another requiem shark living in coral reefs. It inhabits nearshore waters of the tropical and subtropical Indo-Pacific from South Africa to Hawaii (even colonized the eastern Mediterranean Sea by way of the man-made Suez Canal) and prefers more shallow waters than its colleagues grey reef shark and whitetip reef shark. Their nursery grounds (females are viviparous and give birth to living young after feeding them from yolk and via placental connection) can be so shallow that the pups have to swim with their dorsal fins above the water looking “like a flotilla of tiny sailboats” (quote from here).

An expanse of clear water and white sand, and several sharks swimming with their black-tipped dorsal fins protruding above the water
Carcharhinus melanopterus by Leon Brocard from London, UK – PIMG_2915, CC BY 2.0, Link

Not only pups prefer to stay in groups for protection, but also adult Blacktip reef sharks form stabile groups for social purposes and hunting. Their prey are fishes, crustaceans, molluscs and squids, but also terrestrial and sea snakes and even rats and birds. They have got large eyes with a reflective tapetum lucidum like a cat, indicating that they can excellent hunt at night.

Due to their extremely small home ranges and strong site fidelity, Blacktip reef sharks are susceptible to habitat degradation and fishing pressure (only as bycatch by commercial fisheries, but targeted by artisanal fishermen for their meat, liver oil and fins, and also by recreational fishermen and for aquarium trade). Their small litter sizes (only 2 to 5 pups) and long gestation periods (up to 16 month) are no help, either. They are considered “near threatened” by the IUCN. They are normally timid and despite their size (typically up to 5.2 ft -1.6 m-) no danger to humans, but sometimes bite the legs or feet of waders encroaching into their space or spear fishers for their catch.

Sources: here, here and here

Shark of the week: Slender bamboo shark — 9. October 2017

Shark of the week: Slender bamboo shark

The Slender bamboo shark (Chiloscyllium indicum) is a common shark inshore of the tropical western Indo-Pacific. Despite being known since 1789 and thus as long as its cousin Epaulette shark, the only other member of the family bamboo shark known at that time, little is known about this small (up to 26 in -65 cm- long) sluggish bottom-dwelling shark.

Chiloscyllium indicum malaysia 1.jpg
Chiloscyllium indicum by Tassapon KRAJANGDARA – http://www.fishbase.us/photos/thumbnailssummary.php?ID=5900#, CC BY 3.0, Link

Named after its slender body and long slender tail, the Slender bamboo shark has got pattern of darker spots and stripes on its back. Like all bamboo shark it is oviparous, that means females lay eggs. Pups hatch at 5 in -13 cm-, and males mature at 15 to 16.5 in -39 to 42 cm- and females at 17 in -43 cm.

The Slender bamboo shark is considered Near threatened by the IUCN because it is likely to be threatened by overfishing (it is regularly caught for its meat in India, Sri Lanka and Thailand), destructive fishing practices and habitat modification, including the damage and destruction of coral reefs throughout much of its range.

Sources: here and here