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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: Prickly dogfish — 1. January 2018

Shark of the week: Prickly dogfish

My first shark of the week in 2018 is the Prickly dogfish.

First discovered as a cadaver on a beach on Bruny Island off southeastern Tasmania, hence the name Oxynotus bruniensis, this species of the family Roughsharks can be found over the outer continental or insular shelf in southern Australia and throughout New Zealand.

Oxynotus bruniensis.jpg
A prickly dogfish (Oxynotus bruniensis) at the Rumble V submarine volcano (cropped version of original to focus more on the fish) by New Zealand-American Submarine Ring of Fire 2005 Exploration, NOAA Vents Program – http://oceanexplorer.noaa.gov/explorations/05fire/logs/april22/media/dogfish.html, Gemeinfrei, Link

With a prominent “humpback” and extremely rough skin, the Prickly dogfish looks a little different than other Roughsharks, but the shape and the “sailfins” can not be mistaken for something else. It is ovoviviparous with around 7 pups being born at 9.4 in – 24 cm – long. It reaches up to 30 in – 75 cm – in length.

Living in a typical depth range of between 1,150 and 2,130 ft – 350 and 650 m -, the Prickly dogfish is prone to be caught as bycatch by trawlers.

Sources: here and here

Shark of the week: Sailfin roughshark — 25. December 2017

Shark of the week: Sailfin roughshark

Why the uncommon Sailfin roughshark is graced with the scientific name Oxynotus paradoxus may only be known to monsieur Frade who has been founded this name in 1929. I don’t know why this species of roughsharks should be sorted into the category of paradoxical biological species.

Like all roughsharks, the Sailfin roughshark has got rough skin with large denticles, looks like a fleet of sailboats, lives in the deep sea (at depths between 869 and 2,362 ft – 265 and 720 m -) and is ovoviviparous. The pups are born at 9.8 in – 25 cm – length and grow up to at most 3.9 ft – 1.2 m – long.

Oxynotus paradoxus.jpg
Oxynotus paradoxus by © Citron / , CC BY-SA 3.0, Link

Living in the eastern North-Atlantic from Scotland to Senegal, the Sailfin roughshark, like all deepwater species, is a (albeit rare) bycatch of offshore trawling fleets. Since it is poorly known, the IUCN considers its status as data deficient.

Sources: 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: 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: 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: Ornate wobby — 25. September 2017

Shark of the week: Ornate wobby

One of many shark species living solely off Australia is the Ornate Wobbegong (Orectolobus ornatus). This species of carpet sharks is only up to 3.8 ft – 1.17 m – long and had been confused with juvenile Banded Wobbegongs (Orectolobus halei) until 2006.

Orectolobus ornatus great barrier reef.jpg
Banded wobbegong (Orectolobus ornatus) at the Great Barrier Reef, Queensland by Andrew J. Green / Reef Life Survey – http://fishesofaustralia.net.au/home/species/1976, CC-BY 4.0, Link

Ornate wobbegongs are ovoviviparous. Every 3 years around 9 living young are born with a size of 7.9 in -20 cm. The gestation period is only about 10 to 11 month long, but the follicles take two years to enlarge before ovulation.

Since they mature at about 31 in -80 cm- length, measures to protect juveniles of the bigger Banded wobbegong (a minimum catch size limit of 5.9 ft – 1.80 m -), implemented between 2008 and 2013, effectively protected Ornate Wobbegongs of all sizes and allowed their stocks to recover. There are management regulations in place for commercial wobbegong fisheries (where they are taken as bycatch and for their meat for fish and chips and their skin for leather), and a possession limit for recreational fishers of one shark in Queensland and zero sharks in New South Wales. That’s why the IUCN has been considered the Ornate wobbegong since 2015 as “Least Concern” (after “Near Threatened” in 2009).

Sources:here and here

Shark oft the Werk: Leopard shark — 21. August 2017

Shark oft the Werk: Leopard shark

The Leopard shark (Triakis semifasciata) is a beautiful shark of the family of houndsharks. It lives in the northeastern pacific in several distinct populations off Oregon down to Mexico. They like the ground (sandy or muddy) near rocks or kelp forests in depths between 3 and 295 ft -1 and 90 m.

View from above of two leopard sharks lying on the sand side-by-side
By nugunslinger from Lafayette, USofA – Leopard Sharks….NOT Tiger, CC BY 2.0, Link

Female Leopard sharks are ovoviparous and give birth to 4 to 29 living young after a gestation period of 12 month. The 8 in -30 cm- long pups grow slowly and mature only after 10 years. Males are up to 78 in -2 m- and females up to 70 in -1.8 m- long. They live up to 30 years.
Leopard sharks form large groups, even together with different shark species. They search rather buried prey like crabs, worms and clams than feed from fish. Since their habitats are often endangered by human effluent and industrial waste, they tend to concentrate pesticides, polychlorinated biphenyls (PCB’s), and other toxic chemicals in the liver.
Leopard sharks are caught as game fish and fished commercially for their meat. They are often shown in aquaria.

Sources: here, here and here