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shark of the week: knifetooth dogfish — 19. December 2016

shark of the week: knifetooth dogfish

After introducing dangerous sharks, here a harmless one.

Scymnodon ringens
knifetooth dogfish (Scymnodon ringens). Image by Henk Heesen, taken from This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License

But hold on! The teeth look dangerous, doesn’t they?  But this rare, medium sized shark is indeed considered harmless (not that little means automatically harmless, look at the cookiecutter shark), despite its name: knifetooth dogfish.

The knifetooth dogfish (Scymnodon ringens) lives in the deep sea in the Eastern Atlantic and Southwest Pacific and is up to 3.6 ft – 1.1 m – long. It looks, with the exception of its teeth, like a relative of the pocket shark or the ninja lanternshark, but it is actually a species of sleeper sharks (like the Greenland shark). It is used for dried or salted meat and fishmeal.

Teeth must be really something distinctive, since there is a sleeper shark called sparsetooth dogfish, too.

Sources: here and here

Shark of the week: seal shark — 14. November 2016

Shark of the week: seal shark

Did you know that the skin of the seal shark (Dalatias licha) has been made into an expensive leather called shagreen and used in Europe for book binding, sword hilts, luggage and jewelry since the 17th century?

The skin is dark brown (that’s why the seal shark is called Schokoladenhai -schokolade shark- in German) with small and flat dermal denticles. Reaching usually 4.6 ft -1.4 m – in length, the seal shark is with its large teeth and strong bite a powerful predator, but can also take bites out of larger prey, just like its little cousin, the cookiecutter shark (although its lips are not modified to be suctorial).

Dalatias licha head.jpg
Von asobi tsuchiyadeepseashark25, CC BY 2.0, Link

Like all sharks of the family kitefin sharks, the seal shark is a deep sea shark with a slow growth and small reproductive rates. Females give birth to 6 – 8 living young after a gestation period of two years. The embryos hatch inside one of the two functional uteruses (not divided into compartments) and are sustained by yolk (ovovivipary). In between pregnancies the females may have a year of rest. The young are born at a length of 12 to 18 in -30 to 45 cm- and mature at 2.53 to 3.97 ft – 77 to 121 cm- (males) and 3.84 to 5.22 ft – 117 to 159 cm – (females).

The seal shark (also known as kitefin shark) is caught commercially for its liver oil (especially its squalene) and skin (see above) as well as for meat (in Europe and Japan, but also Australia due to the relaxation of regulations regarding seafood mercury content) and fish meal, but also as bycatch in deepwater longline, bottom trawl and gillnet fisheries. Once discarded overboard, deepwater sharks rarely survive. This causes the seal shark to be considered Near Threatened globally and Endangered in European waters. Due to its habitat near the bottom of the sea the seal shark is no danger to humans, but bites occasionally into underwater fiberoptic cables.

Sources: here and here

Shark of the week: rough longnose dogfish — 3. October 2016

Shark of the week: rough longnose dogfish

The classification of shark species can be difficult. Sometimes juvenile and adult specimen of one species are considered different species (Zebra Shark). And sometimes a species is considered equal to another (the sister species birdbeak shark), but the same species found in another place (off Madeira) get its own name, until 3 years later both are deemed the same, new species – the rough longnose dogfish (Deania hystricosa).

Deania hystricosum.jpg
Deania hystricosum, image by Yagogta CC BY-SA 3.0,

The rough longnose dogfish is a medium sized (up to 43 in -110 cm- long) deep sea shark and has been found in the Eastern central and southeast Atlantic around Madeira, the Canary Islands, Namibia and South Africa and the Northwest Pacific around Japan and New Zealand (so far). The species belongs to the family gulper sharks and has got grooved spines in front of both dorsal fins and rough, pitchfork-shaped dermal denticles. It is ovoviviparous with probably around 12 pups per litter. It is taken by deepwater longline fisheries off Madeira and the Canary Islands for its meat and liver oil and as bycatch by bottom trawl fisheries elsewhere.

Sources:here and here


Shark of the week: Gulper shark — 26. September 2016

Shark of the week: Gulper shark

Did you know that a little deep sea shark (classified as “Vulnerable”) is – due to the big portion of high-quality squalene in its liver oil – considered as “the Rolls-Royce of squalene-producing sharks” and targeted purposefully (whether legal or illegal doesn’t seem to matter)? Beside the here mentioned use of squalene in expencive cosmetics and dubious health capsules -where other squalene sources could easily be used instead – is it a (according to GSK without alternative) ingredient to medical products, too, like the flu vaccine adjuvant AS03. I don’t want to discuss the whole purpose of vaccines, or adjuvants, or the in Europe highly controversial pig flu vaccine of 2009, either – but it is to lament that many Gulper sharks (Centrophorus granulosus) were sacrificed pointlessly, since millions of swiftly produced and paid vaccine doses had to be destroyed later. And that in the case of a shark with the probably lowest reproductive rate of all sharks.

A photo of a gulper shark that has been caught.
Centrophorus granulosus, by NEFSC/NOAAPublic Domain

Female gulper sharks reach maturity at 14 and can live between 54 and 70 years long. Since they are pregnant with only one pup for 2 years and take long breaks in between, they typically have only between 2 and 10 pups in their lifetime. They are ovoviviparous and feed the embryo with yolk and unfertilised and also fertilised eggs (oophagy). The egg cell of the gulper shark reaches one of the largest cellular sizes described for any animal, weighing between 143 and 370 grams.

Living in the Atlantic, Pacific and Indian Oceans as well as the Mediterranean Sea, the Gulper shark is caught on longlines and by trawling (together with various other marine animals as bycatch). It is a highly migrating species and forms schools.

Sources: herehere and here


shark of the week: Longsnout dogfish — 5. September 2016

shark of the week: Longsnout dogfish

Did you know that the denticles (tiny skin-teeth) of sharks are different? Some sharks have big denticles, others have got only tiny ones and others none at all. Some have got diamond-shaped denticles, others are  shaped like arrowheads, and this little shark has pitchfork-shaped dermal denticles (just like the Dorian Gray shark): the Longsnout dogfish (Deania quadrispinosa).

Longsnout dogfish (Deania quadrispinosa) in Fishes of Australia, image source: CSIRO National Fish Collection. License: CC BY Attribution – accessed 04 Sep 2016

Like other sharks of the family gulper sharks (Centrophoridae), the Longsnout dogshark is a deepwater shark and ovoviviparous. It has got a extremely long snout (hence the name), and lives in the Atlantic and Indian Oceans and in the South Pacific at depths between 490 and 4460 ft -150 and 1360 m- (but usually between 1310 and 2690 ft -400 and 820 m). Since it only grows to about 45 in -115 cm – it is a rather little shark.

Sources: here and here

shark of the week: Dorian Grey — 29. August 2016

shark of the week: Dorian Grey

Did you know that there is a shark named Dorian Grey? The Deania calcea has got other strange names, too: Birdbeak dogfish, Platypus Shark, Shovelnose Spiny Dogfish and Brier Shark. Some of them I can understand, but others not.

Brier Shark, Deania calcea. Source: Australian National Fish Collection, CSIRO. License: CC by Attribution-NonCommercial

Do they mean the literary character Dorian Gray (with an a instead of an e)? I hope they don’t compare his picture to the image of this shark – what I know of the story indicates that it looked much more gruesome than this little shark. Its face looks a little like the head of a platypus or the beak of a bird, maybe a shovel, but a thicket (brier)?

Names aside, the birdbeak dogfish is a deep water dogfish living in the Pacific Ocean around Japan, southern Australia, New Zealand, and Chile, and in the Atlantic Ocean from Iceland south to the Cape of Good Hope. It has got a very long, narrow snout and pitchfork-like denticles.
Maturing at 17 years (males) and 25 years (females), it has a longevity of about 35 years and reaches up to 48 in -122 cm- length. The female is ovoviviparous with up to 12 pups per litter.

Usually a mid-slope shark (at depths between 239 ft and 4757 ft -73 and 1,450 m-) on the outer continental and insular shelves, birdbeak dogfish seems to migrate vertically and can be found on the upper slopes, too. Since its liver is high in squalene (about 70% by weight), it is targeted for its liver oil and flesh. Catches in Australia have been increasing in the last few years with relaxation of mercury laws and fishers turning to non-quota species. In 2002 regulations in the South East Trawl fishery in Australia prohibit the landings of livers of all sharks unless the accompanying carcass is also landed.

Sources: herehere and here

Shark of the week: Jelly Shark — 1. August 2016

Shark of the week: Jelly Shark

In the family of lantern sharks there are many little-known sharks (the deep sea holds many secrets). One of them is the Bareskin dogfish or Jelly shark (Centroscyllium kamoharai).

This shark species was first named in 1966 and has been found rarely since then. Living in a depth range of 1,600 ft to 4,000 ft -500 m to 1.2 km- in the western Pacific, it is a small shark: mature at 15 in – 40 cm- and a maximum length of 23 in -60 cm. The IUCN considers the Jelly shark like other deepwater dogfishes as highly vulnerable to bycatch, but doesn’t has enough data yet. Since in the meantime monster boats are catching indiscriminately giant amounts of deepwater fish for fish meal every day, there is a risk that enough data may come to late.

The one distinct feature of the Bareskin dogfish are its sparse denticles. I don’t want to expose you to the few images of this shark I found (trust me), so I post an image of normally denticled shark skin instead. This shark doesn’t seem to need that, maybe because it doesn’t swim fast.

shark skin magnified 30 times
shark skin magnified 30 times

Sources: here and here

Shark of the week: longnose velvet dogfish — 25. July 2016

Shark of the week: longnose velvet dogfish

Did you know that nearly every (shark) family has got its longnose cousin? There is a longnose pygmy shark and a longnose spurdog. There are even a longsnout dogfish and a rough longnose shark in a family where all member have got a long snout anyway. There is a longnose sleeper shark: a species that unique that is hasn’t got a description or a scientific name yet.
But I want to introduce the longnose velvet dogfish (Centroselachus crepidater). It is a species of sleeper sharks and can be found in the eastern Atlantic, Indian Ocean and eastern and western Pacific. It lives on or near the bottom of continental and insular shelves.

Centroscymnus crepidater.jpg
By © Citron / , CC BY-SA 3.0,

Reaching up to 4.3 ft -1.3 m- in length and 60 years in age, the longnose velvet dogfish matures at age 15 (males) and 22 (females). After a distinct pairing (with an embrace) the female gives birth to 3 to 9 living young (ovoviviparous).

Despite being fairly common and still classified as least concern by the IUCN, in view of its low productivity the increasing catch numbers (mostly bycatch by deep water trawl and hook and line fisheries, but more and more targeted, too) of the longnose velvet dogfish are cause for concern. It is used not only for fishmeal (to feed our increasing hunger for farmed salmon) and meat (fillets can retail for up to Aus$12/kg in Australia), but also for its liver oil and the squalene within.

Squalene or its saturated form squalane is used as an expensive ingredient in certain cosmetics from anti-aging creme to lip gloss, in squalene health capsules and as an adjuvant in vaccines. As a deep sea shark, the liver of the longnose velvet dogfish contains 61-73% squalene (by weight).
Although plant sources (primarily vegetable oils) are now used as well, Oceana estimated that between three and six million sharks annually are caught for their squalene. Some of them like the gulper shark are now endangered and protected by law (but pirates don’t care for the law).

Even if the longnose velvet dogfish isn’t endangered yet, Australia has been prohibited since 2002, that the liver lands unless the accompanying carcass is also landed (at least by the South East Trawl fishery). In the EU a similar regulation came into effect in 2003, only regarding shark fins (it seems that gutting should be banned like finning). A few years ago the mercury laws in Australia relaxed and allowed to use the meat of longnose velvet dogfish (since its flesh is high in methyl mercury) in addition to using the liver oil (convenient, isn’t it?). Normally I support the usage of all parts to prevent food waste, but in this case?

Sources: herehere and here

Shark of the week: Blackmouth catshark — 20. June 2016

Shark of the week: Blackmouth catshark

Did you know that many oviparous sharks can only lay 2 egg cases at a time (one from each oviduct)? A female blackmouth catshark is able to produce up to 13 eggs at a time (annually up to 100). The cases are bullet-shaped and without any tendrils.

Blackmouth catsharks are abundant in the northeastern Atlantic Ocean and the Mediterranean Sea. The bottom dwelling shark is little: up to 31 in -79 cm- for Atlantic sharks and 25 in -64 cm- for Mediterranean sharks (due to the lack of nutrients in the Mediterranean waters are the sharks there notoriously smaller than their Atlantic Ocean counterparts).

As seen in the picture, the shark has got a unique pattern of dark spots (most probably used for camouflage) on its back, and the inside of its mouth is black (thus its name). Its skin is very thick and covered by well-calcified dermal denticles. While hunting it uses its high number of ampullae of Lorenzini and its large eyes (the rod cells are most sensitive to the wavelengths emitted by bioluminescence, which is exhibited by most of the organisms it hunts).

blackmouth catshark (image source )

Rarely used commercially, blackmouth catsharks are a common bycatch of bottom trawl and longline fisheries. Even discarded the sharks die. There are areas where most of the blackmouth catsharks captured are immature, suggesting there has been a negative impact of fishing pressure. But a ban on trawling deeper than 3,300 ft -1,000 m- in the Mediterranean seems to provide protection for the deep sea species.

Since 2005 the blackmouth catshark (Galeus melastomus) is a species of the shark family pentanchidae instead of catsharks, as still wrongly claimed in English sites.

Sources: herehere and here

Shark of the week: Bluntnose Sixgill — 4. May 2016

Shark of the week: Bluntnose Sixgill

Did you know that there are sharks with six (some even seven) instead of the usual five gill slits on each side? The order is named Hexanchiformes and contains only six living species. They appear like ancient extinct sharks with few modern adaptations. One of them is the bluntnose sixgill shark (closely related to the bigeyed sixgill shark).

They are widely distributed in the Pacific, Atlantic and Indian Oceans and the Mediterranean Sea and usually live at freezing and dark depths of 300 to 6,150 ft -90 to 1,875 m- (as shown here). Known to migrate vertically at night, sometimes bluntnose sixgills seem to stay in shallow waters (despite their usual sensitivity to light). For instance, between August and November each year, several specimen can be found at depths of 70 to 90 ft -21 to 27 m- during the day and as shallow as 10 ft -3 m- during the night, at least at certain locations off the coast of British Columbia, Canada. They are generally curious about divers and, at these depths, can be observed extensively. It is assumed that they followed their prey or sought out safe nursery grounds for their young, utilize plankton blooms to shade their eyes.

Bluntnose sixgills are ovoviviparous and, after probably more than two years gestation, giving birth to large litters of 22 to 108 pups. Newborn sharks are relatively big (28 in -70 cm- long) and developed, but slow and therefore vulnerable. The teeth show a strong sexual dimorphism (the male shark has more erect primary cusps than the female), which fits with the among sharks popular courting behavior of “love nips” (proven by special scars only females exhibit). They are slow-growing and therefore live long (like the Greenland shark).

Like all sharks, bluntnose sixgills have specialized senses. Since their portion of the brain are proportionately small, it seems that electroreception and hearing are relatively unimportant. But their olfactory bulbs are very large (about 8% of total brain weight), and their eyes are well adapted to deep-sea light conditions. Their iris is non-contractile and their retinas are completely populated by rods (and no cones, thus no color vision) “with very long rod outer segments to give it maximal sensitivity at those wavelengths of light that predominate in the deep-sea”. They are predators, but scavenge on whale carcasses, too.

Growing to 16 ft -4.9 m- in length, bluntnose sixgills are captured in commercial fisheries (often incidentally), but also targeted by sport fisheries. Young ones are known to snap and trash when touched or captured, but older ones are lethargic and don’t fight. Bluntnose sixgills pose no danger to humans, despite being one of only 11 shark species that regularly exceed 13 ft -4 m- in length. The catch is used for fresh, smoked, salted or dried food products, as well as liver oil, fish meal and pet food. Their population is near threatened worldwide and severely depleted regionally, e.g. in the Northeast Pacific. Since visiting divers inject between five and ten million tourist dollars into the local economy of British Columbia each year due to the shallow-water bluntnose sixgills there, in 1997 the British Columbia government made it illegal for recreational or commercial fishermen to possess even a single carcass of this species caught by any means. I hope that more governments decide to protect these sluggish “Jurassic monsters”.

Sources: here, herehere and here