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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

Shark of the week: Daggernose shark — 14. August 2017

Shark of the week: Daggernose shark

The genus Isogomphodon of requiem sharks only consists of one living species, but it seems that the Daggernose shark (Isogomphodon oxyrhynchus), known since 1839, soon joins the fate of its extinct sister species Isogomphodon acuarius.

Systematische Beschreibung der Plagiostomen (Plate 15) BHL6353140.jpg
Von Henle, Jacob; Müller, Johannes – http://www.biodiversitylibrary.org/pageimage/6353140, Gemeinfrei, Link

Female Daggernose sharks are viviparous (placental connection after the yolk sac of the embryo is depleted) and give birth to 2 to 8 living young every two years in nursery areas. Newborn sharks measure 15 to 17 in -38 to 43 cm- in length. Males mature at an age of 5–6 years and females with 6–7 years and reach up to 12 years and 4.6 ft -1.4 m- (males) and 20 years and 5.2 ft -1.6 m- (females).

The Daggernose shark lives solely in coastal waters (no rivers since it is intolerant of fresh water) of the Western Atlantic off Trinidad, Guyana, Suriname, French Guiana and northern Brazil. It is highly specialised (tiny eyes but elongated snout to properly use electroreception, a lot of pointed slender teeth -the most teeth of all requiem sharks- to form a fish trap) to hunt small schooling fish in muddy waters. Since humans are after the same fish, the shark gets the short end of the stick – caught as bycatch in gillnets (but also by fishery targeting sharks), it has been considered critically endangered by the IUCN since 2006 as a result. Between 1992 and 2002 it suffered a very large population decline (shown by more that 90% less catches), and moreover, mostly juvenile specimens have been found in the nets. And things may have gotten worse since then. There are simply no new data available regarding daggernose sharks, not for lack of trying but specimen, which means they are (despite being legally protected in Brazil since 2006, which is without enforcement not of much use) in reproductive collapse and on the brick of extinction.

Sources: here, here and here

shark of the week: Whitenose shark — 7. August 2017

shark of the week: Whitenose shark

We know that mutation can be, more often than not, a reason to not function normally and die, but sometimes it can be a way to develop new, successful species. To see evolution at work we only have to look at the Whitenose shark (Nasolamia velox).

See, the Whitenose shark differs from any other requiem sharks in matters of cranial and rostral characteristics so much that is has got its own genus, but all other characteristics are so much like that of the Blacknose shark that scientists began to speculate.

There is this deformation called cyclopia (named after the Greek myth cyclops which is a giant with an eye on its forehead), caused by genetic mutations (or toxins during pregnancy) in mammals and humans (Don’t look at pictures. Seriously, don’t. Look here instead.). Well, incipient cyclopia could explain the differences – so it seems that the Whitenose shark developed from the Blacknose shark, or better ancestors of the Whitenose shark mutated from ancestors of the Blacknose shark, a long time ago. This must have happened before the Isthmus of Panama separated the Pacific and Atlantic oceans, since now the Blacknose shark inhabits only the tropical western Atlantic and the Whitenose shark only the tropical eastern Pacific (so it is an example of Plate tectonics as well).

Nasolamia3.jpg
Nasolamia velox by Mexfish.com – Mexfish.com, CC-BY-SA 4.0, Link

Like other requiem sharks, Whitenose sharks are caught by commercial and artisanal fisheries for their meat and fins.

Sources: here, here and here

shark of the week: Blacknose shark — 31. July 2017

shark of the week: Blacknose shark

The Blacknose shark (Carcharhinus acronotus) is a little (up to 4.3 ft -1.3 m- long) requiem shark. Its name refers to a black spot on the nose of juvenile sharks.

Carcharhinus acronotus noaa.jpg
Carcharhinus acronotus by NOAA – http://www.noaanews.noaa.gov/stories2009/20090724_sharks.html, Public Domain, Link

Blacknose sharks live in tropical or warm-temperate coastal waters of the western Atlantic from the southern USA, through the Gulf of Mexico and the Caribbean all the way to southern Brazil. There is segregation by size and sex: juveniles are typically found in shallow water while adults are located at greater depths. They are known to form large schools.

Blacknose sharks reach a maximum age of 10-16 years for females and 4.5-9 years for males. Both mature at 2 years and are relatively fast growing. Reproduction is viviparous with a yolk-sac placenta. After a 8-11 month gestation period, 3-6 15 to 20 in -38 to 50 cm- long pups are born annually (Gulf of Mexico) or biennially (Northwestern Atlantic) in shallow nursery areas like coastal bays or mangrove swamps, for instance Bulls Bay, South Carolina (U.S.).

Blacknose sharks are fished commercially as target and bycatch, but also as game-fish – as they are deemed decent fighters. This species is considered Near Threatened globally by the IUCN, and benefits from conservation measures (together with other small coastal sharks) in US waters. Although affected by high fishing pressure, Blacknose sharks seem to be in no danger in Brazil (since there are enough mature sharks to be found) but there aren’t sufficient data yet. For the Caribbean, too.

Sources: here, here and here

Shark of the week: Borneo Shark — 24. July 2017

Shark of the week: Borneo Shark

One shark species considered endangered by the IUCN was possibly nearly extinct, since it had not been recorded since 1937 and only five confirmed specimens were known. But in 2004, several specimens of the Borneo Shark (Carcharhinus borneensis) has been found again by scientists in fish markets (just like the Indonesian wobbegong), making images never seen before possible, like color images (of fresh specimens instead of preserved) or of its enlarged denticles.

Denticles c borneensis.png
Denticles borneo shark – By William Toby White i Pieter Last – White, W.T.; Last, P.R.; Lim, A.P.K.. Rediscovery of the rare and endangered Borneo Shark Carcharhinus borneensis (Bleeker, 1858) (Carcharhiniformes: Carcharhinidae), 2010. CSIRO Marine and Atmospheric Research, CC BY 3.0, Link

The Borneo shark has a slender body with a long, pointed snout, rather large, round eyes with nictitating membranes and oblique, slit-like nostrils. Unique is a row of enlarged pores above the corners of its mouth. Like most requiem sharks it is viviparous with up to 6 embryos, provisioned by the mother through a placental connection formed from the depleted yolk sac and born alive at close to 9.4 to 11.0 in -24 to 28 cm- long. Maturing at around 22 to 23 in -55 to 58 cm- in males and 24 to 26 in -61 to 65 cm- in females, the Borneo shark is a rather small shark (the largest known specimen measures 26 in -65 cm- long).

Formerly, the Borneo shark lived in shallow coastal waters of the Indo-West Pacific off Borneo, China and possibly Java. Now it lives maybe solely around the mouth of the Mukah River in Malaysia, where, due to high fishing pressure, it is in grave danger.

Sources: here, here and here

Shark of the week: Carcharhinus tjutjot — 17. July 2017

Shark of the week: Carcharhinus tjutjot

The Carcharhinus tjutjot (sometimes also known as Indonesian whaler shark) is a little known requiem shark. There is no picture of this species to be found, but since it had been confused with juvenile Whitecheek sharks up to 2012, it has to look a lot like them.

Named after one of the Indonesian words for shark, the Carcharhinus tjutjot lives in shallow coastal waters (up to 328 ft -100 m- deep) of the South China Sea off Indonesia, Taiwan and Borneo. This litte shark species (up to 36 in – 93 cm – long) is, like all requiem sharks, ovoviviparous: there have been found 2 late-term embryos measuring 11 in -27.9 cm- length inside one pregnant female.

There is no IUCN Red List Status available, yet, but, since the similar Whitecheek shark faces extinction in its Asian habitats, the risk for the Carcharhinus tjutjot has to be equally high, especially due to the fact that it has got no managed Australian sub-species like its relative.

Sources: here and here

Shark of the week: Finetooth shark — 10. July 2017

Shark of the week: Finetooth shark

Another shark with a name leaving no doubt about the reason is the Finetooth shark (Carcharhinus isodon). Named after its nearly even, rather slender teeth in upper and lower jaw (isodon means „even tooth“), its name is also Eventooth Shark or Smoothtooth Shark, in German Feinzahnhai, in French Requin à Petites Dents and in Spanish Galana Dientefino or Tiburón Dentiliso (however, I don’t know why it is also named Night Shark or Tiburón de Noche).

Carcharhinus isodon in net.JPG
Von National Observer Program, NMFS/SEFSC – http://www.st.nmfs.noaa.gov/observer-home/about-nop/activities/index, Gemeinfrei, Link

The Finetooth shark inhabits extremely shallow coastal waters (no deeper than 33 ft -10 m- in the summer and 66 ft -20 m- deep in the winter) in the Northwest Atlantic, Gulf of Mexico and Southwest Atlantic (off Brazil) in distinct populations. In the past it was known to venture into rivers, though most of its paths are nowadays blocked by dams. That’s a problem, since the females move to special inshore nursery areas. There they give birth to 2 to 6 living, 19 to 25 in -48 to 64 cm- long young every 2 years after a 12 month gestation period. Like all species of the family requiem sharks, finetooth sharks are oviviparous, that means the embryo hatches in the womb and is born alive. Additionally, the depleted yolk sac develops into a placental connection to the mother after the embryos drain their yolk supply during their first 15 weeks (viviparous).

Although having a maximum length of 6.2 ft -1.9 m- (on average males reach only 5.2 ft -1.6 m- in length and females 5.4 ft -1.7 m-), the finetooth shark is no danger to humans. However, it thrashes and snaps at anything within reach when caught. It matures at 40 in -1 m- (at 4-5 years as males and 5-6 years as females, who are growing slower in general) and has a life span of 9 years – males- or 14 years – females.

Off the coast of North America the finetooth shark is known to migrate south seasonally (namely when surface water temperatures drop below 68°F -20°C-). It hunts in large schools for small fish. It is susceptible to habitat degradation (especially it nursery areas) and overfishing (as bycatch and targeted), but is considered (despite its low reproductive rate) as Least Concern – at least in the northern hemisphere, off Brazil however there is only a badly managed fishery which already pushed other shark species near extinction.

Sources: here, here and here

 

shark of the week: Whitecheek shark — 3. July 2017

shark of the week: Whitecheek shark

If you watch the video the name of the Whitecheek shark (Carcharhinus dussumieri) seems fitting (I don’t know about the reason for its other name: Widemouth Blackspot shark).

Whitecheek sharks are, like all other requiem sharks, ovoviviparous with on average two about 15 in -38 cm- long pups annually. Both males and females mature when they are about 28 in -70 cm- long and reach a maximal length of 39 in -100 cm. Only in 2012 it was found out that sharks of the species Carcharhinus tjutjot are no juvenile Whitecheek sharks (as believed since 1982) but a different species.

Living in coastal waters of the Indo-Pacific Ocean, the Whitecheek shark is usually caught as bycatch rather than as the target species, but is nevertheless facing extinction in several Asian regions. But a relatively large and fairly robust population in northern Australia, where it comprises a stable 2-3% of trawl catch (by biomass), saves this species – thus only considered Near Threatened by the IUCN.

Sources: here and here

 

Shark of the week: Whitetip Reef shark — 26. June 2017

Shark of the week: Whitetip Reef shark

Another case of mysterious naming is the Whitetip Reef shark (Triaenodon obesus). Not the English name, which is quite apt due to its white tips on dorsal and caudal fins and its exclusive habitat, but the Latin one is untrue: this slender shark is far from obese. On the contrary, as nocturnal hunter it can detect its prey by electroreception (using its ampullae of Lorenzini) and smell (with unique tubular nasal flaps) and follows it into their resting crevices (well adapted to this hunting practice due to its tough skin, sleek build, blunt snout and ridges to protect its eyes), and some sharks “actually squirm into a hole in one side of a coral head and exit through an opening on the other”.

Three gray sharks lying beside each other on the sea bottom.
By Dorothy from USA – sharks, CC BY 2.0, Link

The Whitetip Reef shark is gregarious (sometimes even hunts in groups) and can be seen resting in groups on the bottom or in caves during daytime. It doesn’t need to swim to breathe, unlike other requiem sharks. Not to be confused with the other Whitetip requiem shark (the Oceanic Whitetip), the smaller Whitetip Reef shark (up to 5.6 ft -1.7 m- long) isn’t dangerous to humans. Sadly, as opportunistic feeder it learnt to associate the sounds of boats and spearfishing with food – the curious shark can become bold and agitated and sometimes bites while trying to steal the fish.

Like all requiem sharks, the Whitetip Reef shark is ovoviviparous: every two years 2 to 3 living young are born at a length of 20 to 24 in -52 to 60 cm. There is a case of Parthenogenesis (asexually reproduction) in Whitetip reef sharks. They grow slowly, mature at about 3.4 feet -1.05 m- and live up to 25 years.

Whitetip Reef sharks live in coral reefs all around the world. They are homebodies and famous for their site fidelity. That means that dangers to their coral reef due to climate change, overheating and pollution have a deep impact on the shark population, too, in addition to commercial and recreational fisheries. They are considered as Near Threatened by the IUCN. Conservation measures like marine protected areas (MPA) seems to help, but only if they are completely no-entry. On the Great Barrier Reef, populations of Whitetip Reef sharks in fishing zones have been reduced by 80% relative to no-entry zones. However, populations in no-take zones, where boats are allowed but fishing prohibited, exhibit levels of depletion comparable to fishing zones, most likely due to poaching (IUU). Demographic models indicate that these depleted populations will continue to decline by 6.6–8.3% per year without additional conservation measures.

Sources: here, here and here

shark of the week: graceful shark — 19. June 2017

shark of the week: graceful shark

The reason for shark names are sometimes mysterious. On example is the graceful shark (Carcharhinus amblyrhynchoides) that has got a similar name in several languages. But, living in the tropical Indo-Pacific from the Gulf of Aden to northern Australia in coastal waters, this rare species of requiem sharks is not graceful but rather tubby.

Carcharhinus amblyrhynchoides phuket.JPG
Von Tassapon KRAJANGDARA – http://www.fishbase.us/photos/thumbnailssummary.php?ID=860#, CC BY 3.0, Link

With a size of up to 5.6 ft -1.7 m- the graceful sharks is potentially dangerous, but has not been implicated in any shark attacks. It is viviparous, and females bear litters of up to 9 living young (with a mean of three) after a 9- to 10-month gestation period every year. Once the developing embryos exhaust their supply of yolk, the depleted yolk sac is converted into a placental connection to the mother.

The graceful shark is regularly caught as bycatch in commercial fisheries and used for its meat, liver oil and fins. In northern Australia it made up 1.5% of the shark catch in gillnets and 0.2% on longlines. Nevertheless, it is only considered near threatened by the IUCN.

Sources: here and here