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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

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Shark oft the week: Silvertip shark — 2. October 2017

Shark oft the week: Silvertip shark

The Silvertip shark (Carcharhinus albimarginatus), despite being easily recognized due to its eponymous white margins on all fins (even pectoral fins, unlike all other “whitetip” shark species), it often confused with its smaller brother Grey reef shark. Both are requiem sharks and can be found on or near coral reefs in the tropical Indian and Pacific Oceans. They are known to form mixed-species aggregations and both perform (slightly different) ritual threat displays to chase away threats (like other sharks or divers) and protect their personal space. The mock attack of an up to 10 ft -3 m- long Silvertip shark (as described here) remind me of male gorillas. While it seems aggressive, I consider this as defensive behavior. Remarkable that they rather flee or chase away instead of attack, in which case they would, more often than not, prevail due to their size.

0979 aquaimages.jpg
Silvertip shark, Image taken by Clark Anderson/Aquaimages – Originaly uploaded to the english wikipedia, Image:0979 aquaimages.jpg, CC BY-SA 2.5, Link

Like all other requiem sharks, Silvertip sharks are viviparous. Females bear every other year litters of around five young after a gestation period of about one year. They grow slowly and mature at around 6.6 ft -2 m- with 20 years.

Silvertip sharks are caught in direct shark fisheries, as bycatch and by illegal practices (IUU) mostly for their fins and meat, but also for cartilage, liver, teeth, jaws and skin. This resulted in areas like Scott Reef off northern Australia, where they are now extinct because of Indonesian fishers. Hence the IUCN considers them as “Vulnerable“. However, there are no species-specific management measures in place.

Despite their inquisitive and aggressive behavior (see above) and being able to take on large prey (due to their dentition, size and power), Silvertip sharks are not known to attack humans. The ISAF lists only 4 provoked, none fatal attacks under this species.

Sources: here and here

Shark of the week: Bignose shark — 18. September 2017

Shark of the week: Bignose shark

There are sharks like last weeks Smoothtooth blacktip shark that are very rare. It is understandable that not much data is available on these species, and the IUCN has to decide if they thus are considered threatened or “only” data deficient.

A taxon is Data Deficient when there is inadequate information to make a direct, or indirect, assessment of its risk of extinction based on its distribution and/or population status. A taxon in this category may be well studied, and its biology well known, but appropriate data on abundance and/or distribution are lacking. Data Deficient is therefore not a category of threat. Listing of taxa in this category indicates that more information is required and acknowledges the possibility that future research will show that threatened classification is appropriate. It is important to make positive use of whatever data are available. In many cases great care should be exercised in choosing between DD and a threatened status. (Source)

Then there is this species of requiem sharks named Bignose shark (Carcharhinus altimus), well known and widespread all around the world on the continental shelf edge in tropical and warm seas, that is considered data deficient, too. How is that possible?  The IUCN suspects that “the overall volume of catch reported to ICCAT does not represent the total removals of these sharks and the data are also very limited with respect to the size-, age- and sex- composition of the catch.”

The Bignose shark is, like all requiem sharks, viviparous. During the 10-month gestation period, 3 to 15 embryos are at first feed by yolk and later via a placental connection. They may be sired by two or more males. Born at 28 to 35 in -70 to 90 cm- long, young sharks stay at first in their nursery areas at depths of around 82 ft -25 m-, but in general the Bignose shark prefers deeper water.

Bignose shark.jpg
Carcharhinus altimus by NOAA – http://www.oceanexplorer.noaa.gov/explorations/05coralbanks/logs/oct17/media/bignoseshark_600.html, Public Domain, Link

Bignose shark are highly migratory: seasonally (traveling distances between 1,000 and 2,000 mi -1,600 and 3,200 km-), but diel vertically, too. By day swimming close to the sea floor at depths of 300 to 1,410 ft -90 to 430 m-, they move at night upwards or toward the coast.

Males reach maturity at 7.2 ft -2.2 m- and females at 7.5 ft -2.3 m-. This species possibly reaches 9.8 ft -3 m- in length. Despite their size, Bignose sharks are no danger to humans. They are taken worldwide as bycatch of gillnet, bottom trawl, and deep-set pelagic longline fisheries and used for their meat and fins or to produce liver oil, shagreen (shark skin leather), and fishmeal. Due to high fishing pressure, for instance in the northwestern Atlantic (where they are considered regionally as Near Threatened), in international waters and around the Maldives, its status is of concern and data collection and precautionary adaptive collaborative management should be a priority. Bignose sharks are considered regionally as Least Concern in Australian waters, and are protected in US waters.

Sources: here and here

Shark of the week: Smoothtooth blacktip shark — 11. September 2017

Shark of the week: Smoothtooth blacktip shark

What do the Smoothtooth blacktip shark (Carcharhinus leiodon) and the Australian blacktip shark have in common? Well, obviously they are both sharks with black tips on their fins. These two species of requiem sharks look similar in other aspects as well, and have both a small range: the first is to be found only off the Arabian Peninsula and the second off Northern Australia. And they are closely related, as shown by genetic tests. How is that possible?
Other close relatives linking the ranges of both species could be the key: the graceful shark inhabiting the whole Indo-Pacific, and the worldwide in tropical waters living blacktip shark.

C. leiodon
By Moore et. al. 2012 – Moore et. al. 2012 New biological data on the rare, threatened shark Carcharhinus leiodon (Carcharhinidae) from the Persian Gulf and Arabian Sea, CC BY-SA 2.5, Link

The Smoothtooth blacktip shark is extremely rare. At first there was only one specimen that had been found in 1902 off Yemen, but in 2008 a few more have been discovered on fish markets in Kuwait. Since then they are known to be at least 3.9 ft – 1.2 m – long. Their eponymous teeth have narrow, smooth cusps, similar only to those from finetooth sharks and juvenile spinner sharks.
Before 2008 the Smoothtooth blacktip shark had been considered as vulnerable by the IUCN. That could change due to the additional finds. Or not, since its small habitat suffers heavily from high fishing pressure and habitat degradation.

Sources: here and here

Shark of the week: Speartooth shark — 4. September 2017

Shark of the week: Speartooth shark

The Speartooth shark (Glyphis glyphis), also known as Bizant river shark or Queensland river shark, is a member of the family requiem sharks. Just like the Ganges shark, this river sharks lives not only in rivers but in the sea, too. It seems that rivers in northern Australia and New Guinea are nursery grounds since newborn and juvenile sharks have been found there. They stay there several years (only shift according to tides or salinity) but up until 2015 no mature specimen had been found. Now it is known that this species mates in rivers, too, but lives also in coastal marine waters.

Speartooth shark melbourne.jpg
Speartooth shark by Bill Harrison from Wellington, New Zealand – Shark attack, CC BY 2.0, Link

Speartooth sharks are named after the shape of their teeth in the lover jaw: they seems topped with a spearhead. Females are viviparous (yolk sac develops into a placental connection) and give birth to up to 6 living young every two years. The pups are 20 to 23 in -50 to 59 cm- long and grow around 7.5 in -19 cm- per year. Mature sharks are up to 8.5 ft -2.6 m- long.

Despite their size, Speartooth sharks pose no threat to humans. They are extremely rare, but are caught incidentally by commercial, artisanal and recreational fishers as bycatch and suffer additionally from habitat degradation. Consequently, they are considered Endangered by the IUCN. Even as Critically Endangered on the 1999 Commonwealth Environment Protection and Biodiversity Conservation Act (but since Commonwealth protection does not apply until a distance of three nautical miles from the coast, which is likely outside the range of this shark, it is of no use). Sadly, there are no regulations restricting the capture of this species in Papua New Guinea, and in Australia a management plan has yet to be enacted. In two Australian National Parks they are protected somewhat from habitat alteration, if not fishing.

Sources: here, here and here

Shark of the week: Grey reef shark — 28. August 2017

Shark of the week: Grey reef shark

The Grey reef shark (Carcharhinus amblyrhynchos) is a common species of requiem sharks living in the Indo-Pacific and the Red Sea. It is a homebody (but not territorial toward other Grey reef sharks) and stays mostly in shallow waters near the drop-off of its coral reef. It has the typical “reef shark” shape, with a broad, round snout and large eyes. Its body is grey above and white below with dusky to black tips on its fins, but will eventually darken in color due to tanning in the shallow water. It is social and forms groups with other Grey reef sharks.

Photo of long-finned shark, swimming
Grey reef shark by CRED/PIFSC, NOAA – http://noaacred.blogspot.com/2009/03/one-fish-two-fish-fish-team-at-wake.html, Public Domain, Link

Female Grey reef sharks are viviparous (embryos feed first from yolk and later via a placental connection) and give birth to one to six living young every other year. Those are 18 to 24 in -45 to 60 cm- long and reach maturity after around 7 years of age (11 years at the Great Barrier reef) when they are 4.3 to 4.9 ft -1.3 to 1.5 m- long (males) or 3.9 to 4.6 ft -1.2 to 1.4 m- long (females). They live up to 25 years.

Usually reaching 6.2 ft -1.9m- in length (although 8.5 ft -2.6m- long specimen have been found), the Grey reef shark is not the biggest shark at the reef. But it is famous for its thread display, which is meant to discourage even bigger enemies or competition from coming near to prevent unnecessary fights and thus possible injuries.

Drawings showing threatening and nonthreatening postures from front and side underlain with a line that is jagged and red on the left and gently curving and blue on the right
By Chris Huh (English Wikipedia user) – English Wikipedia, CC BY-SA 3.0, Link

It works for instance with hammerheads and moray eels and should work with divers, too. But since such a form of interspecies communication is difficult for us humans (we know what a growling and snarling dog means, but we don’t speak Shark), and the Grey reef shark is willing to walk the talk, if needed, there are several reports of shark attacks by Grey reef sharks (thankfully all non-fatal since they are capable of inflicting significant damage, for instance to submersibles). The IUCN considers the Grey reef shark as Near threatened due to low reproduktion rate, site fidelity and habitat degradation, but also high fishing pressure (for meat and fins). At the Great Barrier reef their population has declined by 97 percent in fishing and equally in non-fishing zones, compared to no-entry zones, indicating that poaching is a big problem.

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