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shark of the week: Collared Carpetshark — 20. November 2017

shark of the week: Collared Carpetshark

Did you know that there are sharks with chameleon traits? They are able, to a certain degree, to change their color to hide. One of them (even if it is not observable in this viedeo) is the Collared carpetshark (Parascyllium collare), namesake of its entire family.

Just like last weeks Ginger carpetshark, the Collared carpetshark lives off Australia, more precisely in depths of 180 to 420 ft – 55 to 128 m – near the floor of rocky reefs on the continental shelf off eastern Australia. It is common, but only poorly known. Its maximum length is only 2.79 ft – 85 cm- and females are oviparous.

Collared carpetsharks are not targeted, but often taken as bycatch since they reside in some areas of heavy trawling effort. Then they are mostly discarded (if not, they are later used as bait) and survive this, therefore they are considered least concern. They also benefit from areas with few fishing pressure.

Sources:herehere and here

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Shark of the week: Pacific smalltail shark — 6. November 2017

Shark of the week: Pacific smalltail shark

The Pacific smalltail shark (Carcharhinus cerdale), sister species of the Smalltail shark from last week, is an example for a misguided believe in authorities, in this case the experts Henry Bryant Bigelow and William Charles Schroeder. What happened? After this species was first described in 1898 by Charles Henry Gilbert, colleague Seth Eugene Meek and his assistant Samuel Frederick Hildebrand discovered between 1910 and 1912 several specimen on a fish market in Colón on the Caribbean side of Panama and reasoned that this species lived on both side of the Isthmus of Panama. Maybe they made an error in identification, or wrongly assumed the fish was also caught there, we will never know. Since the Panama Canal had not opened yet, neither the sharks nor the fishing vessel was able to reach the Atlantic side of Panama from the Pacific side afloat, but a fish transporter ashore sure did. Anyway, in 1948 Bigelow and Schroeder not only repeated the mistake of their colleagues, but insinuated that Meek and Hildebrand considered therefore the Pacific smalltail shark not as an own species but as synonymous with the closely related, on the Atlantic side native Smalltail shark. Only in 2011 this mistake was remedied by José I. Castro, but such important websites like the IUCN or the Florida Museum still don’t feature that fact and need an urgent update.

Carcharhinus cerdale SI.jpg
Carcharhinus cerdale by D Ross Robertsonhttp://biogeodb.stri.si.edu/sftep/en/thefishes/species/5290, Public Domain, Link

Just like its sister species,the Pacific smalltail shark is a requiem shark and viviparous. It inhabits coastal waters of the eastern Pacific from the Gulf of California to Peru.

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

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: 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