Angel Shark – Shark of the Month

Angel Shark Project Carlos Suarez, Oceanos de Fuego

Photo credit: Carlos Suarez, Oceanos de Fuego

The angel shark (Squatina squatina) is one of the more unique sharks with its exceptionally flat body and large pectoral fins. It is certainly one of the many favorite sharks that we discuss during our Shark Guardian presentations. And everyone loves a shark that is shaped like an angel, which is how it got its name. Incredibly there are 23 different species of angel sharks!

It seems to resemble more of a ray, than a shark. Its skin is usually  grey to reddish or greenish-brown, scattered with small white spots and blackish dots. Young angel sharks may also have white net-like markings and large, dark blotches, and become more plain as they grow older.  They are masters of disguise and bury themselves in sand or mud waiting for prey, which includes fish, crustaceans, and various types of mollusks.  Don’t be fooled by their grace, their pretty face quickly transforms with its extensible jaws that rapidly snap upward to capture prey with their long, needle-like teeth!

Angel sharks possess simple, whisker-like projections near the nostrils (nasal barbels), which are used to taste and feel and they have large, round eyes with vertical slit pupils. It has no anal fin, and unusually for sharks, the lower lobe of the caudal fin is longer than the upper lobe. Most types grow to a length of 1.5 m (5 ft), with the Japanese angel shark, Squatina japonica, known to reach 2 m.

This video below shows how the angel shark attempts to prey on a horn shark. However, the horn shark got its name for having a special secret weapon against larger predators like the angel shark. A must watch!

 

Angel Shark Distribution & Habitat

Angel Shark found on en.wikipedia.org

Image source: en.wikipedia.org

Angel sharks occur worldwide in temperate and tropical seas. Most species inhabit shallow temperate or tropical seas, but one species inhabits deeper water, down to 1,300 m (4,300 ft).   They are nocturnal and can be found swimming around up off the bottom at night. In the daytime it lies buried in the mud or the sandy bottom, with little more than its eyes protruding.  In the northern parts of its range the Angel Shark is seasonally migratory, and makes northwards journeys during the summer.

 

Angel Shark Population

Image source: elasmodiver.com

Angel sharks were once a common and important bottom dwelling predator over large areas of coastal and sediment habitat in the Northeast Atlantic, Mediterranean and Black Seas. Due to aggressive fishing methods suck as trawling, set nets and longlines, Angel Sharks are highly vulnerable from birth onwards and population has decreased dramatically during the past 50 years. This has resulted in the sad fact that Angel sharks have been declared extinct in the North Sea and they are nearly non-existent in large areas of the northern Mediterranean. Angel sharks are now extremely uncommon throughout most of the remainder of its range, with the possible exception of some areas of the southern Mediterranean and Canary Islands where its status needs to be confirmed and conservation measures introduced as a matter of urgency. Rate of population increase, longevity and mortality are unknown.

bycatch

For more information about the bycatch issue, check out this great infographic here.

Angel Shark Reproduction

Felipe Ravina Olivares-felip

Photo credit: Angel Shark Project

Angel Sharks are ovoviviparous, with both ovaries functional. It has moderate-sized litters of 7 to 25 young, which vary according to the size of the female. Records of size at birth are 24 to 30 cm. Gestation period is 8 to 10 months, born in December to February in the Mediterranean but later in northern parts of its range (July in England). Age at maturity, reproductive age and periodicity are unknown. Females reach maturity at 128 to 169 cm, and males at 80 to 132 cm, with maximum sizes of 183 cm and possibly up to 244 cm, with estimates of less than 240 cm in the Mediterranean Sea.

Conservation Status

Angel Shark by Carlos Suarez, Oceanos de Fuego

Photo Credit: Carlos Suarez

Historic data shows a dramatic decline in angelsharks from tuna traps operating in the Northern Tyrrhenian Sea with catches of the genus Squatina reported at an average of 134 specimens from 1898 to 1905, down to 15 between 1914 and 1922. This early decline probably marks the beginning of trawling activity in the area, to which Angel Sharks are highly susceptible. A low rate of exchange between Squatina populations may makes them especially prone to local depletion and means that recolonisation will be extremely low.  Habitation degradation from humans and tourism are also possible threats.

By 1985, the annual take of angel shark on the central California coast had increased to more than 454 metric tons or an estimated 90,000 sharks. The population declined dramatically and is now regulated. In April 2008, the UK government afforded the angel shark full protection under the Wildlife and Countryside Act. Once considered abundant in the Atlantic Ocean, the angel was classified as “critically endangered” in 2010.

There is an urgent need to confirm the status of this species in the southern Mediterranean, Canary Islands and other areas where populations may still persist. This is why projects like The Angel Shark Project are so important. This project aims to provide vital information on the ecology and distribution of this shark in the Canary Islands, that may be used to implement rapid conservation measures.

angel-shark-concern

Written by Rose Nomura (Shark Guardian Volunteer)

Sources:

http://en.wikipedia.org/wiki/Angel_shark

http://www.iucnredlist.org/details/39332/0

http://www.arkive.org/angel-shark/squatina-squatina/image-G124005.html

Whale Shark – Shark of the month

Slide6

The whale shark (Rhincodon typus) is a slow-moving filter feeding shark and the largest known shark species. The largest confirmed individual had a length of 12.65 m (41.50 ft) and a weight of more than 21.5 metric tons. Unconfirmed reports of considerably larger whale sharks exist. Claims of individuals over 14 m long and weighing at least 30 metric tons are not uncommon. The whale shark holds many records for sheer size in the animal kingdom, rivalling many of the largest dinosaurs in weight. There is only 1 species of whale shark that originated about 60 million years ago.

Whale Shark Distribution and Habitat

800px-Cypron-Range_Rhincodon_typus.svgThe whale shark inhabits all tropical and warm-temperate seas as a pelagic shark. Although typically seen offshore, it has been found closer to land, entering lagoons or coral atolls, and near the mouths of estuaries and rivers. Its range is generally restricted to about 30° latitude. It is capable of diving to depths of at least 1,286 m and is migratory. On 7 February 2012, a large whale shark was found floating 150 kilometers off the coast of Karachi, Pakistan. The length of the specimen was said to be between 11 and 12 m, with a weight of around 15,000 kg.

6a00d8341bf67c53ef01538eb766fe970b-800wiIn 2011, more than 400 whale sharks gathered off the Yucatan Coast. It was one of the largest gatherings of whale sharks recorded.


Whale shark description

Whale sharks have a mouth that can be 1.5 m wide, containing 300 to 350 rows of tiny teeth and 10 filter pads which it uses to filter feed. Whale sharks have five large pairs of gills. The head is wide and flat with two small eyes at the front. Whale sharks are grey with a white belly. Their skin is marked with pale yellow spots and stripes which are unique to each individual. The whale shark has three prominent ridges along its sides. Its skin can be up to 10 cm thick. The shark has a pair of dorsal fins and pectoral fins. Juveniles’ tails have a larger upper fin than lower fin, while the adult tail becomes semilunate. The whale shark’s spiracles are just behind its eyes.

Whale sharks stripes and spots

whaleshark_example_rightThe whale sharks patterns are unique to each animal much like a humans finger prints are each unique. Just like  we use fingerprints for identification, whale shark stripes and spots are also used for identifying individual whale sharks for research purposes. Shark Guardian will soon be launching a campaign to encourage people to submit whale shark photos for ID and conservation purposes. Watch this space!

Whale shark diet

Whale shark teethThe whale shark is a filter feeder – one of only three known filter feeding shark species along with the basking shark and the megamouth shark. It feeds on plankton, krill and small squid or vertebrates. It also feeds on small fish and the clouds of eggs and sperm during mass spawning of fish. The many rows of vestigial teeth play no role in feeding. Feeding occurs either by ram filtration, in which the animal opens its mouth and swims forward, pushing water and food into the mouth, or by active suction feeding, in which the animal opens and closes its mouth, sucking in volumes of water that are then expelled through the gills.

The whale shark is an active feeder, targeting concentrations of plankton or fish. It is able to ram filter feed or can gulp in a stationary position. This is in contrast to the passive feeding basking shark, which does not pump water. Instead, it swims to force water across its gills.

Whale shark reproduction

Tag and release of baby whale shark in the PhilippinesNeither mating nor pupping of whale sharks has been observed.

The capture of a female in July 1996 that was pregnant with 300 pups indicated whale sharks are ovoviviparous. The eggs remain in the body and the females give birth to live young which are 40 to 60 cm long. Evidence indicates the pups are not all born at once, but rather the female retains sperm from one mating and produces a steady stream of pups over a prolonged period. They reach sexual maturity at around 30 years and their lifespan is an estimated 70 to 100 years.

On 7 March 2009, marine scientists in the Philippines discovered what is believed to be the smallest living specimen of the whale shark. The young shark, measuring only 38 cm (15 in), was found with its tail tied to a stake at a beach in Pilar, Philippines, and was released into the wild. Based on this discovery, some scientists no longer believe this area is just a feeding ground; this site may be a birthing ground, as well. Both young whale sharks and pregnant females have been seen in the waters of Saint Helena in the South Atlantic Ocean, where numerous whale sharks can be spotted during the summer.

Whale shark and human interaction

As with all marine animals, a strict code of conduct should be practiced to avoid injuring the animal or yourself. Unfortunately many people have witnessed snorkelers and sometimes divers chasing or making contact with whale sharks. This usually frightens the whale shark and will cause the animal to flee and possible not to return. This could change their migration routes in the future if they feel constantly harassed and threatened.

Good code of conduct

Whale shark conservation status

Slide3The whale shark is targeted by commercial fisheries in several areas where they seasonally aggregate. The population is unknown and the species is considered vulnerable by the IUCN. In 1998, the Philippines banned all fishing, selling, importing, and exporting of whale sharks for commercial purposes, followed by India in May 2001 and Taiwan in May 2007.

Bull Shark – Shark of the month

The Bull Shark, also known as the Zambezi shark, is a large predator that is found in warm coastal waters through out the world. Unlike most shark species, bull sharks have the ability to swim and survive in fresh water and can be found thousands of miles up rivers.

Bull Shark Anatomy and Appearance

Bull SharkBull Sharks are not very long sharks, but they tend to be very thick and stocky. The average size of an adult bull shark is about 2.3 meters in length and 130 kg in weight, but specimens have been found up to 3.5 meters long and weighing in at over 300 kg. Like most sharks, they have counter shading meaning that they are gray on top and white on the bottom.

A key feature that separate bull sharks apart from most shark species is their rectal gland. The rectal gland is used for storing and excreting salt from the body, which allows them to control their salinity levels even when they are in freshwater environments.

Bull Shark Feeding

Due to their ability to feed in both freshwater and salt-water environments, bull sharks feed on a vast array of prey species. They feed on numerous bony fish, stingrays, other sharks, birds, turtles, crustaceans, and some mammals. They tend to hunt in murky waters because it allows them to sneak up on their prey. Pound for pound, bull sharks have the strongest bite force compared to all shark species.

Bull Shark Range and Distribution

800px-Carcharhinus_leucus_distmap

Bull sharks are generally found in warm tropical waters with temperatures around 32 degrees Celsius. They are found along the coasts of southern North America, Central America, South America, Africa, the Indian Ocean, as well as the Indo-Pacific near Southeast Asia. Specimens have been reported being seen nearly 2,000 miles up the Amazon and Mississippi rivers as well as being found in golf course lakes after floods.

Bull Shark Reproduction

Bull sharks are viviparous, meaning that they give birth to free-swimming young. They have a gestation period of about one year and can give birth to one to twelve young per birthing. When they are born, adolescent bull sharks are around 65 to 75 centimeters long. They reach sexual maturity after around fifteen years and are about 270 centimeters long at this age.

Conservation Status of Bull Sharks

Slide4Bull sharks are listed by the International Union for Conservation of Nature as Near Threatened. Bull sharks have long gestation periods, reach sexual maturity at a late age, and give birth to few young making them vulnerable to intense fishing and hunting pressures. They are harvested for their fins and meat, which are used to make the prized Chinese dish shark fin soup. Due to their role as apex predators, bull sharks are very important to the environments they inhabit and are a necessity for a healthy reef and ecosystem.

Thanks very much to Brian Stelmar who completed this as part of his Divemaster training program at Hidden Depths on Koh Lanta in Thailand. Hidden Depths is a Shark Guardian Dive Center

Shark of the month: Blue Shark

Blue Shark

The Blue Shark

With our Shark Guardian tour of UK schools underway we wanted to feature a shark in this months ‘Shark Guardian – Shark of the Month’ on the Blue Shark. A shark that is found in and around the waters of the UK and Europe. The Blue shark is a species of requiem  shark (family Carcharhinidae) that inhabits deep waters in the world’s temperate and tropical oceans. Preferring cooler waters,blue sharks migrate long distances, such as from New England to South America.

Blue sharks can move very quickly. They are viviparous and have been noted for delivering litters of 25 to over 100 pups. They feed primarily on small fish and squid, although they can take larger prey. Blue sharks often school segregated by sex and size, and this behavior has led to their nickname “wolves of the sea”. Maximum lifespan is still unknown, but it is believed that they can live up to 20 years.

Blue shark Anatomy and Appearance

Blue SharkBlue sharks are light-bodied with long pectoral fins. Like many other sharks, blue sharks are counter-shaded, meaning the top of the body is deep blue, lighter on the sides, and the underside is white. The male blue shark commonly grows to 1.82 to 2.82 meters at maturity. Female Blue sharks commonly grow to 2.2 to 3.3 meters at maturity. Large specimens can grow to 3.8 meters long. The Blue Shark is fairly slender in build.

Range and habitat of the Blue Shark

Blue shark Habitat and range

The blue shark is an oceanic pelagic shark found worldwide in deep temperate and tropical waters from the surface to about 350 meters. In temperate seas it may approach shore where it can be observed by divers, while in tropical waters it inhabits greater depths. It lives as far north as Norway and as far south as Chile. Blue sharks are found off the coasts of every continent, except Antarctica.

Feeding

Squid are important prey for blue sharks, but their diet includes other invertebrates such as cuttlefish and pelagic octopus. Whale and porpoise blubber and meat have been retrieved from the stomachs of captured specimens. Blue sharks have been observed and documented working together as a “pack” to herd prey into a concentrated group from which they can easily feed. It is interesting to note that the observed herding behavior was undisturbed by different species of shark in the vicinity that normally would pursue the common prey. The blue shark can swim at fast speeds, allowing it to catch up to prey easily. Triangular teeth allows the Blue shark to easily catch slippery prey.

Conservation and importance

Blue Shark Conservation StatusThe IUCN has classified the Blue Shark as NT (Near Threatened). The Blue Shark is taken in large numbers (an estimated 20 million individuals annually), mainly as bycatch, but there are no population estimates and many catches are unreported. The few fishery assessments carried out suggest relatively little population decline. There is concern over the removal of such large numbers of this likely keystone predator from the oceanic ecosystem.

Shark of the month: Dumb Gulper Shark

Dumb Gulper Shark

 Dumb Gulper Shark  (Centrophorus harrissoni)

The dumb gulper shark is a rare, deep water dogfish found between the east coast of Australia and the north west of New Zealand.  Among other names, it is also known, amusingly, as the dumb shark!

Anatomy and Appearance

These greyish-brown sharks have a slender body and can grow up to 110cm.  They have a long head with a flattened snout, a big mouth and big eyes to help them see at greater depths.

The broad teeth of this species differ between the upper and lower jaws, with the lower teeth being much larger. The teeth also differ between the male and female, with the male having much more erect, upright upper teeth, and upward-curving tips on the lowers.

Feeding

The dumb gulper shark eats most fish, but its most common prey is lanternfish as they are the most abundantly found fish at the depth that the shark lives at.  It also enjoys crustaceans and cephalopods.

Habitat

This shark is found off the east coast of Australia (New South Wales, Victoria, Queensland and Tasmania), and off New Zealand.  It lives in the water column immediately above the seabed on the upper to middle continental slope, usually in depths of 800-1200m.

Reproduction

Females only produce one or two pups every one or two years.

They can live up to 46 years, on average.

Conservation Status

Slide1The dumb gulper shark is listed as critically endangered according to the International Union for Conservation of Nature’s Red List of Threatened Species.  Action is being taken to preserve the shark by the Australian Government, which includes being incorporated into the Environmental Protection and Biodiversity Conservation (EPBC) Act to create a plan to keep this species safe.

 

 

Thank you to Gary Eldridge from Hidden Depths Diving, Koh Lanta, Thailand for this information.

Shark Of The Month: Tiger Shark (Galeocerdo cuvier)

A large macro-predator, the tiger shark derives its name from the dark stripes down its body which resemble a tiger’s pattern. These dark stripes will fade as the shark matures.

Tiger sharks have, over the past 50 years, received a lot of bad publicity due to their perceived aggressive behaviour towards humans. They are second on the list of number of recorded attacks on humans (behind the great white shark). However this is still a very low rate according to researchers. Tiger sharks often frequent shallow coastal waters, including river estuaries, canals and harbours, and are repeatedly found in water almost too shallow for their size. Their preference for this habitat coincides with our penchant for the seaside!

Tiger Shark Anatomy and Appearance

Adult tiger sharks normally grow to a length of 3 – 5m, and a weight of 400 – 650kgs. Even so larger individuals have been recorded. As seems to be the norm with sharks, the females grow larger than the males. It is perfectly camouflaged as, seen from above it is a blue or light green color, and from below it is whitish yellow.

Tiger Shark

It has a wedge-shaped head and this, along with its high back and dorsal fin (located unusually close to its tail). This makes it easy for the shark to turn to the side quickly. Its robust, serrated teeth are specialized to slice through flesh, bone, and other tough substances such as turtle shells.

Tiger Shark Feeding Habits

Tiger Shark Teeth

Tiger Shark Teeth

As an apex predator, it seems the tiger shark will eat just about anything. Fish, crustaceans, sea birds, sea snakes, marine mammals (e.g. dolphins, dugongs, seals and sea lions), turtles, and even other sharks and rays are regularly on the menu. Around the Hawaiian Islands, horses, dogs, sheep, goats, cats and rats are regularly found in their stomachs, mainly due to its scavenging nature. Also scavenged are badly injured or dead whales, and these are found using its excellent sense of smell and great eyesight.

Bizarrely, also found in their stomach are inedible objects such as car number plates, baseballs, oil cans and tyres!

Tiger Shark Habitat

The tiger shark is often found close to the coast mainly in tropical and subtropical waters throughout the world. It is a nomadic shark, guided by warm currents, although preferring to stay nearer the equator in colder months. As previously mentioned, they are regular visitors to the shallows, however they normally live in deep waters that line the reefs, usually around 350m.

Tiger Shark Habitats

Tiger Shark Reproduction

Females mate once every 3 years by internal fertilization. Tiger sharks are ovoviviparous, meaning the eggs hatch inside the female and after a period of 13-16 months young are born live when fully developed. A female can have between 10 to 80 pups, of 51-76cm in length. Tiger sharks reach maturity when they are 2-3m long, and whilst it is not known how long they live for, it is certainly more than 12 years.

Conservation Status of Tiger Sharks

status_nt_on

Tiger sharks are considered a near threatened species due to excessive finning and fishing by humans according to International Union for Conservation of Nature. While shark fin has very few nutrients, shark liver has a high concentration of vitamin A which is used in the production of vitamin oils. In addition, the tiger shark is captured and killed for its distinct skin, as well as by big game fishers. The Tiger Shark is listed as NEAR THREATENED by the IUCN. In 2010, Greenpeace International added the tiger shark to its seafood red list.

Thanks very much to Gary from Hidden Depths Diving on Koh Lanta for this content.

Shark of the month: Swell Shark

The Swell shark, also known as Inflatable or balloon sharks, Cephaloscyllium ventriosum, is a Catshark of the family Scyliorhinidae. A new species of swell shark was only recently discovered in the Philippines in 2011 and locals named it ‘Bubble shark’. When we do our Shark Guardian presentations the children love this name and love the shark as it looks very similar to a leopard shark or juvenile zebra shark.

Swell shark: amazing shark species

Swell shark: amazing shark species

Distribution and Habitat

Swell sharks prefer rocky, algae covered areas

Swell sharks prefer rocky, algae covered areas

Swell sharks are found in subtropical waters close to continental shelves in the eastern Pacific, from central California to the Gulf of California and south to southern Mexico and (possibly) central Chile. This shark prefers rocky, algae-covered areas and are most commonly found at 5 -17m but there are reports of sightings in deeper waters up to 457m deep.

Swell sharks rest during the day in caves and rocky crevices, where they are camouflaged with their surroundings, and then venture out to hunt at night. They feed on small bony fish, mollusks and crustaceans. They actively suck in small fishes or by capturing other prey items by remaining motionless on the bottom with its mouth wide open, waiting for prey to wander in or to be swept in by the currents.

Anatomy and Appearance of the Swell Shark

Swell sharks can grow to about 100 centimeters (39 in) in length and they get their name from their ability to expand their body to about double its regular size. This helps protect it from predators such as seals and larger sharks which may pull it out from rocky reefs, under ledges, and in crevices. If threatened, the swell shark  bends its body into a sharp U-shape, grasps its caudal fin in its mouth and swallows a large quantity of sea water, which makes it swell to twice its normal size. Afterward, they emit a bark-like noise and expel the water returning to their normal size. Are these just the cutest??

Brown blotches and white spots decorate a swell shark’s yellow-brown body with younger swellsharks lighter in color than the older ones. They have a stout body, a flat, broad head, short snout, and a large mouth which contains 55-60 small  teeth in both their upper and lower jaws. Swellshark eyes are large and gold with a nictitating membrane.

A swell shark inflated - its defense mechanism (Photo from mexfish.com)

A swell shark inflated – its defense mechanism (Photo from mexfish.com)

Reproduction

This shark is oviparous (lay eggs) with females laying two flattened green or amber egg sacks which contain the embryo which is attached by two tendrils to a reef. The eggs are pliable and pale at first but develop into hard brown cases within a few hours after birth. The egg cases contain a yolk to provide nutrients to the embryo until it is hatched in 9-12 months. Newborn pups measure 15 cm in length and are able to feed on small mollusks and crustaceans immediately.

A baby swell shark leaving it's egg case

A baby swell shark leaving it’s egg case

Conservation Status and threats

There are no commercial fisheries for the swell shark, however it is considered bycatch in the commercial lobster and crab traps, gill nets, and trawls. This kind of catch has the potential to threaten this shark due to its age at maturity and the low number of young it produces.

The swell shark is sometimes fished recreationally but it is not consumed by humans due to the poor quality of its flesh. Aquariums sometimes include the swellshark in their tanks as it is able to survive captivity for extended periods of time. The World Conservation Union (IUCN) lists the swell shark as a species of “Least Concern”.

Check out this video showing how close a scuba diver can get to these placid sharks:

SOURCES:

Marine Bio

Florida Museum of Natural History

Shark of the month: LEMON SHARK!

Found in sub tropical and tropical waters in the Atlantic and Pacific oceans, the lemon shark can grow to approximately 3 metres in length and weigh approximately 90 kilograms.  It is from the Carcharhinidae family.  There are 57+ species in this family that includes the more popular known Reef and Tiger sharks.

Lemon shark - named because if its skin colour

Lemon shark – named because if its skin colour

Anatomy  and appearance

The lemon shark is aptly named from its body colouration, a soft yellow with white on its belly. This provides a great camouflage for this bottom dweller to hunt its prey of an assortment of bony fish, stingrays, crabs, crayfish and smaller sharks.

There are a few interesting characteristics for lemon sharks, they can adapt to different salinities, their 2nd dorsal fin is as large as their first and they have extremely strong electro-receptor abilities in their snout and generally use this as their guide in hunting as their eyesight is very poor.

Features of a lemon shark

Features of a lemon shark

Reproduction

Females give birth to live young every other year, in shallow nursery waters, up to 17 pups at one time. The lemon shark fends for itself from birth and will stay in the shallow waters until they grow larger. They do not reach sexual maturity for 12-15 years and the average lifespan is only approximately 25 years.

See this video for a lemon shark giving birth!

Habitat

The choice of habitat of the lemon shark may be its worst enemy for two reasons.  They are generally found in shallow to moderate depth (92 metres) mangroves, estuaries and coral reefs so although it is being hunted for meat and shark fin soup; coastal development and the destruction of the mangroves is its biggest threat.  Secondly, the Lemon shark has been found to have a very high survival rate in captivity possibly lessening the responsibility to ensure its natural habitat is preserved.

Lemon Shark

Lemon Shark – found in shallow reefs and mangrove areas

Cosnervation status

The IUCN class the lemon shark as vulnerable and in South East Asia it is endangered. This is believed to be due to the widespread destruction of coral reefs and mangrove habitats as mentioned above.

Written by Sarah Chernecki

PADI Master Scuba Diver Trainer

Resources:

BBC Nature Wildlife

Marinebio.org

Marinelife.about.com

Sharks-world.com

Shark of the month: Thresher Shark

 

Thresher sharks are definitely a favourite amongst school children Shark Guardian present to. With their long, distinctive tails and graceful movement in the water, they are definitely at the top of scuba divers’ lists for sharks they would like to see. There are 3 species of thresher shark and all tend to be solitary creatures.  Although they do not appear to be a threat to humans there have been reports of divers being hit by their tail!

Thresher Shark - Malapascua, Philippines

Thresher Shark – Malapascua, Philippines

Anatomy and Appearance

Its name derives from the greek word for fox. Its impressive caudal fin can be as long as the body itself.  This is its main identifying feature and is easily recognized.  It has a short head with a cone shaped nose and teeth of varying size.  It has 5 gill slits.  The largest species (common thresher) achieves 6m in length.  Overall their colouration is a brown, blueish, purple dorsally and a lighter shade ventrally. The picture below shows the differences in species colour.

Thresher sharks: Shark Guardian presentations

Thresher sharks: Shark Guardian presentations

Feeding

Threshers are known to follow large schools of fish, including tuna, bluefish and mackerel. They also eat squid and cuttlefish.  Their tail is used to herd the fish, using it as a weapon to stun their prey by ‘slapping’ the water.  The thresher shark is a very strong swimmer and are one of the few species known to jump fully out of the water, making turns just like dolphins do. Check out the photo below showing a thresher doing just that, sighted off the coast of Wales by the Sea Trust in the UK in 2013!

Jumping Thresher shark via Sea Trust

Jumping Thresher shark via Sea Trust

 

Habitat

Found in all temperate and tropical oceans of the world, including the eastern and western Atlantic, central Pacific and Indo-west Pacific.  Populations are separated by depth and space according to gender.  Although they can be sighted in shallow inshore waters, they prefer the open ocean at maximum depths of 500m.

Reproduction

The thresher sharks have no distinct breeding season, with litters usually of 2-4 pups and a gestation period of 9 months. They are slow to mature but can live upwards of 20 years.

IUCN Status

Vulnerable

This genus of shark is highly vulnerable to overfishing, hunted for their meat, liver oil and fins.

Clare Walkden

Clare Walkden

Written by Clare Walkden (Divemaster trainee from Hidden Depths Dive Center on Koh Lanta, Krabi, Thailand) with edits by Shark Guardian

To find out more about Thresher sharks follow the blogs of Shark Guardian dive center Malapascua Exotic in the Philippines.

 

April Shark of the Month: ZEBRA (LEOPARD) SHARK

Adult zebra sharks have longitudinal ridges on the body and a spotted pattern

Adult zebra sharks have longitudinal ridges on the body and a spotted pattern

Zebra Sharks (Stegostoma fasciatum or varium)

Known by most of us in Thailand as Leopard sharks, these are one of Shark Guardians favourite sharks due to their beautiful colouring and friendly nature. An experience not to be missed as a diver is when you are able to get close to these sharks whilst moving slowly on the sea bed, as they rest on the sea bed too. Unfortunately sightings of them are becoming more and more rare but we hope with some monitoring by divers, we can play a part of their come back.

Anatomy and Appearance

There is no size difference between males and females and they can grow to a length of 3.5m, with its tail (or lower caudal fin to give it its proper name) making up nearly half of that length.  It has small barbels on its snout, a small mouth, and small eyes.  Its teeth are pointed, with each tooth having two smaller, lateral, flanking points.  Prominent ridges run along its flanks.

The name leopard comes from its’ adult appearance, the beautiful leopard print it displays. In our presentations we love showing the juvenile Zebra shark photos as you get an idea why really they are zebra sharks…

A juvenille zebra shark with a colouring in between that of an adult and a young shark

A juvenille zebra shark with a colouring in between that of an adult and a young shark

Habitat and Migration

Nothing scary about this little baby shark!

Nothing scary about this baby shark!

Found throughout the tropical indo-pacific region on sandy flats, in and around coral reefs, to depths of 62m, the zebra shark gets its name from the juvenile of the species which has a stripped pattern (see above!).  Zebra sharks are more commonly known as leopard sharks in and around the Andaman Sea, but this is confusing as there is another species of shark called the leopard shark (Triakis semifasciata) found along the Pacific North American coast.

 

Diet

Zebra sharks are generally nocturnal hunters, eating crustaceans, molluscs, small bony fish, and they even enjoy the odd sea snake.

Life History

They are solitary creatures although they do form large seasonal congregations of between 20 and 50, with females outnumbering males by three to one.  The reason for this is unknown, although we do know it is definitely not for mating.

In order to mate, males attract females by vigorously biting their fins!  If he is lucky, the male is then allowed to mate with his chosen ‘victim’.  17cm long brown or purplish black eggs are then laid and attached to the substrate.  The eggs can take between 4 and 6 months to hatch.  When the young eventually appear, they are about 20-36cm long.

They can live for up to 30 years.

Conservation

vulnerable

They are preyed upon by larger fish and marine mammals.  However, it is humans that provide the greatest threat to its existence.  Their status with the World Conservation Union is ‘Vulnerable’ due to humans hunting them for their meat, fins and liver oil.  Numbers are dwindling rapidly as they are an easy catch due to their relatively shallow habitat.

Beautiful, elegant swimmers of the sea

Beautiful, elegant swimmers of the sea

Docile and generally slow-moving, they are not dangerous to humans.  However, they have bitten divers who pull their tails and try to ride them.  In fairness, I would get upset if a stranger tried to jump on my back and ride me!

Spot the Leopard Shark: Thailand is a joint venture between Thai researchers (Phuket Marine Biological Center) and Australian researchers (The University of Queensland) and the diving community of Thailand. Shark Guardian is pleased to help promote this project in Thailand. Please get involved, spread the word and contribute your photographs. And of course, if you identify a new shark to the database you get to name it!