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!

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 achieves 6m in length.  Overall their colouration is a brown, blueish purple dorsally and a lighter shade ventrally.

Feeding

They 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.

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.

IUTN Status

Vulnerable

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

Clare Walkden

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

It’s time for shark of the month again! This month we chose a shark is more common along th UK coastline, the Basking Shark!

It is the second largest living fish, after the whale shark, and the second of three plankton-eating sharks, the other two being the whale shark and Megamouth shark. It is a migratory species, found in all the world’s temperate oceans. It is a slow moving and generally harmless filter feeder as it has a greatly enlarged mouth and highly developed gill rakers. The basking shark is usually greyish-brown in colour and often seems to have a mottled appearance. The teeth of the basking shark are very small and numerous and often number one hundred per row.

Basking sharks are a migrating species and are believed to overwinter in deep waters. They may occur in either small schools or alone. Small schools in the Bay of Fundy have been seen swimming nose to tail in circles in what may be a form of mating behavior. Basking sharks are not aggressive and are generally harmless to people.

It has long been a commercially important fish, as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point that some have apparently disappeared and others need protection.

Range, habitat and migration

The basking shark is a coastal-pelagic shark found worldwide warm-temperate waters around the continental shelves. It prefers 8 to 14.5 °C (46 to 58 °F) temperatures, but recently has been confirmed to cross the much-warmer waters at the equator. It is often seen close to land, including bays with narrow openings. The shark follows plankton concentrations in the water column and is therefore often visible at the surface. It characteristically migrates with the seasons. The basking shark is found from the surface down to at least 910 metres.

Satellite tagging confirmed basking sharks move thousands of kilometres during the winter months, seeking plankton blooms. It also found they shed and renew their gill rakers in an ongoing process, rather than over one short period.

A 2009 study tagged 25 sharks off the coast of Cape Cod, Massachusetts, and indicated at least some individuals migrate south in the winter. Remaining at depths between 200 metres (660 ft) and 1,000 metres (3,300 ft) for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of the Amazon River. It is unknown why they undertake this journey. Lead author Gregory Skomal of the Massachusetts Division of Marine Fisheries, suspects it may be related to reproduction.

They are slow-moving sharks (feeding at about 2 knots (3.7 km/h; 2.3 mph)) and do not evade approaching boats (unlike great white sharks). They are harmless to humans if left alone and are not attracted to chum.

Even though the basking shark is large and slow, it can breach, jumping entirely out of the water. This behaviour could be an attempt to dislodge parasites or commensals. Such interpretations are speculative, however, and difficult to verify; breaching in large marine organisms such as whales and sharks might equally well be intraspecific threat displays of size and strength.

Anatomy and appearance

The largest accurately-measured specimen was trapped in a herring net in the Bay of Fundy, Canada in 1851. Its total length was 12.27 metres (40.3 ft), and it weighed an estimated 19 tonnes (19 long tons; 21 short tons). Dubious reports from Norway mention three basking sharks over 12 metres (39 ft), the largest at 13.7 metres (45 ft), dubious because few anywhere near that size have been caught in the area since. On average, the adult basking shark reaches a length of 6–8 metres (20–26 ft) and weighs about 5.2 tonnes (5.1 long tons; 5.7 short tons). Some specimens still surpass 9–10 metres (30–33 ft), but after years of large-scale fishing, specimens of this size have become rare.

Order – Lamniformes
Family – Megachasmidae
Genus - Megachasma
Species - pelagios

Taxonomy
When the first megamouth was captured in 1976, a new shark family, genus and species had to be erected. There are conflicting phylogenetic hypotheses regarding the evolutionary relationships between the Megachasmidae and other shark families. One theory suggests that the Megachasmidae is evolutionary derived and form a monophyletic family with basking shark, Cetorhinidae. Others disagree with this idea and suggest that the Megachasmidae is relatively derived and forms a sister group to the Cetorhinidae, Lamnidae (mako, white and porbeagle sharks) and Alopiidae (thresher sharks). Recent studies suggest that Megachasma pelagios is the most primitive living species within the order Lamniformes, which contains all the aforementioned families, and has independently evolved the filter feeding mode, shared with the basking shark, Cetorhinus maximus.

Geographical Distribution

Although only 49 confirmed sightings (plus 1 unconfirmed sighting) of megamouth shark are reported, this species is now known from Indian, Pacific and Atlantic Oceans. As with the two other filter-feeding sharks, the basking and whale sharks, this species is wide-ranging. However, the megamouth is considered to be less active and a poorer swimmer than the basking or whale sharks. Poor mobility likely is a reflection of its flabby body, soft fins, asymmetrical tail, lack of keels and weak calcification.

Habitat
As its species name (pelagios, the Greek word for “of the sea’) suggests, the megamouth lives epipelagicly (in the upper part of the water column) in open ocean. Although only few sightings of megamouth have been reported, the capture of the 6th megamouth was very important in augmenting our understanding of the ecology of this species. This specimen was tagged and followed for two days, allowing insight into its habitat preference and behavior. It remained at a depth of 15m during the night, then dove to 150m at dawn and returned to shallow waters at dusk. So the megamouth is presumed to be a vertical migrator on a diel cycle, spending the daytime in deep waters and ascending to midwater depths at night.

Behavior
Only two observations of megamouth provide information about this species behavior. The 6th specimen from Dana Point, California (21 October 1990) offered the most important insights into the behavior of this species. The male specimen, with 494cm in total length, was tagged and tracked for two days. One of the conclusions of these observations is that megamouth is probably a vertical migrator on a diel cycle spending the daytime in deep waters and ascending to midwater depths at night. This vertical migration may be a response to the movements of the small animals on which it feeds. The krill that make up part of megamouth’s diet are known to migrate from deep waters to the surface. Megamouth, in contrast to many other deep-water sharks, shows a decrease in specific gravity in the form of a soft, and poorly calcified cartilaginous skeleton; very soft, loose skin; and loose connective tissue and muscles. Others epibenthic (live in the water just above the bottom) and epipelagic sharks often have an enlargement of their abdominal cavity and increased liver volume. The huge liver allows for greater production of liver oil in order to reduce specific gravity and increase hydrostatic support.

The 13th sighted also offered important megamouth behavioral observations. This sighting documented sperm whales attacking megamouth shark. Observers reported that the megamouth was swimming slowly and apparently confused at the surface. The shark showed signs of the whales’ attack, on the base of its dorsal fin and gills.

Reproduction
The megamouths II and VI were both mature males, and both showed evidence of recent mating. The claspers of megamouth II were oozing spermatophores and those of the sixth specimen were abraded and bleeding, a common occurrence in sharks that have just mated. The 6th megamouth also had a fresh wound on the lower jaw, a feature found in other sharks that grasp one another’s mouth during mating. Megamouth II was taken in late November and megamouth VI in late October, these factors make scientists believe that Southern California might be a mating area for megamouth sharks in the fall.

The claspers of the first captured megamouth were fully described. Its claspers were relatively slender, with tip elongated, forming a very narrow, slender process.

The 7th megamouth observed, a female with 471cm in total length was heavily studied and one of the conclusions is that this specimen is immature. This judgement was based on uteri that were enlarged only posteriorly, a poorly developed ovary and ostium and the small size of the oocytes. The ovary of the megamouth is similar to other mackerel sharks and this suggests that megamouth embryos are oophagous (the first well-developed embryo eats the other eggs in the uterus). The 12th megamouth captured is the only known mature female. The total length of this specimen was 5.44m and the expanded uteri measured 260mm. The right ovary possessed a large number of whitish yellow eggs.

Conservation
Due to the lack of information concerning distribution and population status, the megamouth is considered “Data Deficient” by the World Conservation Union (IUCN). The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.

Description
The little-known, deepwater-dwelling frilled shark (Chlamydoselachus anguineus) is one of the most primitive species of living shark. The species name anguineus, which derives from the Latin for snakelike, only partly conveys its bizarre appearance. The head is lizard-like and features a blunt-ended snout and a very large mouth armed with multiple rows of sharp, three-pronged teeth. While each tooth is relatively small, there are around 300 in total, providing almost a thousand sharp hooks on which to trap struggling prey. Behind the head, on both sides of the body, there are six gill slits, which each possess a distinctive frilly margin. The front slits on each side extend beneath the body, meeting under the throat, giving the appearance of a frilly collar. The body is dark brown or grey in color, with a large anal fin and small, paddle-shaped pectoral fins, while the dorsal fin is relatively small and set very far back on the body. The caudal fin has a small, vestigial lower lobe, while in contrast, the upper lobe is very elongated, and further extends the serpentine body.

Biology
Owing to its deepwater habitat, very few observations of the frilled shark have been made in its natural environment. Analysis of stomach contents of frilled sharks brought to the surface indicate that this species mostly preys on deep water squid and a variety of fish, including other sharks. While it is unclear exactly how the frilled shark feeds, its set of needle-sharp, inwardly-pointing teeth, and the fact that its jaws can open extremely wide, suggest that it may actively take prey over one and a half times its own length. Interestingly, this species has also been found higher in the water column at depths between 50 and 200 metres, where it feeds on faster-swimming squid, although these may be taken as carrion or when the squid are exhausted after reproducing.

The frilled shark, like many other sharks, is ovoviviparous, which means that after fertilisation, the embryos develop within the female’s uterus, receiving nourishment from a yolk sac. Once the yolk sac nutrients are exhausted the embryos absorb nutrients from secretions within the uterus until birth takes place. The litter is very small, numbering only two to ten offspring, which each measure between 40 and 60 centimetres in length. Studies of frilled shark embryos indicate that this species could have a gestation period of over 3.5 years, which, if accurate, would be the longest known amongst the vertebrates.

Range
Although the range of the frilled shark extends almost worldwide, it has a very patchy distribution. Populations occur on the outer continental shelves and upper continental slopes off Norway, northern Scotland and western Ireland, south as far as northern Namibia. Other populations occur in the eastern Pacific off southern California and northern Chile; in the west Pacific, off south-east Japan, eastern Australia and New Zealand; and also possibly in the western Indian Ocean, off South Africa.

Habitat
A predominantly deepwater species, the frilled shark typically lives close to the seabed or in the water column at depths between 500 and 1,000 metres, but may be found as deep as 1,500 meters. Individuals are, however, sometimes found higher in the water column at depths between 50 and 200 meters.

Status
The frilled shark is classified as Near Threatened (NT) on the IUCN Red List.

Threats
While the frilled shark is not an important target species for fisheries, individuals are regularly taken as bycatchin some localities by bottom and midwater trawls as well as deep-set longlines and gillnets. When accidently caught, the meat is used for human consumption, fishmeal or is discarded. With almost nothing known about this species’ global population size, it is not clear what effect bycatch may be having. Nevertheless, given this species’ apparent scarcity, long gestation period and low reproductive output, even small losses could have serious consequences for the frilled shark’s survival. Any expansion in deepwater fisheries effort is therefore a cause for concern.

Conservation
There are currently no known conservation measures in place for the frilled shark. While a small number of UN states are developing or have developed shark management plans for commercial fisheries, few, if any, of these employ for managing deepwater fisheries bycatch.

Once again one of out divemaster trainees did a bit of research on a shark we knew little about but heard it is a pretty cool specimen! As you can see from the photos and PDF document, this is yet another example of the wonderful evolution of sharks.

A shark that becomes practically invisible?? Oh yes. Clever? Indeed!! And the dwarf lantern shark is the smallest of all sharks – another amazing feet!

Shame they live so deep, would make a night dive extremely interesting!!

Check out this information: Dwarf lanternshark PDF

We are proud to bring you a new feature at Shark Guardian – Shark of the Month!!! With a little help from our Divemaster trainees who have been researching sharks for us and presenting them to our guests on Bunaken island, we will now bring you new, interesting or unusual sharks to help educate our many followers about sharks even further.

If you have any special requests then do let us know!!

Our first entry is a special shark which i’m sure has caused people to have nightmares throughout the world! A scary looking yet hardly seen shark….

GOBLIN SHARK

Thanks to Lewis Stewart for this information – just click on the link to read all about the goblin and see more pictures like this one!!