Introduction

 On the 22nd August 1995, a Bigeye Thresher shark was landed at Newlyn fish market in Cornwall. It had been caught in an area known as the Porcupine Sea-Bight (51^oN 13^oW) where it had become trapped in a drift net. The drift net had been set at 5 metres in waters more than 2000 metres deep to catch tuna.

Distribution

The Bigeye Thresher is a widespread species; it typically inhabits oceanic and coastal waters from the surface to depths of 500 metres and prefers warm-temperate to tropical waters. It does not generally occur in cold temperate waters (Fig.1). Common Thresher Sharks, Alopias vulpinus, occur widely throughout the north east Atlantic and are commonly found in coastal waters around the British Isles. As Bigeye Threshers are typically found in the open sea, less is known about them. Before this specimen was caught, there had been no confirmed recordings of this species in this region. This was therefore the most northerly confirmed recording of a Bigeye Thresher in the north east Atlantic and coincided with above average sea surface temperatures in the summer of 1995 (Table 1).

 Measurements & Observations

Some measurements for this specimen are given in Table 2. It was easily distinguished from the common Thresher by the comparatively large eyes, the orbits of which extend to the dorsal surface of the head, and the presence of deep horizontal grooves on each side of the head above the dorsal fin. They also have larger and fewer teeth; common Threshers typically have 32 in the upper and 29 in the lower jaw, Bigeye Threshers typically have between 19-24 in the upper and 20-24 in the lower jaw.

TABLE 1. Sea-Surface Temperatures 1993-1995 (^oC) at 50^oN 13^oW in the north-east Atlantic. Data supplied by the Meteorological Office from recordings by automatic buoys and random merchant shipping. Results are expressed as monthly means (n=4).
 

data to be included later, please request EMail Glaucus@hotmail.com
 

It was a female weighing 381 kg (gutted) with a total length of 484 cm which makes it the largest and heaviest record to date for this species. The previous maximum accurately measured specimen was caught off the coast of Cuba, it weighed 284.5 kg and had a total length of 461 cm. Sexual maturity in Bigeye Threshers is reported to occur at approximately 300 cm for males and 350 cm for females, so based upon this assertion, it is likely that this specimen was sexually mature.

Food

Analysis of the stomach contents and internal organs was not possible as the specimen had been gutted at sea by the fishermen. They did, however, report 9 pieces of an unidentified fish species in the stomach which had a total weight of 18 kg. There was also a bite mark on the right flank which was probably sustained from a blue shark, Prionace glauca, while it was trapped in the net.

TABLE 2. Some measurements of the Bigeye Thresher shark from the north east Atlantic. Lengths are in cm and as a proportion of the total length.

 data to be included later, please request   EMail Glaucus@hotmail.com
 

Discussion

Research conducted in the north west Atlantic has shown that Bigeye Threshers tend to be caught in waters where the sea surface temperature ranges from 16^oC-25^oC with a minimum of 14^oC at 75 metres. Results on Table 1 show that these temperature ranges are rarely met at the location where this specimen was caught.

The Bigeye Thresher, in common with a number of other Lamniform species, is able to maintain body temperatures several ° higher than the surrounding water temperature by means of a rete mirabile system.

This is a special tissue consisting of masses of fine intertwined arteries and veins which act as heat exchangers. Arteries, carrying cool, oxygenated blood from the gills, branch into smaller vessels which become entwined with veins. Blood in the veins is warmer due to the activity of muscles and other organs. As the vessels intermingle, the heat from the venous blood is diffused and warms the arterial blood which is bound for the muscles and viscera. In most other fishes, the heat is lost to the surrounding environment as the blood returns more directly to the gills.

The temperature in the muscles and viscera is therefore maintained making the Bigeye Thresher shark effectively "warm bodied." This ability to conserve heat means that the Bigeye Thresher is well adapted to catching fast prey like tuna and facilitates incursions into cooler water. As the known distribution of the Bigeye Thresher clearly indicates that it prefers warm temperate to tropical waters, it is likely that the above average sea temperatures recorded in 1995, to within the lower thermal preference for this species, would explain the unusual occurrence of this specimen in the north east Atlantic.
 

Biogeographical distribution of Alopias superciliosus (Compagno 1984)
 

data to be included later, please request EMail Glaucus@hotmail.com

The Big-eyed Thresher is not included in the 21 species of sharks, excluding the rays and skates, found in the shallow seas (to a depth of 200 metres) around the British Isles. These 21 species belong to 12 different families. The rays and skates, and electric rays, number at least 19 British species in 3 families with the common species featuring in excellent displays in the larger Public Aquaria and Sea Life Centres.

Some of the smaller species like the Common Dogfish, Scyliorhinus stellaris, are often not thought of as members of this group of dangerous animals. The largest shark that inhabits British seas is the harmless plankton eating Basking Shark, Cetorhinus maximus, which in British seas attains a length of 11 metres and is occasionally seen from boats and from the shore.

There have no reports of shark attacks on bathers in British seas.

History

Sharks evolved on this planet about 350 million years ago in the primordial seas of the Devonian period.

Sharks are classified in the Class called the Chondrichthyes which means 'cartilage fishes'. This is because they did not evolve the bony skeleton of the Osteichthyes or 'bony fishes' and their cartilage skeleton and jaws have proved extremely successful. However, their teeth and spines are true bone. All the sharks and rays are classified in the Sub-class Elasmobranchii.

Biology

Over the millions of years sharks have invaded most marine habitats and they are found in many different sizes and forms, some flattened like the rays to live on the seabed and others streamlined and aggressive, culminating in the feared Great White Shark, Carcharodon carcharias, which like the Thresher Shark can maintain a high body temperature. There are no records of the Great White from British seas, but it has been recently discovered still to be breeding in the Mediterranean Sea.

Of the large predatory sharks to visit British seas the streamlined Blue Shark can reach 2.5 metres long around these coasts in which they visit during the summer to feed on mackerel and other shoaling fish. This fish would be dwarfed by the Big-eyed Thresher featured in this article. Common Thresher Sharks are sometimes seen in the English Channel in the late summer and are readily noticed because of their long tail fin. The biggest of the carnivorous British sharks are the Mako, Isurus oxyrinchus, and the Porbeagle, Lamna nasus.
 
 

Further reading:

Field Guide to Sharks of British Coastal Waters
by Philip Vas (Field Studies AIDGAP Guide).

Sharks of Tropical and Temperate Seas
by R. H. Johnson (1992) Les Editions du Pacifique.