Marine Turtles

Leatherback turtle found stranded on Harlech beach, Wales

There is a widespread perception that marine turtles are inhabitants of tropical and subtropical seas, and for the most part this is true. However, records of turtles off the British Isles are not uncommon, particularly around the South West of England, Wales and Southern Ireland.

There are seven species of extant marine turtles, yet the vast majority of records from Britain involve one species: the Leatherback turtle Dermochelys coriacea. Reports of Leatherback turtles in the British Isles are so numerous that the species is widely considered to be a native, at least for part of the year, with significant numbers visiting each summer before returning to warmer climes along the Atlantic coasts of Central and South America in the autumn.

This species differs from all the others in a number of ways. Most conspicuously, as its name implies, it is not a hard shelled turtle. Instead the covering of the carapace is flexible with the texture of hard vulcanised rubber. But what allows it to thrive in the cool waters of the North Atlantic is that, unlike other reptiles, it is at least partially endothermic. Their high metabolic rate produces considerable heat which the animal is able to retain due to several peculiar physiological and anatomical characteristics.

The Leatherback is a particularly oily animal. It has a layer of subcutaneous oil, about five centimetres thick, surrounding the body and permeating the skin, providing very efficient insulation and preventing heat loss to the water.

Additionally, it is easily the largest species of turtle alive today. A male found dead on Harlech beach in Wales in 1988 measured 291cm in length, 277cm from flipper tip to flipper tip and topped the scales at a massive 916Kg. Even the relatively diminutive females regularly reach lengths of 155cms and weights of 365kgs. Such large body size gives it a small surface area to volume ratio, so heat loss occurs more slowly than from a smaller animal.

There is, however, one part of the anatomy that has the potential to lose a lot of heat. The surface area of the flippers is huge, and could be expected to radiate vast amounts of heat from the animal’s body to the surrounding water. To prevent this, the flippers are equipped with a counter current heat exchange system. This is a neat arrangement of the blood vessels at the junction between the flippers and the body, whereby the arteries and the veins are situated in close contact with one another, yet carry blood in opposite directions. This close contact between the two types of blood vessels allows warm blood entering the flippers from the core of the body to give up its heat to the cold blood returning from the flippers to the core of the body. This arrangement ensures that blood circulating through the flippers has already lost its heat, preventing it being radiated out into the water and instead conserving it in the core.

These adaptations allow the species to survive in cool British waters, but what attracts them here is their diet. The species feeds almost exclusively on jellyfish and sea-squirts. During the summer months large swarms of jellyfish appear off the British coast, closely followed by the turtles.

Other species of marine turtles are found around the British coast, but none as frequently as the Leatherback and it is likely that in the majority of cases these other species represent ‘vagrants’ that have been carried here by ocean currents and have not arrived of their own volition.

Although turtles are regularly sighted off the coast of southwest England, they do not appear to be common in Somerset waters, with only two records for the county. This is curious as sightings of turtles are much more numerous along the neighbouring North Devon coast. The marine turtle database maintained by Rod Penrose of Marine Environmental Monitoring shows twenty records for the North Devon coast, a number that is rising every year. This discrepancy may be partially explained by a lack of formal reporting. Derek Purvis who has been running a charter fishing boat from Porlock Weir since the 1960s informs me that he has seen turtles off Somerset in the past, although these have never been formally recorded. However, it is also possible that conditions become less favourable for turtles as the current and turbidity increase further up the Bristol Channel.  

Of the two Somerset records, the first is of a Hawksbill turtle Eretmochelys imbricata captured alive in the River Parret above Bridgwater in 1827, while the second is of a Leatherback found dead on Sand Bay near Weston-Super-Mare in 1996.

The River Parret Hawksbill was originally recorded by Baker in a short note to the periodical ‘Zoologist’ in 1850, then repeated in his article on Somersetshire reptiles published by the Somerset Archaeological and Natural History Society in 1851. This record deserves further analysis.

In his 1972 book ‘ European Atlantic Turtles’, Brongersma  casts doubt on whether any Hawksbill turtle has ever crossed the Atlantic of its own volition. The Hawksbill is a species of warm tropical seas and would be quite out of place in the North Atlantic. In fact, Brongersma has  attempted to check all the historical records of the species in the North Atlantic and has found only one specimen that can be verified as a Hawksbill, all the others were either untraceable or turned out to have been misidentified.  Sadly, the Somerset record referred to by Baker is one of the untraceable specimens.

 In his account of the discovery of the specimen in the River Parret, Baker explains that the turtle was delivered to him, apparently in good health, on the 27th May 1827, a few days after its capture.  He wrote a note to his friend Robert Anstice conveying the news of the animal’s capture and quotes from Mr Anstice’s reply. In this reply Mr. Anstice claims to have seen the species in question around the coast of Portugal. From the quoted letter it would appear that Mr Anstice had not seen the River Parret specimen and was only relying on Bakers identification of it as a Hawksbill. Nevertheless, this is a curious quote, because the Hawksbill turtle is not a species associated with Portugal. The species most commonly encountered around Portugal is the Loggerhead turtle Caretta caretta. This demonstrates some confusion between the two species and therefore casts some doubt on the identification of the River Parret specimen, particularly as records of Loggerhead turtles around the coast of Britain are more numerous than those of Hawksbills. Brongersma concludes by recommending that this specimen be considered unidentified.

No such doubt relates to the identification of the second record of a turtle from Somerset. On August 1st 1996 a dead adult Leatherback was washed ashore on Sand Bay exhibiting injuries probably consistent with an encounter with a boat propeller. In fact the rear portion of the animal was missing altogether, reducing its size by about a third.