Posted by: Footler PJ16 OCT 2013
Horseshoe crabs (Limulus polyphemus) are more closely related to spiders and scorpions than to crustaceans. They are believed to have changed only slightly since their ancestors appeared about 440 million years ago. But, although these so-called living fossils are tough creatures with a proven ability to cope with changes to their environment, their numbers are now dwindling along parts of the eastern seaboard of the US and in Asia.
Thousands are killed every year for bait, mainly to catch eels, and by oystermen because they eat oyster larvae. Their future survival may also be influenced by the biomedical industry which uses an extract of their blood to check that products such as vaccines, intravenous drugs and medical devices are free from bacterial contamination.
The potential use of horseshoe crab blood in a pyrogen test was first noted in the 1960s when researchers discovered that massive clotting occurred when common marine bacteria were injected into a horseshoe crab’s bloodstream. The blood appears bluish because of the copper in the haemocyanin employed instead of haemoglobin to carry oxygen. The blood also contains amoebocytes which play a role in defence against pathogens and which are extracted to make limulus amoebocyte lysate, the substance used to detect bacterial endotoxins.
Harvesting blood involves capturing the animal, taking up to 30 per cent of its blood and then releasing it back into the sea. The blood volume is said to rebound in about a week, although it takes two to three months for the crab’s blood cell count to return to normal. The animals are tagged and should only be harvested once a year. Most of the animals do survive, although mortality rates correlated with the amount of blood extracted and the stress of handling and transportation vary from the 3 per cent claimed by the biomedical companies up to the 10 to 15 per cent reported by some marine researchers.