Horseshoe crabs are ancient chelicerate arthropods with little apparent change in their morphology over the last 150-200 million years and they are often described as ‘living fossils'. Today only four known species remain, with one species inhabiting the eastern coast of North America, while the three other species occur in South East Asia. All four species live in the marine environment and while virtually nothing is known about the physiology of the Asian species, it seems clear that smallest species, Carcinoscorpius rotundicauda, is common in estuaries and river mouths where they may occur at high density.
Our studies show that Carcinoscorpius is an osmoconformer within a wide range of salinities (300-1500 mOsm), although they maintain slightly higher osmolarities of the haemolymph for the first few days after exposure to low salinities. Chloride and sodium are the most important extracellular osmolytes at all salinities. Exposure to low salinity is associated with a pronounced influx of water, while high salinity causes efflux. Body mass may accordingly changes by 20% for many days after transfer.
Air exposure was associated with an uncompensated respiratory acidosis, whereas exposure to hypercapnia (5% CO2) was partially restored by increased HC = 3-levels in the haemolymph. The degree of compensation did not differ between individuals that had been held at high or low salinity suggesting that the availability of acid-base relevant ions plays a minor role in this response.
