Haemoglobin is just one of four respiratory pigments used to transport oxygen in the animal world. Invertebrate organisms such as bristle worms use the green pigment chlorocruorin. Peanut worms (sipunculids), penis worms (priapulids) and brachiopods depend on a blood pigment called haemerythrin. When deoxygenated haemerythrin is colourless, but with oxygen on board it is a violet-pink colour. By contrast, molluscs, spiders, scorpions, crabs, lobsters and cephalopods (such as squids and octopuses) form the blue copper-based pigment haemocyanin. It is thought that in cold water that is low in oxygen, haemocyanin is a more efficient transporter of oxygen than haemoglobin. Nevertheless, at cold temperatures it is difficult for the oxygen in the tissues that has been taken in during respiration to separate from the blue pigment again.
The Antarctic octopus Pareledone charcoti has two ways of offsetting this temperature disadvantage. Firstly, its blood contains up to 46 per cent more haemocyanin than that of related octopuses that live in warmer water. Secondly, a lot of oxygen diffuses directly into the blood in the animal’s gills and dissolves physically there. By these means the little octopod that inhabits the shallow shelf waters of the Antarctic Peninsula ensures that even at temperatures as low as minus 1.9 degrees Celsius its body is supplied with sufficient oxygen right to the tips of its tentacles.
fig. 4.27 > The blood of the Antarctic octopus Pareledone charcoti contains up to 46 per cent more respiratory pigment than that of its warm-water counterparts.