And it does not have to be blue either
Imagine that you caught a fish and want to turn it into a meal. You cut off its head, there is no blood; you cut it open, there is no blood; you slice off a filet and you still don’t see red blood. You’d be surprised, but this could happen to you if you had a so-called bloodless Antarctic ice fish to prepare for dinner – and they are mighty rare and not supposed to be killed. And yet, they do exist in the frigid, but oxygen-rich waters of Antarctica. But actually, they do possess blood, but it’s devoid of red blood cells and thus lacks the oxygen-carrying protein haemoglobin. This rather sluggish fish doesn’t need haemoglobin, because there is sufficient oxygen dissolved in its colourless blood plasma. Amongst the vertebrates, however, these fish are the exception. All other critters with backbones have red blood.
But does it always have to be red? After all. in medical and biological text books the venous blood is usually drawn as blue, but that is of course no reflection of reality as venous blood is as red (only darker) as the oxygen carrying arterial blood, the latter being of a more intense and brighter colour than the former. The oxygen-binding haemoglobin molecule in the blood of vertebrates and even a few invertebrates like earthworms, the larvae of some chironomid midges, Planorbarius snails, etc., contains ferrous iron; without it haemoglobin could not reversibly bind oxygen. That even some plants can contain haemoglobin has been known for a long time, but when it was discovered in the root nodules of legumes, the idea was these plants might originally have obtained it from animals. That this was totally wrong and haemoglobins in plants evolved independently and occur in a wide range of plants including soybean, barley, the casuarina tree, the roots of Lotus corniculatus etc., is now an established fact. Various roles have been suggested for the plant haemoglobins, but whether they function as oxygen scavengers, monitors of oxygen levels, distributors of oxygen in the plant or stress reducers is still far from clear as the haemoglobin could provide different services to different species.
In animals which use compounds other than haemoglobin (or methods) to supply the body tissues with oxygen, blood colours different from red occur. Take crabs and lobsters, for instance. They don’t use haemoglobin, but possess the copper-containing haemocyanin in their blood – and that’s light blue when oxygenated and colourless when de-oxygenated. Insects don’t need any coloured blood at all as the oxygen reaches the tissues via separate, blind ending air containing tubes called the tracheal system. Unusual are also some sedentary tube-worms, which use as their oxygen carrier a green iron-containing compound known as chlorocruorin and sipunculid and priapulid worms that also use iron, but as a violet-pink blood chemical known as haemerythrin to bind oxygen with.
Stranger still are certain ascidians (sea squirts) which are known to selectively absorb vanadium from the oceanic water to levels up to four million times more concentrated in their blood. The idea has been muted that vanadocytes, i.e., the sea squirt’s vanadium containing cells, might possibly be involved in gas exchange as the vanadium could function as an oxygen carrier. However, at least six other functions have been suggested. If only one could ask a sea squirt directly, but the trouble is whenever you probe or push a sea squirt a little too hard, the only response you get is a squirt of water in your face.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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