Blood, how does it work?
d.w.rowlands [at] gmail.com
One of the size limitations on single-celled organisms is that, the larger a cell gets, the larger the distance between the surface and the center. This makes it hard for nutrients to reach the interior of the cell and for waste products to be released. Being made of many small cells instead of one large one allows a much higher ratio of cell surface area to volume. However, when organisms get big enough, getting nutrients to cells in the interior itself starts to be a challenge. This is particularly challenging for metazoans, since they tend to have relatively fast metabolisms to enable them to move around.
In general, there are two separate problems: the transport of organic nutrients, which are generally transported in water-soluble forms, and the transport of oxygen and carbon dioxide gas, both of which are non-polar. Vertebrates, at least, deal with carbon dioxide by converting it to bicarbonate ions, which are water-soluble, but oxygen is a more difficult problem.
Some animals handle these problems separately. Insects, for example, have tubes called trachae which allow air to flow into the interior of their bodies directly, while transporting organic nutrients dissolved in an open circulatory system that mixes with the fluid that fills their body cavities. Other arthropods, such as crustaceans, transport oxygen in their open circulatory systems. Some molluscs do this as well, while cephalopods have closed circulatory systems (where blood is separate from the fluid filling their body cavities, as in vertebrates) that transport organic nutrients and oxygen.
Echinoderms, as usual, are particularly weird. They have vascular systems filled with seawater that are used hydraulically for locomotion as well as for respiration and food transport. They also have open circulatory systems that transport nutrients and sometimes oxygen.
As I noted before, while dissolving organic nutrients in aqueous solution is relatively easy, oxygen gas is not very soluble in water. Vertebrates, as you likely know, deal with this problem by using an iron-containing protein called hemoglobin that binds to oxygen molecules and is itself water-soluble. This significantly increases the oxygen capacity of their blood. A related iron-containing protein called myoglobin is used for oxygen transport in muscles and this, not hemoglobin, is responsible for the red color of meat.
Non-vertebrate chordates, as well as many other invertebrates, either don't transport oxygen in their blood at all, or find that the amount of oxygen that is directly soluble in water is sufficient for them. This generally only works for sufficiently small animals, and works better if their metabolisms are slow. Strangely, one group of Antarctic fish, the crocodile icefish, also use this technique. They have apparently lost the genes for producing hemoglobin, which means their blood is clear and only contains directly dissolved oxygen. They can get away with this by having slow metabolisms and because oxygen is more soluble in water at low temperatures, but this mutation is thought to be maladaptive.
The use of oxygen-binding proteins has evolved a number of times in invertebrates. A number of different such proteins exist. Most similar to hemoglobin are erythrocruorin and chlorocruorin, used by different varieties of annelids. Both, like hemoglobin, have oxygen-binding sites consisting of heme groups that contain iron ions. However, they are less efficient at binding oxygen than hemoglobin, and are generally found free-floating in the blood rather than in the equivalent of red blood cells.
One annelid genus, along with brachiopods, priapulid worms, and peanut worms, use an iron containing protein that does not contain heme groups for oxygen-binding. hemerythrin is also less efficient than hemoglobin but---unlike hemoglobin---it binds oxygen more efficiently than carbon monoxide, so these animals are immune to carbon monoxide poisoning. This is fairly surprising, since carbon monoxide is in general one of the most strongly-binding ligands in inorganic chemistry.
The most common oxygen transport proteins in invertebrates, however, are hemocyanins, which have evolved independently in a number of phyla. They are primarily found in arthropods and molluscs but also turn out to show up in some non-vertebrate chordates. Unlike the above oxygen-transport proteins, these use copper ions for oxygen-binding. Although they are usually less efficient at oxygen transport than hemoglobin, they can be more efficient at low temperatures and in relatively low-oxygen environments, where many of the organisms that use them live. They are responsible for the oxygenated blood of molluscs and many arthropods being blue-green, like Vulcan blood, although the oxygen-carrying protein in Vulcans is supposedly identical to hemoglobin with copper ions in place of the iron.
As for what actually inspired this blog post, it was post on In the Pipeline about the blood of tunicates, which is also blue-green. However, it isn't blue-green because of hemocyanins. Instead, tunicate blood contains proteins called vanabins that bind vanadium ions. It seems that tunicates sequester a lot of vanadium from seawater and concentrate it in their blood. This is fairly unique, as vanadium virtually never shows up in biology, and is a fairly rare element. Although there was some initial thought that they might be oxygen-transport proteins, it turns out they aren't: the current theory is that they are present to deter predation, since vanadium is toxic to most other animals.
A final comment on the bizarre biology of tunicates. Apparently, instead of urinating, they filter nitrogenous waste from their blood and store it as urea crystals. These crystals build up over the course of their lives, because they have no way to dispose of them!