There is a striking convergence between the aorta of the cephalopod and vertebrate heart, notably in its structure and the employment of elastic proteins.

Circulatory, or blood vascular systems have evolved independently many times, and the associated pumps and hearts are also rampantly convergent. A particularly interesting example concerns the similarity between the hearts of cephalopods and vertebrates, which is of course just one more example of the many convergences between these two groups.

Cephalopod and vertebrate blood vascular systems

The cephalopods are the most advanced molluscs, and unlike the great majority of molluscs which have an open system (filled with the haemolymph, and broadly similar to the arrangement seen in the arthropods) in the cephalopods the blood vascular system is closed. There is a striking convergence between the aorta of the cephalopod and vertebrate heart, notably in its structure and the employment of elastic proteins. In addition, similar to the vertebrate vascular system, the cephalopod heart operates a dual system whereby blood pumped to the gills is separate from the system that distributes blood to the rest of the body.

Aorta structure in cephalopods

Amongst the many striking convergences between the cephalopods, notably the octopus, is the structure of the aorta. Active animals, with a closed blood-system that operates under high pressure needs a vessel emerging from the heart that can withstand regular and substantial changes in blood pressure. Not surprisingly, this aorta needs to have an elastic wall, and hence the convergence between cephalopods and vertebrates. Interestingly, the rubber-like proteins employed are different, a nice example of molecular convergence. The similarities are not precise, because the cephalopod aorta has a lower proportion of elastic fibres. The haemodynamics are, therefore, remarkably similar, and in terms of wave propagation also show what are known as strong wave reflections. However, in mammals there are other wave propagation effects. These have not been identified in the cephalopods, but it is speculated they may occur in the giant squid.

Branchial hearts and haemocyanin

The vertebrate and cephalopod systems, however, are not identical. Cephalopods lack true capillaries, have “back-up” hearts associated with the gills (branchial hearts) and use haemocyanin (instead of haemoglobin) as a respiratory based protein. Haemocyanin is, however, convergent with a similar copper based protein found in arthropods.