Fig. 1

Distribution of circulatory systems and respiratory pigments in the metazoan consensus tree. Some species have no known respiratory pigments despite having a circulatory system or at least a fluid filled cavity (nematodes, most echinoderms, urochordates, cephalochordates). It is assumed that in these species, gases either diffuse freely though thin layers of tissues or are freely dissolved in the hemolymph. Some groups have circulating hemoglobins (“red blood”). The status of these hemoglobins is very diverse. They can be extracellular hemoglobins dissolved in the blood, as in many annelids. They can be contained in red cells as in the vertebrates or in some annelids such as capitellids [25]. Some groups have circulating dissolved hemerythrins (“pink blood”), like priapulids, brachiopods and some annelids (Sipunculidae, Magelonidae) [73,74,75]. Mollusks on one side and many arthropods on the other side have circulating dissolved hemocyanins (“blue blood”) of different molecular origins [76]. Circulating respiratory pigments of different types are generally not present together in the blood, suggesting that the recruitment of each type of pigment for the respiratory function occurred multiple times independently in the evolution of bilaterians [76]. HG hemoglobin, HC hemocyanin, mHC molluscan “hemocyanin”, HE hemerythrin, BVS blood vascular system, CCS coelomic circulatory system. Synthetic information on respiratory pigments can be found in [77]