Hemerythrin occurrence

The case of hemerythrin (see, for example, Stenkamp, 1994 Kurtz, 1999) is of particular interest for several reasons. It is clearly distinct from the other hemo-respiratory proteins and also occurs as analogous blood and muscle types (see, for example, Takagi and Cox, 1991). Intriguingly, its occurrence in animals (specifically the brachiopods, sipunculans, priapulids, and a number of annelids) makes no phylogenetic sense in terms of our current understanding of metazoan relationships. This is most obvious in terms of the placement of the priapulids in the ecdysozoans (i.e. related to arthropods), whereas the other three phyla are all lophotrochozoans. Even within this latter superclade, the distribution of hemerythrin does not seem to be phylogenetically informative. 

The biological occurrence of hemerythrins (Hr in Figure 4.8), the third class of dioxygen carriers, is relatively rare, being restricted to the sipunculid family (nonsegmented worms), a few members of the annelid (segmented worm) fam-... 

The distribution of hemerythrin raises the question of whether its occurrence in representatives of these systematically isolated groups has. any phylogenetic significance. As far as annelids and sipunculids are concerned, their relationship has been assumed by the zoologists. No relation, however, exists between the priapulids and the brachiopods. The scattered distribution of hemerythrin in nature is reminiscent of that of hemocyanin. 

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