Respiration
Sipunculids with reduced tentacular crowns – such as P. nigrescens – cannot rely on diffusion through the tentacle-contractile vessel complex as with other sipunculids. Rather, P. nigrescens relies on diffusion through its epidermal membranes. Due to its relatively sedentary life style, and minimal energy needs, this method does not limit P. nigrescens activity (Cutler, 1994).
P. nigrescens uses the molecule hemerythrin, rather than haemoglobin (as in humans), for oxygen transport inside the coelomic and tentacular fluids (Cutler, 1994).
Hemerythrin (Figure 15) is an oligomeric protein, and is unusual in that the oxygen binding site consists of two iron centres bound by carboxylate side chains of glutamate, aspartase and five histidine residues. A hydroperoxide (OOH-) complex is produced after oxygen is uptaken through two-electron oxidation of the diferrous center (Friesner et al., 2003). Hemerythrin is imperative for sipunculid respiration, as it can carry three times more oxygen than sea water: 20 cc/l (Chaphaeu, 1928).
Figure 15: Single oxygenated hemyrithrin protein, from Wikimedia commons
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