Annelid Coelomic Fluid Proteins

The coelomic cavity is part of the main body plan of annelids. This fluid filled space takes up a considerable volume of the body and serves as an important site of exchange of both metabolites and proteins. In addition to low molecular substances such as amino acids and glucose and lactate, the coelomic fluid contains different proteins that can arise through release from adjacent tissues (intestine) or from secretion by coelomic cells. In this chapter, we will review the current knowledge about the proteins in the annelid coelomic fluid. Given the number of more than 20,000 extant annelid species, existing studies are confined to a relatively few species. Most studies on the oligochaetes are confined to the earthworms-clearly because of their important role in soil biology. In the polychaetes (which might represent a paraphyletic group) on the other hand, studies have focused on a few species of the Nereidid family. The proteins present in the coelomic fluid serve different functions and these have been studied in different taxonomic groups. In oligochaetes, proteins involved antibacterial defense such as lysenin and fetidin have received much attention in past and ongoing studies. In polychaetes, in contrast, proteins involved in vitellogenesis and reproduction, and the vitellogenic function of coelomic cells have been investigated in more detail. The metal binding metallothioneins as well as antimicrobial peptides, have been investigated in both oligochaetes and polychaetes. In the light of the literature available, this review will focus on lipoproteins, especially vitellogenin, and proteins involved in defense reactions. Other annelid groups such as the Pogonophora, Echiura, and Sipuncula (now considered polychaetes), have not received much attention and therefore, this overview is far from being complete.