Spicule formation in the New Zealand ascidian Pyura pachydermatina (Chordata, Ascidiacea)

The New Zealand solitary ascidian Pyura pachydermatina (Phylum Chordata, Subphylum Urochordata) is a sessile filter feeder in rocky wave-swept coastal areas. The body is on a long stalk; both are covered by a tough fibrous tunic. Two types of spicules are formed in vascularized areas: "antler-shaped" branched spicules of amorphous calcium carbonate in blood sinuses in the body tissues and "dogbone-shaped" knobbed calcitic spicules in the tunic blood vessels. Both types form extracellularly, contain intraspicular organic components and are covered by an organic matrix coat within an epithelium of sclerocytes. SEM and TEM analysis of the spicules and their formation is included, along with calcein incorporation data used to estimate rate of growth. A comparison with spicules of other ascidians and in selected other organisms is included, with comments on the shared features of biomineralization that these disparate groups exhibit.