EXTRACELLULAR FORMATION OF BODY AND TUNIC SPICULES IN THE NEW-ZEALANDSOLITARY ASCIDIAN PYURA-PACHYDERMATINA (UROCHORDATA, ASCIDIACEA)

The New Zealand ascidian Pyura pachydermatina has a 7-10 cm long body at the end of a stalk up to 1 m long and 1-2 cm in diameter. Two diffe rent spicule types are present: dumbbell-shaped spicules of calcite in the fibrous tunic that covers the body and stalk, and antler-shaped s picules of amorphous calcium carbonate in the soft body tissues. Both types form extracellularly within a closed compartment surrounded by a n epithelium of sclerocytes. In adults the tunic spicules form in 2-3 weeks in the lumen of the tunic blood vessels, as determined by calcei n uptake studies. They add mineral only while surrounded by the sclero cyte epithelium, which is anchored to the vessel wall, Ultimately the sclerocytes rupture at one or more leading points on the spicule. The blood vessel epithelium also becomes very thin at these points and eit her ruptures or the cells separate, allowing the spicules to migrate o ut into the tunic. The sclerocytes degenerate and the blood Vessel clo ses behind the migrating spicule, thus maintaining the vessel's integr ity. Tunic spicules accumulate in the subcuticular region of the stalk , but the outermost layer of tunic covering the body is periodically s loughed off along with some spicules. This gives the ''neck'' between body and stalk a flexibility that allows it to orient to currents, and prevents an accumulation of epizoic organisms on the body. The antler spicules form within blood sinuses of the body tissues, The mineral a nd organic material are arranged in concentric layers. In the branchia l sac, oral tentacles, gut and endostyle, where antler spicules occur most densely, the branches interlock, providing support to the soft ti ssues. They are of many sizes and apparently remain where they form, i ncreasing in number and size throughout the animal's lifespan. (C) 199 7 Published by Elsevier Science Ltd on behalf of the Royal Swedish Aca demy of Science.