ASCIDIAN LARVAE - THE ROLE OF TEST CELLS IN PREVENTING HYDROPHOBICITY

Ascidian test cells co-differentiate on the surface of each ovarian oo cyte beneath the vitelline coat. They become vacuolated and later occu py the perivitelline compartment of each egg and embryo. In some speci es, their vacuoles contain submicroscopic granules or filaments called 'ornaments' and acidic glycosaminoglycans. These test cells deposit t heir products on the surface of the larval tunic in late embryogenesis . In these species, the test cells are lost at hatching. In other spec ies, the test cell vacuoles contain acidic glycosaminoglycans, but no ornaments. In these species, the test cells attach to the larval tunic and probably secrete acidic glycosaminoglycans. We deprived the embry os of seven species of ascidians of their test cells and vitelline coa ts during mid-embryogenesis. After completing their development, the l arvae of both kinds of species were hydrophobic. They easily become tr apped on the surface of sea water in cultures. Normal larvae (controls ), bearing rest cell secretions, are hydrophilic and never become trap ped. We infer that negatively charged secretions of the test cells mak e normal larvae hydrophilic. Some molgulids with direct development ha ve no test cells, no fins and no swimming larva. We reason that the te st cells of these species may have been lost during evolution because they no longer had an important role in preventing hydrophobicity.