Functional role of test cells in swimming larvae of Ascidia malaca: ultrastructural and cytochemical investigations.

Abstract

The functional role played by test cells in larvae of various ascidian species consists in depositing sub-microscopic structures known as ornaments and/or proteoglycan substances on the larval test surface. According to the data reported in the literature, the deposition of ornaments together with proteoglycan substances on the larval test would render the latter hydrophilic and thus allow the larva to swim being immersed in water. Ornament deposition on the larval test does not occur in all the ascidian species. Ultrastructural investigations made on larvae belonging to the Cionidae and Ascididae families, for instance, have failed to evidence the presence of ornaments on the test. For these ascidian families it has been hypothesized that in swimming larvae test cells secrete an amorphous substance that would allow them to adhere to the larval test. In order to ascertain the functional role played by test cells in swimming larvae of the Ascididae family, the presently reported ultrastructural and cytochemical investigations have been made on larvae of Ascidia malaca. Besides suggesting that test cells, tightly adherent to the test surface, present an amoeboidic behaviour, the ultrastructural investigations have evidenced that these cells are still metabolically active. Their cytoplasm, characterized by the presence of a Golgi apparatus actively involved in synthesis, is almost entirely filled with very large granules; some of them gradually empty their contents turning into vacuoles containing scarce residues of electrondense particles. The present ultrastructural observations support the hypothesis that the adhesion of test cells on the larval test could be very likely eased by the secretion of substances synthesized by the Golgi and released through pseudopodes which test cells then wedge into the test. The cytochemical investigations were based on a reaction (fixation in glutaraldehyde-tannic acid) which evidences the presence, at the ultrastructural level, of proteoglycan substances such as glycosaminoglycans (Singley and Solursh, 1980). The reaction has given positive results in test cell granules undergoing emptying, on the outer membrane of the same cells, and on the outer cuticular layer C1 of the larval test. The present investigations, besides confirming the absence of ornament deposition on the test surface by test cells of Ascidia malaca swimming larvae, have evidenced that the secretion products deposited on the larval test surface by test cells consist of glycosaminoglycans, i.e. proteoglycan substances. In agreement with the data reported in the literature, it is hypothesized that the deposition of glycosaminoglycans on the surface of Ascidia malaca larval test makes the larval tunic hydrophilic and thus the larva is able to swim being immersed in water.