ETHMODISCUS REX COLLECTED BY NET SAMPLING OFF THE COAST OF NORTHERN CHILE

Abstract

Ethmodiscus is an oceanic taxon found widely scattered throughout the intertropical and temperate zones of all the world's oceans. Although the two taxa to which references are more frequently made, namely E. gazeilae and E. rex, have been reported for the southeastern Pacific, this study reveals that only E. rex is present in Chilean waters, as far as 26° 20'S; it is always associated with subtropical waters. Most of the investigations on Ethmodiscus have been made on fragments or isolated valves, and thus do not provide enough information about the morphology of the genus. In the present study the examination of more than seventy whole frustules by SEM and TEM has given relevant additional information about the essential characteristics of E. rex. The shape of the cell, the orientation, size, shape and structure of the areolae, the distribution of the rimoportulae on both valve centres, and the morphological features of the bands are recognized as fairly constant characteristics. Heterovalvy, suggested to occur by other authors, has been confirmed in this study. The special structure of the areolae and of the marginal rimoportulae, reported previously for E. gazellae, make it difficult to establish the correct taxonomie position of the genus.