Morphology and Systematics

Abstract

Radiolaria are holoplanktonic protozoa widely distributed in the oceans, including arctic, subtropical, and tropical waters. They occur throughout the water column from the near surface to hundreds of meters depth. As with many planktonic organisms, their abundance in a geographical region is related to the quality of the water mass, including such variables as temperature, salinity, productivity, and available nutrients. They are largely nonmotile organisms, and their general morphology clearly reflects an adaptation for a floating existence. Various structures in the cytoplasm enhance buoyancy, including bubble-like alveoli in the peripheral rhizopodia of some species (e.g., Thalassicolla nucleata, Fig. 1-1A) and lipid globules localized in the dense central cytoplasm or dispersed within the surrounding complex cytoplasmic network. The skeleton, when present, is composed of amorphous silica, and is probably the most widely recognized morphological feature of the radiolaria (Figs. 1-1B-D and 1-2). It is also a major feature distinguishing the radiolaria from their close relatives, the Acantharia, possessing strontium sulfate skeletons (Fig. 1-3). Acantharian skeletons, moreover, are often much more massive with intersecting rods joined at the center of the skeletal array arranged as a set of Cartesian axes. Some Acantharia, however, possess very delicate spines and great care must be taken to distinguish them from the elaborate glassy skeletons of the radiolaria.