Radiolaria and Acantharia
(Contributed by O. Roger Anderson, Columbia University, NY, USA)
Acantharia (Fig. 1) and radiolaria (Fig. 2) are planktonic (floating), single-celled
organisms dwelling largely in the open ocean. Some are carried by ocean currents
toward islands and continental margins, but most do not survive in shallow water
near land. Acantharia produce skeletons composed of strontium sulfate forming
a set of rod-like spines that are anchored to one another at a central point
of attachment or node (Fig. 3). Radiolaria produce skeletons composed of silicate
(glass-like substance) and are highly variable in shape and size – often
of very beautiful ornamentation (Fig. 4). Acantharia and radiolaria are related
to the amoebae and produce long, narrowly tapered and stiffened pseudopodia
called axopodia that radiate out from the cell body. The body of these single-celled
organisms is divided into two regions: 1. A central dense mass of cytoplasm,
known as the central capsule, contains the nucleus (or nuclei) and other major
subcellular structures, and 2. A halo of external frothy or web-like cytoplasm
known as the extracapsular cytoplasm. The axopodia extend outward through the
extracapsular cytoplasm. Algal symbionts are sometimes found in the extracapsular
cytoplasm of the acantharian or radiolarian host (Fig. 2). These algae are held
within vacuoles of the host cytoplasm and provide a food source to the host
through photosynthesis. Acantharia and radiolaria consume a wide variety of
prey ranging from very small algae to small crustacea such as copepods that
swim freely in the water and are captured by the sticky axopodia radiating out
from the central capsule. The skeletons of radiolaria are deposited in the sediments
of the ocean. The layers of skeletons that build up over many millions of years
contain a rich source of information about the kinds of radiolaria that have
lived during these vast time periods. Each species of radiolarian lives in a
particular environment characterized by optimal temperature and salinity conditions
for their growth. Therefore, the kind of radiolaria found in each layer of the
sediment can be used to interpret the climate of the ocean at the point in time
when the radiolaria were alive. The field of micropaleontology is a study of
the history of the earth as revealed through the varying kinds of radiolarian
skeletons and other microfossils deposited in the ocean sediments during each
major time period of the earth’s geological history.