Abstract
Thalassicolla nucleata is a skeletonless, large spumellarian radiolarian (ca 3 mm diameter) occurring abundantly in the surface water of the Sargasso Sea. It has a complex cellular organization consisting of (1) a well-defined central capsule surrounded by a non-living capsular wall perforated by numerous cytoplasmic strands and (2) an extracapsulum containing a frothy layer of alveolate cytoplasm penetrated by numerous rhizopodia. Evidence is presented for physiological specialization complementing this structural compartmentalization. Acid acryl phosphatase activity (a digestive vacuole marker enzyme) is approximately 3 times greater in the extracapsulum compared to the intracapsulum; whereas, cytochrome oxidase (a respiratory marker enzyme) is approximately 1.5 times greater in the intracapsulum compared to the extracapsulum. Furthermore, cytochemical and electron microscopic evidence indicates that the extracapsulum, containing large vacuoles and copious amounts of acid phosphatase reaction product, is a major site of catabolism; while the intracapsulum, containing the nucleus, numerous Golgi bodies, and a rich supply of mitochondria, is a substantial site of anabolism, lysosomal secretory activity, and food product storage. This marked level of cellular specialization in T. nucleata may enhance its biological efficiency and thus partially account for its abundance in oligotrophic environments.
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