Fine structure of a collodarian radiolarian ( Sphaerozoum punctatum Mu¨ller 1858) and cytoplasmic changes during reproduction

Abstract

Sphaerozoum punctatum is a colonial radiolarian possessing siliceous spicules. Numerous cells are embedded within a common gelatinous matrix. A large vacuole, apparently containing a lipid reserve, occurs at the center of the cell. The peripheral cytoplasm contains smaller vacuoles, tubular mitochondria, Golgi bodies, single membrane organelles resembling peroxisomes and numerous nuclei possessing prominent patches of condensed chromatin. The central capsule membrane fine structure resembles that in Collozoum inerme Mu¨ller 1858 (a skeleton-less colonial radiolarian). Dinoflagellate zooxanthellae are enclosed within ectoplasmic sheaths and the spicules are surrounded by a thin cytoplasmic envelope, a feature also observed in noncolonial, skeleton-bearing Radiolaria. During reproduction, the endoplasm forms flagellated swarmers containing prominent nuclei surrounded by a thin layer of cytoplasm with lipid inclusions. Dividing nuclei contain internal microtubules attached to the inner surface of the nuclear membrane at each pole. The central capsule membrane persists during early stages of swarmer development.