Evidence of binary division in mature central capsules of a collosphaerid colonial radiolarian: Implications for shell ontogenetic patterns in modern and fossil species

Abstract

Evidence is presented from fossil shells and living species of the colonial radiolarian Acrosphaera that maturer central capsules with shells can produce daughter central capsules and shells by binary fission. These data indicate that in colonial collosphaerid radiolaria, at least, proliferation of central capsules can occur after maturation and may account for rapid increase in biomass and population size in response to favourable environments. This augments prior evidence that central capsules of non-shelled colonial radiolarin can proliferate by binary fission. Also, these observations extend our understanding of the pattern of silicification and shell formation to include two possibilities: (1) the current view of "simultaneous shell deposition" where all of the skeletons are deposited at nearly the same time following multiple divisions of skeletonless central capsules and (2) "successive shell depositon," where mature central capsules with shells give rise to additional ones through delayed binary fission. These observations have interesting implications for life cycle dynamics of radiolaria in modern and paleo-oceanic environments