Plain-Language Summary:
Colonial radiolaria are among the largest and most elegant of radiolaria, consisting of a clear gelatinous envelope (sometimes several centimeters in diameter) enclosing numerous spheroidal central capsules, each surrounded by an elaborate siliceous shell in skeletal-bearing species. A few entire colonies are illustrated at: http://palaeo-electronica.org/1998_2/boltovskoy/fig1c.htm. For illustrations of the isolated shells of colonial radiolarians, see the entries in the family Collosphaeridae of the order Spumellaria in the 'Identification of species' section of Boltovskoy's article at: http://palaeo-electronica.org/1998_2/boltovskoy/issue2 .htm. Abundances of colonies can reach several hundred per cubic meter in the oceans, and the siliceous skeletons are widely represented as microfossils in marine sediments. While much attention has been given to their sedimentary distribution and abundance over geological time, less is known about how their skeletons are formed and to what extent mature central capsules are capable of dividing to increase the size of the colony.
It is known that during the development of many species, the central capsules divide repeatedly before all of them secrete a skeleton nearly simultaneously. This is called simultaneous central capsule proliferation and maturation. Moreover, non-skeletal bearing species can increase their size and number of central capsules through binary fission (splitting into two) of the central capsule. Do skeletal-bearing central capsules divide or is the enclosing skeleton the final stage of development?
Data reported here from fossil shells and living colonies collected in the Sargasso Sea indicate that in the genus Acrosphaera that we have been able to observe, mature central capsules can reproduce by a fission process known as budding. A second shell is sometimes produced at one side yielding an hour-glass shape in fossil specimens. Likewise, similar hour-glass shaped, or separated, paired-shells are observed in living individuals. Putative early stages of budding in living individuals have also been observed as bud-like protrusions of the central capsule cytoplasm. This would indicate a "successive" shell deposition as the colony matures, with additional central capsules added at later times during subsequent proliferation within the gelatinous envelope. This may be significant in understanding how colonial radiolaria proliferate asexually and help to explain the relatively large size and high abundances of colonial radiolaria in the surface water of some oceanic locations.
Central capsule: In an individual living radiolarian, a dense central mass of protoplasm which includes the nuclear material, as well as constituents important in respiration, digestion and food storage. A central capsular membrane separates this intracapsular material from the extracapsular protoplasm. In a colonial radiolarian, each central capsule defines a single organism, and the extracapsular protoplasm holds the central capsules (with their shells, when present) together to form the colony.
Diagenesis: Chemical change after sedimentation.
Ontogenesis: The course of development of a single organism.
Phylogeny: The evolution of a genetically related group of organisms.
Polycystinea: Radiolaria in which the skeleton is of simple opaline silica, not of a combination of silica and organic material as it is in the Phaeodaria.
O. Roger Anderson, Biological Oceanography, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, U. S. A.
Copyright: Palaeontological Association, 1 September 1998