Recent knowledge on radiolarians (Polycystina) based on live cell observation and culture experiments

Abstract

Radiolarians are a major group of Rhizaria. Three orders of Radiolaria are characterized by a siliceous skeleton, and are called polycystines (Polycystina). The skeletons of polycystines are well preserved in sediments as fossils. These fossils are important for paleontological analysis; thus, many stratigraphic, paleoceanographic and taxonomic analyses of polycystine fossils have been conducted from the 19th century to the present day. However, there is a lack of data on living polycystine cells, and little is known about their biology and ecology because of the difficulties in collecting and culturing cells. Despite these difficulties, several reports have succeeded in examining living cells. These studies have clarified cell structure, pseudopodial activity, nutritional uptake, skeletal growth, and survival strategies. In this review, recent progress in experimentation on living polycystine is summarized, and unsolved problems are discussed. Polycystines inhabit an unusually long geological range from the Cambrian to the Present, and their evolutionary processes can be revealed by fossil analysis, which is an advantage in researching relationships among the paleoenvironment, paleoecology, and evolution. Further research into living polycystine cells would encourage new developments in both biological and paleontological researches.