Shell variability of Pseudocubus obeliscus Haeckel in the early autumn radiolarian fauna off Tassha, Sado Island, central Japan

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Shell variability of Pseudocubus obeliscus Haeckel within the early autumn radiolarian fauna collected from the Sea of Japan, off Tassha, Sado Island in 2001 is demonstrated based on detailed scanning electron microscopic observations and comparisons among morphologically varied specimens. Pseudocubus obeliscus has an inverted truncated-pyramidal skeleton consisting of an initial spicule, rings formed by several arches, columellae, and other secondary spines. The initial spicule is comprised of a median bar, an apical spine, a dorsal spine, two primary lateral spines, and an axial spine. The ventral spine and secondary lateral spines are absent. Comparative observations of morphologically varied specimens with reference to highly symmetrical, well-regulated specimens show that morphologic variation in P. obeliscus is caused by the transformation of angles between columellae and a primary ring, shape of the primary ring and columellae, and their combination. In addition, some secondary arches are formed for rebuilding a distorted pyramidal skeleton. Therefore, it is concluded that shell variability is expressed by the skeletal instability of some specific parts and self-organizing formation of secondary arches.
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