Cytoplasmic and shell fine structure of Tetrapetalon elegans (Polycystinea) and comparisons to Hexacontium spp. with implications for phylogeny and taxonomy of the Spumellarida

Abstract

The cytoplasmic fine structure of Tetrapetalon elegans (synonym?: Hexalonche amphisiphon) is characterized by loosely arranged radial lobes extending from a centrally located, lobate nucleus. The capsular wall is thin (ca. 250 nm) composed of a membranous envelope, and there is no evidence of fissures as occur in thicker central capsular walls, e.g., Thalassicolla sp. The fusules project through a thin, slightly flared collar that appears to make cytoplasmic bridges to the fusule strand at places, but is sometimes unconnected, leaving small openings. These openings may permit exchange of fluid and gases between the central capsule and the surrounding space containing the extracapsulum. The fusule strands are connected to the intracapsular lobes by a short cytoplasmic segment ca. 300 to 500 nm in length, and each extends through the fusule collar that projects above the surface of the capsular wall forming a tube-like extension. The fine structural characteristics of the cytoplasm and of the porous, laminar skeleton are very different from other genera with porous skeletons (e.g., Hexacontium) and supports current revisions in taxonomy placing Tetrapetalon and Hexacontium in separate families in contrast to Haeckel's original designation. These findings are discussed in relation to toxonomy of the Spumellarida.