The marine planktonic diatom Rhizosolenia robusta (Bacillariophyta): morphological studies support its transfer to a new genus, Calyptrella gen. nov.

Abstract

The marine planktonic diatom Rhizosolenia robusta Norman is widely distributed in tropical and subtropical areas and occurs occasionally in more temperate ones. Traditionally it has been considered a species of the genus Rhizosolenia, but its taxonomic status has not been reviewed. The present study is based on light and electron microscopy of net plankton samples front several tropical and Subtropical locations. including Pacific and Atlantic coasts of Mexico and the Indian Ocean. Features unique for this species include Undulated and ribbed valves, the Structure Of the valve wall, the morphology of the process at the apex of each valve, the location and shape of the labiate structure, and the arrangement of the girdle hands. These features Support transfer to a new genus Calyptrella gen. nov. its Calyptrella robusta (Norman) Hernández-Becerril et Meave. Morphological variability includes Shape Of file cell and length of the external tube. The term 'calyptra' is reintroduced for it structure analogous to the process of Rhizosolenia sensu stricto. The morphology of the complex labiate structure-external tube is discussed, and the term 'conflatuportula' (United rimoportula) is proposed for this Structure. Further studies are necessary to decide whether Rhizosolenia firma Karsten belonggs to the new genus.