New observations on the large hemidiscoid diatom Palmerina ostenfeldii and its symbiotic ciliate Vaginicola collariforma sp. nov. from subtropical Australian waters

The large hemi-discoid diatom Palmerina ostenfeldii was common in subtropical Moreton Bay, Australia, following a flood in January 2011. All diatom cells exhibited diagnostic subapical folds settled by loricate peritrich ciliates, but which readily abandoned stressed diatom cells. We characterized both diatoms and ciliates by morphological and molecular analyses, including careful video observations on non-preserved cells immediately after collection. The fold in the diatom cell wall comprises a narrow shelf upon which the ciliates attach (on average seven per fold, and similar for the two folds of a single diatom cell) but without penetrating the diatom wall itself. Folds were fully developed in newly formed internal valves, indicating that the ciliates play no role in their morphogenesis. SSU rRNA sequences of P. ostenfeldii from Moreton Bay (with ciliates) differed by 25 bp (1.5%) from those of P. hardmaniana from Texas (without ciliates), but surprisingly rbcL chloroplast sequences for both diatom species were indistinguishable. The ciliate species epiphytic on P. ostenfeldii, previously referred to as the cold-water tintinnid Amphorella borealis could not be assigned to any known species. We formally describe it here as Vaginicola collariforma sp. nov. within the crown clade of peritrichs which also includes the closely related genus Cothurnia (but distinguished as having an external stalk). Beating of the peritrich oral cilia was observed to drive rotational movement of the large discoid diatoms as in a ferris wheel. The observed diatom-ciliate symbiosis may represent a unique evolutionary adaptation in which the ciliate is host-specific and attaches in a specific locality on the host cell.