Studies on the benthic genus Sinophysis (Dinophysales, Dinophyceae) II. S. canaliculata from Rapa Island (French Polynesia)

The poorly known dinoflagellate Sinophysis canaliculata, which was originally described from the Indian Ocean, was abundant in a collection from 2015 from Rapa Iti, a small, remote island of French Polynesia. Since the taxonomic description of this species was incomplete, a detailed morphological study was carried out. The theca was studied with epifluorescence microscopy using Solophenyl Flavine 7GFE500, a fluorescent dye specific to cellulose. This fluorophore was used successfully for the first time with a thecate dinoflagellate. It has the advantage of using blue excitation light and avoids the fading of fluorescence encountered with other dyes with long excitation time. Using this technique and high-resolution field emission scanning electron microscopy, the thecal plate pattern was clarified, and the epitheca had six major plates, 4E and 2A. A conspicuous apical pore was present on the left side of the epitheca. A smaller pore was located on the ventral left side of the apical pore. The apical plates A(1) and A(2) formed prominent projections which encircled and partly covered the pores. This thecal pattern agreed with the recent emended description of the genus. Two additional platelets, visible only from inside the cell, were possibly present in contact with the pores. In addition, the 'canal' cut on the left hypothecal plate H-2 had a slit at its base. Seen from the inside, the slit was actually a sieve-like area comprising several small pores with a diameter of 80-100 nm. We hypothesise that this novel structure functions in the extrusion of mucus threads.