Dactylodinium arachnoides sp. nov. (Borghiellaceae, Dinophyceae): a new marine dinoflagellate with a loop-shaped apical structure complex and tubular membranous extrusomes

The morphology and phylogeny of an enigmatic marine dinoflagellate were examined using LM, SEM, TEM, and molecular phylogeny inferred from rDNA. Cells were elliptical with a conical epicone and a hemispherical hypocone. They measured c. 24.7 ?m long, with a cingulum descending approximately twice its own width. Amphiesmal vesicles were thick and transparent, and were supported by translucent ridges at the suture, which were distinctive and visible by light microscopy. A large peduncle extending to the anterior was accommodated in the anterior sulcal extension. A nucleus was usually located in the hypocone, and chloroplasts were distributed peripherally. The apical structure complex was loop-shaped, originating from the right side of the anterior sulcal extension and extending upward to encircle the dorsal side of the cell apex in an anticlockwise direction. TEM showed trichocysts with lateral hairs, a pyrenoid penetrated by cytoplasmic insertions containing tubules, and an eyespot comprising globules within the chloroplast. Cells also had tubular extrusomes, unique in dinoflagellates, composed of a membrane stacked in a vesicle, and formed a long membranous tube with 2?4 finger-like terminal branches after being released from the vesicle. Molecular phylogenies showed the affinity of the species to Dactylodinium pterobelotum in the family Borghiellaceae. We propose Dactylodinium arachnoides sp. nov. for the dinoflagellate described here. It shares the characteristic traits of amphiesmal vesicles, a peduncle, a pyrenoid, and trichocysts with D. pterobelotum, whereas the number of amphiesmal vesicles, the loop-shaped apical structure complex, eyespot type A, and tubular membranous extrusomes are distinct.