SURFACE-MORPHOLOGY OF THE MARINE DINOFLAGELLATE SINOPHYSIS-MICROCEPHALUS (DINOPHYCEAE) FROM A MANGROVE-ISLAND, TWIN CAYS, BELIZE

Sinophysis microcephalus Nie and Wang 1944 is a nonphotosynthetic, tropical, benthic, dinophysoid dinoflagellate. I isolated it from floating detritus on a subtropical mangrove island, Twin Cays, Belize, Central America, and describe its micromorphology from light and scanning electron micrographs. Cells of S. microcephalus are circular to subcircular and compressed laterally with a cell size of 42-44 mum long and 33-35 mum wide and with a length / width ratio of 1.25-1.28. Areolae are numerous, 368-550 per valve, ranging in size from 0.75 to 2.0 mum. Pores are oblong and deeper at the valve's center and pentagonal-shaped at the plate margin. The well-defined cingulum is narrow and deeply incised with a smooth surface. The epitheca is small, moderately convex, and divided into two large, highly ornate, asymmetrical plates: the left and right epithecal plates. The left epithecal plate bears two slightly curved, upright anterior projections located dorsally adjacent to the epithecal list, a relatively large opening, and three smaller openings compressed against the sagittal suture. The right plate contains a wide megacytic zone with two parallel ridges, a fairly large oblong apical pore in ventral position adjacent to the cingulum, and eight areolae each with a round, uniform-sized pore opening. There are two long and narrow sulcal lists, gently convex with a smooth edge without structure or ribs. The left sulcal list has an ear-shaped lobe, a form of a primitive dinophysoid list. The megacytic zone is smooth and expands unevenly during cell division. The epitheca and sulcus distinguishes S. microcephalus from all examined Dinophysis.