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, tro pical, benthic, dinophysoid dinoflagellate. I isolated it from floatin g detritus on a subtropical mangrove island, Twin Cays, Belize, Centra l America, and describe its micromorphology from light and scanning el ectron micrographs. Cells of S. microcephalus are circular to subcircu lar 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. Po res are oblong and deeper at the valve's center and pentagonal-shaped at the plate margin. The well-defined cingulum is narrow and deeply in cised with a smooth surface. The epitheca is small, moderately convex, and divided into two large, highly ornate, asymmetrical plates: the l eft and right epithecal plates. The left epithecal plate bears two sli ghtly curved, upright anterior projections located dorsally adjacent t o the epithecal list, a relatively large opening, and three smaller op enings compressed against the sagittal suture. The right plate contain s a wide megacytic zone with two parallel ridges, a fairly large oblon g apical pore in ventral position adjacent to the cingulum, and eight areolae each with a round, uniform-sized pore opening. There are two l ong 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 exp ands unevenly during cell division. The epitheca and sulcus distinguis hes S. microcephalus from all examined Dinophysis.