Tursiocola gracilis

This species grows exclusively on the skin of West Indian manatees.

Frustules are linear-rectangular in girdle view, with a narrow cincture and porose copulae. Valves are linear to linear-lanceolate, with weakly produced obtuse apices. The raphe is straight, with straight proximal raphe ends. Proximal raphe ends terminate within a central area that appears stellate in outline in LM because of optical interactions with the closely underlying butterfly structure. The central area is intersected by a very narrow, linear stauros. Transapical striae are slightly convergent in the middle of the valve becoming parallel toward the apices. Pseudosepta extend deeply into the frustules for approximately 1/3 of the valve length from each apex.

SEM: Externally, transapical striae are uniseriate. Proximal raphe ends are slightly expanded and open into spathulate grooves. Distal raphe ends are deflected toward the secondary side of the valve. Internally, pseudosepta extend from the apices concealing the distal raphe ends and most of the valve interior. Pseudosepta are narrow strips along the valve margin and widening at the butterfly-like structure. Internally, the raphe fissure is slightly expanded in the central area and is positioned along the center of a raphe rib. Two C-shaped knobs are present on the raphe rib on opposing sides of the raphe at the valve center.