LM morphology: frustules are rectangular in girdle view with bluntly rounded ends and porose girdle copulae (Figs 1–4). Pseudosepta are evident in girdle view and are deflected towards the interior of the frustule at their termination (Figs 1–2, 4). Valves are isopolar to slightly heteropolar and narrowly lanceolate, tapering gradually from the middle of the valves to the juncture of the pseudosepta and the valve margin and then tapering more rapidly towards sub-acute apices (Figs 5–13). Valve morphometrics are as follows: length 10–20 μm, width 2.0–2.9 μm, length to width ratio 4.1–7.4, n = 30. Heteropolarity of the valve, when present, is evidenced by small differences in the lengths of the valve halves and differences in the shape of the apices (i. e., one valve apex, usually the shorter valve half, may be more bluntly rounded than the opposing end). The valve face is slightly asymmetric around a narrow axial area with 1/2 of the valve face slightly wider than the other (Figs 5–7, 9, 11–13). The raphe is straight and the central area is formed by a wide stauros that extends to the valve margins (Figs 5, 7, 9, 11–13). The transapical striae are radiate around the central area becoming undiscernible in LM at the apices, 26–34 in 10 μm, n = 30 (Figs 5, 7, 9, 11–13). By focusing through the valve, each pseudoseptum can be seen to extend over ca. 1/6 of the valve length from the apices and then continues as narrow strips along the valve margin. These strips then widen at the central area where they fuse with the stauros (Figs 8, 10). SEM morphology: Externally, the valve face has uniseriate transapical striae composed of transapically elongated areolae (Figs 16–21). The striae are radiate and more distantly spaced in the middle of the valve, 26–34 in 10 μm. n = 5 (Figs 16–18, 20), becoming less radiate to parallel and more closely spaced towards the apices, 37–43 in 10 μm, n = 5 (Figs 16–17, 19, 21). The striae extend partly onto the valve mantle terminating before a hyaline mantle margin (Fig. 15). The areolae are arranged in slightly undulate longitudinal rows along the transapical axis, 33–37 areolae in 10 μm, n = 5 (Figs 16–21). The valve mantle slopes steeply without a sharp transition between the valve face and mantle (Fig. 15).The mantle margin is undulate (Fig. 15). A straight and narrow rib lies within the axial area (Figs 16–21). The raphe is slightly eccentric and straight (Figs 16–21). A large, non-punctate, rectangular to bow tie-shaped stauros that extends to the valve margins is located in the central area (Figs 16–18, 20). The external proximal raphe ends are straight, opening into asymmetric spathulate grooves, and terminate where the stauros extends towards the valve margin (Figs 16–18, 20). The distal raphe ends are hooked but are obscured by overhanging siliceous flaps that bend towards the same side of the valve at both apices (Figs 19, 21). Internal views of the valve reveal the butterfly-like structure that connects the pseudosepta to the central area and stauros (Figs 22–24). The pseudosepta extend from the apices as siliceous plates for ca. 1/6 of the valve length, and then continue as narrow strips that run along the valve margins before expanding into the “wings” of the butterfly-like structure in the central area (Figs 22–24). The wings fuse with the large oval central area close to the raphe sternum (Fig. 23). The wings of the butterfly widen at their junction with the central area forming narrow concave “wings” on each side of the raphe sternum (Figs 22–24). Internally, the raphe slits open along the middle of a strong siliceous rib that widens slightly in the central area (Figs 22–24). A single knob-like structure is present on the rib at the valve center (Figs 22–24). The bands of the cingulum are differentiated into two types: closed valvocopulae (Figs 25–27) and multiple copulae (up to 4 observed) that are open at one end (Figs 25, 29, 30). The valvocopula is flanged inward on the pars interior where it attaches to the valve (Fig. 26). The flange widens into 3 pairs of opposing tabs (Figs 26–27). One pair of tabs is located at the valve middle underneath the butterfly structure, whereas the other two pairs are located near the poles underneath the valve where the pseudosepta start to widen at the valve apices (Figs 24, 26–27). The valvocopula has a single row of ovoid pores around the perimeter, pore density = 37–45 in 10 μm (Figs 26–28), and a second row of similarly spaced pores located abvalvar to the complete row only at the poles (Fig. 28). Views of the pars exterior side of the valvocopula show a thickening of the polar tabs and extension of the pores through the tabs (Figs 24, 27). The copulae have only a single row of ovoid pores, pore density = 39–54 in 10 μm, n = 2 (Figs 29–30).

Type:— UNITED STATES. Florida: Florida Bay, samples removed from the skin in the dorsal neck area of loggerhead sea turtles Caretta caretta, 24º 55’ 01” N, 80º 48’ 28” W, B. A. Stacy, 24 June 2015 (holotype CAS! 223049, illustrated as Figs 1–4, 6, 12, 15–30, paratypes ANSP! GC 59142, BM! 101 808, illustrated as Figs 7–10, 14, BRM! ZU10/31, Figs 5, 11, 13).

Etymology:— the epithet honours Dr. Luc Denys (Research Institute for Nature and Forest, Brussels, Belgium), in recognition of his important research on epizoic diatoms.

Taxa relative abundances:— 46 taxa from 23 genera were observed from the neck skin of 4 loggerhead turtles. The relative abundances of the newly described Tursiocola denysii ranged from 5 to 43%. The most relatively abundant taxon was Hyalosynedra cf. laevigata (Grunow 1877: 166) D.M. Williams & F.E. Round (1986: 316), comprising 2– 62% of the valve counts. The next most relatively abundant taxon was an undescribed species of Poulinea Majewska, De Stefano & Van de Vijver (in press), comprising 13–47% of the valve count. We were unable to identify ca. 7% of the valves to genus or species. Only four other taxa (i. e., Mastogloia cuneata (Meister 1937: 268) Simonsen (1990: 134), Navicula sp., Achnanthes cf. pseudogroenlandica Hendey (1964: 177), and Brachysira aponina Kützing (1836: 3)) exhibited mean relative abundances>1%. The remaining taxa observed are common benthic forms. Only a single valve of Tursiocola ziemanii Frankovich & M.J. Sullivan in Frankovich et al. (2015: 34) of the seven previously described Tursiocola species was observed in the material.