Prorocentrum formosum Faust, 1993b
Species Overview:
Prorocentrum formosum is an armoured, marine, benthic dinoflagellate species. This species is associated with floating detritus and sediment in tropical embayments of the Caribbean Sea.
Taxonomic Description:
Prorocentrum formosum is a bivalvate species often observed in valve view. Cells are 25-28 µm long and 15-16 µm wide. They are small and ovate with a flattened and angled anterior (Figs. 1,2), and a smooth surface (Figs. 4,5). The relatively thick thecal valves (Fig. 9) reveal two different sized valve pores: small marginal pores (< 0.1 µm in diameter) and large furrowed trichocyst pores (0.2 µm in diameter) (Fig. 6). These pores are unevenly distributed on the cell surface and form a characteristic pattern. The large trichocyst pores number approximately 42-55 per valve, are even in size, and round to oblong in shape with a smooth edge (Fig. 8). The small marginal pores are also even in size, and round with smooth edges (Figs. 6,8). The center of each valve is devoid of pores. The intercalary band is transversely striated (Figs. 7,8) (Faust, 1993b).
The periflagellar area on the right valve is inclined and off-center. It is a broad, V-shaped, highly ornamented shallow depression (Figs. 1,4-6). It is made up of 5-6 apical platelets. The periflagellar area houses a large and prominent curved periflagellar collar on one side, and opposite is a smaller angled periflagellar plate. The periflagellar collar can be recognized under LM and may appear as a spine. Between the two periflagellar structures lies a single large pore, the flagellar pore (Figs. 4-6). A protuberant periflagellar flange borders the flagellar pore opening (Fig. 6). The anterior end of the left valve is flattened and oblique (Figs. 2,3) (Faust, 1993b).
Morphology and Structure:
Cells of P. formosum are photosynthetic and contain distinctly pigmented cinnamon-brown chloroplasts. The nucleus is situated anteriorly. Cells of this species are easily distinguishable from other Prorocentrum species because of their distinct color (Faust, 1993b).
Reproduction:
Prorocentrum formosum reproduces asexually by binary fission.
Species Comparison:
P. formosum has a valve pore pattern and cell shape similar to P. elegans (Faust, 1993a); however, P. formosum cells are larger with a more complex periflagellar architecture. These two species share a number of characteristics: a.) straight sides; b.) anterior end of the left valve is oblique, although the anterior end of P. elegans can also be flattened (Faust, 1993a); c.) a smooth thecal surface with two pore types, situated in similarly arranged arrays: P. formosum has 42-55 large pores per valve, while P. elegans bears 20-22 large pores per valve; and d.) a smooth transversely striate intercalary band with broad evenly spaced bands (Faust, 1993a, Faust, 1993b).
The valve pore pattern of P. formosum differs from that of other small Prorocentrum species with smooth thecal surfaces: P. venetum has valve pores that radiate perpendicularly from the posterior cell margin (Tolomio and Cavolo, 1985a); P. ovum has irregular scattered valve pores (10-14 per valve) (Schiller, 1918; Dodge, 1975); and P. nanum has only a few pores (1-4 per valve) (Schiller, 1918; Dodge, 1975).
The periflagellar area of P. formosum is different from that of P. mexicanum (Faust, 1990b) and P. elegans (Faust, 1993a). The periflagellar area of the latter two species has 8 platelets and two pores, while P. formosum has only 5-6 apical platelets and one flagellar pore. P. triestinum, a planktonic species, also displays just one pore in the periflagellar area (Loeblich et al., 1979b). In P. formosum the flagellar pore is bordered by a periflagellar flange not observed in any other species of Prorocentrum (Faust, 1993b).