Dinophysis sacculus Stein, 1883
Species Overview:
Dinophysis sacculus is an armoured, marine, planktonic dinoflagellate species. It is a toxic species associated with DSP outbreaks in Europe.
Taxonomic Description:
Species in this genus are laterally compressed with a small, cap-like epitheca and a much larger hypotheca (dorso-ventral depth of epitheca is 1/3 to 1/2 hypotheca). The shape of the cell in lateral view is the most important criterion used for identification (Taylor et al., 1995).
Cells of Dinophysis sacculus are long and oval with a rounded posterior (Figs. 1,2). It is typically sack-like in shape and highly variable in width (Figs. 4-6, 12,13). A short left sulcal list (about 1/2 length of the cell) extends midway down the hypotheca (Figs. 1-3). Occassionally cells are found with a few small blunt spines on the posterior end (Figs. 7-9,11) (Larsen and Moestrup, 1992, Taylor et al., 1995, Zingone et al., 1998).
The thecal surface is covered with small unevenly distributed pores; however, the surface texture can vary from completely smooth (Figs. 9,10,14) to coarsely areolated (Figs. 1-3,15,16). Pores are not found in the megacytic zone (Fig. 9). Cell size ranges: 40-60 µm in length and 20-40 µm in width (Larsen and Moestrup, 1992, Taylor et al., 1995, Zingone et al., 1998).
Thecal Plate Description:
The small epitheca is made up four plates nearly totally obscured by the well developed cingular lists. The cingulum (Fig. 3) is bordered by two cingular lists (Fig. 2): a wide list formed by the epithecal plates, the anterior cingular list (ACL), and a smooth list formed by the hypothecal plates, the posterior cingular list (PCL) (Zingone et al., 1998).
The sulcus is comprised of four irregularly shaped plates. The flagellar pore is housed in the sulcal area. The left sulcal list generally reaches the middle of the cell, however, the length can vary (Figs. 1-3). Three strong supporting ribs are thin and smooth, and in general, are without ornamentation (Figs. 4-8). The right sulcal list is also visible (Fig. 3) (Zingone et al., 1998).
The large hypotheca is made up of four plates. The dorsal and ventral margins of the hypotheca are important morphological characteristics used to identify this species (Zingone et al., 1998). The dorsal margin of the hypotheca is straight or undulating: convex below the cingulum, slightly concave in the middle, and convex again posteriorly. The ventral margin also displays some undulation: convex at the middle, and concave below the middle. The shape of these margins is also variable in this species (Figs. 11-13). The convexity of the ventral margin generally corresponds to the region where the third rib of the left sulcal list is inserted (Taylor et al., 1995).
Morphology and Structure:
D. sacculus is most likely a photosynthetic species; Larsen and Moestrup (1992) state that 'chloroplasts are probably present'. Moreover, Giacobbe (1995) found the possible presence of chlorophyll and phycobilin pigments in cells of D. sacculus using epifluorescence microscopy.
Giacobbe and Gangemi (1997) have shown that the concavity of the dorsal margin can vary in the life history of the species; e.g. the development of the megacytic zone (the intercalary band connecting the two halves of the cells). This area can increase before cell division or following gamete fusion (Giacobbe and Gangemi, 1997). Biological factors (i.e. life history and nutrition) can explain the presence of different morphotypes in the same locality (Zingone et al., 1998).
Reproduction:
D. sacculus reproduces asexually by binary fission (Taylor et al., 1995). Giacobbe and Gangemi (1997) reported sexual reproduction in this species.
Species Comparison:
D. sacculus is most often misidentified as D. acuminata. The major difference between these two species is the shape of the large hypothecal plates: in D. sacculus they are long and sack-like, whereas in D. acuminata they are shorter, more convex dorsally and often more slender posteriorly. D. acuminata also exhibits more pronounced thecal areolation and sulcal list ornamentation, but these are variable characteristics. Moreover, since D. sacculus and D. acuminata rarely occur in the same area with the same importance, the possibility of misidentification is reduced (Zingone et al., 1998).
Surface thecal ornamentation in this species is similar to a number of other Dinophysis species: D. acuta, D. caudata, D. norvegica and D. fortii (Hallegraeff and Lucas, 1988).
Remarks:
D. sacculus has a history wrought with identification problems mainly attributable to the morphological variability of this species. This problem is enhanced by the many synonyms and questionable identifications that have accumulated in the literature over the years (see Zingone et al., 1998).
Many authors consider Phalacroma to be synonymous with Dinophysis (Steidinger and Tangen, 1996).