Species Description: The weakly silicified cells of L. danicus are cylindrical, and occur in chains of up to 20 or more cells. Cell length (pervalvar axis) is normally 3-5 times the cell width (diameter), but occasionally up to 10 times the cell diameter (Figure 2). Adjacent cells are closely abutted. The valve of one cell is slightly convex, while the adjacent valve is slightly concave (Figures 2 & 5), giving a characteristic appearance in the light microscope (Figures 1 & 2).
Individual valves are circular, ornamented with uniseriate rows of poroid areolae radiating from the valve center, with a ring of short triangular spines at the junction of the valve face and mantle (Figures 3-5). The poroid rows continue on the mantle to the edge of the valve. There is a narrow annulus (sometimes visible in the light microscope) around the valve center (Figures 3 & 5). Most valves also have a single pore near the center but outside the annulus, some cells have two pores, and rarely there is no pore at all (Figure 4). Based on culture studies, French & Hargraves (1985) suggested that the presence of this pore is correlated with the ability to form resting spores. There are many girdle bands that are approximately trapezoidal in shape and perforated by tiny areolae (Figures 4 & 5). Each cell has many small rounded or elongate chloroplasts (Figure 1).
Many of these morphological features are visible only with electron microscopy.
Resting spores, occurring rarely (see below), are morphologically dissimilar from vegetative cells. Spores are formed within an auxospore envelope
(Figures 6 & 10), and are composed of two dissimilar valves (Figure 6). The larger valve is adorned with numerous elongate spines (Figures 6 & 7). Resting spore formation may be initiated by nutrient depletion, especially of nitrogen (French & Hargraves 1986).
Habitat & Regional Occurrence: Leptocylindrus danicus is a coastal and estuarine species. It has a worldwide distribution except for the Arctic and Southern Oceans, occasionally in bloom proportions.
IRL Distribution: This species is present throughout most of the year in the central IRL (Hargraves & Hanisak 2011), and infrequent in the northern IRL (Badylak & Phlips 2004). In general, L. danicus is more abundant near the inlets where exchange with coastal water is more extensive, suggesting that populations are frequently renewed by tidal action.
Size: According to literature records, cell diameters are 5-16 µm.
Reproduction: In diatoms, auxospore formation is a form of sexual reproduction (Round et al. 1990), and infrequent in occurrence. It is also the primary means of restoration of maximum size after diminution through vegetative growth. The co-occurrence of resting spore formation, as part of the sexual process in Leptocylindrus danicus (Figures 8-10), is almost unique among diatoms (French & Hargraves 1985). Spores are rarely seen within the IRL, but occasionally found in adjacent coastal waters (Hargraves, pers. obs.)
Parasites: The effect of parasites on the wax and wane of L. danicus populations is unknown, but this diatom can be infected by the nanoflagellates Pirsonia formosa and Cryothecomonas longipes (Kühn et al. 1996; Schnepf & Kühn 2000).
Badylak, S & EJ Phlips. 2004. Spatial and temporal patterns of phytoplankton composition in a subtropical lagoon, the Indian River Lagoon, Florida, USA. J. Plankton Res. 26: 1229-1247.
French, FW III & PE Hargraves. 1985. Spore formation in the life cycles of the diatoms Chaetoceros diadema and Leptocylindrus danicus. J. Phycol. 21: 477-483.
French, FW III & PE Hargraves. 1986. Population dynamics of the spore-forming diatom Leptocylindrus danicus in Narragansett Bay, Rhode Island. J. Phycol. 22: 411-420.
Hargraves, PE & MD Hanisak. 2011. The significance of chlorophyll size fractionation in the Indian River Lagoon, Florida. Fla. Sci. 74: 151-167.
Kühn, SF, Drebes, G & E Schnepf. 1996. Five new species of the nanoflagellate Pirsonia in the German Bight, North Sea, feeding on planktonic diatoms. Helgoländer Meeresuntersuchungen 50: 205-222.
Hendey, NI. 1964. An Introductory Account of the Smaller Algae of British Coastal Waters. Part V. Bacillariophyceae. Ministry of Agriculture, Fisheries and Food. Fisheries Investigations, Series IV. London, IMSO. 317 pp.
Round, FE, Crawford, RM & DG Mann. 1990. The Diatoms - Biology and morphology of the genera. Cambridge University Press, Cambridge. 760 pp
Schnepf, E & SF Kühn. 2000. Food uptake and fine structure of Cryothecomonas longipes sp. nov., a marine nanoflagellate incertae sedis feeding phagotrophically on large diatoms. Helgoland Mar. Res. 54: 18-32.