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Figure 1. Line drawing of colony of Cerataulina bicornis/pelagica with individual cells rotated 90º, multiple chloroplasts, and narrow aperture between cells.

Figure 2. Line drawing of Cerataulina pelagica in valve view.

Figure 3. Line drawing of Cerataulina pelagica in oblique valve view.

Figure 4. Single cell, likely Cerataulina bicornis because of the size of the linking spine. Chloroplasts are plasmolysed around the nucleus.

Figure 5. Colony of Cerataulina bicornis with partially plasmolysed cells. Arrows indicate 90º rotation of individual cell, as indicated by location of linking spines.

Figure 6. Colony of Cerataulina pelagica with rows of lipid droplets around the internal periphery of the valves (arrowheads).

Figure 7. Cerataulina pelagica valve, SEM image. Rimoportula is near the valve center and areolae are radially arranged around the rimoportula.

Figure 8. Detail of Cerataulina pelagica rimoportula and radial areolae.

Figure 9. Cerataulina bicornis valve, SEM image. Rimoportula is close to valve margin and areolae are randomly distributed.

Figure 10. Auxospore formation in Cerataulina.

Figure 11. Resting spore (cyst) of Cerataulina bicornis.

Species Name: Cerataulina pelagica (Cleve) Hendey
Cerataulina bicornis (Ehrenberg) Hasle
Common Name: Diatom
Synonymy: Cerataulina pelagica (Cleve) Hendey
Zygoceros pelagicum Cleve, 1889
Cerataulus bergonii Peragallo, 1892
Cerataulina bergonii (Peragallo) Schütt 1896 Cerataulina bicornis (Ehrenberg) Hasle
Syringidium bicorne Ehrenberg, 1845
Syringidium americanum Bailey, 1862
Syringidium daemon Greville, 1866
Cerataulina compacta Ostenfeld, 1901
Cerataulina daemon (Greville) Hasle

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Protista Bacillariophyta Mediophyceae     Cerataulina

    Use your mouse to rollover the terms in purple for their definitions. If this feature is not supported by your browser, please refer to the accompanying glossary for terminology.

    Both species (Cerataulina pelagica & Cerataulina bicornis) are discussed here because they are found in the Indian River Lagoon (IRL) in uncertain proportions, not easily separable in water mounts.

    Species Descriptions

    Both species have cylindrical cells, with the pervalvar axis 2-3 times the diameter (Figures 1, 5 & 6). They form straight chains that are rather delicate and weakly siliceous, so turbulence often results in colony breakage, and unicells become common (Figure 4). They are most often seen in girdle view. Within a chain, individual cells are twisted about 90° in the pervalvar axis (Figures 1 & 5), so that half the cell is in broad girdle view and the other half is in narrower girdle view, indicated by the appearance of the elevated ornamentations on opposite sides of the valve (sometimes called ‘horns’ or ‘linking spines’). These elevations consist of a small raised area perforated by a series of narrow slits (referred to as a costate ocellus; Figures 2, 7 & 9), terminating at the valve margin in a broadly triangular spine (Figures 3, 7 & 9). Colony formation is effected by the fitting of these spines into complementary triangular grooved areas on the mantle of the adjacent valve, which are thus slightly misshapen. Laterally, there are wing-like projections on either side of the spine (Figures 7 & 9). Adjacent valve surfaces are closely opposed, resulting in a narrow aperture between cells (Figures 1, 5 & 6). Details of these structures are thoroughly discussed in Hasle & Syvertsen (1980) and Round et al. (1990). Living colonies in girdle view often have concentrations of lipid droplets near the apices of the cells (Figure 6).

    The valves are circular or nearly so, with the linking spines visible via light microscope (Figures 4, 5 & 10) but more clearly so in SEM (Figures 7 & 9). There is a single rimoportula, which is centrally located in C. pelagica, and near the margin in C. bicornis (Figures 7, 8 & 9). The areolae are arranged radially, centered on the rimoportula in C. pelagica (Figure 8), and irregularly arranged in C. bicornis (Figure 9). The cingulum consists of a large number of collar-shaped bands with open ends (Figures 7 & 9); the more bands, the greater the pervalvar axis. A characteristic resting spore is formed by C. bicornis (Figure 11); no spore has been seen in C. pelagica. Both species have many small chloroplasts distributed in the periphery of the cytoplasm. The geological antiquity of C. bicornis is indicated by the presence of the resting spores (as Syringidium bicorne) in Miocene sediments (Hasle & Sims 1985).

    Potentially Misidentified Species

    Differentiation between these two species can be difficult in living samples, since cells are almost invariably seen in girdle view. Permanent mounts in a medium of high refractive index, or electron microscopy, are frequently necessary. Further complicating identification is the possibility that a third species known in this genus, C. dentata Hasle (Hasle & Syvertsen 1980), may occur in the IRL based on its known distribution in tropical and subtropical waters. In most of the easily measured criteria, the three species overlap considerably (Table 1) and the linking spines (nominally larger in C. bicornis) are somewhat variable in size. Presence of the resting spore is definitive for C. bicornis but their occurrence is only an occasional event. Location of the rimoportula on the valve is useful to separate C. pelagica from C. bicornis (compare Figures 7 & 9), though C. dentata (if it is found in the IRL) also has its rimoportula near the valve margin.

    Nucleotide sequence data are available at for several samples identified as Cerataulina.

      Diameter Pervalvar Axis Valve Striae Per 10 µm
    C. pelagica 7-56 µm 55-120 µm 14-25
    C. bicornis 5-75 µm 87-200 µm 18-30
    C. dentata 12-50 µm 26-88 µm ~20
    Table 1. Morphometric data for Cerataulina species, adapted from Hasle & Syvertsen 1980.

    Habitat & Regional Occurence

    Although species differentiation may be difficult in biogeographic transition zones such as the IRL, known distribution records from the Atlantic and Pacific Oceans indicate that C. pelagica is a globally cosmopolitan species, and C. bicornis is restricted to warmer coastal waters.

    Indian River Lagoon Distribution

    While C. pelagica has been identified as a major constituent of phytoplankton in the northern IRL (Badylak & Phlips 2004; Phlips et al. 2010, 2011), it appears that Cerataulina bicornis is more frequent in the central and southern IRL. Most published research on phytoplankton in the IRL has not mentioned the presence of C. bicornis (e.g. Hargraves 2002; Hargraves & Hanisak 2011). Thus, it would be appropriate to verify whether C. bicornis has been overlooked in the northern and southern IRL system. While C. bicornis is tolerant of estuarine and brackish environments, C. pelagica prefers higher salinity, and is more common near the inlets and offshore.



    As with most diatoms, Cerataulina can undergo auxospore formation (Figure 10), a sexual process that restores maximum cell size to a diatom species. Resting spore formation is an asexual process, often induced by nitrogen limitation (Hargraves & French 1982). In Cerataulina, resting spore formation is confined to C. bicornis (Figure 11).


    It has been reported that C. pelagica is a cause of harmful algal blooms. A significant marine mortality in New Zealand was attributed to this species (Taylor et al. 1985), though no toxins have been demonstrated and mortality may have been a consequence of hypoxia.


    Cerataulina pelagica is vulnerable to parasitic infection by the nanoflagellate Pirsonia diadema (Kühn et al. 1996) but there is no indication that this parasite is either present or impacts populations in the IRL system.


    No information is available at this time


    No information is available at this time


    No information is available at this time


    Badylak, S & EJ Phlips. 2004. Spatial and temporal patterns of phytoplankton composition in a subtropical coastal lagoon, the Indian River Lagoon, Florida, USA. J. Plankton Res. 26: 1229-1247.

    Hargraves, PE. 2002. Diatoms of the Indian River Lagoon, Florida: an annotated account. Fla. Sci. 65: 225-244.

    Hargraves, PE & FW French. 1982. Diatom resting spores: significance and strategies. 49-68. In: Fryxell, GS (Ed.). Survival Strategies in the Algae. Cambridge Univ. Press. NY.

    Hargraves, PE & MD Hanisak. 2011. The significance of chlorophyll size fractionation in the Indian River Lagoon, Florida. Fla. Sci. 74: 151-167.

    Hasle, GR & PA Sims. 1985. The morphology of the diatom resting spores Syringidium bicorne and Syringidium simplex. British Phycol. J. 20: 219-225.

    Hasle, GR & EE Syvertsen. 1980. The diatom genus Cerataulina: Morphology and taxonomy. Bacillaria 3: 79-113.

    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.

    Phlips, EJ, Badylak, S, Christman, MC & MA Lasi. 2010. Climatic trends and temporal patterns of phytoplankton composition, abundance, and succession in the Indian River Lagoon, Florida, USA. Estuar. Coasts 33: 498-512.

    Phlips, EJ, Badylak, S, Christman, M, Wolny, J, Brame, J, Garland, J, Hall, L, Hart, J, Landsberg, J, Lasi, M, Lockwood, J, Paperno, R, Scheidt, D, Staples, A & K Steidinger. 2011. Scales of temporal and spatial variability in the distribution of harmful algae species in the Indian River Lagoon, Florida, USA. Harmful Algae 10: 277-290.

    Round, FE, Crawford, RM & DG Mann. 1990. The Diatoms - Biology and morphology of the genera. Cambridge University Press, Cambridge. 760 pp.

    Taylor, FJ, Taylor, NJ & JR Walsby. 1985. A bloom of the planktonic diatom Cerataulina pelagica, off the coast of northeastern New Zealand in 1983, and its contribution to an associated mortality of fish and benthic fauna. Internationale Revue der geampten Hydrobiologie 70: 773-795.

Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat
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Page last updated: 25 September 2011


In Cerataulus, a thickened hyaline or poroid siliceous area often slightly raised above the valve surface.


A tubelike opening through the cell wall with an internal flattened tube or lip-like slit; also called labiate process.


A regularly repeated perforation through the cell wall.


A special cell that restores cell size; normally a result of sexual reproduction.

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