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Diatom Glossary

Species Name: 

Rhizosolenia setigera

Common Name:      Diatom



Bacillariophyta Coscinodiscophyceae Rhizosolenia

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Figure 1. Valve apex of Rhizosolenia setigera, transitional to forma pungens. Image captured via scanning electron microscope (SEM).

Figure 2. Valve apex of R. setigera f. setigera (SEM).

Figure 3. A monospecific bloom of R. setigera in the central IRL, cell diameters 6-9 µm.

Figure 4. Resting spore pair in R. setigera.

Figure 5. R. setigera, individual girdle band segment (copula). Image captured via transmission electron microscope (TEM).

Figure 6. R. setigera. Girdle band segments in two dorsiventral columns (SEM).

Figure 7. R. setigera. One spine typical of f. setigera, the other transitional to f. pungens. Cell diameter is 12µm.

Figure 8. Bloom of R. setigera f. setigera and f. pungens (some in clusters). Cell diameters 4-19 µm.


Rhizosolenia setigera Brightwell


Rhizosolenia setigera was described by T. Brightwell in 1858. Generally accepted synonyms are Rhizosolenia japonica Castracane 1886 and Rhizosolenia hensenii Schuett 1900. Rhizosolenia pungens was described by Cleve-Euler in 1937. Rhizosolenia crassispina Schroeder shares many features with both var. setigera and var. pungens, and may also be synonymous. The resting spores were originally described as Pyxilla baltica Grunow in Van Heurck 1881. According to Hustedt (1930), R. setigera var. kariana Henckel 1925 is also subsumed into var. setigera. Sundstrom (1986) postulated that R. setigera is not a "true" Rhizosolenia species, a conclusion supported by Hernandez-Becerril (1995), who also maintains separate species identities for R. setigera, R. pungens and R. crassispina. The inclusion of R. pungens into R. setigera was first made by Brunel (1970) as Rhizosolenia setigera Brightwell forma pungens (Cleve-Euler) Brunel. Rhizosolenia setigera and R. pungens have alternately been considered separate species, varieties, or forms. The primary distinctions are the shape of the spine (gradually tapering in setigera; basally thickened, then abruptly hairlike distally in pungens) and the presence of resting spores (setigera only). These differences are clear, but when many cells are present (e.g. bloom conditions) one may easily find cells that are indeterminate between R. setigera and R. pungens. These indeterminate variations sometimes include cells with setigera spine structure on one theca, and pungens on the other theca. One may thus question separation at the species level. Whether the ternary names are appropriately 'subspecies', variety' or 'form' becomes a matter of personal conviction. In this discussion, 'variety' is used for convenience. Certainly the ability/inability to form resting spores implies a yet to be defined genetic differentiation at some level. A number of nucleotide and protein sequences exist for R. setigera (, but there is some question as to whether accurate identification preceded the sequencing, or whether the name includes a complex of cryptic species.

The general shape is tubular, with a length (pervalvar axis) to width (diameter) ratio often in excess of 20:1. Each valve terminates in a spine (the external part of the rimoportula) of variable length and morphology.  Each cell contains many irregularly elliptical chloroplasts, distributed parietally throughout the cell. The nucleus is also parietal, and usually located approximately mid-length. Cytological events during valve morphogenesis in cell division have been documented by Van De Meene and Pickett-Heaps (2004). These authors found that after cytokinesis, the rimoportula (labiate process) rotates 5-8 turns in one direction, pauses, then rotates similarly in the opposite direction. Similar post-cytokinesis events have not been seen in other diatoms.

Cells are straight or slightly curved and solitary, except in rapidly growing populations where several cells may be clustered in parallel. The valves are conical and terminate in long, slightly oblique spines.


Depending on the variety, the external part of the rimoportula (spine) may be slightly wider at the base for some distance, then tapering gradually to a fine, hair-like tip (var. setigera). In var. pungens, the spine is initially narrow, widens more or less abruptly for a portion of its length before narrowing to a hair-like tip. The interior part of the spine is a rimoportula. In contrast with most other Rhizosolenia species, the valves lack otaria, and claspers are rudimentary or lacking. These are features that aid in linking adjacent cells together after cell division. The copulae (girdle bands) are in two dorsoventral columns. Each band is approximately trapezoidal in shape and perforated by many tiny poroid areolae. In lateral view, bands appear as a zig-zag line. Occasionally after cell division, the imprint of the long spine is visible in several of the copulae in a pervalvar direction (see Sunesen & Sar 2007).

In publications where var. setigera and var. pungens are differentiated by size, either as varieties or as separate species, the size distinction involves a smaller (pungens) and larger (setigera) separation. In practice, there is a complete size overlap between the two varieties. According to literature reports, var. setigera varies in width (diameter) from 2-50 µm, with a length (apical axis) of ~100-725 µm (without spine, and >1 mm including spine). Variety pungens varies from 4-20 µm in width, with a length of 116-450 µm (without spine, and a total length of 370-760 µm including spine).



R. setigera has been classified as a species with north temperate distribution. However, it has been found throughout the world's oceans both from tropical to cold temperate latitudes, apparently excluded only from polar seas. It is primarily a species of coastal and estuarine environments, but is occasionally found in the open ocean. Variety pungens appears to prefer lower salinity than var. setigera.
Based on culture experiments and distribution records, R. setigera is eurythermal and euryhaline. It has been recorded in the temperature range of 2 to 34 °C, and salinities of 1.5 to 37 psu. In culture experiments testing growth from -1.5 to 26 °C, Baars (1988) found that a North Sea strain grew best at 6 to 12 °C, but did not grow beyond 20 °C.

There are reports of R. setigera reliably occurring in the late Pleistocene (Jouse & Mukhina 1978; Ryu et al. 2005) and from 2-3 million years ago (Koizumi 1992). It is unclear whether these reports are based on the presence of the theca & spine, or whether the spore (as Pyxilla baltica) was identified. If these reports are accurate, the species already had a global distribution about two million years ago.

In the IRL, where the annual temperature range is about 10-32 °C, Phlips et al. (2011: Figure 5) found R. setigera to be most abundant in late Spring.  It can reach concentrations exceeding 4 million cells l-1 in the central IRL  (Hargraves, pers. obs.).   Guillard and Kilham (1977) found substantial differences in growth rates over a temperature gradient, between tropical and temperate strains of R. setigera. These disparities suggest substantial variability in ecological preferences among geographically separated strains.



Resting spores are formed in var. setigera, but have not been confirmed in var. pungens. Spores are formed in pairs, are torpedo-shaped and smooth, and may occur with or without a small proboscis on the spore epitheca. Details of spore structure are found in Hargraves (1976). R. setigera var. setigera cells produce resting spore pairs in nature and in culture, probably due to nutrient (especially nitrogen) depletion (Hargraves & French 1984). R. setigera var. pungens apparently does not produce resting spores. For this species, resting spore formation is an asexual process.
Sexual reproduction is
oogamous with uniflagellate sperm (up to 32 spermatozoids per cell). The zygote forms an auxospore that restores the maximum size dimension of the species. Details of sexual reproduction in diatoms can be found in Round et al. (1990).

Pathogens of Rhizosolenia setigera include fungi (Johnson 1967), viruses (Nagasaki et al. 2004) and protistan flagellates of uncertain affinity (Kuehn et al. 1996). 

There are scattered reports of marine mortalities associated with natural populations dominated by R. setigera. Although the species is known to produce monocyclic alkenes (Masse et al. 2004), it is likely that any marine mortality is due to oxygen depletion following bloom decay.



Baars, JWM. 1988. Autecological investigations on marine diatoms. 5: Coscinodiscus concinnus W. Smith and Rhizosolenia setigera Brightwell. Hydrobiol. Bull. 22: 147-155.

Brunel, J. 1970.  Le phytoplancton de la Baie des Chaleurs. Montreal, Canada: Les Presses de L'Universite de Montreal. 365 pp.

Guillard, RRL & P Kilham. 1977. The ecology of marine planktonic diatoms. In: D Werner (Ed.). The Biology of Diatoms. pp. 372-469. Berkeley: University of California Press.

Hargraves, PE. 1976. Studies on marine plankton diatoms. II. Resting spore morphology. J. Phycol. 12: 118-128.

Hargraves, PE & FW French. 1983. Diatom resting spores: Significance and strategies. In: GA Fryxell (Ed.). Survival Strategies of the Algae. pp. 49-68. Cambridge: Cambridge University Press.

Hernandez-Becerril, DU. 1995. Planktonic diatoms from the Gulf of California and coasts off Baja California: The genera Rhizosolenia, Proboscia, Pseudosolenia, and former Rhizosolenia species. Diatom Res. 10: 251-267.

Hustedt, F. 1930. Die Kieselalgen Deutschland, Oesterreichs und der Schweiz. Dr. L. Rabenhorst's Kryptogamen-Flora von Deutschland, Oesterreich und der Schweiz. Band VII, 1.Teil. 920 pp. Leipzig, Germany: Akademische Verlagsgesellschaft m.b.H.

Johnson, TW. 1967. Monocentric fungi on species of Rhizosolenia from saline habitats. Mycopathologia. 32: 281-290.

Jouse, AP & V Mukhina. 1978. Diatom units and the paleogeography of the Black
Sea in the late Cenozoic. DSDP Reports 42: 903-950.

Koizumi, I. 1992. Diatomacean sediments along the Pacific coastal areas of South America and their evaluation. J. Faculty Sci. Hokkaido University, Series 4: Geology and Mineralogy. 23: 227-295.

Kuehn, SF, Drebes, G, & E Schnepf. 1996. Five new species of the nanoflagellate Pirsonia in the German Bight, North Sea, feeding on planktonic diatoms. Helgolander Meeresun. 50: 205-222.

Masse, G, Belt, ST & SJ Rowland. 2004. Biosynthesis of unusual monocyclic alkenes by the diatom Rhizosolenia setigera Brightwell. Phytochemistry. 65: 1101-1106.

Nagasaki, K, Tomaru, Y, Katanozaka, N & Y Shirai. 2004. Isolation and characterization of a novel single-stranded RNA virus infecting the bloom-forming diatom Rhizosolenia setigera. Appl. Environ. Microb. 70: 704-711.

Phlips, E, 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. 747 pp. Cambridge: Cambridge University Press.

Ryu, E, Yi, S & S-J Lee. 2005. Late Pleistocene-Holocene paleoenvironmental changes inferred from the diatom record of the Ulleung Basin, East Sea (Sea of Japan). Mar. Micropaleontol. 55: 157-182.

Sundstrom, B. 1986. The Marine Diatom Genus Rhizosolenia.Department of
Systematic Botany. PhD, 117 + 39 plates. Lund, Sweden: Lund University.

Sunesen, I & EA Sar. 2007.  Marine diatoms from Buenos Aires coastal waters (Argentina). IV. Rhizosolenia s.str., Neocalyptrella, Pseudosolenia, Proboscia. Phycologia. 46: 628-643.

Van De Meene, A & JD Pickett-Heaps. 2004. Valve morphogenesis in the centric diatom Rhizosolenia setigera. Eur. J. Phycol. 39: 93-104.





Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat

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Page last updated: 14 June 2011

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


Sexual reproduction in which the sperm is small and motile, and the egg is larger and nonmotile.


Regularly repeated perforations through the cell wall.


Simple unchambered hole through a diatom valve.


In the genus Rhizosolenia, small structures clasping the adjacent valve of linked cells.


Membranous costae that occur opposite each other on a valve, especially the genus Rhizosolenia.


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


A developmental sequence that causes an organism to develop its shape.