Semiorbis rotundus

G.Reid and D.M.Williams 2010      Category: Eunotioid

REPORTED AS: Semiorbis hemicyclus (Siver et al. 2005, p. 184, Pl. 27, Fig.6 , Pl. 77, Figs. 1-2) | Semiorbis hemicyclus (Camburn and Charles 2000, p. 21, Pl. 15, Figs. 22-24) 

Semiorbis catillifera


Simonsenia delognei

LM scalebar = 10 µm = 40 pixels.


Contributor: David R.L. Burge | Mark Edlund - July 2016
Length Range: 33-45 µm
Width Range: 3.9-4.8 µm
Striae in 10 µm: 12-13


Valves are strongly curved and semi-circular. Apices are reduced in breadth (3.0-3.8 µm) and subrostrate. Valves at the small end of the size range have a more pronounced dorsal shoulder. Apices are slightly recurved in some specimens. Striae are nearly perpendicular to the dorsal margin, numbering 12-13 in 10 µm. The striae are separated by externally thickened, transapical costae. The costae terminate on the dorsal margin as prominent, planar wing- or spine-like projections. A reduced raphe is present on the ventral valve mantle interface at the apices, but it may not be discernable in LM. Areolae are not visible in LM. Rimoportulae are absent.

There are a number of reports of Semiorbis hemicyclus in North America (Patrick and Reimer 1966, Camburn and Charles 2000, Siver et al. 2005). However, Reid and Williams (2010) examined specimens of Semiorbis from North America and Europe, concluding that S. hemicyclus occurs only in Europe and Greenland, but not in North America. In that work, Reid and Williams described S. rotundus, and determined that this taxon occurs in North America and Japan. We concur that some North American records conform to S. rotundus (Camburn and Charles 2000, p. 21, Pl. 15, Figs. 22-24, Siver et al. 2005, p. 184, Pl. 27, Fig.6 , Pl. 77, Figs. 1-2). We did not, however, examine additional voucher specimens of North American S. hemicyclus. Further work is needed to determine the morphological differences and the geographic distribution of S. hemicyclus and S. rotundus.

Original Description

Author: G.Reid and D.M.Williams 2010
Length Range: 21-45 µm
Width Range: 4-5.2 µm
Striae in 10 µm: 12-13

Original Description

Valves curved, strongly semicircular, 21-45 µm long, 4.0-5.2 µm wide, externally with transverse ridges of silica, extending upwards at both margins, terminating in spine-like protrusions, 12-13 per 10 µm. Spines hollow slits, maximum length 2.4 µm at centre of valve margin, shorter, ca. 1.5 towards apices. Stria situated between silica ridges, uniseriate at ventral margin, biseriate at dorsal margin; central hyaline area between striae running the length of valve (Fig. 20). Internally raphe is a small straight slit, one at each pole, on the ventral margin of the valve, terminating in a small (reduced) helictoglossa (Figs 18, 19). Externally, raphe terminates in a fissure, curving towards and onto valve face (Figs 15, 19). A row of pores situated at poles, directly above raphe endings (Figs 15, 18). Girdle bands narrow, striated (Figs 19,21).

Original Images

Cite This Page:
Burge, D., and Edlund, M. (2016). Semiorbis rotundus. In Diatoms of the United States. Retrieved April 22, 2018, from

Species: Semiorbis rotundus

Contributor: David R.L. Burge | Mark Edlund

Reviewer: Jen Beals


Camburn, K.E. and Charles, D.F. (2000). Diatoms of Low-Alkalinity Lakes in the Northeastern United States. Academy of Natural Sciences of Philadelphia, Special Publication 18, 152 pp.

Patrick, R.M. and Reimer, C.W. (1966). The Diatoms of the United States exclusive of Alaska and Hawaii, V. 1. Monographs of the Academy of Natural Sciences of Philadelphia 13.

Reid, G. and Williams, D.M. (2010). Notes on the genus Semiorbis Patrick with a description of a new species. Diatom Research 25: 355-365.

Siver, P.A., Hamilton, P.B., Stachura-Suchoples, K. and Kociolek, J.P. (2005). Diatoms of North America. The Freshwater Flora of Cape Cod. Iconographia Diatomologica 14: 1-463.

Links & ID's

Index Nominum Algarum (INA)

Original INA

California Academy of Sciences (CAS)

Semiorbis rotundus CAS

North American Diatom Ecological Database (NADED)


Autecology Discussion

Semiorbis rotundus is known to occur in low pH ponds of New Jersey (Siver and Hamilton 2011) and low alkalinity lakes of the Adirondacks (Camburn and Charles 2000), reported as Semiorbis hemicyclus in both references.