Sellaphora capitata D.G. Mann & S.M. McDonald 2004

Introduction

Sellaphora capitata was described formally by D.G. Mann & S.M. McDonald in Mann et al. (2004). It was previously referred to as phenodeme 4 (Mann 1984) or the 'capitate' deme of the morphospecies S. pupula sensu lato (e.g. Mann 1989, 1999, Mann et al. 1999). Sellaphora capitata, like S. blackfordensis, seems to be common in the epipelon of muds that are rich in organic matter, in eutrophic lakes and ditches. It was studied first in a small urban pool (Blackford Pond) in Edinburgh. Detailed studies of the mating system and life cycle were made by Mann et al. (1999).

Characteristics

• Valves linear-elliptical, with broad, subcapitate poles (capitate in the longest specimens)
• Dimensions: 19–44 µm × 7.0–9.3 µm (the longest valves, which are also the widest, are rarely seen in natural populations)
• Striae radiate, curved and usually becoming angled near the poles, with some intercalated short striae at the centre, 18.2–19.3–20.5 (rarely as low 16 or as high as 22) in 10 µm
• Areolae invisible in LM
• Axial area narrow
• Central area expanded (to c. 55–70% of the valve width), somewhat irregular, mostly bow-tie–shaped or transversely rectangular
• No grooves visible alongside the raphe-sternum in LM, which is therefore not obviously differentiated from the remainder of the valve face; however, SEM sometimes reveals a slight step at the edge of the raphe sternum externally
• Conopeum absent
• Raphe slightly sinuous
• Central external raphe endings expanded, turned towards the primary side
• Terminal fissures bent above the helictoglossa, turned towards the secondary side
• Polar bars present, usually parallel
• Allogamous, strictly heterothallic

Type Data

DNA Barcode (cox1): GenBank EF164947.

Holotype:  specimen at England Finder R41/0 (bottom and right-hand edges of the finder co-located with the top and left-hand edges of the the specimen slide, when the label is to the left) on slide E16/4 "Blackford Pond, Dec. '83, mud (tissue)" in the herbarium of the Royal Botanic Garden Edinburgh (E). Holotype valve can be viewed on-line at http://rbg-web2.rbge.org.uk/algae/research/types/Sellaphora_capitata_type.html

Similar Species

Many Sellaphora species have a linear-elliptical outline and broad subcapitate poles; Evans et al. (2007, 2008) and Mann et al. (2008) illustrate several species visually very similar to S. capitata. For example, S. capitata resembles S. blackfordensis and often grows alongside it. Careful measurements of valve width and striation density can help to achieve a reasonably certain identification in LM, but for critical work confirmation should be made by DNA barcoding (Evans et al. 2007).

Ecology

Found so far only in the epipelon of eutrophic lakes, ponds and ditches. A preliminary ecological analysis of some British Sellaphora species has been carried out by Poulíčková et al. (2008).

Geographical Distribution

Distributional data are difficult to obtain, because the species is easily confused with other Sellaphora species. Barcode confirmations are available for several Scottish and English lakes, and S. capitata has also been found in Belgium and Australia [barcode] and the Crimea (Ukraine) [morphology and mating experiments].

References

Evans, K.M., Wortley, A.H. & Mann, D.G. (2007). An assessment of potential diatom “barcode” genes (cox1, rbcL, 18S and ITS rDNA) and their effectiveness in determining relationships in Sellaphora (Bacillariophyta). Protist 158: 349–364.

Evans, K.M., Wortley, A.H., Simpson, G.E., Chepurnov, V.A. & Mann, D.G. (2008). A molecular systematic approach to explore diversity within the Sellaphora pupula species complex (Bacillariophyta). Journal of Phycology 44: 215–231

Mann, D.G. (1984). Observations on copulation in Navicula pupula and Amphora ovalis in relation to the nature of diatom species. Annals of Botany, 54, 429-438.

Mann, D.G. (1988). The nature of diatom species: analysis of sympatric populations. In F.E. Round (ed.) Proceedings of the 9th International Diatom Symposium, 317-327. Biopress, Bristol and Koeltz Scientific Books, Koenigstein.

Mann, D.G. (1989). The species concept in diatoms: evidence for morphologically distinct, sympatric gamodemes in four epipelic species. Plant Systematics and Evolution, 164, 215 237.

Mann, D.G. (1989). The diatom genus Sellaphora: separation from Navicula. British Phycological Journal, 24, 1-20.

Mann, D.G. (1999). The species concept in diatoms (Phycological Reviews 18). Phycologia, 38: 437-495.

Mann, D.G., Chepurnov, V.A. & Droop, S.J.M. (1999). Sexuality, incompatibility, size variation, and preferential polyandry in natural populations and clones of Sellaphora pupula (Bacillariophyceae). Journal of Phycology, 35, 152-170.

Mann, D.G., McDonald, S.M., Bayer, M.M., Droop, S.J.M., Chepurnov, V.A., Loke, R.E., Ciobanu, A. & du Buf, J.M.H. (2004) Morphometric analysis, ultrastructure and mating data provide evidence for five new species of Sellaphora (Bacillariophyceae). Phycologia 43: 459-482.

Mann, D.G., Thomas, S.J. & Evans, K.M. (2008). Revision of the diatom genus Sellaphora: a first account of the larger species in the British Isles. Fottea 8: 15–78.

Poulíčková, A., Špačková, J., Kelly, M.G., Duchoslav, M. & Mann, D.G. (2008). Ecological variation within Sellaphora species complexes (Bacillariophyceae) – specialists or generalists? Hydrobiologia