Arachnoidiscus Deane ex Shadbolt, 1852, nom. cons.
Holotype species: Arachnoidiscus japonicus Shadboldt ex Pritchard
Original publication and holotype designation: Shadbolt, G. (1852). On the structure of the siliceous loricae of the genus Arachnoidiscus. Transactions of the Microscopical Society of London 3: 49-54, pl. 11.
Description: Cells discoid, epiphytic on seaweeds. No information on plastids or other aspects of the live cell. Also fossil, extending back to the Palaeocene. Frustules heterovalvar; valves petri-dish shaped. One valve with a plain central area, the other with the plain centre ringed by elongated radial slits, which are the openings of rimoportulae. Striae radiating, some short, broken by a plain region at valve rim. Pattern somewhat sectored (especially obvious in light and transmission electron microscopy). One valve, which may be the plain-centred or the radial-slit type, has a distinct step in the mantle. Areolae occluded by volae, in depressions of a massive costate system. Valve mantle areolae smaller and without surrounding costae. Fine diagonally crossed rows of 'pin-holes', which do not penetrate to the inside, are sometimes observable at the junctions of the external costae. Internally, massive radial and tangential ribbing occurs, superimposed on which is a system of costae radiating from a flange around the central ring, or from intermediate points, to a flange on the valve mantle edge. The internal and external flanged portions have been termed the umbilicus by Helmcke & Kreiger (1953-77, pl. 321) but this does not seem an appropriate term though their reconstruction is fairly accurate. One valve has a ring of slits internally in the depressed centre, which may be interpreted as rimoportulae, whilst the other valve has a series of openings in its sunken centre, which do not seem to penetrate through to the outside. The cingulum appears plain but requires further study.
The most recent alteration to this page was made on 2024-10-01 by G.M. Guiry.Taxonomic status: This name is of an entity that is currently accepted taxonomically.
Gender: This genus name is currently treated as masculine.
Most recent taxonomic treatment adopted: Nikolaev, V.A., Kociolek, J.P., Fourtanier, E., Barron, J.A. & Harwood, D.M. (2001). Late Cretaceous diatoms (Bacillariophyceae) from the Marca Shale member of the Moreno Formation, California. Occasional Papers of the California Academy of Sciences 152: 1-119, 39 plates.
Comments: The genus is not often encountered, occurring mainly on seaweeds, especially on the tropical coasts around the Pacific, where it sometimes occurs in such abundance that the dried seaweed appears covered with a white powder. Hustedt (1927-66) doubted its occurrence in European waters and it may be virtually confined to the Pacific. Several fossil species are recorded. We assume that the ring of slits around the centre of one valve are rimoportulae though they do not have very typical form. However, P.A. Sims (personal communication) confirms this with excellent illustrations of rimoportulae in A. cf. rssicus. Stictodiscus is possibly the only near relative.
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Contributors
Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera,
organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA)
and intended to be published in CD format.
These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above.
The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.
Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.
Linking to this page: https://www.algaebase.org/search/genus/detail/?genus_id=45124
Citing AlgaeBase
Cite this record as:
G.M. Guiry in Guiry, M.D. & Guiry, G.M. 01 October 2024. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 22 November 2024
