Lithothamnion Heydrich, 1897, nom. et typ. cons.

Holotype species: Lithothamnion muelleri Lenormand ex Rosanoff

Original publication: Heydrich, F. (1897). Melobesiae. Berichte der deutsche botanischen Gesellschaft 15(7): 403-420, pl. XVIII [18].

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Type designated in: Woelkerling, W.J. (1983). A taxonomic reassessment of Lithothamnium (Corallinaceae, Rhodophyta) based on studies of R. A. Philippi's original collections. British Phycological Journal 18(2): 165-197, 33 figures, 4 tables.

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Precise date of publication: 7 September 1897 (see Woelkerling & Lamy 1998: 680 for details). The requirements for valid publication are specified in the ICBN(International Code of Botanical Nomenclature).

Description:

Plants calcified, lacking genicula, entirely pseudoparenchymatous; encrusting to warty, lumpy or fruticose; epigenous and growing partially to completely attached to the surface of various substrates (e.g. algae, molluscs, rock), or growing unattached and free-living as rhodoliths; haustoria unknown.

Thallus organization generally dorsiventral in crustose portions but more or less radial in branches; thallus construction monomerous throughout, consisting of a single system of branched laterally coherent filaments that contribute to a ventral or central core and a peripheral region where portions of core filaments or their derivatives curve outwards towards the thallus surface; coaxial growth (in which cells of adjacent filaments in core region are aligned in arching tiers) apparently not recorded. Most filaments usually terminating at the thallus surface in one or more epithallial cells (generally one per filament); epithallial cell shape variable, outermost walls rounded to flattened, with or without flared corners, even within one specimen; cell elongation occurring mainly within actively dividing subepithallial initials that are usually as long as or longer than their immediate inward derivatives. Cells of adjacent filaments linked by fusions; secondary pit-connections unknown. Trichocytes, when present, single or paired, at surface and/or buried.

Gametangia (where known) and carposporangia (where known) developing in uniporate conceptacles. Spermatangia (male gametangia) and carpogonia (female gametangia) almost always produced in separate conceptacles (very rarely in the same conceptacle); male and female conceptacles occasionally formed on the same plant but much more commonly formed on different plants. Spermatangia (where known) formed on unbranched filaments that arise from the conceptacle chamber floor, walls and roof; spermatangial initials (where known) apparently not overlain with protective cells during early stages of development; spermatangial conceptacle roof formation (where known) occurring centripetally from groups of vegetative filaments peripheral to developing spermatangial filaments on the conceptacle chamber floor. Unfertilized carpogonial branches (where known) across floor of chamber. One or two-celled vegetative filaments each bear a 2-4 celled carpogonial branch, consisting of a hypogenous cell, a sterile cell, often absent, that is an undeveloped carpogonium, and a terminal carpogonium that extends to form a trichogyne. Fertilization appears restricted to peripheral carpogonia; apparently multiple fertilization events occur, each resulting in different (discontinuous) carposporophytes in the same conceptacle. From each fertilized carpogonium five- to eight-celled, unbranched gonimoblast filaments arise directly with terminal cells becoming carposporangia; fusion cell absent.

Tetrasporangia/bisporangia formed in multiporate conceptacles on separate plants from gametophytes. Tetra/bisporangial roof formation from vegetative filaments both peripheral to and interspersed among tetr/bisporangial initials. Lowermost cells of these vegetative filaments elongate, some degenerate to form the conceptacle chamber, while uppermost cells form the conceptacle roof. Tetra/bisporangia zonately divided and produce apical plugs that block roof pores before spore release.

Emended description (Gabrielson et al., 2023).

Information contributed by: Wm. J. Woelkerling; updated by Paul Gabrielson. The most recent alteration to this page was made on 2023-11-26 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

Gender: This genus name is currently treated as neuter.

Most recent taxonomic treatment adopted: Schneider, C.W. & Wynne, M.J. (2007). A synoptic review of the classification of red algal genera a half a century after Kylin's "Die Gattungen der Rhodophyceen". Botanica Marina 50: 197-249.

Comments: Information on the taxonomic history, nomenclature, and other matters associated with the name Lithothamnion Heydrich is contained in Woelkerling (1988: 171-175). Growth form terminology (encrusting, lumpy, fruticose, etc.) follows Woelkerling et al. (1993).

An account of the lectotype specimen of Lithothamnion muelleri, the lectotype species of Lithothamnion, is presented in Woelkerling (1983: 190-192, figs 29-33); additional figures of the lectotype are included in Wilks & Woelkerling (1995: 572-573, figs 1A, 2A), who also provide a detailed account of the species.

As of November, 2023, only four species are confirmed to belong in Lithothamnion, L. muelleri, the generitype, L. kraftii, L. saundersii, and L. woelkerlingii all of whose type specimens have been sequenced. None of these species can be distinguished by morpho-anatomical features from each other; DNA sequences are needed to correctly identify these species. All other species currently considered to belong in Lithothamnion need to be sequenced to determine if they belong in the genus.

Biogeographically, Lithothamnion appears to be a mostly southern hemisphere genus.

The lists below of diagnostic characters of Lithothamnion, and of the higher taxa to which it belongs, are derived from data in Woelkerling (1988), Harvey, Broadwater, Woelkerling & Mitrovski (2003), Harvey, Woelkerling & Millar (2003), Le Gall & Saunders (2007), Woelkerling et al. (2008: 282) and/or Le Gall et al. (2009). Diagnostic characters are those that taken together distinguish a taxon from others of the same taxonomic rank (e.g. characters distinguishing Lithothamnion from other genera of the Hapalidiaceae, subfamily Melobesioideae). Harvey, Woelkerling & Millar (2003: 653) also provide a diagnostic comparison of Lithothamnion with other currently recognized non-fossil genera of Melobesioideae.
Lithothamnion (Hughey et al. 2008).

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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.

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M.D. Guiry in Guiry, M.D. & Guiry, G.M. 26 November 2023. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 04 March 2024

 
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