Neogoniolithon erosum W. H. Adey 1970
Creators: name: Maneveldt, G. W.
name: Merwe, E. Van Der
name: Keats, D. W.
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Plantae
Rhodophyta
Cryptonemiales
Corallinaceae
Neogoniolithon
Neogoniolithon erosum

description: Neogoniolithon erosum (Foslie) W.H. Adey, 1970: 8   
(Figs 36–43)  
 Basionym:— Lithophyllum erosum Foslie, 1906: 20   
Synonyms:—None  
 Holotype:—TRH!, A2-101; originally unnumbered. Previous references to typification were by Adey & Lebednik (1967: 16), Woelkerling (1993: 85) (as Lithophyllum), and Adey (1970: 8) (as Neogoniolithon).   
Type Locality:—Magens Bay (as Magenbay), St Thomas Island, US Virgin Islands.  
 Etymology:— ‘ erosum ’, erosus = erose, having an irregularly toothed or apparently gnawed margin (Stearn 1973). Foslie (1906) did not explain the origin of the epithet, but it presumably makes reference to the highly irregular nature of the margins.   
Distribution:—The species has only been reported for the tropical and subtropical western Atlantic (Foslie 1906, Taylor 1960, Wynne 2011).  
Appearance and vegetative anatomy:—The following description is based solely on the holotype material from TRH as no representative specimens for this taxon were available for verification. Holotype material contains several thin, adherent, epilithic individuals (Fig. 36) that measure 50–300 μm thick. Thalli encrusting (smooth), lacking protuberances, and have adherent, highly irregular margins that are entire to lobed, but lack orbital ridges.  
Thallus monomerous and dorsiventrally organised (Figs 37, 39). Medullary region consists of a central plumose (non-coaxial) core (Fig. 39), with square to elongate cells that measure 7–31 μm in length and 4–7 μm in diameter. Cortical filaments comprise squat to rectangular cells that measure 5–11 μm in length and 4–7 μm in diameter. Cell fusions abundant (Figs 38, 39); secondary pit connections not seen. Subepithallial initials square to rectangular (Fig. 38), and measure 4–12 μm in length and 5–6 μm in diameter. Epithallial cells squat to elliptical, measure 4–6 μm in length and 5–9 μm in diameter, and occur singly (up to 2 when shedding) (Fig. 38). Rectangular to elongate trichocytes occur singly at the thallus surface (Fig. 37). Trichocyte chains typically comprise 2 cells; a megacell and a support cell. Individual trichocytes measure 19–27 μm in length and 6–12 μm in diameter. Buried trichocytes not observed. Data on measured characters in the holotype are summarized in Table 1.  
Reproduction:— Holotype lacked gametangial material.  
Tetrasporangial conceptacles flush to only slightly raised above the rest of the thallus surface (Fig. 40), and measure 105–275 (340) μm in external diameter. Conceptacle chambers elliptical to spherical (Fig. 41), and measure 79–188 (202) μm in diameter and 49–78 (98) μm high. Conceptacle roof 19–37 μm (2–4 cells; mostly 3 cells incl. epithallial cell) thick, comprising a single squat to elliptical to spherical epithallial cell, a single columnar meristematic cell that is 2.5–5 times the length of the epithallial cell, and with or without 1–2 small inner cells (Fig. 43). Conceptacle floor located 8–15 cells below the surrounding thallus surface. A ring of enlarged, domed cells lines the base of the pore canal and is oriented more-or-less perpendicular to the surrounding roof surface (Figs 41, 44); these cells do not project into the pore. Central columella not observed although zonately divided tetrasporangia occur peripherally in conceptacle chambers (Fig. 42). Tetrasporangia measure 34–62 μm in length and 15–37 μm in diameter. Bisporangia not observed. Buried tetrasporangial conceptacles not observed. Data on reproductive characters in the holotype are summarized in Table 2.  
 Remarks:—Similarly as with L. caribaeum, L. erosum was transferred to the genus Neogoniolithon by Adey (1970: 8). Lithophyllum erosum has never undergone any taxonomic revision since Adey’s (1970) characterisation of this species.   
 From the description above, L. erosum belongs within the genus Hydrolithon as it is currently characterised. While the mode of development of the tetrasporangial conceptacle roof is not evident, the presence of the enlarged cells lining the pore canal and the orientation (perpendicular to the roof surface) of these cells are all characters diagnostic of the genus Hydrolithon (Penrose & Woelkerling 1992, Penrose & Chamberlain 1993, Kato et al. 2011). The presence of a tetrasporangial conceptacle roof that is commonly composed of 3 cell layers (a single epithallial cell subtended by an elongate [columnar] meristematic cell that in turn is subtended by a small inner cell) ascribes this specimen to H. samoënse.   
 From Foslie’s (1906: 20) descriptions of L. erosum and L. samoënse there appears to be no logical reason for the separation of these taxa. Foslie’s (1906: 20) descriptions of the gross morphology of the two taxa are virtually identical with both taxa forming thin crusts with irregular, crenulating margins. The only difference in Foslie’s descriptions is those of the cell dimensions of the cortical filaments (perithallial cells). According to Foslie (1906: 20) L. erosum bears predominantly vertically elongated cortical cells while those of L. samoënse are roughly square. It is now widely accepted that such differences used as sole characters are no longer valid (Woelkerling & Irvine 1986; Woelkerling & Campbell 1992; Woelkerling et al. 1993, Braga & Aguirre 1995). Furthermore, Foslie (1906: 20) concluded his description of L. samoënse by stating that: “A couple of specimens from Tahiti, that I have earlier, with doubt, described as L. decipiens, belong to the same species (herb. Bornet)”. Foslie (1906) was referring here to L. caribaeum (as L. decipiens f. caribaeum), which incidentally was separated from L. erosum and L. samoënse by the former taxon having predominantly horizontally elongated cortical cells.   
 Foslie (1906: 20) recorded and described L. erosum and L. samoënse in the same publication. All species epithets from the publication, including erosum and samoënse, thus have equal nomenclatural priority (see Art. 11.5 [Ex. 20 and Ex. 21] of the ICBN), irrespective of the order in which they appear in the publication. Here we are combining the species for the first time and choose to use the epithet samoënse because that epithet is in current common use. Lithophyllum erosum is therefore considered a heterotypic synonym for H. samoënse.   
 FIGURE 37. Vertical fracture of the thallus under SEM showing the monomerous construction, cortical filaments, medullary filaments (M), solitary bottle-shaped trichocytes (t), and cell fusions (f) between adjacent filaments (scale bar = 20 μm). FIGURE 38. Vertical section of the outer thallus showing the single epithallial cell layer (e), the subepithallial initial (i), the first cortical cell (c), and cell fusions (f) between adjacent cortical filaments (scale bar = 15 μm). FIGURE 39. Vertical fracture of the lower portion of the thallus under SEM showing plumose medullary filaments with abundant cell fusions (f) between adjacent filaments (scale bar = 20 μm). FIGURE 40. Thallus surface under SEM showing flush to slightly raised tetrasporangial conceptacles (arrows) (scale bar = 100 μm). FIGURE 41. Vertical fracture through a tetrasporangial conceptacle under SEM showing the pore canal (arrow) and an elliptical conceptacle chamber (K). Note the remnants of the enlarged cells (arrowhead) that typically line the base and length of the pore canal (scale bar = 60 μm). FIGURE 42. Magnified view of two buried tetrasporangia (t) (scale bar = 30 μm). FIGURE 43. Magnified view of the roof of a tetrasporangial conceptacle showing the roof comprised of mostly 3 cells (e, i, c) (scale bar = 15 μm). FIGURE 44. Magnified view of the pore canal (P) showing the remains of the enlarged cells (arrowheads) that line the base of the pore canal (scale bar = 15 μm).
description: Published as part of Maneveldt, G. W., Merwe, E. Van Der & Keats, D. W., 2015, Taxonomic review of Hydrolithon samoënse (Corallinaceae, Corallinales, Rhodophyta) and other taxa found to be conspecific, pp. 230-253 in Phytotaxa 192 (4) on pages 242-244, DOI: 10.11646/phytotaxa.192.4.2, http://zenodo.org/record/13641914