Neogoniolithon caribaeum (Foslie) W.H. Adey, 1970: 8

(Figs 29–35)

Basionym:— Lithophyllum decipiens f. caribaeum Foslie, 1906: 18 (‘ caribaea ’).

Homotypic synonyms:— Lithophyllum caribaeum (Foslie) Foslie, 1907: 22; Heteroderma caribaeum (Foslie) Segonzac, 1984: 101.

Lectotype: TRH!, A1-8; originally unnumbered. Lectotype designated by Woelkerling (1993: 48). Previous references to typification were by Adey & Lebednik (1967: 14), Masaki (1968: 31), Woelkerling (1993: 48) (as Lithophyllum) and Adey (1970: 8) (as Neogoniolithon).

Type locality:—The harbour, St. Thomas Island, US Virgin Islands.

Etymology:— ‘ decipiens ’ means deceiving (used of a species closely resembling another) (Stearn 1973); ‘ caribaea ’, Foslie (1906) did not explain the origin of the epithet, but it presumably makes reference to the Caribbean Sea from where this specimen was collected.

Distribution:—The species has been reported from the north Atlantic, subtropical western Atlantic, the Indian and the western Pacific Oceans. See Guiry & Guiry (2014) for a detailed distribution with sources.

Appearance and vegetative anatomy:—The following description is based solely on the lectotype material from TRH (Figs 29–35) as no representative specimens for this taxon were available for verification. Lectotype material contains several epilithic individuals that are thin and adherent, measuring 75–240 μm thick. Thalli encrusting (smooth), lacking protuberances and have adherent margins that are entire to lobed, but lack orbital ridges.

Thallus monomerous and dorsiventrally organised (Fig. 31). Medullary region consists of a central plumose (non-coaxial) core with square to rectangular cells that measure 3–16 μm in length and 3–8 μm in diameter. Cortical filaments comprise small, squarish to bead-like cells, and measure 3–7 μm in length and 3–7 μm in diameter. Cell fusions abundant; secondary pit connections not seen. Subepithallial initials square to rectangular (Fig. 32), and measure 4–11 μm in length and 4–6 μm in diameter. Epithallial cells squat to elliptical, occur singly (up to 2 when shedding), and measure 3–6 μm in length and 3–7 μm in diameter (Fig. 32). Squarish to elongate trichocytes occur singly (rarely paired) at the thallus surface (Fig. 32). Trichocyte chains typically comprise 2 cells; a megacell and a single support cell. Trichocytes measure 8–27 μm in length and 5–10 μm in diameter. Buried trichocytes not observed. Data on vegetative characters in the lectotype are summarized in Table 1.

FIGURE 29. Lectotype fragments and specimen packaging (scale bar = 20 mm). FIGURE 30. Slide and specimen label (scale bar = 20 mm).

Reproduction:— Lectotype lacked gametangial material.

Tetrasporangial conceptacles flush to only slightly raised above the rest of the thallus surface (Fig. 33), and measure 73–155 μm in external diameter. Conceptacle chambers elliptical to spherical (Fig. 34), and measure 61–85 μm in diameter and 37–66 μm high. Conceptacle roof 10–17 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.35). Conceptacle floor located 12–17 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; these cell do not project into the pore canal (Fig. 35). Central columella not observed although zonately divided tetrasporangia occur peripherally in conceptacle chambers. Tetrasporangia measure 25–50 μm in length and 6–34 μm in diameter. Bisporangia not observed. Buried tetrasporangial conceptacles rare, containing apparently viable tetrasporangia. Infilling of buried conceptacles not observed although crescent-shaped scars have been observed, which suggest shedding of conceptacles. Data on reproductive characters in the lectotype are summarized in Table 2.

FIGURE 31. Vertical fracture of the thallus under SEM showing a monomerous thallus construction, and the cortical (C) and medullary filaments (M) (scale bar = 20 μm). FIGURE 32. Vertical section of the outer thallus showing the location of the epithallial cell layer (arrowhead), a solitary elongated bottle-shaped trichocyte (t), subepithallial initials (i) and the first cortical cells (c) (scale bar = 15 μm). FIGURE 33. Thallus surface under SEM showing flush to slightly raised tetrasporangial conceptacles (arrows) (scale bar = 100 μm). FIGURE 34. Vertical section through a tetrasporangial conceptacle showing an elliptical chamber (K), the pore canal (p) and enlarged cells (arrowheads) lining the base of the pore canal (scale bar = 30 μm). FIGURE 35. Magnified view of the pore canal (P) and roof of a tetrasporangial conceptacle showing the roof comprised of mostly 3 cells (e, i, c) and the remains of the enlarged cells (arrowheads) that line the base of the pore canal (scale bar = 15 μm).

Remarks:— Lithophyllum caribaeum was transferred to the genus Neogoniolithon by Adey (1970: 8) because Adey considered Neogoniolithon to be characterised by, among other features, the presence of a monomerous (“multilayered hypothallium”) thallus as opposed to the dimerous (“single-layered hypothallium”) thallus of Hydrolithon (sensu Adey 1970: 11). Since Adey’s (1970) descriptions, both genera have undergone substantial revision and characterisation based solely on thallus construction no longer holds. 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) and the crescent-shaped infilling scars notably left by shed conceptacles, ascribes this material to H. samoënse. Neogoniolithon caribaeum is therefore considered a heterotypic synonym for H. samoënse, the latter species having nomenclatural priority.

Masaki (1968: 31–32) described a new form of L. caribaeum that he called Lithophyllum caribaeum f. boreale Masaki. However, from Masaki’s descriptions and diagrams, and from our current understanding of the generic characterisation within the Corallinales, Masaki’s (1968) species belongs to the genus Spongites and is more closely related to S. yendoi (Foslie) Y.M.Chamberlain and S. decipiens (Foslie) Y.M.Chamberlain (see Chamberlain 1993).