Hydrolithon samoënse (Foslie) Keats & Y.M. Chamberlain, 1994: 15

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, Phytotaxa 192 (4), pp. 230-253 : 232-239

Hydrolithon samoënse (Foslie) Keats & Y.M. Chamberlain, 1994: 15

( Figs 1–28 )

Basionym:— Lithophyllum samoënse Foslie, 1906: 20 ( Figs 1–11 ).

Homotypic synonyms:— Pseudolithophyllum samoënse (Foslie) Adey, 1970: 13 .

Heterotypic synonyms:— Lithophyllum illitus Lemoine, 1929: 54 ; Neogoniolithon illitus (Lemoine) Afonso-Carrillo, 1984: 133 ; Spongites wildpretii Afonso-Carrillo, 1988: 99 .

Lectotype:—TRH!, A2-111; originally unnumbered. Lectotype designated by Woelkerling (1993: 193). Previous references to typification were by Dawson (1960: 50), Adey & Lebednik (1967: 17) (as Lithophyllum ), Adey (1970: 13) (as Pseudolithophyllum ) and Chamberlain (1994: 126) (as Hydrolithon ). Lectotype illustrated in Printz (1929: pl. 53, Fig. 19 ) (as Lithophyllum ).

Paratype material also exist in PC (unnumbered, Thuret-Bornet Herbarium [box collection, filed under Lithothamnion decipiens ]) (see Woelkerling & Lamy 1998).

Type locality:—Satana, Saraii Island, Western Samoa.

Etymology:— ‘ samoënse ’ refers to the type locality in the Samoan island group.

Specimens examined:—In addition to the lectotype, 14 representative specimens were examined during this study.

FIJI. Suva Point , Suva (D Keats, 3.viii.1994, UWC 94 /1146 B) ; Suva Lagoon, Suva (D Keats, 23.xi.1995, UWC 95 /1515) ; Fish Patch, Suva Barrier Reef (D Keats, 26.xi.1995, UWC 95 /1523) .

SOUTH AFRICA. Kwazulu-Natal: Jesser Point, Sodwana Bay (D Keats , 7.xi.1991, UWC 91 /179 A) ; Western Cape: Holbaaipunt, Cape Hangklip (D Keats, 18.v.1992, UWC 92 /98; D Keats, 1.vi.1992, UWC 92 /146; D Keats & G Maneveldt, 21.v.1993, UWC 93 /58; D Keats & GW Maneveldt, 21.vi.1993, UWC 93 /70) ; Western Cape: Nature’s Valley, Tsitsikamma ( GW Maneveldt & U van Bloemestein, 4.iv.2008, UWC 08 /05) ; Western Cape: Struisbaai ( GW Maneveldt & E van der Merwe, 6.vi.2009, UWC 09 /102) ; Western Cape: Skulpiesbaai, Stilbaai ( GW Maneveldt & E van der Merwe, 12.vi.2010, UWC 10 /107; UWC 10 /110, UWC 10 /113) .

TAIWAN. Wanlitong Bay, southern Taiwan (D Keats, GW Maneveldt & J Lewis, 17.ix.1998, UWC 98/322).

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

Habitat:—On tropical to subtropical reefs the species has been recorded to 15 m depth, occurring epilithically on boulders and epizoically on dead coral skeletons. On temperate shores the species has been recorded to occur epilithically on boulders in high and mid-shore rock pools and epilithically on boulders and the primary substratum (bedrock) on the low shore and the shallow subtidal zone to 6 m depth.

Appearance and vegetative anatomy:— Lectotype collection contains several epilithic individuals that are thin and adherent, measuring up to 20 mm in diameter and up to 375 μm thick (see also Printz 1929: pl. 53, Fig. 19 ). Thalli encrusting (smooth), lacking protuberances, individual margins adherent, lacking orbital ridges. Small ridges (upgrowths), however, occur where margins of two or more individual thalli meet. Surface texture is matt.

Lectotype thallus fragments monomerous and dorsiventrally organised ( Figs 1, 3 ). Medullary region consists of a central plumose (non-coaxial) core ( Figs 1, 3 ), with square to rectangular cells that measure 5–19 μm in length and 3– 17 μm in diameter. Cells of cortical filaments squat to rectangular appearing bead-like and measure 3–15 μm in length and 3–9 μm in diameter ( Fig. 4 ). Cell fusions abundant ( Figs 3, 4 ); secondary pit connections not seen. Subepithallial initials square to rectangular, and measure 5–12 μm in length and 5–7 μm in diameter ( Fig. 4 ). Epithallial cells solitary (up to 2 when shedding), squat to elliptical ( Fig. 4 ), and measure 4–6 μm in length and 5–7 μm in diameter. Trichocytes common, occurring singly, in pairs and/or in small vertical rows ( Figs 2, 4 ). Trichocyte chains comprise 2–5 cells; a megacell, and up to 4 support cells. Trichocytes rectangular to bottle-shaped to elongate, and measure 16–31 μm in length and 7–10 μm in diameter. Trichocytes overgrown and buried in the thallus ( Fig. 2 ). Data on measured characters in the lectotype are summarized in Table 1.

otherwise stated, all measurements are in micrometres.

FIGURE 1 . Vertical fracture through the thallus margin under SEM showing a monomerous thallus construction, the location of the epithallial cell layer (arrow) and cortical ( C) and medullary filaments ( M) (scale bar = 60 μm). FIGURE 2 . Vertical fracture of the outer thallus under SEM showing numerous solitary trichocytes in short vertical rows ( T) both at the surface and buried in the thallus (scale bar = 30 μm). FIGURE 3 . Vertical fracture of the ventral region of the thallus under SEM showing the plumose medullary region with extensive cell fusions (f) between adjacent filaments (scale bar = 30 μm). FIGURE 4 . Vertical section of the outer thallus showing a single epithallial cell layer (arrowhead), a solitary trichocyte ( T), a subepithallial initial (i), a first cortical cell (c) and cell fusions (f) between adjacent cortical filaments (scale bar = 15 μm).

Features evident in the lectotype are present in various other specimens examined ( Figs 12 , 15–17 ). Thalli monomerous, dorsiventrally organised, and have similar features and proportions (but greater ranges) of medullary and cortical filaments, subepithallial and epithallial cells, and trichocytes. Data on measured characters are summarized in Table 1. Greater variation occurs in growth form and habitat across the species as a whole. Individual thalli measure up to 100 mm in diameter and are variably thin (75 μm) to 800 μm thick. Thalli mostly encrusting (smooth) ( Fig. 12 ) to minutely warty. Colour in living thalli generally gleaming purplish-red to brilliant pink. Craters of old conceptacles and swirls of sloughing epithallial cells commonly observed at the surface ( Fig. 13 ). Conceptacles crowded across the thallus surface appearing as tiny, white discs in fresh material ( Figs 13, 14 ).

Reproduction:—Gametangial thalli from the lectotype are dioecious.

Spermatangial conceptacles small and inconspicuous, and measure 75–150 μm in external diameter ( Fig. 5 ). Conceptacle roofs more-or-less flush with the surrounding thallus surface. Conceptacle chambers wide and shallow to elliptical, and measure 37–62 μm in diameter and 19–29 μm high, with the roof 10–16 μm thick. Simple spermatangial systems restricted to the conceptacle chamber floor. Spermatangial conceptacles buried in the thallus.

Carpogonial conceptacles small and inconspicuous, and measure 145–200 μm in external diameter. Conceptacle roofs slightly raised above the surrounding thallus surface and measure 11–30 μm thick. Conceptacle chambers elliptical to spherical to flask-shaped ( Fig. 6 ), and measure 56–80 μm in diameter and 25–30 μm high. Carpogonial branches 3–4 celled, occurring centrally on the chamber floor. Carpogonial conceptacles appear to be shed as none have been observed buried in the thallus.

Carposporangial conceptacles have not been observed in the lectotype.

Tetrasporangial conceptacles small, more-or-less flush with the thallus surface, to slightly raised above it, and measure 140–190 μm in external diameter ( Figs 7, 8 ). Conceptacle chambers elliptical to spherical ( Figs 8, 10 ), and measure 56–130 μm in diameter and 31–60 μm high, with the roof 15–28 μm thick. Conceptacle roof 2–4 (mostly 3) cells thick, comprising a single squat to elliptical to spherical epithallial cell, a single columnar meristematic cell

FIGURE 5 . Vertical section of the outer thallus through a spermatangial conceptacle showing simple spermatangial systems restricted to the conceptacle floor (arrowheads) (scale bar = 30 μm). FIGURE 6 . Magnified view through the chamber of a carpogonial conceptacle showing carpogonial branches (arrow) distributed across the chamber floor (scale bar = 15 μm). FIGURE 7 . Thallus surface under SEM showing a single flush tetrasporangial conceptacle (arrow) (scale bar = 100 μm). FIGURE 8 . Section through a mature tetrasporangial conceptacle showing enlarged cells (arrowheads) lining the base of the pore canal. No central columella is evident (scale bar = 20 μm). FIGURE 9 . Magnified view through a tetrasporangial conceptacle roof showing enlarged cells (arrowheads) lining the base of the pore canal and the roof comprised of 3 (1–3) cells (scale bar = 10 μm). FIGURE 10 . Section through the thallus showing a buried, un-infilled tetrasporangial conceptacle ( K) with the remains of the enlarged cells (arrowheads) that line the base of the pore canal (scale bar = 30 μm). FIGURE 11 . Section through the thallus showing crescent-shaped scars (i) indicating infilling of areas previously occupied by sporangial conceptacles (scale bar = 60 μm).

that is 2.5–5 times the length of the epithallial cell, and with or without 1–2 small inner cells ( Fig. 9 ). Conceptacle floor located 8–16 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 (vertically orientated) to the roof surface; these cells do not project into the pore canal ( Fig. 9 ). Tetra/bisporangia not observed. Both shedding and burying of tetrasporangial conceptacles observed. Buried conceptacles not infilled ( Fig 10 ). However, infilling of scars left by shed conceptacles is evident ( Fig. 11 ). Data on measured reproductive characters in the lectotype are summarized in Table 2.

FIGURE 12 . Typical encrusting (smooth) growth form ( UWC 93/70) (scale bar = 20 mm). FIGURE 13 . The craters of shed senescent conceptacles are commonly observed at the thallus surface giving the surface a pock-marked appearance ( UWC 93/58) (scale bar = 1 mm). FIGURE 14 . Conceptacles are crowded across the thallus surface, appearing as tiny, white discs in fresh material ( UWC 93/58) (scale bar = 500 μm).

Reproductive features evident in the lectotype are present in various other specimens examined. Gametangial thalli are dioecious.

Spermatangial ( Fig. 18 ) and carpogonial conceptacles ( Figs 19 , 20 ) have similar features and proportions, but only slighter greater ranges compared to the lectotype material. Data on reproductive characters are summarized in Table 2. Unlike the lectotype, carposporangial material has been observed in representative material.

Carposporangia develop in carpogonial conceptacles after presumed karyogamy. Carposporangial conceptacles only slightly larger than carpogonial conceptacles, and measure 175–234 μm in external diameter ( Fig. 21 ). Conceptacle

FIGURE 15 . Vertical section of the outer thallus showing a single epithallial cell layer (e), a subepithallial initial (i), a first cortical cell (c) and a solitary trichocyte ( T) ( UWC 95/1515) (scale bar = 15 μm). FIGURE 16 . Cortical filaments often comprise small, squat to squarish, bead-like cells with abundant cell fusions (f) between adjacent filaments ( UWC 92/146) (scale bar = 15 μm). FIGURE 17 . Magnified view of the ventral region of the thallus showing a plumose medullary region with extensive cell fusions (f) between adjacent filaments

( UWC 92/146) (scale bar = 15 μm).

FIGURE 18 . Vertical section of the outer thallus through a spermatangial conceptacle showing simple spermatangial systems restricted to the conceptacle floor (arrowheads) ( UWC 10/113) (scale bar = 10 μm). FIGURE 19 . Vertical section of the outer thallus through a carpogonial conceptacle ( UWC 95/1515) (scale bar = 30 μm). FIGURE 20 . Section through a carpogonial conceptacle showing carpogonial branches bearing trichogynes (arrowheads) and carpogonia (c), distributed across the chamber floor ( UWC 95/1515) (scale bar = 15 μm). FIGURE 20 (insert). Magnified view of some carpogonial branches comprised of a carpogonium (c) terminating in a trichogyne, a support cell (s) and a sterile cell (arrow) ( UWC 95/1515) (scale bar = 10 μm). FIGURE 21 . Vertical section through a carposporangial conceptacle showing the pore canal (p), a thick and narrow, discoid, central fusion cell (arrow) bearing peripheral gonimoblast filaments terminating in carpospores (c) ( UWC 91/179) (scale bar = 30 μm).

FIGURE 22 . Section through a tetrasporangial conceptacle primordium showing early roof development from filaments (arrowheads) peripheral to the fertile area and the developing tetrasporangial initials (t – t). A single layer of epithallial cells (E) persists as a protective covering (UWC 09/102) (scale bar = 10 μm). FIGURE 23 . Section through a tetrasporangial conceptacle primordium showing later but still early roof development from filaments peripheral to the fertile arrow (arrowheads) and from filaments interspersed (arrowheads) among the sporangial initials (t – t). A layer of epithallial cells (E) still persists as a protective covering over the developing conceptacle (UWC 09/102) (scale bar = 10 μm). FIGURE 24 . Section through a young tetrasporangial conceptacle showing continued roof development from filaments peripheral to the fertile arrow (arrows) and from filaments interspersed (arrowheads) among the sporangial initials (t – t). A layer of epithallial cells (E) continues to persists as a protective covering over the young developing conceptacle (UWC 10/113) (scale bar = 10 μm). FIGURE 25 . Section through a young tetrasporangial conceptacle showing more advanced roof development from filaments peripheral to the fertile arrow (arrows) and from filaments interspersed (arrowheads) among the sporangial initials (t – t). Note the absence of the formation of a central columella as tetrasporangial initials (t – t) are distributed across the centre of the chamber floor. A ring of enlarged cells (e) has already formed where the pore canal will be situated (UWC 10/113) (scale bar = 10 μm). FIGURE 26 . Section through a mature tetrasporangial conceptacle showing the pore canal (p), the remains of filaments (arrowheads) that gave rise to the pore canal cells, and zonately divided tetrasporangia (t). Chambers lack a central columella (UWC 98/322) (scale bar = 30 μm). FIGURE 27 . Magnified view of the pore canal of a tetrasporangial conceptacle showing the enlarged cells (arrowheads) that typically line the base and length of the pore canal and the sterile filaments (arrows) that had given rise to them (UWC 98/322) (scale bar = 15 μm). FIGURE 28 . Magnified view of the roof of a tetrasporangial conceptacle showing the roof comprised of mostly 3 cells (e, i, c) (UWC 92/146) (scale bar = 15 μm).

chambers substantially larger than those of carpogonial conceptacles. Chambers elliptical to spherical, and measure 80–140 μm in diameter and 40–100 μm high with the roof 16–26 μm thick. Conceptacle pore lined with small papillate cells that tilt slightly into the pore canal. Central fusion cell discoid and continuous, and narrow and thick with gonimoblast filaments borne peripherally ( Fig. 21 ). Gonimoblast filaments 4 cells long including a terminal carpospore that measures 25–43 μm in length and 20–50 μm in diameter. Buried carposporangial conceptacles not observed.

Tetrasporangial features evident in the lectotype are present in the various specimens examined ( Figs 22–28 ) and are summarized in Table 2. Greater variation, however, exists in the ranges of the measurements for this species as a whole. Conceptacle roof formed by filaments peripheral to the fertile area and interspersed among the tetrasporangial initials ( Figs 22–25 ). Some of these filaments persist in mature conceptacle chambers ( Figs 26, 27 ). Central columella not observed. In young developing tetrasporangial conceptacles, tetrasporangial initials distributed across the conceptacle chamber floor ( Figs 24, 25 ). In mature conceptacles, however, zonately divided tetrasporangia generally located peripherally in the conceptacle chamber ( Fig. 26 ), but often expand across the chamber floor as they fill the chamber completely. Tetrasporangia measure 18–60 μm in length and 6–35 μm in diameter. Bisporangia not observed. Senescent tetrasporangial conceptacles shed leaving craters (scars) that become infilled.

Remarks:— Hydrolithon samoënse is reported to be common throughout the north eastern Atlantic, Indian and Pacific Oceans ( Keats & Chamberlain 1994). Keats & Chamberlain (1994) transferred the species to the genus Hydrolithon (sensu Penrose & Woelkerling 1992) and distinguished it from other species of the genus on the basis of two tetrasporangial characters. Primarily, H. samoënse possesses a tetrasporangial conceptacle roof that is more 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. Secondarily, the tetrasporangial conceptacles are evident as tiny ‘pinpricks’ at the thallus surface that are usually shed individually or in small groups on senescence (some become buried), leaving small scars at the thallus surface; scars left by shed conceptacles eventually become infilled. While the former character separates Hydrolithon samoënse from all other species of Hydrolithon , the latter character separates it from the very similar H. superficiale Keats & Y.M.Chamberlain in which tetrasporangial conceptacles are produced in a superficial layer that is shed in its entirety upon senescence ( Keats & Chamberlain 1994).

Figures

FIGURES 1–4. Vegetative anatomy of the lectotype of Hydrolithon samoënse (TRH A2-111).

FIGURES 5–11. Reproductive anatomy of the lectotype of Hydrolithon samoënse (TRH A2-111).

FIGURES 12–14. Gross morphology of representative specimens of Hydrolithon samoënse.

FIGURES 15–19. Vegetative anatomy of Hydrolithon samoënse.

FIGURES 18–21. Gametangial anatomy of Hydrolithon samoënse.

FIGURES 22–28. Tetrasporangial anatomy of Hydrolithon samoënse.