Hydrolithon rupestre (Foslie) Penrose, 1996: 265

Wolf, M. A., Maneveldt, G. W., Kaleb, S., Moro, I. & Falace, A., 2015, Morphological and molecular characterization of Hydrolithon rupestre (Corallinaceae, Corallinales, Rhodophyta): first report from the Mediterranean Sea, Phytotaxa 224 (1), pp. 59-71 : 61-65

Hydrolithon rupestre (Foslie) Penrose, 1996: 265 (as “ rupestris ”) ( Figs 1–10 )

Basionym:— Lithophyllum rupestre Foslie, 1907: 26 .

Holotype:— C. J. Gabriel; April 1905; TRH ( A 3-149) ( Penrose 1996: 266). Depicted in Printz (1929, pl. LIV, fig. 1 as Lithophyllum ) and in Penrose (1990, fig. 54A-D). Additional slides in a cardboard slide holder marked ‘Slides prepared by Gavin W. Maneveldt, Univ. of Western Cape, South Africa’ were deposited at TRH in April 2002 ( Woelkerling et al. 2005).

Holotype locality:— Ocean Beach, Phillip Island, Victoria, Australia ( Woelkerling & Campbell 1992: 100). Additional data on the type are by Woelkerling et al. (2005: 41).

Specimens examined:— Vis Island ( Croatia), Mediterranean Sea (43°03’12.69’’N, 16°02’37.73’’E), 12 m depth, leg. S. Kaleb, 05.viii.2008 ( UWC 14/ V 3). Only one sample bearing multiple thalli was examined.

Description

Thalli are non-geniculate, encrusting (smooth) to warty, and firmly attached to the rocky substrate ( Fig. 1 ). Freshly collected material pink in colour. Thalli monomerous and dorsiventrally organised with a thin plumose (non-coaxial) medullary core ( Fig. 2 ) and thick cortical region. Cells of adjacent medullary and cortical filaments are joined by cell fusions ( Figs 2 & 3 ); secondary pit connections were not observed. Subepithallial initials are squat to slightly rectangular and measure 5–8 μm in length and 4–6 μm in diameter ( Fig. 3 ). Epithallial cells occur in a single layer, are rounded to elliptical and measure 3–5 μm in length and 4–6 μm in diameter ( Fig. 3 ). Trichocytes occur solitary, paired and in small horizontal rows (but with normal vegetative filaments between them) at the thallus surface ( Fig. 3 ), and sometimes also buried in the thallus ( Fig. 4 ).

Gametangial thalli are monoecious. Spermatangial (male) conceptacles are small and slightly raised above the surrounding thallus surface. Their chambers are generally elliptical and measure 20–34 μm in height and 44–61 μm in diameter ( Fig. 5 ). Unbranched (simple) spermatangial systems are confined to the chamber floor.

Carpogonial (female) conceptacles were not observed. After presumed karyogamy, carposporophytes develop in female conceptacles and form carposporangial conceptacles. Carposporangial conceptacles are slightly raised above the surrounding thallus surface ( Fig. 6 ). The roof is 3–5 cells thick. Chambers are spherical to elliptical and measure 46–58 μm in height and 64–87 μm in diameter. The chamber floors are located 12–13 cells below the thallus surface. Most carposporangial material was senescent but it was possible to observe a continuous central fusion cell with peripherally arranged gonimoblast filaments. Gonimoblast filaments are many-celled (due to the deteriorated nature of the material, we did not observe intact filaments), including terminal carposporangia. Unfertilised carpogonia persist across the surface of the fusion cell.

Tetrasporangial conceptacles are uniporate and are slightly raised to flushed with the surrounding thallus surface ( Fig. 7 ). Their chambers are spherical to elliptical and measure 41–53 μm in height and 69–82 μm in diameter. The chamber floors are located 12–16 cells below the surrounding thallus surface. The base of the pore canals is lined by a ring of enlarged, domed cells that do not protrude into the pore canal, but are oriented more-or-less perpendicularly to the roof surface ( Figs 8–10 ). Conceptacle roofs are 3–4 cell layers thick ( Fig. 8 ). Chambers lack a central columella of sterile filaments and in mature viable conceptacles, zonately divided tetrasporangia are arranged across the chamber floor ( Fig. 7 ). Conceptacle primordia have not been observed, but based on the orientation of peripheral roof cells and the persistence of sterile filaments interspersed throughout the chamber ( Fig. 10 ) it is evident that the roof is formed from filaments both peripheral to the fertile area and interspersed among the developing tetrasporangia. Conceptacles of all types become buried in the thallus.

Distribution and habitat

Hydrolithon rupestre appears to have a tropical to subtropical affinity ( Maneveldt 2005). It is known from Southern and South-Eastern Australia and New Zealand ( Penrose 1996), Pacific Islands ( Payri et al. 2000, N’Yeurt & Payri 2010), Mexico (Pacific) ( Fragoso & Rodríguez 2002), South Africa and Southern Taiwan ( Maneveldt 2005). More recently the species was described from the equatorial Atlantic ( Crespo et al. 2014). Here we report for the first time its presence in the Mediterranean Sea as epilithic, encrusting to warty specimens found at 12 m depths.

In Southern and South-Eastern Australia H. rupestre is epilithic or epizoic, encrusting to warty, and occurs commonly on intertidal rock platforms and to water depths of 3 m ( Penrose 1996) or to depths of 17 m ( Harvey et al. 2006). Atlantic specimens are epilithic, encrusting, and were found to water depths of 49 m ( Crespo et al. 2014), while in French Polynesia the species is found on hard substrates in lagoons ( N’Yeurt & Payri 2010). In Fiji, South Africa and Taiwan, H. rupestre is encrusting to warty, and occurs both epizoically and epilithically throughout the intertidal and to water depths of 15 m ( Maneveldt 2005).

Phylogenetic analyses

The nSSU and psb A sequences, obtained for the nine isolates examined in this study were all identical. For this reason only one sequence for each marker from the same specimen was included in the following molecular analyses.

Alignments of 742 positions for the nSSU dataset and 786 positions for the psb A dataset were used for the phylogenetic analyses.

The ML tree obtained using the nSSU marker shown in Fig. 11 detects 5 separate groups of sequences representing 5 different Corallinaceae subfamilies ( Corallinoideae , Lithophylloideae, Metagoniolithoideae, Hydrolithoideae and Porolithoideae ). Within each subfamily the representative genera are distinguishable by well-supported clades including the type species of the genus (evidenced in bold). Species representing the genus Porolithon (subfamily Porolithoideae ), namely P. gardineri (Foslie) Foslie and P. onkodes (Heydrich) Foslie (the type species of the genus), form a well-supported clade (99 ML /98 MP) distantly related to the clade representing H. reinboldii (Weber-van Bosse & Foslie) Foslie (100 ML /100 MP), the generitype of Hydrolithon (subfamily Hydrolithoideae ).

* genicula composed of untiered multicellular filaments

**trichocytes in large pustulose horizontal fields

***genicula absent in the genus Lithophyllum Philippi

The sequence of our Mediterranean sample forms a clade with a sequence belonging to the species Hydrolithon samoënse ( AY 234236). Similarly, other sequences of species included in the analyses and attributed to the genus Hydrolithon ( H. murakoshii AB 576012, H. improcerum EF 628239) group in a non-supported clade.

Figure 12 represents the ML tree inferred from psb A gene sequences. Phylogenetically distinct clades are represented by 5 subfamilies of Corallinaceae ( Mastophoroideae sensu lato Harvey et al. 2003, Lithophylloideae, Corallinoideae , Porolithoideae and Hydrolithoideae ). The clade that includes species of the subfamily Corallinoideae is basal to those representing the other subfamilies. In this case the sequence of our isolate falls in a clade including sequences of specimens attributed to the genus Hydrolithon ( H. improcerum DQ 168006) and Pneophyllum ( P. conicum (E.Y.Dawson) Keats, Y.M.Chamberlain & M.Baba AB 576040).

Figures

FIGURE 11. ML tree inferred from nSSU gene sequences, calculated using the GTR+G+I model of evolution. Numbers near nodes indicate bootstrap values (>50%) for ML analysis (left), and MP analysis (right). The Adriatic species sequence determined in this work is indicated as LN849720 ‘Hydrolithon rupestre’. The type species for each genus is bolded.

FIGURE 12. ML tree inferred from psbA gene sequences, calculated using the GTR+G+I model of evolution. Numbers near nodes indicate bootstrap values (>50%) for ML analysis (left), and MP analysis (right). The Adriatic species sequence determined in this work is indicated as LN849721 ‘Hydrolithon rupestre’.

FIGURES 1–4. Gross morphology and vegetative anatomy of Hydrolithon rupestre from the Mediterranean (UWC 14/V3). 1. Encrusting to warty habit (scale bar = 20 mm). 2. Vertical section of the inner thallus showing plumose medullary filaments. Note the cell fusions (f) between contiguous filaments (scale bar = 20 μm).3. Vertical section of the outer thallus showing a single epithallial cell layer (arrowheads) and solitary to paired trichocytes (T). Note the cell fusions (f) between contiguous cortical filaments and the normal vegetative filaments (arrows) between trichocytes (scale bar = 20 μm). 4. Vertical section through the outer thallus showing buried trichocytes (T) (scale bar = 20 μm).

FIGURES 5–10. Reproductive anatomy of Hydrolithon rupestre from the Mediterranean (UWC 14/V3). 5. Vertical section of the thallus through a buried spermatangial conceptacle showing unbranched (simple) spermatangial systems restricted to the conceptacle floor (arrowheads) (scale bar = 20 μm). 6. Vertical section of the outer thallus through a carposporangial conceptacle showing a continuous central fusion cell (arrowhead) bearing peripheral gonimoblast filaments each terminating in a carposporangium (g). Note the remains of unfertilised carpogonia across the dorsal surface of the fusion cell (arrow) (scale bar = 20 μm). 7. Vertical section of the outer thallus showing a flush (left) and a raised (right) tetrasporangial conceptacle (T), as well as a buried tetrasporangial conceptacle (t). Note the absence of a central columella and the arrangement of the tetrasporangia across (right conceptacle) the chamber floor (scale bar = 50 μm). 8. Vertical section through the roof of a tetrasporangial conceptacle showing the enlarged, domed cells (arrows) lining the base of the pore canal. Note that the roof is comprised of 3–4 cells (scale bar = 20 μm). 9. Vertical fracture through the roof of a tetrasporangial conceptacle under SEM showing the enlarged, domed cells (arrow) lining the base of the pore canal (scale bar = 30 μm). 10. Vertical section through a senescent tetrasporangial conceptacle showing the remains of filaments (arrows) that gave rise to the roof and pore canal cells. Note the enlarged, domed cells (arrowheads) lining the base of the pore canal and the absence of a central columella (scale bar = 20 μm).