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Meloneis Gen. Nov., a New Epipsammic Genus of Rhaphoneidaceae (Bacillariophyceae)

The diatom family Rhaphoneidaceae is characterized by high generic diversity and low species diversity with most genera known to have long stratigraphic ranges. The genera within this family are neritic marine, and mostly epipsammic. A new modern and epipsammic genus, Meloneis gen. nov., is described herein and is compared to all genera within Rhaphoneidaceae and especially to Rhaphoneis Ehrenberg s.l. Within Meloneis three new species and one variety are distinguished and described herein: M. mimallis sp. nov., M. mimallis var. zephyria var. nov., M. akytos sp. nov., and M. gorgis sp. nov.

In this paper, a new genus of Rhaphoneidaceae, named Meloneis, is established, with four infraspecific taxa. Genus Meloneis is taxonomically distinct from the related genera Rhaphoneis, Neodelphineis and Perissonoë, or the superficially similar genera Adoneis and Dickensoniaforma. The new taxa of Meloneis were found as epipsammic around submarine hydrothermal vents of Milos Island (Greece). Another genus of Rhaphoneidaceae, i.e. Detonia Frenguelli, was taxonomically rearranged from epipsammic material of the same area ( [22]). A catalogue of epilithic diatoms found distant of the vents is also available [23], together with euendolithic chlorophytes and cyanobacteria (see also [24]).

Materials and Methods
Milos is an island in the middle of the Hellenic Volcanic Arc with some 35 km 2 of geothermally active seabed [25], and Palaeochori Bay (24u31.009E; 36u40.119N) on the southeastern Milos is one of the most active geothermal submarine areas of the Aegean Sea [26]. The hydrothermal fluids of the submarine vents in Palaeochori are warm (max. 115uC), acidic (min. pH 3.54), highly saline, generally enriched over seawater in chloride, calcium, strontium, barium, sodium, potassium, lithium, silicon, iron, manganese, zinc, cobalt, lead, nickel, yttrium, vanadium but depleted in magnesium and sulphate [27]. The gases in these fluids contain mainly carbon dioxide up to 91.9%; methane, hydrogen, and hydrogen sulphide are also released at concentrations of up to 9.7%, 3.0%, and 8.1% respectively [25].
Sediment samples were collected from submarine hydrothermal vents in Palaeochori Bay during two multidisciplinary field trips of some European institutions in June 1996 and June 1997 in the framework of EU-funded programmes. The study area is open to the public and is not under any protection act, therefore, no specific permits were required for visiting the area, working in the field and collecting samples. Furthermore, the collection did not involve in any way endangered or protected species of any kind. Each vent is surrounded by three characteristic concentric zones of distinctive color precipitates [25][26][28][29]: yellow (Y), white (W), and brown (B). The yellow color of the innermost zone results from sulfur condensing on sand grains; the nature of the white precipitates of the intermediate zone is a mixture of amorphous silica, Si nodules and hollow tubes containing elemental sulfur on the outer surfaces; the outermost brownish zone of the vent system consists of Mn-oxides which predominately precipitate at increasing distances from the vent outlet. Material was studied from all the three color precipitates in both collections (in 1996 at a depth of 7 m; and in 1997 at a depth of 4 m and 7 m), and additionally from a control site (C) outside of the vents (in 1997, at a depth of 4 m and 7 m). The collected samples were preserved in formaldehyde solution. Material was oxidized and slides were prepared for diatom analyses according to standard procedures [30]. Observations were made using Zeiss Axiolab microscope equipped with a Sony DSC-S85 digital camera and Jeol JSM-35 Scanning Electron Microscope equipped with Adda 3 Olympus Soft Imaging Solutions and Scandium Universal SEM Imaging Platform. Description. New genus in Rhaphoneidaceae described herein. Valves with flat surface, shallow mantles, broadly lanceolate outline, smoothly convex sides and slightly produced apices. Axial area hyaline, distinct, narrow, linear to slightly lanceolate. Valve surface covered with round to ovate rota-type areolae, radiating in curved transverse striae from the axial area. Transverse striae not aligned across the axial area. A single continuous row of areolae runs along the mantle; mantle areolae aligned with the transverse striae. A single large rimoportula placed in-between the areolae of the last transversal stria at each valve end. A pseudocellus consisting of 2 to several fine pores in a rather circular pattern at each valve end. Edge of the valve face ornamented by an irregular row of low siliceous dome-like papillae.
Description. Valves morphologicaly identical to the species M. mimallis but with broader produced apices and with loosely spaced longitudinal rows of areolae, 4-4.5/10 mm at the middle.
DERIVATION OF NAME: ze.phy.ri'.a. femin., N.G. femin.  Figure 2B. Detail of the valve apex showing the circular pattern of pseudocellus consisting of 6 pores (black arrow); grey arrow indicates the external opening of the rimoportula; black arrow with white outline points to the extra incomplete row of small papillae (see also the complete rows in Fig. 2C). Figure 2C. Note the presence of an extra short continuous row of fine papillae at each pole (arrows) (cf. Fig. 2B). Figure 2D. Description. Valves with flat surface, shallow mantles, broadly lanceolate outline smoothly convex sides and acutely produced apices; length 21.5 to 32 mm, width 14 to 19.5 mm. Axial area hyaline, distinct, narrow, linear to slightly lanceolate. Transverse striae composed of very large circular to ovate areolae with rota-type vela, not aligned across the axial area. Transverse striae radial, in the middle 4.5-6/10 mm (mainly 5/10 mm), at the margin 5-6.5/10 mm (mainly 6/10 mm). Areolae arranged also in slightly curved longitudinal rows, 5-6/10 mm at the middle. Short marginal striae present between complete striae. A single continuous row of areolae runs along the mantle of the valve; mantle areolae aligned with the transverse striae. Rimoportulae one at each pole, positioned diagonally or on the same side of the apical axis. Rimoportulae with a round external pore, and an elongate slit-like internal opening. A reduced pseudocellus at each valve end consisting either of 3 and 3 or 3 and 4 (rarely 2 and 2) fine pores. Edge of the valve face ornamented by an irregular row of low siliceous dome-like papillae.
HOLOTYPE: Botanical Museum of the Athens University, Greece; ADH slide 1318, England Finder Ref. T31/1; Figure 1G in this paper.
TYPE LOCALITY: Palaeochori Bay, Milos Island, Greece; marine, neritic, epipsammic. Description. Valves with flat surface, shallow mantles, broadly lanceolate outline, smoothly convex sides and acutely produced apices; length 17 to 25 mm, width 11 to 14 mm. Axial area hyaline, distinct, linear and narrow at the valve ends and slightly widened at the middle. Transverse striae composed of circular to ovate areolae with rota-type vela, not aligned across the axial area. Transverse striae radial, in the middle 10-12/10 mm, at the margin (8)
The detailed morphology of the new genus Meloneis is characterized by (i) valves broadly lanceolate with smoothly convex sides and slightly produced ends, forming an outline similar to that of Adoneis, some Rhaphoneis, and the fossil Dickensoniaforma, (ii) areolae circular or ovate, containing solid, simple rotae connected to the valve by two or scarcely three struts aligned parallel to the margin of the valve closest to the areola, a feature reminiscent of Adoneis, Delphineis, Neodelphineis, Diplomenora; areolae externally showing central pits like those of Adoneis and Perissonoë, (iii) transverse rows of areolae fully nonaligned across the axial area, as found in Neodelphineis, Adoneis, Dickensoniaforma, Diplomenora, Detonia, and some Rhaphoneis, (iv) a single row of areolae at the mantle below the marginal ridge in alignment with the transverse rows of areolae, a common feature of many genera such as Delphineis, Neodelphineis, Perissonoë, Adoneis, Diplomenora, and Rhaphoneis (at least sensu Round et al. 1990 [1]), (v) rimoportulae one at each pole, diagonally or laterally positioned in relation to the longitudinal axis and lying at the last transverse row of areolae, as observed in Neodelphineis, Perissonoë (with 1-4 rimoportulae per valve), Rhaphoneis (?), and Lancineis (?), (vi) pseudocelli one at each pole, consisting of two to seven very fine pores in a rather circular pattern between the last transverse row of areolae and the marginal ring of papillae, a feature unique for Meloneis and vaguely similar to those of Neodelphineis (with one pore), Perissonoë (with pores in a rather radiating pattern), and Rhaphoneis and Lancineis (with many pores in a rather disorganised pattern), (vii) lack of surface furrows along the transapical striae, similarly to the genera Adoneis, Detonia, Dickensoniaforma, Diplomenora, Lancineis, Perissonoë, Sceptroneis and most possibly Rhaphoneis, in contrast to the genera Drewsandria, Neodelphineis and most Delphineis having grooved external valve face (the valves of the Delphineis surirella group are usually smooth or very slightly grooved [6,31]); (viii) presence of a ring of papillae at the valve margin, a feature also found in Perissonoë, some Delphineis, and partly in Adoneis (Neodelphineis bears short spines).
The above descriptive comparison shows a closer similarity of Meloneis to the genera Rhaphoneis, Neodelphineis, and Perissonoë especially concerning the fine structure of the apex (Table 1).
-Concerning genus Rhaphoneis sensu lato (see [32]) there is a lack of knowledge about the fine structure of some taxonomic features in several species. Therefore, comparison can be made to Rhaphoneis amphiceros (Ehrenberg) Ehrenberg [18,19] Dickensoniaforma are considered as rather superficial since they are restricted mainly to the general valve appearance and outline.
Specifically, genus Adoneis bears an apical rimoportula at each pole positioned among the pores of pseudocellus, and additional rimoportulae at each lateral valve margin; genus Dickensoniaforma lacks pseudocelli and the apical pore fields, ocelli, appear to be consiting of vestigial (?) areolae i.e. fine areolae.
Therefore, the unique combination of features cited above and especially the morphology of pseudocellus and papillae suggest a new taxon deserving a distinct taxonomic status, i.e. Meloneis gen. nov. Within genus Meloneis, three species and one variety (i.e. M. mimallis, M. mimallis var. zephyria, M. akytos, M. gorgis) are discernible (see Table 2) mainly differing in size and polar outline of the valves, size of areolae, pore number in pseudocelli, and densities of transapical striae and areolae. SRa = presence of a single row of areolae even around the apices, SRna = presence of a single row of areolae but not around the apices. 5 Ps = pseudocellus, RP 1 = apical pore field reduced to 1 pore. 6 x = number, M = many, F = few. 7 Ci = rather circular, Di = rather disorganised, Rd = rather radiating.