Aulacoseira principissa, Vijver, 2012

Vijver, Bart Van De, 2012, Aulacoseira principissa sp. nov., a new ‘ centric’ diatom species from the sub- Antarctic region, Phytotaxa 52 (1), pp. 33-42 : 35-38

Aulacoseira principissa sp.nov. ( Figs 2–25 )

Frustula cylindrica, rectangularia in aspectu cinguli. Valvae circulares. Dimensiones valvarum: diameter discorum 4.0– 16.3 μ m, limbus valvarum 4.2–6.8 μ m. Discus planus, tectus areolis aequaliter dispersis sed saepe valvae tectae seriebus areolarum unis ac duabas in margino disci adsunt. Iunctura faciei valvae limbique leviter curvata. Striae limbi, 15–16 in 10 μ m, parallelae. Areolae 18–20 in 10 μ m. Collum paene angustum, sulcus indistinctus, pseudoseptum moderate latum praesens interius in limbo. Spinae marginales positae in iunctura faciei valvae limbique, bifurcatae, in omni costa pervalvari. Valvae separandae nonnullae adsunt, sine spinis. Rimoportula una observata in limbo, supra pseudoseptum.

Frustules cylindrical, rectangular in girdle view. Valves circular, 4.0–16.3 µm in diameter. Mantle height 4.2– 6.8 µm. Discus flat with evenly arranged areolae but valves with only one to two rows of areolae near the discus margin likewise present. Valve face-mantle junction gently curved. Mantle areolae arranged in parallel rows, 15–16 in 10 µm. Areolae 18–20 in 10 µm. Collum rather narrow, sulcus indistinct, moderately thick Ringleiste present inside the mantle. Linking spines on each mantle costa, bifurcated. Separation valves completely spineless. One rimoportula present on the mantle, above the Ringleiste.

Type: — KERGUELEN ISLANDS. Île Kerguelen: Val Studer , 04 February 1998, B. Van de Vijver sample BW390, slide no. BR-4263 (holotype BR), slide PLP-208 (isotype University of Antwerp), slide BRM-ZU8/ 33 (isotype BRM) .

Ecology:— The new Aulacoseira is a typical constituent of the aquatic diatom flora on the sub-Antarctic Iles Kerguelen. Almost all large populations (>20% of all counted valves in a sample) have been found in larger lakes, ponds and occasionally in small bog ponds. The pH of the sampled localities ranges between 6 (rarely 5) and 7 (rarely>7) indicating the preference of the species for slightly acid conditions. All populations were found in sites with low to very low specific conductance values (>80 µS cm -1). When measured, nutrient values were always very low. In mosses, the species was found only in low abundances and when observed, mosses were usually located close to the water edge of larger lakes and pools indicating a possible contamination from the aquatic habitat at high water levels. In purely terrestrial mosses (semi-wet to dry), the species was never observed. The communities with A. principissa were usually dominated by Stauroforma exiguiformis ( Lange-Bertalot 1993: 45) Flower, Jones & Round (1996: 53) and Psammothidium abundans (Manguin in Bourrelly & Manguin 1954: 19) Bukhtiyarova & Round (1996: 22).

Distribution: — Aulacoseira principissa is widely distributed in the sub-Antarctic Region ( Fig. 1 ). Large populations have been found on most islands in the southern Indian Ocean ( Van de Vijver et al. 2001, 2002, 2008) but also on South Georgia in the southern Atlantic Ocean ( Van de Vijver & Beyens 1996). Only on Heard Island, the most southern of all Indian Ocean islands, only one small population was found ( Van de Vijver et al. 2004). The species has however not (yet) been found on localities in the Maritime Antarctic Region such as the South Shetland Islands, nor on the Antarctic Continent ( Kellogg & Kellogg 2002). This distribution pattern indicates that A. principissa is most likely a typical sub-Antarctic endemic. The species is most probably also present on Macquarie Island, a sub-Antarctic island in the southern Pacific Ocean (Krystyna Saunders, pers. comm.).

Etymology: —The specific epithet principissa (latin for princess) refers to the delicate shape of the valve resembling a princess’ crown.

Observations: —LM ( Figs 2–14 ): Frustules in long chains of up to 40 cells ( Figs 2–4 ). The valve diameter is 4.0–16.3 µm (mean 10.3 ± 3.5 µm; N = 25), the mantle height is 4.2–6.8 µm (mean 5.7 ± 0.8 µm; N = 25) and the ratio of mantle height to diameter varies between 0.4 and 1.2 (mean 0.74 ± 0.23; N =10). The mantle striae density is 15–16 in 10 µm (N = 10). SEM ( Figs 15–25 ): Mantle: The collum, the lowest part of the mantle lacking areolae makes up only 10–15% of the mantle height ( Figs 15, 17, 18 ). The entire collum is covered by narrow, raised ribs ( Figs 17, 18 ). Between the striae, the entire mantle is covered by small, siliceous plaques ( Fig 19 ). The pores in the girdle bands are very fine and typical of those found in other Aulacoseira species ( Crawford & Likhoshway 1999) ( Fig 16 ). The boundary between the areolated mantle and the collum, the sulcus, is almost indistinct (arrows in Fig 18 ). Areolae: The mantle striae consist of relatively small, round areolae, arranged in well separated rows, parallel to the pervalvar axis ( Figs 17, 18 ). Areola density per stria varies between 18 and 20 in 10 µm (N = 10). The striae continue without interruption over the mantle/valve face margin onto the valve face ( Figs 20, 21, 22 ). Discus: The discus is generally flat, never verrucose lacking papillae, with a gentle transition towards the mantle ( Figs 20, 21, 22 ). The discus is usually entirely covered with rounded, evenly distributed areolae ( Figs 5, 6, 7 , 20, 22 ). However, some valves show large hyaline areas on the discus apparently lacking areolae although shallow traces of areolae still can be visible ( Figs 9, 10, 11 , 21 ). Internally, the areolae on both the mantle and the valve face are covered individually by irregular discrete vela, in the shape of a short funnel with the narrow ends pointing into the areolae ( Crawford & Likhoshway 2002) ( Figs 24, 25 ). Separation valves: a few separation valves were observed. These valves lack the typical separation spines which are commonly observed in Aulacoseira ( Crawford & Likhoshway 1999) . Separation valves show no trace of spines along the smooth valve face/ mantle rim, indicating spines were not eroded but are simply lacking ( Fig. 22 ). The valve face is completely areolated ( Fig. 22 ). Ringleiste: The Ringleiste is a rather narrow, bulge-like broadening ( Figs 14 , 17 , 23, 24 ) located at the junction between the areolated mantle and the collum. The width of the Ringleiste is approximately 5–10 % of the valve diameter (N = 8). Spines: At the junction of the mantle and the valve face, a marginal ring of well developed linking spines can be seen, separated by one single areola ( Figs 15, 17, 18 , 20, 21 ). Spines are straight, run parallel to the pervalvar axis and clearly bifurcate at the end. Spines vary only rarely in shape, although some valves tend to have less bifurcating, irregularly shaped spines ( Fig. 16 ). Spine length varies around 1.1 µm. Typical separation valves with characteristic long, pointed separation spines were never observed. Rimoportula: only one rimoportula was observed, located above the Ringleiste, replacing an areola in the normal stria ( Figs 14 , 24 , see arrows). The only observation of a rimoportula made concerned an eroded valve making a description of its correct morphology impossible ( Fig. 24 ). Since a large number of smaller and larger valve fragments (>50) have been observed without recording traces of a rimoportula, it is highly likely that rimoportulae are quite rare in this species.

Figures

FIGURE 1. The Antarctic region with the location of the islands and archipelagos where Aulacoseira principissa was found. Macquarie Island, in the southern Pacific Ocean where the species might have been observed too is lacking.

FIGURES 2–14: LM images of Aulacoseira principissa. All pictures taken from the holotype population (sample BW390). Figs 2–4 show girdle views whereas Figs 5–14 show valve views. Fig. 2: Chain of six entire frustules and two valves in girdle view, with clearly visible pervalvar striae. Fig. 3: Girdle view with cross-section focus. Fig. 4: Chain of two entire frustules and one valve. Figs 5–11: Decreasing size range of valve views. Note the irregular areola pattern in Figs 9–11. Figs 12–13: Valve views showing the Ringleiste. Fig. 14: Photograph taken using the method explained in Houk & Klee (2007) showing the possible position of the rimoportula (rp?). Scale bars represent 10 µm.

FIGURES 15–19: SEM images of Aulacoseira principissa. All pictures taken from the holotype population (sample BW390). Fig. 15: Chain of two entire frustules showing the linking spines, parts of the girdle bands and Ringleiste. Fig. 16: Frustule still enclosed in the (eroded) girdle bands. Note the fine pores in the girdle. Fig. 17: Two valves linked with bifurcated linking spines. Fig. 18: Valve showing linking spines, absence of sulcus (see arrow), parallel striae with rounded to square areolae and mantle plaques. Fig. 19: Detail of valve mantle with plaques. Scale bar represents 1 µm except for Fig. 15 where scale bar = 10 µm.

FIGURES 20–25: SEM images of Aulacoseira principissa. All pictures taken from the holotype population (sample BW390). Fig. 20: Valve view of a linking valve with marginal ring of spines and regular areolae pattern. Fig. 21: Valve view of a linking valve with marginal ring of spines but irregular areolae pattern showing only open areolae near the valve margin. Fig. 22: Separation valve lacking linking spines. Fig. 23: Inside view with the narrow Ringleiste. Fig. 24: Detail of a broken valve part with the Ringleiste and the (eroded) rimoportula (see arrow). Fig. 25: Detail of a fragment showing vela from the inside. Scale bars represent 1 µm.