Aulacoseira coroniformis Pearce & Cremer, 2010

Pearce, Christof, Cremer, Holger & Wagner-Cremer, Friederike, 2010, Aulacoseira coroniformis sp. nov., a new diatom (Bacillariophyta) species from Highlands Hammock State Park, Florida, Phytotaxa 13, pp. 40-48 : 42-44

Aulacoseira coroniformis Pearce & Cremer , sp. nov. ( Figs 2 – 37 )

Frustula cylindrica, rectangularis in aspectu cinguli. Valvae circulares. Dimensiones valvarum: diameter discorum 6–15 µm, limbus valvarum 4–8 µm. Discus planus, tectus areolis aequaliter dispersis. Iunctura faciei valvae limbique leviter curvata. Striae limbi, 22–28 in 10 µm, parallelae, continuantes in peripheriam faciei valvae. Areolae 35–45 in 10 µm. Collum latum, sulcus distinctus, pseudoseptum latum praesens interius in limbo. Spinae marginales positae in iunctura faciei valvae limbique, in terminali omni vel fere omni costae pervalvaris. Rimoportula una observata in parte interna pseudosepti.

Frustules cylindrical, rectangular in girdle view. Valves circular, 6–15 µm in diameter. Pervalvar axis (mantle) 4–8 µm in length. Discus flat with evenly arranged areolae. Valve face-mantle junction gently curved. Mantle areolae arranged in parallel rows (22–28 in 10 µm), continuing onto the periphery of the valve face, 35–45 in 10 µm. Collum broad, sulcus distinct, thick ringleiste present inside the mantle. Linking spines on each to every third mantle costa. One rimoportula at the inner side of the ringleiste.

Type: — Highlands Hammock State Park , Sebring, Florida, U.S.A., 27°27’47.45’’N, 81°32’21.48’’W, Cleaned sediment from 74 cm depth from a 78 cm long sediment core (HHA 3), drilled on the 1 st of April 2008. Slide BR – 4199 (holotype BR, Figs 2–6 ) .

Habitat: —Epiphytic in modern swamps; fossil in Holocene lake sediments.

Distribution: —Central Florida, U.S.A.

Etymology: —From the Latin corona (crown) and forma (form), referring to the crown-shaped appearance of the valve.

Observations: — Valve outline and dimensions: The valve face is generally flat with a gentle transition towards the mantle ( Figs 11–13 ). The discus-mantle boundary is difficult to recognize but usually characterized by a predetermined breaking line (arrows in Figs 12, 14 and 15 ) along which the discus regularly breaks ( Fig. 12 ). The valve diameter is 6–15 µm (mean 9 µm; N = 48), the mantle height is 4–8 µm (mean 5.5 µm; N = 35) and the ratio of mantle height to diameter varies between 0.3 and 1.1 (mean 0.6; N = 35). Mantle: Approximately one quarter of the mantle height consists of the collum which possesses no areolae or other ornamentation. The boundary between the areolated mantle and the collum, the sulcus, is indicated as a local narrowing of the valve (arrows in Figs 3 , 18 ). Areolae: The mantle striae consist of relatively small round ( Figs 17, 18 ) to more or less rectangular ( Figs 26, 27 ) areolae that are arranged in rows parallel to the pervalvar axis. The mantle striae density is 22–28 in 10 µm (N = 18) and the areola density per stria varies between 35 and 45 in 10 µm (N = 16). The striae continue over the mantle-valve face edge onto the periphery of the valve face ( Fig. 11, 13 ). The discus itself is covered with mostly circular, evenly distributed areolae ( Figs 14 – 16 ). The areola density on the discus is 98–108 areolae per 10 µm 2 (N = 8). On the valve face, the line between the peripheral striae and the evenly arranged areolae in the centre marks the boundary between the discus and the mantle ( Figs 11, 12, 14, 15 ; see also Krammer 1991a, p. 91). Vela, which are rather common in the genus Aulacoseira ( Crawford & Likhoshway 2002) were not observed in the examined specimens of A. coroniformis . This could mean that the vela in A. coroniformis are too delicate to be preserved after acid-cleaning of the raw material. Ringleiste: The ringleiste is a thick bulge-like broadening ( Figs 29–31 ) located inside the valve opposite from the sulcus, at the junction between the areolated mantle and the collum. The width of the ringleiste is approximately 10–15 % of the valve diameter (N = 6). Spines: At the junction of the mantle and the valve face, linking spines arise usually from a single pervalvar costa and are present on each to every third costa ( Figs 17 – 22 ). Spines are usually straight, lacking any side spines and run parallel to the pervalvar axis. Spines vary considerably in shape being relatively long with blunt or pointed distal ends ( Figs 17, 21, 22 ) or comparably shorter and club-shaped ( Figs 26–28 ). Also spines having anchorshaped distal ends were observed ( Fig. 11 ). The spine length ranges from 1.1–2.0 µm (mean 1.6 µm; N = 16). Separation valves with characteristic separation spines were not observed. Rimoportula: The labiate process is often difficult to recognize in Aulacoseira . A few valves of Aulacoseira coroniformis were observed having a rimoportula near the inner side of the ringleiste ( Figs 32–37 ). The rimoportula is circular, stalk-less and surrounded by a thickened rampart-like siliceous wall ( Figs 35, 37 ). Valves with more than one rimoportula were not observed. The external opening of the rimoportula which very often resembles a regular areola is difficult to recognize in A. coroniformis . The enlarged and somewhat isolated pores next to the sulcus visible in many valves (for example Figs 18, 22, 25 ) might correspond with the external rimoportulae openings. Likhoshway & Crawford (2001) provide an excellent overview on the variability of the morphology and position of rimoportulae in Aulacoseira .

Figures

FIGURES 2–16: Aulacoseira coroniformis. Figs 2–10: LM images, all in girdle view apart from Figs 5, 6 & 9 which are in valve face view. Figs 2–6: Specimens from the holotype slide. Figs 2 & 3: Two spine-linked valves belonging to two frustules shown in two different foci. Arrows in Fig. 3 indicate the sulci of both valves. Figs 5 & 6: Two different foci of a valve in face view. Fig. 5 emphasizes the marginal ring of spines, Fig. 6 the areolae on the valve face periphery. Figs 4 & 7: Two complete frustules in girdle view. Fig. 8: Chain of four valves in girdle view, the middle two forming one frustule, with clearly visible, closely spaced pervalvar striae. Figs 11–16: SEM images documenting variation of discus areolation and spine morphology. The arrows in Figs 12, 14 & 15 indicate the border between discus areolae and peripheral striae belonging to the mantle. Scale bars: Figs 2–10: 10 µm; Figs 11–15: 5 µm; Fig. 16: 2 µm.

FIGURES 17–28: Aulacoseira coroniformis. SEM images, all in girdle view. Figs 17–25: variability of habitus, size and height-to-diameter ratio in selected valves. The black arrow in Fig. 18 points to the sulcus. White arrows in Figs 18, 22 & 25 point to possible external rimoportula openings. Figs 26–28: close-ups of Figs 23–25, respectively, showing the spine linkages between two valves of two different frustules. Scale bars: Figs 17, 19, 21, 23, 24, 25: 5 µm; Figs 18, 20, 22, 26, 27, 28: 2 µm.

FIGURES 29–37: Aulacoseira coroniformis. SEM images. Figs 29–31: size and position of the ringleiste (arrows in Figs 29 & 30). Figs 32-34: position of the rimoportula near the inner ringleiste (arrows). Figs 35–37: close-ups of Figs 32–34, respectively, showing the shape, size and positioning of the rimoportula (arrows). Scale bars: Figs 29–34: 5 µm; Figs 35–37: 1 µm.