Luticola laeta Spaulding & Esposito

Kohler, Tyler J., Kopalová, Kateřina, Vijver, Bart Van De & Kociolek, J. Patrick, 2015, The genus Luticola D. G. Mann (Bacillariophyta) from the McMurdo Sound Region, Antarctica, with the description of four new species, Phytotaxa 208 (2), pp. 103-134 : 112-118

Luticola laeta Spaulding & Esposito in Esposito et al. (2008: 1383, figs. 28–36) ( Figs. 30–47 )

LM observations:— ( Figs. 30–46 ) Valves elliptical-lanceolate to linear-lanceolate with convex margins. Apices rounded at smaller size ranges ( Figs. 38–40 ), rostrate at upper sizes ( Figs. 30–36 ). Valve dimensions (n=13): length 17.4–35.6 μm, width 7.1–8.8 μm. Axial area linear, expanded near central area and apices. Central area rectangular to almost bow-tie shaped, bordered by shortened striae on both sides. An isolated pore present in the central area, halfway between valve center and valve margin. Raphe straight, with external distal and proximal endings deflected into the opposite direction of the isolated pore. Transapical striae radiate throughout, more broadly spaced in valve center, 18–21 in 10 μm.

SEM observation:— ( Fig. 47 ) Striae composed of 4–6 areolae, 3–4 at the apices. Areolae rounded, elongated at axial area, sometimes extending past distal raphe endings. Isolated pore rounded. Central area bordered by 1–2 rows of four rounded areolae on the isolated pore side, a single row on the opposite side. Proximal and distal raphe endings deflect away from isolated pore, terminating into small pores on valve face. Clear separation between valve face areolae and valve margins. A single row of rounded areolae present along entire valve mantle, interrupted at apices.

Habitat, ecology and distribution:— At present, this species is known only from the Antarctic continent, in the MDVs from which it was described. While it is ubiquitous throughout the Dry Valleys, albeit at low relative abundances, the greatest populations have been observed in moss samples from Canada Stream (INSTAAR #2257) and Bowles Creek (INSTAAR #566), the latter of which is the type locality (though not type-habitat) of this species. Luticola laeta is also a common Luticola species in cryoconite holes, especially on Canada Glacier ( Stanish et al. 2013), though it appears to be rare in ponds and absent from Ross Island ( Sakaeva 2014).

INSTAAR herbarium:— Aiken Creek 901–902, 2224–2228; Von Guerard Stream 905–908, 911–913, 916– 919, 923–926; Harnish Creek 928; Bowles Creek 933, 935, 938–940; Commonwealth Stream 946, 948, 952–954; Wormherder Creek 955; Wharton Creek 982; Delta Stream 984, 986–987, 990–992; Onyx River 995; Canada Stream 1010, 2257; Green Creek 1018–1019, 1021, 1025–1026, 2315–2318; Lawson Creek 1031–1033, 1038, 1041; Miers Stream 1046–1047, 1049; Adams Stream 1051–1055, 1064; Relict Channel 1066–1071, 1077–1081, 1084, 1087.

Observations:— The smaller size range of this species has probably been identified as L. mutica or L. cohnii , which are species complexes similar to that of L. muticopsis ( Kopalová et al. 2011) , where small rounded cells have been assigned taxonomically. Described from the Northern Hemisphere, L. mutica is more oval-shaped and has eccentric proximal raphe endings terminating in recurved hooks ( Levkov et al. 2013), a characteristic not present in any MDV Luticola species. Luticola cohnii is also a species described from Europe (as Stauroneis cohnii Hilse in Rabenhorst 1860). Similar to L. laeta , it is 19–40 μm in length, and 8.5–11.0 μm in width, but is always ovular in outline ( Levkov et al. 2013). Most importantly, the external distal raphe endings in L. cohnii do not terminate until shortly before valve edge ( Van de Vijver et al. 2011: figs. 30–31), unlike in L. laeta , where the external distal raphe ends terminate on the valve face.

Among other Luticola from the Antarctic Region, L. vermeulenii Van de Vijver in Van de Vijver et al. (2011: 145, figs. 16–29) has a similar length range (24–50 μm), but is generally wider than L. laeta (9.5–14.0 μm vs. 7.5–10.0 μm) with denser striae (14–17 in 10 μm) and more elongated areolae per stria (5–7; Van de Vijver et al. 2011). Luticola australomutica Van de Vijver in Van de Vijver & Mataloni (2008: 458) is another similar species both in valve outline and dimensions, with length 17–35 μm, width 6–8 μm, and stria density 16–21 in 10 μm. However, L. austalomutica has only 2–4 areolae per stria (compared to 4–6 in L. laeta ), and the raphe terminates at the apices before the last row of areolae ( Van de Vijver & Mataloni 2008), whereas it continues to or past the last row in L. laeta ( Esposito et al. 2008: fig. 35). Valve sides are also generally more convex for L. laeta , while one side of L. austalomutica is flattened ( Van de Vijver & Mataloni 2008).

Among MDV taxa, L. laeta shares similarities with L. spainiae Kohler & Kopalová , sp. nov. and L. murrayi (W. & G.S. West 1911: 285) D.G. Mann in Round et al. (1990: 671), especially in length and width ratios. Luticola laeta differs from these taxa primarily in terms of valve outline. Luticola spainiae has more broadly rostrate apices, and generally has fewer areolae per stria than L. laeta (4–5, 2–4 at apices). Luticola murrayi is widest in the very center of the valve, gradually tapering toward the apices, while L. laeta have more convex margins. Furthermore, the areolae and isolated pore in L. laeta are rounded, while in L. murrayi the outside row of areolae and isolated pore are elongated. Finally, L. laeta shares similarities in valve outline to L. bradyi , sp. nov., but L. bradyi is both wider (10.5–12.5 μm) and longer (28.3–44.8 μm) than L. laeta , and has broadly rostrate to sub-capitate apices with more areolae per stria (5–7).

Luticola murrayi (W. & G.S. West 1911: 285, pl. 26, fig. 129) D.G. Mann in Round et al. (1990: 671) ( Figs. 48–56 , Fig. 48 is the original illustration by West & West (1911))

LM observations:— ( Figs. 49–54 ) Valves rhombic-elliptic to elliptical-lanceolate with convex margins, rapidly attenuating to broadly rounded non-protracted apices. Valve dimensions (n=52): length 28.1–40.4 μm, width 6.8–10.2 μm at valve center, 4.5–5.8 μm at apices. Axial area linear, only slightly expanded near central area and apices. An isolated pore present in central area, located halfway between the valve center and valve margin. Central area rectangular to bow-tie shaped, bordered by shortened striae on both sides. Raphe branches straight, deflecting away from isolated pore at both distal and proximal ends. Transapical striae irregular, radiate throughout, 12–16 in 10 μm, 14–18 in 10 μm towards apices.

SEM observations:— ( Figs. 55–56 ) Externally ( Fig. 55 ), striae composed of 3–5 areolae, 2–3 at apices. Areolae elongated and slit-like throughout, becoming more rounded towards central area, extend past distal raphe endings. Isolated pore slit-shaped. Central area bordered on both sides by a single row of 3–4 rounded areolae positioned closer to valve margin than other striae. Distal raphe endings terminate on valve face on before margins, deflected away from isolated pore. Proximal raphe endings defect away from isolated pore, terminate as small pores. Clear separation between valve face and margin, with clear open area at apices around distal raphe endings. A single row of rounded areolae visible along valve mantle. Internally ( Fig. 56 ), central nodule slightly thickened. Both proximal and terminal raphe endings slightly deflected towards isolated pore. Marginal canal slightly visible. Areolae covered by hymenes. Isolated pore covered by solid silica flap.

Habitat, ecology and distribution:— Luticola murrayi was described from Cape Royds, Ross Island, as Navicula murrayi by West & West (1911). Although commonly referred to in the literature and widely reported from a large number of Antarctic localities ( Prescott 1979), this species is often confused with other Luticola species and reported distributions should be interpreted with caution. Illustrated specimens from the literature that match our concept of L. murrayi include reports from Mirny and Molodezhnaya Stations (as Navicula muticopsis f. murrayi , Fukushima 1966: fig. 1d) and the greater Ongul Islands at Lützow Holm Bay (as Navicula globiceps var. elongata, Ko-Bayashi 1963 , pl. 4, fig. 2, and as Navicula muticopsis var. muticopsis f. murrayi , Fukushima et al. 1974: pl. 2 fig. D). This species has been commonly reported from Southern Victoria Land (as Navicula muticopsis f. murrayi, Seaburg et al. 1979 : pl. 22, fig. 91) and the MDVs, though it has probably been confused with L. laeta ( Van de Vijver et al. 2012) . While more rare in the MDVs as compared to Cape Royds where it was described ( Sakaeva 2014), it has been observed in Taylor Valley streams. In all populations, it is present alongside abundant Achnanthes taylorensis Kellogg et al. (1980: 174 , pl. 1, figs. 3–4, pl. 2 figs. 1–2) and Navicula shackletoni W. & G.S. West (1911: 286, pl. 26, figs. 136–138).

INSTAAR herbarium:— Pony Lake 2354, 2355 ; Coast Lake 2467–2468 ; Green Lake 2473–2475, 2503 ; Blue Lake 2487, 2497 ; Picture Pond 2666–2667, 2670 .

Observations:— There is a lack of consensus on the concept of L. murrayi , probably because the original species description, as well as for its variety, Navicula (Pinnularia) murrayi var. elegans ( West & West 1911: 285, pl. 26, fig. 130), was made from the measurement of one valve. Dimensions are given as single values; length 45 μm, width 11.5 μm and 14 striae in 10 μm. No type specimen or type slide has been recovered, and only one illustration is given in the manuscript (reproduced here as Fig. 48 ). The illustration furthermore lacks an isolated pore, and the absence of this feature diagnostic for the genus has been a source of speculation ( Ko-Bayashi 1963, Van de Vijver et al. 2012, Levkov et al. 2013). One year after its description, Fritsch (1912) reported Navicula murrayi from Gap Pond, Winter Harbor, and gives a similarly vague description; length 43 μm, 10 μm width (no stria density given), and comments that it is rare. A half century later, Ko-Bayashi (1963: 13, pl. 4) reported several species from the Ongul Islands as different forms of N. murrayi , with the result being several new concepts that differ from the original illustration. In a classic case of species drift, one concept of N. murrayi by Ko-Bayashi (1963, pl. 5–7) was later reported by Hustedt (1966), Sabbe et al. (2003), and Ohtsuka et al. (2006) before finally being described as a new species, L. pseudomurrayi , by Van de Vijver et al. (2012) from material gathered from Lützow Holm Bay, across the continent from Ross Island.

In a recent investigation of the Cape Royds type material, Van de Vijver et al. (2012) reported that they were unable to find a single valve that matched the description of L. murrayi in West & West (1911). They proceeded to create a new “ lectotype ” (BM 34129) for L. murrayi , even though L. murrayi was not observed on the slide. Further, Van de Vijver et al. (2012) suggested that the true L. murrayi is likely a synonym with L. laeta , which is commonly found in the MDVs and roughly matches the illustration. However, neither Van de Vijver et al. (2012) nor the authors of this paper have observed any valves of L. laeta on Cape Royds or Ross Island. As a result, there is no clear evidence to support the hypothesis that the two species are conspecific.

Another concept of L. murrayi comes from Levkov et al. (2013), who, unlike Van de Vijver et al. (2012), report finding the true L. murrayi on the original West & West (1911) slides from the moraines on Mt. Erebus (BM 34124 and BM 34126), with an original note beneath two slides indicating that the species was present as evidence ( Levkov et al. 2013: 539, pl. 129, figs. 29–30). On this slide were numerous capitate diatoms, along with what the authors explain to be a number of initial cells. It is these initial cells, according to Levkov et al. (2013), that most closely fit the original description and illustration, and the capitate diatoms were likely confused with L. muticopsis , which are also noted to be present on the slides. Therefore, in the concept of L. murrayi advocated by Levkov et al. (2013), the original illustration depicts only an initial cell, and they argue that this would make sense given the lack of an isolated pore, which may be possible in initial valves. They then designated a new “ lectotype ” for L. murrayi , slide BM 34126.

However, the L. muticopsis specimens illustrated on the same plate in West & West (1911: figs. 123–124) also do not have an isolated pore (see below, Figs. 64–65 ) and are obviously not initial cells. The presence of an isolated pore is also discussed in the original description of L. murrayi , and along with the illustrated straight proximal raphe endings as noted in Van de Vijver et al. (2012), the omission of an isolated pore was most likely an oversight and not intentional. Furthermore, the initial cell presented in Levkov et al. (2013) is capitate, like the smaller-sized cells, and this is inconsistent with both the illustration and the description of L. murrayi . It is our opinion that the species pictured as L. murrayi in Levkov et al. (2013) better fit the variety “ elegans ,” which is illustrated directly beside the nominate species in the original publication ( West & West 1911: 285, pl. 26, fig. 130).

We examined the slides with the original notes (BM 34124 and BM 34126) from the London Natural History Museum, and like Levkov et al. (2013) found a capitate Luticola to be the dominate species on both slides, along with a few initial valves that have a similar morphology to the illustration in West & West ( Fig. 57 ). Based on this information alone, it is reasonable to speculate that L. murrayi is a drawing of an initial valve and the capitate diatom is instead the real species. However, as part of our sampling of recent material on Cape Royds, we found a large population of Luticola that we identify as L. murrayi from the margin of Green Lake (INSTAAR #2475–19). These specimens are not initial valves as suggested by their integrity in valve outline, uniform striae, and diminution in size. At the lowest size range, this specimen overlaps with the capitate diatoms in Levkov et al. (2013), although never presenting capitate apices as part of its morphology. As a result of these observations, we suggest our concept of L. murrayi better fits the original description than do previous interpretations.

Luticola murrayi differs from other Antarctic species such as L. australomutica and L. laeta primarily in its valve outline, which is widest in the center and tapers to its apices into broadly rounded to sub-rostrate ends. Both the former taxa resolve into more rostrate endings, with margins of L. laeta being both convex, and one convex and one flattened in L. australomutica . L. murrayi furthermore has slitted outer rows of areolae, whereas areolae are generally rounded in L. laeta . Lastly, the distal raphe endings terminate at the last row of areolae in L. murrayi , unlike L. australomutica where the raphe terminates long before. Luticola murrayi further differs from the diatom it was long confused with, L. pseudomurrayi , by not having the broad, capitate apices present in the latter ( Van de Vijver et al. 2012). Luticola pseudomurrayi is additionally wider than L. murrayi (7.5–12.0 μm), and has more areolae per striae at apices (3–5 vs. 2–3).

Luticola muticopsis ( Van Heurck 1909: 12, pl. 2, fig. 181) D.G.Mann in Round et al. (1990: 671) ( Figs. 58–74 , Figs. 65–68 — first four pictures of the second row, left to right — are original illustrations by West & West (1911))

LM observations:— Figs. 58–64, 69–73 . Valves linear-elliptical, with the pore-bearing valve margin straight, the opposite convex. Apices broadly capitate, becoming truncate in smaller specimens ( Figs. 69–70 ). Valve dimensions (n=10): length 13.9–31.4 μm, width 6.7–10.2 μm. Axial area present, linear, expanding toward the central area. An isolated pore present in the central area, located halfway between the valve center and valve margin. Central area rectangular to bow-tie shaped, bordered by shortened striae on both sides. Raphe straight, both proximal and distal raphe endings strongly deflecting away from the isolated pore. Transapical striae radiate throughout, 15–19 in 10 μm.

SEM observations:— ( Fig. 74 ) Striae composed of 3–5 areolae, 1–3 at apices. Areolae rounded, becoming elongated at valve margins, sometimes extending beyond distal raphe endings. Isolated pore rounded. Central area bordered by 1–2 rows of three rounded areolae on isolated pore side, one row on the opposite side. Both distal and proximal external raphe endings terminate into slits, deflecting away from isolated pore at strong, 45-degree angles on valve face. Clear separation between areolae and valve margin visible. A single row of areolae present along entire valve mantle, not interrupted at apices.

Habitat, ecology and distribution:— Commonly reported from the entire Antarctic Region, such as Cape Royds on Ross Island ( West & West 1911: pl. 26, figs. 121–124), the Ongul Islands ( Fukushima 1973: figs. A–E), Southern Victoria Land ( Seaburg et al. 1979: pl. 22, fig. 90), James Ross ( Kopalová et al. 2012: fig. 4.5x) and Deception Islands ( Van de Vijver & Mataloni 2008: figs. 71–90) from the Maritime Antarctic, and the Kerguelen Islands ( Levkov et al. 2013: 534, pl. 127, figs. 20–37) and Île de la Possession (Van de Vijver et al. 2002: 244, pl. 57, figs. 1–6) from the sub-Antarctic. Although widely reported, this species is probably rare in the MDVs ( Sakaeva 2014).

INSTAAR herbarium:— Pony Lake 2352, 2354–2355, 2662 ; Coast Lake 2466–2467, 2502; Green Lake 2473, 2475 .

Observations:— Luticola muticopsis is a catch-all species complex for capitate Luticola species ( Kopalová et al. 2011). The identity of this diatom is ambiguous due to only a single specimen present on the type slide ( Van de Vijver & Mataloni 2008), and therefore has historically been a place to put all Luticola valves with capitate apices ( Van de Vijver & Mataloni 2008) or a starting point for creating forms and varieties (i.e. West & West 1911, Ko-Bayashi 1963, 1965). After examining the Van Heurck type slide, Van de Vijver & Mataloni (2008) provided a more narrow species concept for L. muticopsis , and we base our concept of this diatom on their description of this valve and the Deception Island specimens included in the paper. Despite being widely reported in the MDVs (i. e. Stanish et al. 2011, 2012), L. muticopsis is actually observed rather infrequently in Dry Valley streams ( Sakaeva 2014). This is probably due to its confusion with L. austroatlantica , which may have begun with the publication by Kellogg et al. 1980, where an image of what appears to be L. austroatlantica is labeled as L. muticopsis . Therefore, reports of this species prior to the description of L. austroatlantica in Esposito et al. (2008) should be interpreted with caution.

Luticola cognata Levkov et al. in Levkov et al. (2013: 90, pl. 130, figs. 21–38) is morphologically very similar to L. muticopsis . Levkov et al. (2013) indicate that L. muticopsis has more capitate valve apices, and that L. cognata is consistently wider. However, the dimensions given are 7.0–9.5 μm wide for L. cognata vs. 7.0–9.0 μm for L. muticopsis , and the differences in morphology are not easily observed in the type material as illustrated by Van de Vijver & Mataloni (2008), as well as in the Levkov et al. (2013) images of L. muticopsis . Therefore, it is possible that L. cognata is conspecific with L. muticopsis . Luticola muticopsis is also similar to L. truncata Kopalová & Van de Vijver in Kopalová et al. (2009: 118, figs. 34–50), especially at the lower end of the size range. However, L. truncata always maintains its rostrate, truncated apices, whereas L. muticopsis remains distinctly capitate throughout its entire size range. Luticola muticopsis differs from other MDV species such as L. austroatlantica and L. permuticopsis primarily by having one distinctly straight valve margin, whereas both margins in L. permuticopsis and L. austroatlantica are convex. Luticola austroatlantica furthermore has more elongated valve outline than L. muticopsis . Finally, valve apices in L. austroatlantica are more rounded and constricted at the neck, while L. muticopsis has more flattened apices that are less constricted.

Figures

FIGURE 57. Initial cell from BM 34124, an original West & West (1911) slide, LM. Scale bar = 10 μm.

FIGURES 30–47. Luticola laeta Spaulding & Esposito. Light (LM) and scanning electron micrographs (SEM). 30–46. Specimens from a confirmed locality, Canada Stream, Taylor Valley, LM. 47. External valve view from Canada Stream, SEM. LM scale bar = 10 μm. SEM scale bar = 5 μm.

FIGURES 48–56. Luticola murrayi (W. & G.S.West) D.G.Mann. 48. Original illustration from West & West (1911: pl. 26, fig. 129). Light (LM) and scanning electron micrographs (SEM). 49–54. LM of specimens from Green Lake, Cape Royds. 55. SEM of external valve view from population at Green Lake. 56. SEM of internal valve view from a population from a pond on the McMurdo Hut Ridge. LM scale bar = 10 μm. SEM scale bars = 10 μm.

FIGURES 58–74. Luticola muticopsis (Van Heurck) D.G. Mann. 65–68. Original illustration from West & West (1911: pl. 26, figs. 121–124). Light (LM) and scanning electron micrographs (SEM). 58–64 and 69–73. Specimens from Green Lake, Cape Royds, LM. 74. External valve view of specimens from Green Lake, Cape Royds, a confirmed locality in West & West (1911), SEM. LM scale bar = 10 μm. SEM scale bar = 5 μm.