Fragilaria crocodylus Van de Vijver & C.E.Wetzel, 2022

Vijver, Bart Van De & Wetzel, Carlos E., 2022, A new Fragilaria Lyngbye species (Fragilariaceae, Bacillariophyta) from a historic Grunow sample from the Attersee, Austria, Phytotaxa 561 (2), pp. 210-214 : 210-213

Fragilaria crocodylus Van de Vijver & C.E.Wetzel sp. nov. ( Figs 1–19 LM, 20–25 SEM)

Frustules rectangular in girdle view ( Fig. 1 ), solitary although valves connected to each using small conical spines also observed ( Fig. 21 ). Valves linear to weakly linear-lanceolate with almost parallel margins. Valves occasionally bent. Apices clearly protracted, typically capitate in longer valves ( Figs 2–19 ). Smaller valves with more rostrate apices ( Figs 18–19 ). Continuous series of small, solid, conical spines, each located in a pit-like depression, present on the valve margin ( Figs 20, 23–24 ). At the apices, short series of spines present above the apical pore field ( Fig. 22 ). Large mantle plaques present on the mantle edge ( Figs 22, 25 ). Valve dimensions (n=25): length 40–110 µm, width 2.5–3.0 µm. Sternum narrow but distinct, linear, very gradually widening towards the central area. Central area small, asymmetrical with a rectangular unilateral fascia on one side and slightly shortened striae on the opposite side ( Fig. 24 ). Striae uniseriate, composed of relatively large, rimmed rounded areolae externally covered by individual vola ( Fig. 22 ). Near the sternum, striae terminating in short shallow slits ( Fig. 24 ). Mantle striae composed of 3–4 rounded areolae ( Figs 20–21 ). Apical pore field large, well, delimited, of ocellolimbus type, composed of up to 7 rows of small, squarish pores ( Figs 22, 23 ). One rimoportula present, located in a fairly deep depression, transapically elongated ( Figs. 22, 23 ). Internally, rimoportula large, raised ( Fig. 25 ).

Due to the rarity of the species in the sample, the dominance of other needle-shaped Fragilaria species in the sample and the age of the sample (late 19 th century), we did not find any valves with girdle bands. Observations on the girdle bands were therefore not possible.

Type:— AUSTRIA. Attersee , Upper-Austria, Grunow sample 2646 (acc. number W0127051), coll. date VII.1862, leg. Von Mörl (n°48) (holotype slide BR-4736= Fig. 6 , isotype slide 2646a in W, W0164874). PhycoBank registration: http:// phycobank.org/103275

Etymology:— The specific epithet “ crocodylus ” refers to the continuous series of conical marginal spines and the 5 spines at the apices, resembling the teeth of a crocodile ( crocodylus ).

Ecology & associated diatom flora:— Sample 2646 was collected from stones in the Attersee, the largest lake in the province of Upper-Austria ( Austria). The lake, situated at 470 m a.s.l., has a total surface of 45.9 km 2 with a maximum depth of 170 m ( Dokulil & Teubner 2002). The sample is dominated by several Achnanthidium spp. , Brachysira neoexilis LangeBertalot (in Lange-Bertalot & Moser 1994: 51), Denticula tenuis Kützing (1844: 43) , Eucocconeis flexella ( Kützing 1844: 80) F. Meister (1912: 95) , Fragilaria perdelicatissima Lange-Bertalot & Van de Vijver (in Lange-Bertalot & Ulrich 2014: 19) and Humidophila perpusilla ( Grunow 1860: 552) R.L. Lowe et al. (2014: 358) . Following the ecological preferences of the observed species, based on literature data in Lange-Bertalot et al. (2017), this diatom flora points to oligotrophic, calcium bicarbonate enriched, low saprobity lake conditions. The Attersee has always been ultra-oligotrophic and therefore been considered a reference ecosystem for alpine lakes in the Austrian Salzkammergut district following the rules of the European Water Framework Directive (WFD) ( Dokulil & Teubner 2002). Unfortunately, recent diatom data are lacking that could be used to compare the present-day diatom flora with the historic (19 th century) flora. The (European) distribution of the new species is unclear, most likely due to confusion with similar, needle-shaped Fragilaria species.

Taxonomic comments:— Fragilaria crocodylus can hardly be confused with any other needle-shaped Fragilaria species. Fragilaria tenera lacks the distinctly developed, capitate apices but instead had narrow, weakly capitate apices ( Lange-Bertalot & Ulrich 2014, Almeida et al. 2016). Moreover F. tenera has narrower valves (1.8–2.5 µm versus 2.5–3.0 µm) and therefore also a more needle-shaped valve outline with gradually tapering margins, contrary to the almost linear valve outline with parallel margins in F. crocodylus . The spines in F. tenera are shark-tooth like and are directly connected to the valve margin, whereas in F. crocodylus , the spines are conical and located in pit-like depressions ( Almeida et al. 2016). Other longer Fragilaria species such as F. neotropica P.D.Almeida, E.Morales & C.E.Wetzel (in Almeida et al. 2016: 171) and F. salvadoriana K.J.Krahn & C.E.Wetzel (in Krahn et al. 2021: 5) sufficiently differ by their thin, linear-lanceolate valve outline, their lower valve width (<2.5 µm) and the structure of their spines (never acute conical in pit-like depressions), to exclude conspecificity.

Figures

FIGURES 1–19. Light micrographs of Fragilaria crocodylus Van de Vijver & C.E.Wetzel sp. nov. LM images taken from the holotype population (BR-4736, Grunow sample 2646, Attersee, Austria). 1. Frustule in girdle view. 2–19. LM views of the population arranged in decreasing length. Scale bar = 10 µm.

FIGURES 20–25. Scanning electron micrographs of Fragilaria crocodylus Van de Vijver & C.E.Wetzel sp. nov. SEM images taken from the holotype population (BR-4736, Grunow sample 2646, Attersee, Austria). 20. SEM external view of an entire valve. Note the continuous series of marginal spines running from apex to apex and the lack of rimoportula at one apex. 21. SEM girdle view of two valves connected to each other via the short, conical spines. 22. SEM side view of the valve apex showing mantle plaques, the apical pore field, five protruding conical spines at the apex and the depressed rimoportula. 23. SEM external view of a valve apex showing the rimoportula, the conical spines on their pit-like depressions and the uniseriate striae with the combination of rounded and slit-like areolae. 24. SEM external detail of the central area. Note the slit-like areolae terminating the striae at the sternum. 25. SEM internal detail of a valve apex with the large rimoportula. Scale bars = 10 μm (Figs 20–21), 1 μm (Figs 22–25).