Ardissoneopsis appressata Lobban & Ashworth sp. nov.

Figs 15, 16

Ardissonea fulgens gigantea Ardissonea fulgens var. gigantea (Lobarzewsky) Rabenhorst sensu Lobban et al. 2012, p. 260, pl. 1, figs 1, 2, pl. 16, figs 6-8. Synonym.

Diagnosis.

Differentiated from congeners in the straight valve with inflated center and apices, and valve-appressed colony formation.

Description.

Cells valve-appressed to form flat colonies from sturdy mucilage pads, cells toward the outside increasingly curved in the apical axis (Fig. 15A-D). Nucleus in widest part of the cell and appressed to one side, numerous lenticular plastids (Fig. 15B). Valves very long and narrow, inflated at the middle and poles (Fig. 15E-G); frustule also inflated in the middle in girdle view because of deeper mantle and wider girdle bands (Fig. 15C vs D). Length 560-900 µm, width at center 9-12 µm, poles 7-10 µm, 5 µm in between; striae 18 in 10 µm (Figs 15F, G, 16A-C). Annulus along junction of valve face and mantle except at poles, where it is further back (Fig. 16B, C); some thickening under annulus but not as thick as transverse costae (Fig. 16E). Striae parallel except radiate at poles (Fig. 16B, C). Internally, costae on most virgae except near poles (Fig. 16C, E, F). Spines absent. No pseudosepta (Fig. 16C); transverse costae on the mantle diminishing toward margin (Fig. 16E arrowhead; contrast Fig. 13J). Three girdle bands present (Figs 15D, 16A): fimbriate valvocopula (Fig. 16A, F) with two rows of pores around the poles, decreasing to one toward the basal pole, along with one row along bottom edge of pars interior; fimbriate copula (Fig. 16A, D) with six rows of pores in a decussate pattern, decreasing to four near basal pole; pleura (Fig. 15D) very narrow, pars interior not observed.

Holotype

(designated here). Slide 449, multiple specimens present. Slide deposited at ANSP accession # ANSP-GC20111. Representative specimen published in Lobban et al. 2012, Micronesica 43: pl. 16, figs 6, 7.

Registration.

Phycobank http://phycobank.org/103242.

Type locality.

Guam: Apra Harbor, GabGab reef. 13.443°N, 144.643°E, sparse filamentous algal turf in farmer fish territory ( Plectroglyphidodon lacrymans), depth ca. 5 m., collection number GU44Y-13, 10 May 2009. C. Lobban and M. Schefter leg.

Additional materials examined.

Guam: GU68H-1!, GU44Z-15!, GU44T-1!, GU44BJ-4!, inter alia. Federated States of Micronesia: Yap: Y42-1!; Chuuk: TK28!.

Etymology.

Adjective (L.) appressus, appressed, with reference to the colony formation.

Taxonomic comments.

We previously (Lobban et al. 2012) identified this species as Ardissonea fulgens var. Ardissonea fulgens gigantea (Lobarzewsky) Rabenhorst, based on literature. Starting from Peragallo and Peragallo (1897-1908), that taxon has been described as extremely long (400 µm to 1 mm), with a broad central portion joined to capitate poles by very narrow linear portions, the striae 13-14 in 10 µm, but not forming distinctive colonies. Cells from Guam matched this description, except for somewhat higher stria density, but adhered valve to valve while still attached to massive mucilaginous stalks, giving colonies a unique form, a characteristic not mentioned in the literature. The variety is distinctive enough by the shape and colony formation to differentiate it from the nominate variety but has been even less studied than A. fulgens .

However, drawings of Synedra gigantea Lobarzewsky (1840, p. 276, pl. 6, fig. a-c) show a highly elongated diatom occurring in valve-appressed clusters, somewhat similar to those seen in Guam, but not showing any bulge in the middle and being apparently very flexible; as far as one can tell, the frustule is uniformly linear in both views, but he stated that the side (i.e., valve) view is 3-4 times narrower and with a wavy outline. This does not well accord with valve views of Ardissonea fulgens var. Ardissonea fulgens gigantea, which has never been depicted as wavy but has been shown with a strong central inflation. Lobarzewsky’s drawings look more like Licmophora flucticulata Lobban, Schefter and Ruck (2011), which is wavy and very narrow and occurs in valve-appressed clusters, except that the clusters of that species in girdle view are very obviously tapered. The descriptions of this taxon were carried forward into Rabenhorst (1864: 140) and De Toni (1892: 674) but there is no mention of the valve outline. The comparison of its shape to Toxarium hennedyanum (W. Gregory) Pelletan seems to have begun with Peragallo and Peragallo (1897-1908), who clearly describe the valve shape and stria density for the first time, but show a smooth valve outline. This taxon was said to be frequent in the Mediterranean (Peragallo and Peragallo 1897-1908), yet neither they nor Hustedt (1931-1959) refer to valves sticking together in distinctive fan-shaped clusters. The extreme length is the sole character that could link the acid cleaned frustules so accurately described by the Peragallos to the taxon drawn by Lobarzewski (1840), and that is insufficient, so that the identity of Synedra gigantea is a mystery, while the lack of colony information and the lower stria density leave doubt about how the Guam population is related to any European taxa. Given findings of Kanjer et al. (2021) on the nominate variety, we reject our earlier identification and propose a new species for our material.