Dictyoneis apapae Lobban & Witkowski 2023, sp. nov.

Dictyoneis apapae Lobban, sp. nov. Figs 44–50, 52–55, 57, 58

Diagnosis: Differing from Dictyoneis marginata in the broadly lanceolate outline, absence of field of small pseudoloculi around the central area, and smaller outer openings of the pseudoloculi, especially along the margin.

Holotype: Specimen at 10.2 mm E and 6.0 mm S of the mark on slide 3056, deposited at Diatom Collection, Academy of Natural Sciences of Drexel University, Philadelphia, accession number GC20106. Fig. 44.

Type locality: Guam, Apra Harbor, Scuba Beach, 13°27.840', 144°39.360', scarce in biofilm on calcareous sand ca. 10 m deep, in communities with Arcuatasigma spp. (Lobban & Reid 2018), Progonoia spp. (Lobban 2015b), and several species listed below. Sample GU52X-5b, 10 May 2015, coll. C.S. Lobban & M. Schefter

Etymology: Apapa, genitive noun in apposition, for the type locality in Apra Harbor, not far from Cabras Island. Ayong (2023) explains that, “Apra is a corruption of the Chamorro word ‘ apapa ’ which means ‘low.’ Apapa is the original name for what is now known as Cabras Island.”

Morphology: Valves broadly lanceolate (Figs 44–46), 94–115 µm long, 22–26 µm wide, outer pseudoloculi in rows giving apparent stria density, especially in LM, of 8–12 in 10 µm, (Figs 44, 45) but true striae measured on interior views 24–26 in 10 µm (Figs 52, 55). External raphe slits straight, bordered by thin ridges of silica (Fig. 48, arrow), terminal endings deflected in opposite directions. Inner layer of pores completely overlain by pseudoloculate framework forming larger pores, except in a variable zone near the sternum and at the apices where the outer openings are also small (Figs 48, 49 and cf. fracture wall, Fig. 54). The outermost areolae are very large and in LM give the impression of chambers (cf. Mastogloia) (Figs 44, 45) but are simply part of the outer layer (Figs 46, 47,49). There is also a long groove on each side in the outer layer near the apex (Figs 47, 50, 57). Figs 50 and 51 compare exterior features of the new species with a specimen of D. marginata (F.W.Lewis) Cleve from Georgia. Interior surface shows regular striae opening by transapical slits (Figs 52–55); many short striae and a few longer ones are interpolated on the mantle forming a denser fringe (32 striae in 10 µm), followed by a hyaline border (Fig. 53). Raphe bordered by thick ribs, central and terminal endings simple, the latter with a very small helictoglossa. Wall structure (Fig. 54) shows deep pseudoloculate outer network over the basal striate layer. Figs 55 and 56 compare the similar internal surfaces of the new species with D. marginata. Girdle bands (Figs 47, 57, 58): copulae open: valvocopula with deep notch at the closed end filled by ligula on 3 rd copula (Fig 57, arrow), with two rows of pores along advalvar edge, scattered pores/pits elsewhere; two other copulae with similar structure.

Additional records: GUAM: GU52V-1! (biofilm); YAP, FSM: Y-D2 (seagrass leaves)

Registration: http://phycobank.org/104437

Comments: Dictyoneis apapae differs markedly from D. marginata [Hustedt 1931 –1959, p. 576, fig. 1009; Montgomery 1978 pl. 81C–F; Round et al. 1990 p. 468 (implicitly showing D. marginata); Hein et al. 2008, p. 51, pl. 26, fig 2, pl. 27 fig. 1] but there seem to be additional discrepancies among the images in the literature. The apical groove in Round et al. (1990) is long like that in D. apapae, and they also note that the external raphe slits are bordered by thin ridges of silica, which can be seen in our images of D. apapae but not in our D. marginata specimens (Figs 50 vs. 51). Moreover, Round et al. (1990, p. 469, fig. h) show thick transverse costae and note that “the small poroids of the inner layer are difficult to distinguish because of the strong development of the ribs,” a description that does not fit D. apapae (Fig. 54) or even our Georgia specimens (Fig. 56). These seem to be small discrepancies compared to the differences in the central area, marginal pore size and constriction (the latter two used by Cleve 1890 as taxonomic criteria); we are therefore confident that the Guam species cannot be included in D. marginata, but not so confident in our identification of the D. marginata specimen.

When Cleve (1890) erected the genus, he provided a key to the known species, of which only two were lanceolate, and only one of those had marginal pores larger than the ones on the valve face, that was D. thrumii Cleve, described in the same article. However, he gave no drawings and the description, based on a single specimen from a sea cucumber gut in China, mentions little of the structure. His valve was larger than our specimens, 150 µm long, 32 µm wide. Montgomery (1978, pl. 81A, B) showed an unnamed lanceolate species from Florida with broad semilanceolate zones of smaller pseudoloculi on each side of the raphe, clearly different from ours. There is need for further study of this genus, and the assertion by Round et al. (1990, p. 468) that, “only the type [species] is recorded at all frequently” should not be a license for assigning every constricted specimen to D. marginata.