Rhoiconeis pagoensis Lobban, 2015

Lobban, Christopher S., 2015, Benthic marine diatom flora of Guam: new records, redescription of Psammodictyon pustulatum n. comb., n. stat., and three new species (Colliculoamphora gabgabensis, Lauderia excentrica, and Rhoiconeis pagoensis), Micronesica 2015 (2), pp. 1-49 : 14

Rhoiconeis pagoensis Lobban n. sp. Figs 131–138

NAVICULALES Bessey : Naviculaceae Kützing

Samples: GU7X-2, GU7X-7

Dimensions: Length 16–49 µm, width 6.7–7.9 µm; striae 16–22 in 10 µm

Diagnosis: Differing from the other two species in the stria density.

Holotype: Slide # 1319 from GU7X-7, Fig. 133 . Deposited at California Academy of Sciences , accession # 627426 slide # 220342.

Type locality: Pago Bay , Guam, 13°25'40" N, 144°47'57" E, on shore in University of Guam Marine Laboratory seawater outfall stream, collected by C. Lobban, 3 June 2013 .

Paratypes: Slides #1265, 1266, 1270–1272 in UOG Diatom Herbarium.

Etymology: named for the type locality, Pago Bay, itself probably from pagu, the Chamorro name for the tree Hibiscus tiliaceus L., whose bark was traditionally used to make rope.

Description: Live cells motile, two plastids lying along the girdle faces as in Navicula ( Fig. 131, 132 ). Frustules flexed in girdle view, the central areas of the two usually different, with the broader area on the concave side ( Figs 136, 137 ). Striae lineate, radiate except for short ones bordering the central area ( Figs 135, 137 ). Valvocopula in two segments, with a sigmoid junction near one apex

( Fig. 138 ); the smaller segment reminiscent of the end plate in Rhabdonema arcuata ( Pocock & Cox 1982) . There were slight differences in stria count common between apices and center, the two apices and the two valves. Frustules did not come apart even after double acid cleaning and I was not able to get SEM of the internal valve face.

Comments: Samples were collected over several days from a diatom bloom in an outfall stream from the U. Guam Marine Laboratory seawater system. The bloom developed soon after the seawater flow was restarted, epiphytic on a population of the siphonous green alga Trichosolen sp. , itself known to be rare except for blooming after a major disturbance ( Lobban & Tsuda 2003).

Rhoiconeis Grunow was resurrected by Medlin (1985) and currently includes two species ( Table 1 ). The genus is characterized by naviculoid frustules flexed in girdle view and having lineate striae and internal siliceous sheets joining the outer parts of the interstriae. There is a distinctive pattern in the girdle bands, with a segmented valvocopula and two pleural bands, as shown in Medlin (1985, fig. 38; compare with my Fig. 138 ). In Rhoiconeis both valves are raphid, in contrast to Achnanthes ; isopolar, in contrast to Rhoicosphenia ; and distinguished from Campylopyxis by the pores, girdle bands, and internal valve features. Both known species of Rhoiconeis have radiate, lineate striae and slightly different central areas on the convex and concave valves. Thus, the present material can be positively identified as Rhoiconeis even without good internal views, and can be distinguished from the other species on the basis of valve size and stria density ( Table 1 ).

Figures

Figures 127–138. Figs 125–126. Psammodictyon pustulatum (GU66A-3) (SEM). Fig. 127. Oblique view of frustule showing linear pores on the dorsal valve face (arrow). In both Figs 127 and 128 the transition from the ventral to dorsal parts of the valvocopula is visible (arrowheads). Fig. 128. Apical view of frustule showing the distinctive topography of the ventral side of the raphe-keel, along with the linear pores on the dorsal side of the other valve (arrow) (two stacked images). Figs 129, 130. Pteroncola inane (SEM). Fig. 129. Frustule in girdle view (GU41D-1A). Fig. 130. Broken frustule showing internal and external valve faces, with the single row of internal pores (arrows) (Jordan collection, image by Misaki Ishizawa) (SEM). Figs. 131–138. Rhoiconeis pagoensis. Figs 131, 132. Same live cell in valve and girdle view showing plastids (GU7X-8) (DIC). Figs 133, 134. Holotype (valve view) and a girdle view from the same slide (GU7X-7) (DIC). Fig. 135. Frustule in valve view (GU7X-7) (SEM). Fig. 136. Frustule in girdle view; note the very short striae at the central area on the concave valve (GU7X-2) (SEM). Fig. 137. Detail of a pair of frustules showing difference in striae lengths at central area on convex and concave valves (GU7X-2) (SEM). Fig. 138. Apex of a frustule in girdle view showing the segmented valvocopula (S1 and S2) and two pleural bands (P1 and P2) (GU7X-5) (SEM). Scale bars: Fig. 136 = 10 µm, Figs 127–128, 131–135 = 5 µm, Figs 129, 130, 137, 138 = 2 µm.