Rickia platessa Santam., Enghoff, & Reboleira, 2016

Santamaria, Sergi, Enghoff, Henrik & Reboleira, Ana Sofia P. S., 2016, Hidden biodiversity revealed by collections-based research-Laboulbeniales in millipedes: genus Rickia, Phytotaxa 243 (2), pp. 101-127 : 117-118

Rickia platessa Santam., Enghoff, & Reboleira , sp. nov. (Figs 42–46)

Mycobank MB 815408

Diagnosis:—Receptacle biseriate, a: 5, p: 6. Perithecium nearly completely embedded between the two series of cells. Antheridia very abundant, forming series of vertical rows on both sides of the thallus. Brown septa only under secondary appendages and not below the antheridia.

Type: — SINGAPORE. Seletar reservoir, Forest, 1°24’N 103°48’E, on Trigoniulus corallinus (Gervais 1841) (Spirobollidae, Pachybolidae ), 7 November 1991, C-F-92262, C-F!, holotype designated here; BCB-SS ·E580a, c, BCB!, isotypes designated here.

Etymology:— platessa , refers to the superficial similarity of this species to a flatfish, especially the plaice, Pleuronectes platessa .

Thallus hyaline to reddish brown; if hyaline, then with the exception of the brown foot, the tan trichogyne scar and the brown septa separating the appendiculate cells from appendages; if reddish brown, then uniform without darker colour of parts as mentioned for other thalli. Total length 131–257 μm. Receptacle biseriate. Basal cell about 2.5–3

times as long as broad, clavate, with a narrow, even acute base above a diminutive, point-sized foot. The basal cell borders distally on the lower cells of the two series, flanked by these cells and immersed between them for 2/3 of its total length.

Anterior series consisting of 5 cells (Fig. 42). Cell a 1 triangular in section, opposite and nearly identical to p 1, rarely with one appendage and one antheridium. Cells a 2–5 on their upper corners with 3–8 lateral antheridia each, and sometimes also 1–2 secondary appendages below. Cell a 3 giving rise to the perithecium.

Posterior series consisting of 6 cells (Fig. 44). Cell p 1 almost identical to the opposite symmetrical a 1, rarely with one antheridium or one secondary appendage on its upper corner. Cells p 2–5 on their upper corners with 3–8 lateral antheridia each, and sometimes also 1–2 secondary appendages. Cell p 6 supporting the primary appendage including its dome-shaped and very small basal cell (Fig. 42, arrow), separated by a constricted and slightly darkened septum from the inconspicuous distal elongated cell, which is nearly identical to secondary appendages; rarely, cell p 6, also supports one secondary appendage. Primary septum constricted and brown-colored.

Antheridia 18–22 × 4–7 μm, hyaline (Figs 42–44) to reddish brown (Fig. 46), above squarish stalk cells, partially immersed in the thallus, with curved to straight, long efferent necks where rows of spermatia are clearly seen in the discharge channel. Septum separating the stalk cell from the appendiculate cell hyaline and not constricted.

Secondary appendages 14–17 μm, hyaline, short, clavate, above the constricted and brown septa which separate them from the embedded appendiculate cells.

All the appendiculate cells are superposed in vertical-diagonal series and are embedded in the thallus (Figs 43,

45).

Perithecium 77–138 × 30–52 μm, solitary, broadly fusiform, more curved on the dorsal margin, asymmetric, with an acute apex. Surrounded along both margins by the anterior and posterior series of cells, with only the tip free on the posterior margin, and the 1/5 of its length on the anterior margin. A persistent brown trichogyne scar is seen at the upper anterior margin of the neck (Fig. 42, trs). Perithecial basal cells indistinguishable. Perithecial stalk cell ( VI) very flattened, triangular in section, located in the upper inner corner of cell a 3 (Fig. 42, VI).

Position on host:— In some specimens of T. corallinus from the same sample as the type host, there is an almost complete, regular complement of thallus feet on virtually all body rings of the millipede, but apart from the feet, the thalli are lost. This gives the millipede such a distinctive appearance that it was first assumed by HE to represent a new species.

Notes on hosts:— Trigoniulus corallinus is a tropical ‘tramp’ species that has been recorded from numerous places in the tropics, especially from islands where it has obviously been introduced by man ( Shelley & Lehtinen 1999). The unidentified Thai pachybolid is one of many undescribed species currently under study by Piyatida Pimvichai.

Discussion:— This species belongs to the group of species of Rickia with biseriate thalli. Although the number of cells per series seems rather constant, the quantity of antheridia, and to a lesser degree of the appendages, is heavily increased in some unusual, maybe teratological thalli.

At the first glance, the two collections included here were considered different species, especially because of the conspicuous differences in the colour of the thalli, hyaline vs. reddish-brown. But in spite of this difference, the morphology is almost identical in the two collections with only minor differences relating to the number of appendages and antheridia above the cells of the receptacle series. Both collections came from the same host family and from neighboring regions which support the idea of co-specificity.

Additional collections examined: — THAILAND. Tham Suae Temple, Krabi, on Pachybolidae indet.

( Spirobolida ), 14 January 2013, Piyatida Pimvichai leg., BCB-SS·E590ac (BCB!), C-F-95100 (C-F!). Hosts at Chulalongkorn University Museum of Zoology coll.