Cothurnia buetschlii Zelinka, 1914

(Figs 5J, 6–9; Tables 1, 3, 4, 6)

Material examined

In total, 14 specimens of Cothurnia buetschlii were found on five males of Setaphyes kielensis mounted for light microscopy and an one female and one male of the same species mounted for SEM: All specimens were collected 2016, and the basibiont S. kielensis deposited at the Museum für Naturkunde Berlin in the collection "Vermes" under the catalogue numbers ZMB 12468, 12474, 12486–12489, and 12499. For collecting details see Table 1.

Emended diagnosis

Vaginicolid with bulbous lorica basally and narrowing anteriorly, lorica slightly dorso-ventrally compressed with anterior margin recessing on opposing sides laterally, short stalk of about 10 µm, stalk and endostyle with internal longitudinal striae, mesostyle lacking, zooid with weak circular annulation, C-shaped or serpentine macronucleus orientated longitudinally in periphery of zooid, epibiontic on pycnophyid Kinorhyncha.

Description

The zooid was withdrawn into its lorica in all 14 specimens except one (Fig. 5J). The kinorhynchs exhibited also numerous suctorian ciliates (Figs 7, 8, 9A; Tables 3, 4).

Each lorica with its bulbous posterior end was slightly dorso-ventrally compressed and contained a single zooid (Figs 6, 7, 9). An operculum or a valve was missing. The lorica measured 46–58 µm in length, 21–23 µm at its largest diameter, and 10–19 µm at its aperture (Table 6). Basally, the lorica wall thickened slightly with distinct outer and inner borders depending on the angle of the specimen to the DIC prisms (Fig. 9D, E, white and black arrows). A lorica attached to the cuticle of a kinorhynch with a short stalk (Fig. 9D–G). The anterior margin of a lorica recessed on opposing sides laterally (Figs 6, 7). The surface of the lorica appeared smooth in SEM but was sometimes covered with detritus (Figs 7, 8; ZMB 12489). In two specimens, epibiontic filamentous bacteria grew on the lorica (Fig. 8B).

The withdrawn zooid reached a length of 22–37 µm and revealed a weak circular annulation on its outside around its entire body (ZMB 12486–12489; Figs 6, 9C, D; Table 6). The external stalk showed conspicuous, internal, longitudinal striae continuing into the endostyle and attached to the cuticle of the basibiont with a kind of cement with an almost central pore (ZMB 12486–12489; Figs 6, 9D–G). An endostyle with longitudinal striae extended between the base of the lorica and the zooid, a mesostyle was missing (ZMB 12486, 12489; Figs 6, 9D–F). The considerably elongated, more or less C-shaped or serpentine macronucleus meandered longitudinally in the periphery of the zooid and reached a length of up to 30 µm (ZMB 12486–12489; Figs 6, 9E–G; Table 6). One or at least five micronuclei were observed (ZMB 12486; Figs 6, 9E–G). A large spherical cavity seemed to occur below the infundibulum at the anterior end of the zooid and may or may not represent a contractile vacuole (ZMB 12486; Figs 6, 9G). No information became available about the peristomial lip and disc because of the withdrawn zooids. Peristomal cilia may appear as a row of bristle-like projections at the anterior end of the withdrawn zooid (ZMB 12486, 12489; Fig. 9B).

A single specimen with an extended zooid occurred ventrally on segment 10 of one basibiont of S. kielensis (ZMB 12486; Fig. 5J, right specimen). The zooid (51 µm x 16 µm) exhibited a basal attachment area with longitudinal striae, a long endostyle (ca. 18 µm x 2–5 µm) not clearly separated from the zooid apically, a large serpentine macronucleus (15 µm x 4 µm), and anteriorly a large spherical structure (13 µm x 8 µm), possibly a contractile vacuole (Fig. 5J). The endostyle did not show any longitudinal striae but a kind of inner annulation, which was regarded as a fixation artefact.

Diagnostic characters of Cothurnia buetschlii .

Zelinka (1914, p. 680–682) described Cothurnia buetschlii inadvertently as a new species by mentioning several morphological characters and providing measurements for the lorica (53 µm x 21 µm) and stalk (length 10 µm). In his monograph, Zelinka (1928, pp. 194, 195, text figs 37a, b, figs 9, 10 in pl. 7, fig. 8 in pl. 14) provided much more detailed information about the epibiont: He found up to 10 or more specimens of C. buetschlii on adult but also on juvenile specimens of Pycnophyes communis and other, unnamed species of Kinorhyncha, mainly on the ventral side of the trunk or on the lateral terminal spines. The lorica (53.1 µm x 21 µm, 19.2 µm at aperture) appeared recessed on two opposing sites. The stalk (8.5–12.6 µm x 5.3 µm) showed an internal longitudinal striation and a striated endostyle (Zelinka 1928, text fig. 37a; the very weak striation is only recognisable in the original book but not necessarily in xerocopies). The contracted zooid reached a length of 27.7 µm. The macronucleus was C-shaped and extended in longitudinal direction (Zelinka 1928, text figs 37a) but was mentioned by Zelinka (1928, p. 197) as “quergestellt” (= transversely oriented). Measurement data of the inadvertent original description (Zelinka 1928) differed slightly from those of his intended species description (Zelinka 1928), the latter being more precise, and also from those given by Warren and Paynter (1991, pp. 21, 22, 29, figs 47, 48: lorica 53.2 µm x 27 µm, stalk 10 µm x 4 µm). Their descriptive text disagreed from their identification key in that the mesostyle was stated to be absent and an endostyle to be present, whereas the identification key stated the opposite. The latter was erroneous, because it did not agree with Zelinka’s (1928) description and with our observations.

All observations of Zelinka (1914, 1928) were based on living specimens of C. buetschlii, but the zooids never raised above the lorica, so not all characters became available for study. Our preserved specimens of C. buetschlii agreed with Zelinka’s (1914, 1928) descriptions in the sizes of the lorica, stalk, and length of the zooid (Table 6), the shape of the lorica with two opposite recessed anterior sides (Figs 7, 8), the existence of an endostyle, the longitudinal striae in both the stalk and endostyle (Figs 6, 9D–F), and the longitudinal, C-shaped macronucleus (Figs 6, 9E–G). Our observations of a weak surface annulation of the zooid (Figs 6, 9C) was not reported by Zelinka (1914, 1928) but may have been overlooked by him.

Referrence to C. buetschlii in the literature was rare. Besides the review of species of Cothurnia by Warren and Paynter (1991), Kahl (1933, p. 138) mentioned C. buetschlii briefly and erroneously cited the species as C. buetschlii Zelinka, 1885 . It remains a bit mysterious why Kahl claimed that one of the broader sides of a specimen would be longer than the opposite side and would cover elastically the opening (Kahl 1935, p. 780, fig. 143.35–35a). This was not reported by Zelinka (1914, 1928) at all and could not be confirmed by us as well. To our knowledge C. buetschlii has not been mentioned otherwise and was not redescribed, yet.

Distribution of Cothurnia buetschlii on Kinorhyncha in European waters.

Zelinka (1928) reported C. buetschlii from Pycnophyes communis and also from similar species of Kinorhyncha without providing their species names. The distribution areas of the basibiont P. communis and of the here reported Setaphyes dentatus and S. kielensis overlap widely (Fig. 1). Whereas P. communis was reported from the Skagerrak, Baltic Sea, North Sea, Irish Sea, Mediterranean Sea, and Black Sea, S. dentatus was mentioned from the Baltic Sea, North Sea, Irish Sea, Mediterranean Sea, and Black Sea, and S. kielensis showed records from the White Sea, Baltic Sea, North Sea, British Channel, and Black Sea (Fig. 1; González-Casarrubios et al. 2023; Neuhaus 2023a –c). With this overlap of distribution areas it is highly likely that an exchange of epibionts between P. communis, S. dentatus, and S. kielensis would be possible.