Trematosoma husselae sp. nov.

(Figs 2–5, 7, 8; Tables 1–5, 7)

Synonymy

Neuhaus 2013, p. 331, fig. 5.8.1D, E: Suctoria, possibly Acineta sp.

Diagnosis

Material examined

Altogether, 705 specimens of Trematosoma husselae sp. nov. were studied on its basibionts Setaphyes kielensis and S. dentatus ( Kinorhyncha), of these 308 specimens by light microscopy and 397 specimens by SEM. The basibionts were deposited at the Museum für Naturkunde Berlin in the collection "Vermes" under the catalogue numbers ZMB 12294–12313, 12359–12418, and 12454–12514. The holotype represents the specimen with a swarmer ventrally on segment 1 of the basibiont ZMB 12499. All light microscopical epibionts on the basibiont S. kielensis ZMB 12479–12505 represent paratypes. For collecting details see Table 1.

Type locality

Intertidal mud at Kampen, east coast of the island of Sylt, German Bight, North Sea, 54°57’15”N, 008°21’34”E, precision 50 m.

Type basibiont

Setaphyes kielensis (Zelinka, 1928), Kinorhyncha, Pycnophyidae

Additional basibiont

Setaphyes dentatus (Reinhard, 1881), Kinorhyncha, Pycnophyidae

Etymology

The species is named after Birgit Hussel, previously technician at the Litoralstation Sylt, in recognition of her help to B.N. and H.Y. with collecting the specimens and her engagement over many years in sending meiofauna samples from Sylt to the first author for his public outreach activities in Germany and Sweden during the 2000s.

Diagnosis

Acinetid with calyciform, dorso-ventrally compressed lorica of about 35 µm x 30 µm size, short stalk with internal longitudinal striae, one left and one right row of tentacles ca. 5 µm long, tentacles separated by small gap, epibiontic on pycnophyid Kinorhyncha.

Description

Specimens of Trematosoma husselae sp. nov. sat on a short stalk and revealed a calyciform, dorso-ventrally compressed lorica (lorica holotype: 37 µm length x 31µm width; range 30–47 µm x 26–34 µm, thickness 9–16 µm) with the zooid (Figs 2, 3, 4B–F, 5A–C, 7, 8, 10C). The zooid usually filled the lorica except for a small basal cavity (Figs 2, 4D, F, 5B, L). The stalk (length 5–7 µm, width 3–4 µm) contained few longitudinal striations recognisable only in favourable light microscopical specimens (Figs 4F, 5G). The apical edge appeared more pronounced in SEM mounts and sometimes more rounded in light microscopical preparations depending on the degree of flattening on the slide (comp. Figs 3, 7 and 8 with Figs 2A, 4B–F and 5A). The surface of the lorica showed slightly elevated circular rings at irregular intervals around the body in SEM (Figs 3D, 8A). In optical section, the lorica wall appeared much thicker (about 4 µm) in its basal two thirds than apically (Figs 2, 4C–F). The tentacles (length 4–6 µm) could be withdrawn, and the terminal end of the lorica revealed a cleft (Figs 2B, 3G, 4A). The tentacles occurred in one left and one right row separated by a small gap on one of the broader sides of the zooid (Figs 2, 4C), but seemed sometimes to form a single row of tentacles (Figs 3, 4E, 7B, 8). An epibiont exhibited 8–16 tentacles in each row (Figs 2, 3C–F, 4C, E, F, 8A). Each capitate tentacle terminated in a small blunt knob with a minute central depression (Figs 2, 3D–F, 4C). Occasionally, empty shells of T. husselae sp. nov. were found (ZMB 12462, 12488; Fig. 4A).

Specimens of Trematosoma husselae sp. nov. possessed usually one central spherical macronucleus and occasionally a more ovoid macronucleus or two spherical macronuclei, which did not fuse at a different focal level (ZMB 12499; Fig. 5C). In many cases, no micronuclei could be observed, but often 2–7 spherical micronuclei of the same yellowish colour as the macronucleus were found (e. g., ZMB 12479, 12480, 12500; Figs 2, 4C, D). A contractile vacuole? may appear apically in an epibiont, but could only be traced in few specimens, possibly because of insufficient conservation.

Eleven epibionts harboured a single, inner, ovoid structure covered by two membranes, with a nucleus (ZMB 12365, 12397, 12489, 12492, 12498, 12499, 12500; Figs 4D, 5B; 13–15 µm x 8–10 µm). This ovoid structure was interpreted as a developing swarmer. Trematosoma husselae sp. nov. seemed to develop from a significantly smaller spherical stage (Fig. 5D, E; 12 µm x 10 µm) with a minimum of about four tentacles and a short stalk (3 µm x 2 µm) via small calyciform and laterally compressed stages to much larger calyciform and laterally compressed specimens with up to 25 tentales (comp. Fig. 4C–F with Fig. 5D, E). One young attached specimen (16 µm x 13 µm) seems to possess parallel rows of surface structures (Fig. 5F).

A small spherical structure with one macronucleus and two micronuclei seemed to be held by the tentacles of a specimen of T. husselae sp. nov. (ZMB 12492; Fig. 4E). This was interpreted as a swarmer recently born from its trophont.

On one specimen on S. dentatus, a small spherical Trematosoma husselae sp. nov. with few tentacles appeared inside the empty shell of a larger empty lorica (ZMB 12503; Fig. 5G). This specimen was probably re-arranged during mounting of the slide and is regarded as an artefact.

In specimens collected 1998, the cytoplasm was often found withdrawn from the base to the apical part of the lorica and revealed almost no substructures except the macronucleus and micronuclei; tentacles could usually not be traced (Fig. 5M). This situation is interpreted as a fixation artefact. Few specimens were preserved more satisfyingly similar to the ones collected 2016, e. g., specimens on ZMB 12397 and 12399 (Fig. 5L).

Diagnostic characters of Trematosoma husselae sp. nov.

All species of Trematosoma possessed a compressed lorica or zooid and tentacles arranged in rows and originating from a fold, the latter characters beeing regarded as genus-specific (Batisse 1972; Jankowski 1978; Curds 1985; Dovgal 2002). The compressed body was not totally clear for T. ovata (Pritchard, 1852) because of the insufficient description of the species (Alder 1851; Pritchard 1852). Trematosoma husselae sp. nov. agreed with T. amphiasci (Precht, 1935) and T. pusilla (Maupas, 1881) in the calyciform shape of the lorica and with most species in the general size of the body except for the much larger T. constricta (Collin, 1909) and T. pusilla (Table 7). Measurements of length and width of stalks were missing in several species, so only a more general statement can be given about this organ. The new species could be distinguished from T. amphiasci, T. complatana (Gruber, 1884), T. constricta, T. falcata (Jankowski, 1981), T. ovata, and T. pusilla by the short length of its stalk and agreed with T. bocqueti (Guilcher, 1950) and T. rotunda (Allgén, 1951) in this character (Table 7). Longitudinal striae were reported for the stalk of T. constricta and T. husselae sp. nov., transversal striae for T. rotunda, whereas information about these characters was missing for the remaining species (Table 7). A basal disc was mentioned for T. bocqueti and T. falcata but did not seem to exist in T. husselae sp. nov. (Table 7; Batisse 1972; Jankowski 1981; this paper).

Tentacles were arranged in all species in a row with a broad central gap in T. complatana, a small or a broad gap in the Australian population of T. bocqueti, and a small gap in all remaining species except T. ovata and T. pusilla, for which information was limited in this respect. Tentacles occurred also laterally in T. complatana and in the Australian population of T. bocqueti, but not in any other species of the genus. Trematosoma husselae sp. nov. agreed with most species in the number of tentacles from 8–16 on, but T. falcata and T. pusilla had only 4–6 tentacles (Table 7). Short tentacles of up to about 10 µm length were reported for T. amphiasci, T. bocqueti, T. falcata, T. pusilla, T. rotunda, and T. husselae sp. nov., whereas much longer tentacles appeared in T. complatana and T. constricta; T. amphiasci and T. bocqueti possessed both short and long tentacles (Table 7).