Myriotrochus (Oligotrochus) meteorensis Bohn, 2005, spec. nov.

Myriotrochus (Oligotrochus) meteorensis spec. nov.

(Figs 1 A–M, 2)

Material. Holotype (Ψ anterior fragment, ZSM 20043112), 8 paratypes (2 ɗ anterior fragments, 2 Ψ anterior fragments, 2 juv. [?] anterior fragments, 2 posterior fragments, ZSM 20043108), FS "Meteor", station M 36/99, Agassiz trawl 151, 21°36.2' N, 18°40.6' W, 2786–2843 m, 26.II.1975.

Description. The seven anterior fragments are only few millimeters long and 6.0– 9.2 mm wide (close to the calcareous ring). The larger of the two posterior fragments is about 25 mm long. The anterior fragments are circular to slightly oval in cross­section, the posterior fragments have a slightly tapering posterior end.

There are 12 conical tentacles with 4–5 pairs of lateral digits. Both, the left and right lateral interradius have three tentacles, while all other interradii only have two. The tentacles lack calcareous deposits.

Body wall deposits are myriotrochid type wheels (Fig. 1 L, M), which are scattered in the anterior part of the body and get scarce towards the median body region. The posterior end of the body lacks wheel deposits.

The calcareous ring is stout (Fig. 1 A–C), and ventrally much higher than dorsally. The posterior margin of the ring is undulating, in particular on the lateral and ventral side, due to flat posterior processes on the plates (Fig. 1 F–H). Five radial and five interradial plates build up the calcareous ring. The two dorso­lateral radial plates each have two anterior processes (Fig. 1 E), while all other plates only have one anterior process (Fig. 1 D, F–I), which are all of about the same height. The radial plates are perforated for the passage of the nerve.

The gonad consists of two tufts of few branching tubules. There is a single tubular polian vesicle. The stone canal, which is in close contact to the dorsal interradial plate, consists of a short convoluted tube embedded in a prominent calcareous madreporite body (Fig. 1 J). The intestine has a large loop.

The myriotrochid type wheels are circular or oval and range from 111 to 316 µm in diameter. The hub is due to the fusion of proximal parts of adjacent spokes of variable outline (Fig. 1 M) and size and reaches from about one­sixth to more than one­third the size of the wheel diameter. There are 10 to 18 spokes per wheel, fused in their proximal part with their neighbors, thus enlarging the hub. They are more or less straight in their free part, and close to the rim the majority of the spokes branche into two (Fig. 1 M, see also Tab. 1). The mean ratio of branched spokes to spokes varies in the seven investigated specimens between 75.8–100 % (Tab. 1). Usually, there are about twice as much teeth projecting from the rim of the wheel as there are spokes. The teeth are triangular in outline and have a bifid tip (Fig. 1 K). Detailed measurements of wheel parameters for seven specimens are given in Table 1 and are summarized in Table 2.

There is a significant linear relationship between the number of spokes (p <0.0001), the number of teeth (p <0.0001), the hub diameter (p <0.0001) and the diameter of the wheels (Fig. 2). With increasing size of the wheel, the number of spokes and teeth as well as the size of the hub increase.

Remarks. Based on the shape of the calcareous ring (stout, with undulating posterior margin) and the morphology of the tentacles (conical, with 4–5 pairs of lateral digits) this new species has to be placed in the subgenus Oligotrochus sensu Heding (1935). Currently there are five species included in Oligotrochus (see Smirnov 1999)— Myriotrochus (Oligotrochus) vitreus (M. Sars, 1866), M. (O.) bathybius H. L. Clark, 1920, M. (O.) clarki Gage and Billett, 1986, M. (O.) neocaledonicus Smirnov, 1999 and M. (O.) rotulus Smirnov, 1999.

Good characters separating the new species together with M. (O.) vitreus and M. (O.) clarki from M. (O.) bathybius, M. (O.) neocaledonicus and M. (O.) rotulus are the morphology of the wheels and the calcareous ring.

According to Smirnov (1999), the last­mentioned three species have wheel deposits with a hub perforated by a more or less complete circle of holes and a calcareous ring where the ventral pieces are less than two times as high as the dorsal pieces (not known in M. (O.) rotulus). In contrast, M. (O.) meteorensis, M. (O.) vitreus and M. (O.) clarki have wheels without perforations on the hub and the ventral pieces of the calcareous ring are about two times higher than the dorsal pieces (see Heding 1935, Gage & Billett 1986, Smirnov 1999). Though quite similar, the new species also differs in several characters from M. (O.) vitreus and M. (O.) clarki.

Parameter Specimen

ZSM ZSM 20043108

20043112

The wheels of M. (O.) vitreus are considerably smaller, with a diameter ranging from 55 to 95 µm (Østergren 1902, Heding 1935) compared to those found in M. (O.) meteorensis which have a mean diameter of 260.6 µm (range: 111–316 µm, see Tab. 2). Other wheel parameter separating both species are the number of teeth, which usually is lower in M. (O.) vitreus and the ratio of spokes to teeth, which is considerably higher in M. (O.) vitreus (see Tab. 2). A character, both species have in common are their tentacles with four to five pairs of lateral digits.

Myriotrochus (O.) clarki differs by several wheel parameters from the new species (see Tab. 2). Most obvious is the smaller size of the wheel deposits, which have a mean diameter of 121.7 µm and range from 56 to 244 µm (Gage & Billett 1986) compared to those found in M. (O.) meteorensis (mean: 260.6 µm, range: 111–316 µm). Also the mean number of spokes and teeth per wheel are lower in M. (O.) clarki (see Tab. 2). The size differences observed in the wheels of both species cannot be ascribed to size differences of the investigated material, because the current specimens (calcareous ring diameter: 6.0– 9.2 mm) are within the range of the specimens described by Gage & Billett (calcareous ring diameter: 1.6–9.2 mm). Furthermore, both species differ in their tentacle morphology: Myriotrochus (O.) clarki has two pairs of lateral digits per tentacle (Gage & Billett 1986), while the new species has four to five pairs.

Parameter Species There can be little doubt, that both species are closely related. Some presumable synapomorphic features are their unique wheel deposits. (1) Spokes are fused in their proximal part with their neighbors, thus enlarging the hub. (2) Close to the rim spokes branche into two. (3) The teeth projecting from the rim of the wheel often are bifid. According to Gage & Billett (1986) these characters are only present in the larger wheels of M. (O.) clarki, while they are found in nearly all wheels of M. (O.) meteorensis with a diameter larger than 210 µm. Out of 177 wheels measured, there were only 4 wheels which had no branched spokes, and each one, with only 1, 2 or 3 branched spokes. Five of these seven wheels have a diameter of less than 210 µm. Similar to small wheels in M. (O.) clarki, small wheels in M. (O). meteorensis may lack this character, though all of them have bifid teeth projecting from the rim.

Myriotrochus (O.) clarki has been found off NW Ireland and off NW United Kingdom at depths between 1040 and 2907 m (Gage & Billett 1986, Harvey et al. 1988). Recently, it also has been described from off NW Spain, at a depth of 480–520 m (Smirnov 1999).

In conclusion, M. (O.) meteorensis may be distinguished from the closely related M. (O.) clarki by its tentacles, which bear four to five pairs of lateral digits, instead of only two in the latter species, and by its considerably larger wheels.

Distribution. Only known from the type locality, off NW Africa, NE Atlantic Ocean, 2786–2843 m.

Etymology. The name, meteorensis, refers to the German research vessel FS "Meteor" the service of which contributed much to our current knowledge on marine biodiversity.