Loxosomella unicornis Borisanova & Krylova, 2014, sp. nov.

Loxosomella unicornis sp. nov.

(Figs 2, 3)

Material examined. Holotype: ZMMU No. Uk-5 (an adult), from Kandalaksha Bay, White Sea (66°34' N, 33°08' E), 24 m, 26 June 2013, on the bryozoan Cribrilina sp. living as an epibiont on a shell of Mytilus edulis Linnaeus, 1758. Paratypes: ZMMU No. Uk-6 (7 paratypes, all adults), from the same locality as holotype. Additional material examined: the same locality, 5 m, 27 August 1983, on the bryozoan Electra sp., 20 adult specimens.

Etymology. Greek unicornis, horn, alluding to the horn-shaped appendage on the top part of the calyx.

Diagnosis. Adults: Medium-sized species, total length of body from end of foot to base of tentacles 300–350 µm (up to 650 µm) (Table 2). Calyx length 130–215 µm, width 120–230 µm. Calyx compressed in anteroposterior axis, about twice as wide as thick. Stalk length varies from 90 to 320 µm, stalk width about 70–105 µm.

There is pair of thin, transparent lateral wings on sides of calyx making it rounded in frontal view. Calyx with 8 rather short tentacles, directed forward; 2 pigmented semicircular strips located on lateral margins around tentacle crown. Top part of back side of calyx above tentacle ring bearing a conspicuous horn-shaped appendage about 50–60 µm in length. Two pairs of ciliated lateral papillae are located on each side of calyx. Stomach about twice as wide as high, with lateral lobes. Stalk with prominent longitudinal muscles. Foot reduced.

Reproduction: Buds emerge from paired laterofrontal areas at lower level of stomach, one bud on each side. Some specimens have 1–2 developing larvae in brood chamber. Larva with well-developed frontal and apical organs, prototroch diameter about 85 µm and episphere height about 110 µm.

Specimen 1 2 3 4 5 6 * * Total length 296 250 289 370 398 355 377 515 Loxosomella unicornis sp. nov. differs from all described loxosomatid species in having a large horn-shaped appendage on top of the calyx. It is known that some loxosomatids have conspicuous appendages on their calyces (Table 3), but these appendages in the other species are thinner and longer, and have different locations, than in L. unicornis.

Some individuals of L. vivipara Nielsen, 1966 bear an enigmatic organ on the back side of the calyx, above the anus, in the same position as the horn-shaped appendage in L. unicornis. However, there are structural differences between these organs in the two species—in L. vivipara the enigmatic organ consists of a ring of cells radiating from an opening that leads into a hollow, ball-shaped structure whilst in L. unicornis the appendage is solid and lacks an opening.

Another remarkable feature of L. unicornis is the presence of two pigmented semicircular strips around the tentacle crown. A very similar structure was recorded in L. ameliae Sánchez-Tocino & Tierno-de-Figueroa, 2009. There is a field of well-marked gland cells along the base of the tentacle crown, but this species lacks the horn-like structure on top of the calyx, and additionally differs in tentacle number (10–12), foot structure (having a foot groove and glandular cells) and body size (520–820 µm).

Loxosomella unicornis sp. nov. appears to be an epibiont of bryozoans. Indeed, most Loxosomella species are epibionts of different invertebrate animals such as polychaetes, bryozoans and sponges. Individual species of Loxosomella are usually associated with only one type of host animal (Nielsen 1964). About 20 loxosomatid species have been found associated with bryozoans. Five of them are similar to L. unicornis in having lateral compressions (wings) on the calyx and a similar number of tentacles, viz L. nordgaardi Ryland, 1961, L. marisalbi Bagrov & Slyusarev, 2002 and three species of uncertain position that were described as Loxosoma but which probably belong to Loxosomella species: L. cingulata Kluge, 1946, L. loricatum Harmer, 1915 and L. rotunda Kluge, 1946. All of these species differ from L. unicornis in different respects. L. nordgaardi lacks lateral papillae; L. marisalbi has a calyx with three pairs of sensitive receptors; and Loxosoma cingulata lacks stomach lobes and bears 8–10 glandular cells along lateral wings; L. loricatum differs in size (its total length is up to 210 µm) and stomach shape (without lobes) and has only one pair of papillae; L. rotunda is very small (total length about 270 µm) with a very short stalk and no lateral papillae.