Zelentia ninel Picton 2017, sp. nov.

Zelentia ninel sp. nov.

(Figures 1; 3 A–H; 4 A; 5)

http://zoobank.org/urn:lsid:zoobank.org:act:9B3BAEC2-85DC-442C-967C-001732761896 Type material. Holotype, ZMMU Op-509, 6 mm in length, 1.2 mm in width, Barents Sea, Dalne-Zelenetskaya Bay (near Dalnie Zelentsy settlement, 69° 07´N 36° 03´E), intertidal, collected by T.A. Korshunova, A.V. Martynov, 19.08.2012. Paratypes, ZMMU Op-187, 1 specimen same locality, collected by T.A. Korshunova, A.V. Martynov, 0 1.08.2005. ZMMU Op-188, 1 specimen, same locality, collected by T.A. Korshunova, 17.08.2006. ZMMU Op-400, 1 specimen, 4.5 mm in length, 0.8 mm in width, live, same locality, collected by T.A. Korshunova, A.V. Martynov, 19.08.2012. ZMMU Op-401, 1 specimen (5 mm in length, 0.6 mm in width, live), same locality, collected by T.A. Korshunova, A.V. Martynov, 19.08.2012. ZMMU Op-402, 1 specimen (4.8 mm in length, 1 mm in width, live), same locality, collected by T.A. Korshunova, A.V. Martynov, 0 3.08.2012.

Type locality. Dalnie Zelentsy, Barents Sea.

Etymology. This species is named in honour of the scientist and marine biologist Ninel Panteleeva (Murmansk Marine Biological Institute), who for a long time lived in the Dalnie Zelentsy settlement, close to the type locality of this species. Ninel also generously helped us with the field work at Dalnie Zelentsy.

Description of adults. External morphology. The length of the holotype is 6 mm and width 1.2 mm (Fig. 1A– B). The length of more than one hundred observed living mature specimens ranged from 4 to 6 mm, the width ranged from 0.6 to 1.2 mm. The body is narrow. The rhinophores are similar in size to the oral tentacles, smooth. The cerata are relatively long, thin, finger-shaped. Ceratal formula of the holotype: right (2,2,2; anus, 2,2,1) left (2,3,3; 3,2,1). The foot is narrow, anteriorly rounded, no foot corners.

Colour. The ground colour is whitish. The digestive branches in the cerata are pale brownish to pinkish. A few small opaque white spots are scattered at the ceratal tops. There are no white spots throughout most of the length of the cerata.

Anatomy. Digestive system. The jaws are ovoid (Fig. 1E, G) The masticatory processes of jaws bear a single row of simple denticles (Fig. 1F). The radular formula in two studied specimens (4–5 mm in length) is 17–19 x 0.1.0. The radular teeth are slightly yellowish. The central tooth is narrow, elongated, with low cusp and four lateral denticles (plus sometimes very small outer denticles) (Figs. 1D).

Reproductive system. (Figs. 4A). The ampulla is moderately short and swollen (Fig. 4A, a). The prostate is a slightly convoluted tube (Fig. 4A, pr). The prostate transits to a penial sheath, which contains a conical penis with a curved chitinous stylet (Fig. 4A, p). A supplementary (“penial”) gland inserts into base of the penis (Fig. 4A, pg). The seminal receptacle is small, rounded, on a stalk (Fig. 4A, r). The female part includes mucous and capsular glands (Fig. 4A, fgm).

Description of egg masses and early juveniles. Eggs mass is ovoid or semi-spiral. Number of eggs ca. 10–60. Eggs develop within 3–4 weeks. This new species is invariably characterized by direct development with hatching shell-less early postlarval specimens (Fig. 3). The early juveniles with small oral veil and only single pair of short cerata. No oral tentacles and rhinophores. Anus lateral on the right side. The digestive gland without numerous branches. Radula and jaws present. The anterior part of reproductive system is absent.

Habitats. Inhabits the stony intertidal. Feeds on campanulariid hydroids, including Gonothyraea loveni (Allman, 1859) and Obelia longissima (Pallas, 1766).

Distribution. To date known only from the southern coast of the Barents Sea in the Murmansk region. Potentially can be discovered in the neighboring parts of Norway.

Remarks. Uncorrected p-distances for the COI gene show significant differences between Zelentia ninel sp. nov. and Zelentia pustulata, being 13.83% ± 1.45%. Uncorrected p-distances within Z. ninel sp. nov. are within 0.1%. Uncorrected p-distances within Z. pustulata are up to 0.27% but uncorrected p-distances between Z. ninel sp. nov. and Z. pustulata reach 14.01%. Uncorrected p-distances between Z. ninel sp. nov. and Z. fulgens are 12.91% ± 1.3%, between Z. pustulata and Z. fulgens are 10.49% ± 1.19%. Importantly, in the present study several specimens of Z. pustulata were collected in the British Isles (ie. in the same geographic area as the type locality of “ Eolis ” pustulata at Cullercoats, north east England, (Alder & Hancock, 1854) and one UK specimen and one Irish specimen have been sequenced and confirmed as belonging to the same species as specimens of Z. pustulata from the Barents Sea. At the same time, p-distances between Z. ninel sp. nov. and Z. pustulata (ranged from 14.01- 14.18%) clearly indicate that Z. ninel sp. nov. is a distinct species. Z. ninel sp. nov. belongs to the same clade as Z. pustulata (Alder & Hancock, 1854) comb. nov. (Fig. 2) according to the molecular phylogenetic data (Cella et al. 2016: 9, as “ Tenellia sp. J”) with significant molecular distance. Z. ninel sp. nov. morphologically differs considerably from Z. pustulata by having fewer cerata, the absence of white spots on the cerata (except for the apical parts), fewer lateral denticles on the radula teeth, and a thinner prostate in adult specimens (the width of prostate is 0.125 mm in Z. ninel sp. nov. specimen of 5 mm length and 0.250 mm in Z. pustulata of 15 mm length). Z. pustulata is larger than Z. ninel sp. nov.: the new species usually reaches not more than 6 mm (Fig. 1), whereas Z. pustulata reaching at least 18 mm in length (Fig. 2) and differences in prostate thickness thus can be therefore also size-depended. However, in the reproductive system particularly evident differences between Z. pustulata and Z. ninel sp. nov. are highlighted by the shape of the penial stylet (compare Fig. 1H, I and 2L). Furthermore, the two species are well distinguished ecologically and behaviorally: Z. ninel sp. nov. exclusively inhabits the intertidal zone and commonly occurs under stones where it feeds on the hydroids Gonothyraea loveni and Obelia longissima, whereas Z. pustulata inhabits depths below 10 m, and lives on bushes of the hydroid Halecium muricatum (Ellis & Solander, 1786) and possibly other Halecium species Results obtained by PopART showed a network of haplotypes that clearly clustered into two groups coincident with Z. ninel sp. nov. and Z. pustulata (Fig. 5). The most drastic difference between the two related species is in their early development: while Z. pustulata has veliger larvae, Z. ninel invariably develops through a crawling shell-less postlarvae. In accordance with these differences egg masses of Z. ninel sp. nov. are much smaller in size and in number of eggs (ca. 10–60), ovoid or semi-spiral, compared to the spiral egg masses of Z. pustulata, comprising several whorls and significantly more eggs (up to 200- 300 eggs at least). Importantly, more than one hundred egg masses of Z. ninel sp. nov. have been observed in the field and in the laboratory, and crawling shell-less postlarvae with a small oral veil and a single pair of already formed cerata hatched from them all (Fig. 3 F–H). In contrast, observed egg masses of the real Z. pustulata at the Barents and White seas had considerably more eggs than in Z. ninel sp. nov. (compare Figs 3 A, B and 3 I, J) from which veligers with well-defined oval shells invariably hatched (Fig. 3K). At the southern coast of the Barents Sea Z. ninel sp. nov. and Z. pustulata are geographically sympatric, but ecologically and ontogenetically very different since Z. ninel inhabits exclusively the intertidal zone and develops through a crawling postlarvae, whereas Z. pustulata inhabits subtidally, deeper than 10 m, and develops through planktonic veligers. Specimens of Z. ninel sp. nov. and Z. pustulata from almost exactly the same locality (around cape Probny and Nemetskyi Island in the Dalne-Zelenetskay Bay) but taken from the intertidal and 12 m depth respectively are figured precisely at the time when they started to lay their very different egg mass (Fig. 3A for Z. ninel sp. nov. and Fig. 3I for Z. pustulata), highlighting this remarkable case of narrow niche differentiation in two apparently sympatric species.