Amphorina linensis (Garcia-Gomez, Cervera & Garcia, 1990)

Korshunova, Tatiana, Malmberg, Klas, Prkic, Jakov, Petani, Alen, Fletcher, Karin, Lundin, Kennet & Martynov, Alexander, 2020, Fine-scale species delimitation: speciation in process and periodic patterns in nudibranch diversity, ZooKeys 917, pp. 15-50 : 15

Amphorina linensis (Garcia-Gomez, Cervera & Garcia, 1990) Figs 1 , 2 , 3 , 4d-e , 7d

Eubranchus linensis Garcia-Gomez, Cervera & Garcia, 1990: 585-593.

Amphorina linensis (Garcia-Gomez, Cervera & Garcia, 1990): Martynov 1998: 775.

Eubranchus tricolor : sensu Trainito & Doneddu, 2014: 118, non Forbes, 1838.

Eubranchus sp. 1: Prkić et al., 2018: 353-357.

Material examined.

NE Atlantic, Skagerrak, Sweden, Västra Götalands län, Bohuslän, Väderöarna Islands (58°33'00"N, 11°02'30"E), 19 m depth, 09.04.2017, coll. Klas Malmberg, one specimen (GNM Gastropoda -9392, 10 mm in length, live, preserved length 4.2 mm). Mediterranean Sea, Croatia, Iž Island, Svežina (44°03'55"N, 15°07'15"E), 5 m depth, 13.01.2018, coll. A. Petani and Đani Iglić, two specimens (ZMMU Op-706, preserved length 6.5 mm, ZMMU Op-707, preserved length 6 mm).

Diagnosis.

Body up to 30 mm; dorsal spots, if present, reddish orange; in specimens with dorsal and ceratal spots distinct colouration of tail absent; completely pale specimens lack tail stripe or spot; light pinkish subapical ring on cerata absent; presence of distinct line of white pigment, sometimes punctuated, on the edge of the foot; cerata commonly broad with distinctly attenuated apices; digestive gland in cerata relatively thin without distinct short branches; up to six anterior rows of cerata; radular formula 38-61 × 1.1.1, copulative stylet relatively long, slightly bent at the middle, seminal receptacle elongate oval with moderate distinct stalk between reservoir and rapidly widened base.

Description.

External morphology. The length of adult specimens may reach 30 mm. The body is narrow. The rhinophores are smooth and 1.5-2 times longer than the oral tentacles. The cerata are relatively long, very broad, with distinctly attenuated apices. Ceratal formula of the specimen from Sweden (GNM 9392): right (2, 3, 3, 4; anus, 3, 2, 2, 1) left (2, 3, 4; anus, 3, 2, 2, 1). The foot is narrow, anteriorly without foot corners.

Colour. There are three main and eight subdivisions of colour variations (Fig. 3 ), from completely pale body and cerata without pigment spots to specimens with very distinct reddish orange pigment spots on the body. In the specimens with distinct dorsal spots, there is never any pigmentation on the tail. A distinct white line is present on the external edge of the foot, although this could be broken or punctuated. No specimens found with blackish non-transparent pigmentation on the body, nor any homogeneously orange specimens. Absence of light pinkish subapical ring on cerata. Small white pigment dots of various density can be present on the cerata. The upper part of the rhinophores is commonly covered with white relatively dense dots with small insertions of yellowish brown pigment in some specimens, without the formation of ring-shaped colouration. The oral tentacles are similarly coloured.

Anatomy. Digestive system (Fig. 4 , e3-e8). The jaws are triangularly ovoid. The masticatory processes of the jaws bear a single row of ca. 25 distinct denticles. The radular formula in the specimen studied from Croatia (Op-707) is 38 × 1.1.1. The radular teeth are yellowish. The central tooth is narrow, with a low cusp and four or five lateral denticles (three or less on anteriormost eroded teeth), including smaller intercalated denticles that may occur in different parts of the tooth.

Reproductive system. (Fig. 7D ). The ampulla is relatively small, not distinctly swollen (Fig. 7D , am). The prostate is distinct, relatively short and narrow (Fig. 7D , pr). The prostate transitions to a penial sheath, which contains a conical penis with a chitinous, very short, broadly conical stylet (Fig. 4 , e9, e10). A supplementary ( “penial”) gland is relatively long and inserts into the base of the penis (Fig. 7D , pg). The seminal receptacle is elongate oval (Fig. 7D , rs) with a distinct stalk between the reservoir and the widened base. The female gland mass includes mucous and capsular glands (Fig. 7D , fgm).

Distribution and habitats.

Mediterranean Sea and all European Atlantic coasts to Sweden and Southwest Norway. At the Swedish west coast, living below the halocline.

Remarks.

Morphologically A. linensis differs from A. farrani , A. viriola sp. nov., A. andra sp. nov., and A. pallida by having reddish orange and not orange-yellow pigment spots (in spotted forms), the presence of a distinct, sometimes dotted white line along the foot edge, the shape of the cerata with attenuated apices, and a small ampulla. The present materials are well consistent with the original description of A. linensis (Garcia-Gomez et al. 1990) in such key characters as the shape of the cerata, the presence of a distinct white dotted line along the foot and reddish spots in some specimens, and the shape of the seminal receptaculum and prostate, but there are some differences in the number of rows of the radula, most likely due to specimen size differences. Adriatic specimens differ from the Atlantic ones in smaller size and different colouration. There is often a light blue pigmentation that covers cerata, rhinophores and oral tentacles, partially or completely, often the whole animal has a bluish appearance. No specimens with dorsal reddish dots have been found so far at the Adriatic coast. Mediterranean specimens of A. linensis have been frequently misidentified in the literature as Eubranchus tricolor .

Minimum uncorrected p-distances of the COI marker which separate the A. linensis from A. viriola sp. nov., A. andra sp. nov., A. farrani , and A. pallida are 9.15%, 11.42%, 10.05%, and 13.70% respectively.

Figures

Figure 1. Phylogenetic relationships of nudibranchs Amphorina based on COI + 16 S + H 3 concatenated dataset inferred by Bayesian Inference (BI). The posterior probabilities from BI / bootstrap values for Maximum Likelihood (ML) are shown.

Figure 2. The haplotype network based on cytochrome c oxidase subunit I (COI) molecular data showing genetic mutations occurring within species of the genus Amphorina (A). Statistical test of the reliability of the bathymetric distribution patterns (and correlated with depths of brackish and marine environments) of A. viriola sp. nov. (red bar) and A. andra sp. nov. (blue bar) in Swedish waters (B). All specimens of A. viriola sp. nov. occur strictly in a very shallow brackish water layer above the halocline (salinity usually ca. 24 - 25 ‰), whereas in the same geographic region A. andra sp. nov. occur only below the halocline (at ca. 15 m depth) in waters with more stable oceanic salinity at 34 - 35 ‰.

Figure 3. Periodic-like presentation of the colouration variation among all species of the genus Amphorina, representing vertical rows. Three main periods (horizontal rows), each with several subperiods, are recognised according to the principle of spotless body / colourless forms at the bottom to forms with the maximal number of spots / coloured body at the top. Note that different species display fundamentally similar colouration forms, but not all, so some morphs in particular species (e. g., forms with extensive surface pigmentation and dark body in A. farrani, A. linensis, and A. pallida) can either be eventually discovered or do not exist, by some further constraints of the developmental system. Non-observed forms for each particular species are indicated as " unkn " = " unknown ").

Figure 4. Amphorina farrani (Alder & Han cock, 1844) (a-c), A. linensis (Garcia-Gomez, Cervera & Garcia, 1990) (d, e) and A. pallida (Alder & Hancock, 1842) (f-g). (a) A. farrani GNM 9268, UK, a 1, head; a 2, cerata; a 3, tail; a 4, posterior part of radula (SEM, scale bar 20 μm); a 5, posterior part of radula (10 μm); a 6, anterior part of radula (10 μm); a 7, jaw (light microscopy); a 8, jaw (SEM, 100 μm); a 9, jaw details (20 μm); a 10, details of stylet (3 μm); a 11, stylet (30 μm). (b) A. farrani, image from original description of Eolis farrani in Alder, Hancock, 1844 (not in copyright currently), b 1, head; b 2, tail with orange-yellow colouration; b 3, cerata. (c) A. farrani, France, Mediterranean, (external data - ZMMU Op- 702), c 1, head; c 2, tail; c 3, cerata; (internal data - GNM 9278), c 4, posterior part of radula (50 μm); c 5, anterior part of radula (20 μm); c 6, anterior part of radula (20 μm); c 7, jaw (100 μm). (d) A. linensis GNM 9392, Sweden, d 1, head; d 2, tail and cerata. (e) A. linensis ZMMU Op- 707, Mediterranean, Croatia, e 1, head; e 2, tail and cerata; e 3, posterior part of radula (20 μm); e 4, anterior part of radula (20 μm); e 5, anterior part of radula (20 μm); e 6, jaw (light microscopy); e 7, jaw (SEM, 200 μm); e 8, jaw details (50 μm); e 9, stylet details (10 μm); e 10, stylet (100 μm). (f) A. pallida ZMMU Op- 710, Norway, f 1, head; f 2, tail and cerata; f 3, radula (100 μm); f 4, stylet (100 μm); (g) A. pallida ZMMU Op- 712, Norway, g 1, head; g 2, tail and cerata; g 3, posterior part of radula (100 μm); g 4, anterior part of radula (30 μm); g 5, jaw (light microscopy); g 6, jaw (SEM, 100 μm); g 7, jaw details (20 μm).

Figure 7. Reproductive systems, schemes. (A) Amphorina farrani. (B) Amphorina viriola sp. nov.; (C) Amphorina andra sp. nov. (D) Amphorina linensis. (E) Amphorina pallida. Abbreviations: am ampulla, fgm female gland mass, pg penial gland, pr prostate, psh penial sheath, rs receptaculum seminis.