Pleurobranchaea britannica, Turani & Carmona & Barry & Close & Bullimore & Cervera, 2024

Turani, Martina, Carmona, Leila, Barry, Peter J., Close, Hayden L., Bullimore, Ross & Cervera, Juan Lucas, 2024, First occurrence of the genus Pleurobranchaea Leue, 1813 (Pleurobranchida, Nudipleura, Heterobranchia) in British waters, with the description of a new species, Zoosystematics and Evolution 100 (1), pp. 49-59 : 49

Pleurobranchaea britannica sp. nov.

Material examined.

Holotype: NHMUK 20230085, 18 mm preserved length, (49°54'5.306"N, 6°45'7.056"W), southern England, 103 m depth, Apr 2019 . Paratypes: NHMUK 20230087, 19 mm preserved length, (49°35'59.389"N, 4°39'48.485"W) southwest England, 91.98 m depth, Mar 2018 ; NHMUK 20230086, 18 mm preserved length, (49°42'13.429"N, 4°6'28.514"W) southwest England, 81.12 m depth, Mar 2018 ; NHMUK 20230091, 22 mm preserved length, (50°5'10.929"N, 3°41'34.436"W) southwest England, 68.94 m depth, Mar 2018 ; MNCN 15.05/200180, 24 mm preserved length, (50°2'41.978"N, 4°3'33.805"W) southwest England, 75.69 m depth, Mar 2018 , dissected specimen; NHMUK 20230090, 18 mm preserved length, (49°54'5.306"N, 6°45'7.056"W), southwest England, 103 m depth, Apr 2019 ; NHMUK 20230089, 19 mm preserved length, (49°54'5.306"N, 6°45'7.056"W), southwest England, 103 m depth, Apr 2019 ; NHMUK 20230088/1, 20 mm preserved length, (49°54'5.306"N, 6°45'7.056"W), southwest England, 103 m depth, Apr 2019 , dissected specimen; MNCN 15.05/200181, 7 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 ; MNCN 15.05/200182, 8 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 ; MNCN 15.05/200183, 11 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 ; MNCN 15.05/200184, 10 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 ; MNCN 15.05/200185, 11 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 ; MNCN 15.05/200186, 9 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Feb 2020 . Additional material: MNCN 15.05/94837, 41 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Mar 2020 ; MNCN 15.05/94838, 43 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Mar 2020 ; MNCN 15.05/94839, 48 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Mar 2020 ; MNCN 15.05/94840, 42 mm preserved length, (36°16'19.56"N, 7°32'52.8"W) Gulf of Cadiz , 555 m depth, Mar 2020 .

Diagnosis.

Body oval, large, translucent with a minute cream/ochre pigmentation. Some specimens with opaque white specks irregularly spread all over mantle, oral veil, gill and posterior region of the foot not covered by the mantle. Rhinophores with dark spots on the front and white ones on the back. Gill bipinnate, with 15-18 pairs of pinnules and smooth rachis. Caudal spur absent. Outermost radular teeth bicuspid. Seminal receptacle short; bursa copulatrix at the end of the vagina and directly fused to it.

Description.

External morphology (Fig. 2 ). Body oval and large, with a rough mantle forming irregular polygons delimited by shallow grooves (Fig. 2A, B ). Base colour translucent with a minute cream to ochre pigmentation, which may not always be present. Opaque white specks might appear irregularly spread all over mantle, oral veil, gill and posterior region of the foot not covered by the mantle. Speckles density variable. Viscera partially visible through mantle in lighter individuals. Posterior part of foot round with no caudal spur (Fig. 2A ). Moreover, no pedal gland was observed. Sole patterned. Anterior part of rhinophores brown and posterior covered with close white dots (Fig. 2A ). Oral veil with trapezoid shape and fused with mantle where rhinophores are inserted. Veil front edge not smooth, but slightly irregular. Some specimens with series of white specks at veil corners. Gill located on the right side of the body, clearly visible and not covered by the mantle. Gill bipinnate, with 15-18 pairs of pinnules and smooth rachis. Gill with same base colour, white grains almost always present and variable in density, being visible on rachis and pinnules. Genital openings in front of gill and nephropore, clearly visible since it is covered by a circular fleshy papilla, which may have white dots. Anus opens above the 6th and 7th pinnule of the gill.

Internal anatomy (Figs 3 , 4 ) Radula almost rectangular with no rachidian teeth. Radular formulae are: 35 × (53-50).0.(53-50) (NHMUK 20230088/2); 33 × (54-56).0.(54-56) (MNCN15.05/200180) (Fig. 3A ). All teeth with two long and blade-shaped cusps. The outermost cusp is larger, while the one facing the centre of the radula is smaller and sometimes covered by the next tooth. Innermost teeth slightly more elongated and with finer tips, while outermost teeth with rounder tip (Fig. 3B ). Jaws elongated. Anterior part of jaw elements hexagonal, hand-shaped, with 4 to 9 denticles along anterior edge. The jaw elements have a depression in the middle (Fig. 3C, D ).

Reproductive system (Fig. 4 ) begins with the hermaphroditic duct which first widens into the ampulla and thereafter narrows and divides into two parts: one entering the prostate gland and the other continuing to oviduct. The prostate gland is composed of small and pyramidal-shaped papillae. Exiting from prostate, vas deferens entering the penial sac, anchored by a retractor muscle to the inner body wall. Inside the sac, penis relatively straight, with a couple of twists, but not coiled and apparently not cuticularised. The oviduct widens slightly forming a bilobed seminal receptacle, narrowing before entering laterally into the muscular vagina. Copulatory bursa spherical, placed at the distal end of vagina. Copulatory bursa not very muscular, its wall being delicate and thinner than the vagina’s. Female gland and vagina join laterally, very close to the female orifice. There are two different genital openings: the opening closest to the nephropore is the female one, the further one is male.

Etymology.

The species name in Latin refers to the British waters where this species was initially found.

Distribution.

The species has been found in a number of locations in the southwest of UK waters and the Gulf of Cadiz, see Fig. 1 , but we hypothesise that it could probably be distributed throughout the Atlantic coast of Spain, Portugal and France up to the southwest approaches to the English Channel.

Type locality and habitat.

South-western England (see Fig. 1A ). Collected from a range of depths (70-110 m) and a range of substrates that include areas of mosaic rock and mixed sediments and areas of muddier sediments.

Figures

Figure 1. Sampling stations where Pleurobranchaea britannica sp. nov. material was collected. A. The map on the left shows the south of England: the red dots refer to the 2018 campaign and the green dots to the 2019 campaign; B. The map on the right shows part of Spain and the orange dot is where samples were collected in 2019.

Figure 2. A. Living specimens of Pleurobranchaea britannica sp. nov. collected on Survey CEND 0518, southwest England. Photo by Ross Bullimore (NHMUK 20230085); B. Two young individuals of P. britannica sp. nov. from the Gulf of Cadiz, Spain (MNCN 15.05 / 200181; MNCN 15.05 / 200182); C. Specimen of P. meckeli from Morocco, Mediterranean Sea (MNCN 15.05 / 94845).

Figure 3. Pleurobranchaea britannica sp. nov. Scanning electron micrographs of radula and jaw. A. Complete radula (MNCN 15.05 / 200180); B. Lateral teeth of radula (MNCN 15.05 / 200180); C. View from above of the anterior part of the jaw (MNCN 15.05 / 200180); D. Lateral view of the anterior part of the jaw (MNCN 15.05 / 200180).

Figure 4. Pleurobranchaea britannica sp. nov. Reproductive system (NHMUK 20230088 / 5). Abbreviations: a - atrium; am - ampulla; bc - bursa copulatrix; dd - deferent duct; fg - female gland; hd - hermaphroditic duct; ov - oviduct; p - penis; pr - prostate; r - retractor muscle; ps - penis sac; sr - seminal receptacle; v - vagina.