Swennenia jabae ( Swennen, 2001 ) Buatip & Tan, 2020

Buatip, Somsak & Tan, Siong Kiat, 2020, A new genus Swennenia for Gascoignella jabae Swennen, 2001, a small rare mangrove dwelling sea slug in the Gulf of Thailand, with notes on the species (Gastropoda: Sacoglossa: Plakobranchoidea), Raffles Bulletin of Zoology 68, pp. 629-635 : 630-633

publication ID	https://doi.org/ 10.26107/RBZ-2020-0078
publication LSID	lsid:zoobank.org:pub:92F62756-C89F-415C-AAEE-40C76B82FA6C
persistent identifier	https://treatment.plazi.org/id/EA4187E1-FFAD-9E6D-D3C3-F8C57462474D
treatment provided by	Diego
scientific name	Swennenia jabae ( Swennen, 2001 )
status	comb. nov.

Treatment

Swennenia jabae ( Swennen, 2001) View in CoL , new combination

( Figs. 2–6 View Fig View Fig View Fig View Fig View Fig )

Gascoignella jabae Swennen, 2001: 78 , fig. 2; Jensen, 2007: 278; Kohnert et al., 2013: 584, 600; Krug et al., 2018: 704, fig. 2.

“ Gascoignella View in CoL ” jabae – Krug et al., 2015: 996, 997, fig. 3; Filho et al., 2019: 543, 544, 553, 564, fig. 3.

Material examined. 5 exx. (2 specimens dissected, not preserved; 1 specimen sent to Dr Patrick J. Krug for DNA studies; 2 specimens kept for biological observations eventually died, not preserved), in mangroves, Ban Bang Ta Wa , Pattani, Thailand, coll. October 2008 ; 1 ex. (dissected, not preserved), mangroves, Ban Bana , Pattani, Thailand, coll. June 2014 ; 2 exx. (dissected, not preserved), same locality data as previous, coll. July 2015 .

Live colouration. Slug translucent yellow with a small fine patch of light brown pigment on the frontal part of the head, and a large dark brown to black transverse patch over the head, with frontal open areas for the eyes, that extend longitudinally along the lateral sides of the notum. There is a similarly coloured longitudinal band over the central part of the notum up to the posterior end where it merges with the lateral bands. The cerata also bear a dark band on the dorsal side. Dark green digestive gland discernible under the skin of the body, along the body to the tips of the cerata ( Fig. 2 View Fig ).

External characteristics. Slender, rather flat, smooth slug. Total length, including cerata, up to 10 mm (adult specimens 6–10 mm). Body length about 3–4 times its width. Head with a small velum, without rhinophores or oral tentacles. Posterior end blunt, with two cerata maintained in the rearward-facing position as extensions of the body. The cerata, typically with a length that is around or more than half of the body, are rather stiff, slightly contractible and have blunt tips ( Figs. 2 View Fig , 3A View Fig , 5 View Fig ).

Anus on the dorsum, right of the pericardium. Renal pore on the left side of the pericardium. Genital apertures triaulic, with penal just behind and positioned much lower than the right eye, oviducal shortly posterior of the penal, and vaginal aperture more posterior and more dorsal in position than oviducal ( Fig. 3A View Fig ).

Internal characteristics. Eyes nearly spherical, with clear lens ( Fig. 3C View Fig ). In a large specimen (total length of body with cerata 9.8 mm) eye height 100 µ m, width 95 µ m; in a small specimen (total length 6.5 mm) eye height 63 µm,

width 60 µm. Oral tube below the eyes. Pharynx situated just posterior of the eyes, slender, length 242 µm, height 133 µm ( Fig. 3B View Fig ). Radula with 6 teeth in ascending and 5 teeth in descending row, about 20 partly broken teeth in ascus ( Fig. 3B View Fig ). Teeth length 75–86 µm, basal parts 34–49 µm with large articulation knobs ( Fig. 3E View Fig ).

Oesophagus goes straight to the stomach, from which the green digestive gland branches on both lateral sides, both further branching in the anterior and posterior directions, the latter to the posteriormost end of the body where they go via a constriction that is not green, into a ceras where the branch widens and is green again ( Fig. 3F View Fig ). The intestine runs from the dorsal side of the stomach to the anus ( Fig. 3F View Fig ). Heart and kidney in posterior half of body, anterior of anus ( Fig. 4A View Fig ).

Reproductive system. The hermaphrodite follicles are in the posterior part of the body on both sides with an open area in between ( Fig. 4B, F View Fig ). It may locally have a partial septum ( Fig. 4B, C View Fig ). The specimen examined has 52 follicles ( Fig. 4E, F View Fig ) with a mean length of 225.9 ± 16.4 µm (194–245 µm) and mean width of 160.1 ± 27.0 µm (111–199 µm). Hermaphrodite ductules unite into a main duct, a widening or ampulla is not noted. The duct splits into a short oviduct and vas deferens. The vas deferens receive the prostate duct shortly after the split. The duct is connected with two large prostate glands each consisting of two parts, the part connected with the duct has small, pale globules and the distal part has large, white globules ( Fig. 3G View Fig ). The penis has a small, curved spine with a width at the tip of about 3 µm ( Figs. 3D View Fig , 4D View Fig ). Short oviduct goes to the central canal that receives the albumen duct which is connected with the branches of the albumen gland ( Fig. 4F View Fig ), which closely join the distal branches of the digestive gland that extends from the stomach into both cerata.

The vagina opens via a narrow tube, enters a seminal receptacle and goes into the central canal ( Fig. 3G View Fig ). In this area an indeterminate gland was found, but for which the connection could not be established. A genital receptacle originates from the central canal. The other female glands (capsule and mucus glands) could not be separated. They envelop the central reproductive organs and connect with the oviducal aperture.

Ecology and biology. Based on collections thus far, this small slug seems to be restricted to areas in the mangroves that are only occasionally inundated during the highest tides at certain times of the year. Hence, they appear to be capable of long periods of dormancy, hiding most of the time inside stiff clayey mud. Their apparent association with the algae Boodleopsis cf. pusilla , suggest that it is possibly their primary food source. In the field no specimens were seen in the open during low tide. It is not known if the animals only appear when the ground is submersed or wet during high tides.

The two specimens kept for observations did not hide and were seen seemingly sucking on the algal threads. Black faecal matter was expelled in thin strings from the anus ( Fig. 5A View Fig ). In one individual that had lost a ceras during initial handling, the regeneration of the lost ceras is apparent after two days ( Fig. 5B View Fig ). Egg strings were produced overnight; the narrow strings found in a knot on the algae. Egg capsules were packed in single row, arranged spirally in the string ( Fig. 6A, B View Fig ). Fresh ova yellow, with one ovum per capsule, un-cleaved spherical ova with a mean diameter of 54 µm, SD ± 3 (N=8). Capsule size 120 ± 9 mm × 98 ± 8 mm (N=10). Veligers hatched after 4 days in a mean temperature of 28°C ( Fig. 6C View Fig ).