Plakobranchus papua Meyers-Munoz & van der Velde

Meyers-Munoz, Maria Angelica, van der Velde, Gerard, van der Meij, Sancia E. T., Stoffels, Bart E. M. W., van Alen, Theo, Tuti, Yosephine & Hoeksema, Bert W., 2016, The phylogenetic position of a new species of Plakobranchus from West Papua, Indonesia (Mollusca, Opisthobranchia, Sacoglossa), ZooKeys 594, pp. 73-98 : 78-81

Taxon classification Animalia Sacoglossa Plakobranchidae

Plakobranchus papua Meyers-Munoz & van der Velde sp. n. Figures 2, 3, 4, 5, 6, 7a

Type material.

Holotype RMNH MOL.336417, length 35 mm (COI, anatomy. Genbank Accession number: KU934191). Paratype RMNH MOL.336418, length 30 mm (COI, anatomy, radula. Genbank Accession number: KU934192). Paratype RMNH MOL.336419, length 34 mm (COI, anatomy. Genbank Accession number: KU934193).

Additional material.

RMNH MOL.336420, length 28 mm (anatomy, radula). RMNH MOL.336421, length 25 mm (anatomy, radula, penial bulb). RMNH MOL.5010422-5010434, slides of a single specimen, length 30 mm, northeast of Pulau Mansuar (S00°34.083', E130°38.525), Expedition Sta. RAJ.38, 30 November 2007 (histology). RMNH MOL.336423 (n = 7), length 15-26 mm, east side of Pulau Kri, Sorido Resort lagoon, near Jetty (S00°33.347', E130°41.225), Expedition Sta. RAJ.02, 4 December 2007 (anatomy, penial bulb). RMNH MOL.336424, length 41 mm, south side of Pulau Kri (S00°33.536', E130°41.258'), Expedition Sta. RAJ.03, 4 December 2007 (anatomy, penial bulb). RMNH MOL.336425 (n = 6), length 15-26 mm, west side of Pulau Yeben Kecil (S00°29.344', E130°30.081'), Expedition Sta. RAJ.48, 6 December 2007 (anatomy, radula).

Type locality.

Mangrove Creek, south Gam Island (0°30'403"S, 130°38'986"E), West Papua, Indonesia, 25 November 2007.

Habitat.

The specimens were collected in sea grass beds and on coral reef sand between 1 and 10 m depth.

Etymology.

The specific name papua of this species is based on the name of the Indonesian part of New Guinea (provinces Papua and West Papua) where the type material was collected.

Description.

External morphology (Figure 3). Body elongated, dorsoventrally flattened with wide parapodial flaps folding along the dorsal midline (Figure 3a). Rhinophores extended from lateral edges of the head long, smooth, rolled (Figure 3b). An eye pair belonging to the central nervous system (CNS) is visible on the head. Eyes situated very close to each other, in front of a prominent, elongated white spot (Figure 3c). The voluminous renopericardial prominence is short, whitish (Figure 3d), and covered by the parapodial flaps. The folded parapodia show a smooth surface from a dorsal view. The parapodia margin at the mid-line shows short yellow rod-like spots along the complete parapodial border (Figure 3a) such as those visible in ' Plakobranchus ocellatus ' ( Wägele et al. 2011: fig. 1a). When the parapodia are open, the edges appear as soft yellow pectinate margins (Figure 3d). Internally, the parapodia have thick parallel longitudinal lamellae. Only the mid-central lamellae connect directly to the renopericardial prominence and run parallel to each other towards the posterior side of the body. The most external anterior lamellae are shorter, connected through anastomosing lamellae with the most internal lamellae and through them connected with the renopericardial prominence (Figure 3d). These fine dorsal anastomosing lamellae can only be recognized in live and in freshly collected, non-preserved specimens. In preserved contracted specimens it is not possible to distinguish these dorsal anastomosing lamellae. The lamellae possess visible internal granulations corresponding to the digestive gland ramifications, which contain chloroplasts. The genital opening is situated on the right anterior side of the body, just in front of the anterior part of the parapodia, and immediately behind the rhinophore. The anal opening is located on the right anterior side of the pericardium. Ventrally a bilobed oral prominence (Figure 3e) with a very fine, undulating black line boarding the upper lip is present. Pedal tentacles are short (Figure 3c). The narrow foot has a smooth surface and truncated tail (Figure 3e).

Colouration (Figure 3). Intense ochre body with white spots scattered all over the dorsum, head, and flanks: closest to the head region, on the anterior side corresponding to 1/4 of the body length, the spots are largest, on 2/4 and 4/4 of the body length the dots are smaller, and on the 3/4 of the body length they are larger, but not as large as on the anterior part of the body. Spots are pale white with a thin yellow outline (Figure 7a). Rhinophores black, the outer tips purplish. Internal parapodial flaps ridges bright green to olive green in colour, owing to chloroplasts in the digestive gland. The pericardium region is hyaline white. The pedal tentacles are translucent white. The foot sole is white with a black tail.

Digestive system (Figure 4). The masticatory apparatus was studied in four preserved specimens (RMNH.MOL.336418, 336420, 336421, 336425). The pharynx is connected to the stomach through a short muscular lightly bent oesophagus. The stomach is approximately 25% smaller than the pharynx. A pair of salivary glands inserted and extended along the oesophagus, reaching the first third of the stomach. The pharynx has prominent dorsal septate muscles. It consists of a large cuticular pharyngeal cavity, the radular sac, and one large ascus, which are also embedded in cuticular material. Uniserial radula, small, approximately 0.50 to 0.75 mm in length with 15 teeth, eight in the ascending series and seven in the descending series (Figure 4b), and an ascus-sac which varies in size (Figures 4b, c). Teeth are sharp with triangular cusps and 10-14 denticles at each margin side, the teeth measuring 70-75 μm (Figure 4 d–g).

Reproductive system (Figures 5-6). Occupies almost the central anterior part of the body, at nearly 1/4 of the body length. The distal part of the reproductive system is situated below the heart and directly behind the central nervous system. The penial bulb, which is only approximately 0.5 mm long (Figure 5: p), is situated below the rhinophores at the same level as the eyes. It possesses a sharp cuticular stylet, which is hollow, with an oblique orifice at the tip (Figure 6). The vas deferens (Figure 5: vd) bends over the mucus gland (Figure 5: m), and is orientated towards the proximal area. The distal part of the vas deferens, which is in direct connection with the penial bulb, is muscular and arched. Its proximal part is thin and coiled and is connected to two spherical genital vesicles (Figure 5: gv). After this intersection the vas deferens continues in a proximal direction, where it appears as a short bulky extension, continuing in a thin duct attached to numerous ramifications, the follicles. Underneath this vesicle one large piriform white genital receptacle is present (Figure 5: gr), connected directly to the mucus gland. The mucus gland is large and divided in two lobes. The distal lobe is smaller, a little narrow and coiled. The prostate (Figure 5: pr) and albumen glands (Figure 5: ag) are extensively branched. The terminology used here is similar to the one in the schematic drawing of the reproductive system of Plakobranchus ocellatus by Jensen (1992: fig. 22B).

Central nervous system. The central nervous system is located at the anterior part of the oesophagus and forms a circumoesophageal ring, consisting of a very small pair of buccal ganglia, a large pair of fused cerebro-pleural ganglia, and a pair of pedal ganglia. The CNS consists for the main part of the cerebral and pleural ganglia. The eyes of the CNS are situated very close to each other.

DNA analyses (Figure 8). The molecular phylogeny reconstruction of the genus Plakobranchus contains all the currently available sequences on GenBank. The analyses in MrBayes (Bayesian inference) and Phyml (maximum likelihood) resulted in trees in which the specimens were assigned to the same clades. In Phyml the tree formed a polytomy, whereas the analyses in MrBayes showed a topology with highly resolved clades. Ten different clades can be distinguished, of which three are represented by single specimens. The largest clade contains 49 sequences. Our new species groups with Plakobranchus sp. 1 of Krug et al. (2013) from Sulawesi, Indonesia and Panglao, Philippines. Over 580 base pairs there is a difference of 1.2% (7 bp) between our specimens and the specimens of Plakobranchus sp. 1 from Krug et al. (2013).

The ABGD analysis resulted in prior maximal intraspecific divergence of ca. 0.07. Values higher than the maximal intraspecific divergence resulted in 10 Molecular Operational Taxonomic Units (MOTUs) in both the recursive and initial partition. Each of these MOTU’s corresponds to a clade in the phylogeny reconstruction (Figure 8). The three singletons (GQ996679, KC573731, KC573734) from the ABGD analysis were also retrieved as singletons in the molecular phylogeny.