Siphopteron dumbo Ong & Gosliner, 2017

Location: Yaene, Hachijo Island, Tokyo, Japan
Date: 2018/05/15
Length: 8mm
Depth: 15.0m
Water temperature: 23.0℃

Description

A small siphopteran 3 mm in length. Parapodia, head shield and foot pale yellowish over entire surface. Head shield and parapodia with pale blue marginal line. Additional irregularly shaped pale blue network of interconnected squiggles present on parapodia and head shield. Pale blue pigment also found on sides and centre of head shield. Pale blue also present as medial line on posterodorsal portion of foot.
Head shield roughly triangular, tapering posteriorly to siphon. Siphon with medial white ridge along posterior edge, extending to well-elevated apex that is much higher than lateral margins. Two black lines found on posterior end of head and lateral margins of siphon. Dorsally, black lines merging on to siphonal ridge and continuing to apex. Additional black lines also found on dorsal margins of siphon. Flagellum situated to right of midline on visceral hump. Flagellum elongate and acutely pointed. Scattered black pigment found at the base of flagellum. Outer two-thirds of flagellum with solid black pigment. Gill very small with two simple folds.
Shell: No trace of a shell found in specimen dissected. Radular formula 15 × (4·1·0·1·4) in one specimen (NMP 041187). Inner lateral teeth broad with single primary cusp. Masticatory margin of inner laterals broad with large triangular denticle flanked on either side by series of crowded smaller denticles. The penis is complex with short, curved prostate. Penial sac straight, containing arc of 34 small spines. Secondary papilla found at the anterior end of penial sac with fleshy blunt apex devoid of armature.

Distribution

Type locality: La Laguna, Puerto Galera, Mindoro Oriental, Philippines, at 20 m depth. Thus far, known only from the Philippines and probably Japan (Ono, 1999).

Etymology

Verbatim from the original description (Ong et al., 2017, p.780):

The species epithet, dumbo, refers to the similarity of this species to the Disney character, Dumbo the elephant, as it swims through the water.

Remarks

Originally described as Siphopteron dumbo Ong & Gosliner, 2017. What appears to be Siphopteron dumbo has been confused previously with S. flavum (Rudman, 1999, Sea Slug Forum: lower photo).
Siphopteron dumbo has a colour pattern similar to several other species that have a yellowish ground colour: S. flavum (Tokioka & Baba, 1964), Siphopteron citrinum (Carlson & Hoff, 1974), Siphopteron brunneomarginatum (Carlson & Hoff, 1974) and S. nigromarginatum. S. flavum, originally described from Japan, has a bright yellow body with black pigment on the siphon and flagellum. The black pigment on the siphon is found along the posterior margins and on the entire apex and anterior face of the siphon in S. dumbo. The pigment is a much finer line only on the posterior side of the siphon. In addition, S. flavum has a short apex to the siphonal crest, whereas it is elongate in S. dumbo. The flagellum is short and blunt and situated medially in S. flavum, whereas it is elongate and located well to the right of the midline in S. dumbo.
Internally, there are differences between the two species. The radula of S. flavum has an even row of small denticles on the inner lateral tooth and only three rows of outer lateral teeth. In S. dumbo, the inner lateral tooth has a prominent large triangular denticle with smaller denticles flanking either side and has four rows of outer lateral teeth. Both species have a similar penial morphology with an arch of small penial spines, but with several key differences. In S. flavum, the secondary penial papilla is large and pointed, whereas in S. dumbo it is short and blunt. The prostate is more elongate and curved in S. flavum than in S. dumbo. Siphopteron citrinum has a pink body colour with yellow spots and patches on the head shield, posterior shield and parapodia, and has black pigment on the very tip of the siphon and on the entire flagellum.