Siphopteron fuscum (Baba & Tokioka, 1965)

Description

A tiny gastropterid, about 4 mm in length when fully extended. The body is very narrow with a long tapering tail. The cephalic shield is elongate-rectangular, posteriorly forming a funnel supported by a dorso-median crest; paired black eyes lie in the middle of the shield. The dorsal hump is elongate-oval with smooth integument. A single long horn-like protuberance arises near the rear right of the dorsal hump. The parapodial lobes are freely expandable, with a long tapering tail. The free mantle margin is very short, and the ctenidium is rudimentary.

In life: dorsal hump deep chocolate brown sparsely spotted with white; lower parapodia deep chocolate brown with bold irregular white markings toward the free margin; horn-like protuberance vermilion distally; narrow vermilion band along the parapodial margin and tail edge; a transverse vermilion band on the cephalic shield, with paired longitudinal vermilion bands converging distally and holding a snow-white streak between; lateral edges of the shield deep chocolate; funnel margin vermilion; sole and tail underside translucent whitish.

Distribution

Type locality: Ayukawa, Etizen coast (Japan Sea side of Honshu, Fukui Pref.), 2 m depth.

Etymology

The specific epithet fuscum is from Latin fuscus ("brown"), in reference to the dominant chocolate-brown coloration of the dorsal hump and parapodial undersides.

Remarks

Originally placed in Gastropteron, later transferred to Siphopteron. Most similar to Siphopteron flavum: both share a short free mantle margin, reduced ctenidium, and a single dorsal-hump protuberance. S. fuscum is distinguished by (1) a sharp horn-like (not blunt mammiform) protuberance and (2) chocolate-brown (not lemon-yellow) body colour. The diagnostic colour characters apply only to live specimens; preserved animals are visually indistinguishable.

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References

• Ayukawa-umikotyo, Baba K. & Tokioka T. (1965). TWO MORE NEW SPECIES OF GASTROPTERON FROM JAPAN, WITH FURTHER NOTES ON G. FLAVUM T. & B. (GASTROPODA : OPISTHOBRANCHIA). Publications of the Seto Marine Biological Laboratory. 12(5): 363-378. https://doi.org/10.5134/175379
• アユカワウミコチョウ, 小野篤司. (1999). ウミウシガイドブック. TBSブリタニカ.
• 高岡生物研究会. (2002). 日本海のウミウシ. 第2版.
• アユカワウミコチョウ, 小野篤司. (2004). 沖縄のウミウシ. ラトルズ.
• アユカワウミコチョウ, 中野理枝. (2004). 本州のウミウシ. ラトルズ.
• アユカワウミコチョウ, 小野篤司 & 加藤昌一. (2009). ウミウシ. 誠文堂新光社.
• Siphopteron fuscum, Terrence Gosliner, Ángel Valdés and David Behrens. (2015). Nudibranch and Sea Slug Identification Indo-Pacific. New World Pubns Inc.