Siphopteron brunneomarginatum (Carlson & Hoff, 1974)

Description

A small cephalaspidean reaching only 3–5 mm in length. The body is uniformly light greenish yellow, with chocolate-brown lines running along the margins of the parapodia onto the foot, as a transverse band across the visceral hump, and onto the flagellum (when present) and the apex of the siphon.
The head shield is short and triangular, with an involuted siphon at its posterodorsal end; the siphon bears a medial crest whose apex is slightly elevated above the rest. The visceral hump is elongate and posteriorly rounded. An elongate, conical flagellum is present on the right side of the visceral hump in most individuals, but is occasionally absent. The gill is small and consists of only 3 or 4 simple plicae.
The radular formula is 23 × 3-1:0-1-3. The inner lateral teeth bear 5–7 small, irregular denticles along the inner margin of the elongate central cusp; the outer laterals are narrower and lack denticles.
Among congeners that also lack chitinous spines on the penial bulb (S. citrinum, S. pohnpei), S. brunneomarginatum is uniquely distinguished by its dark-brown parapodial margins.

Distribution

Type locality: Guam. Also recorded from Enewetak Atoll, Papua New Guinea, the Philippines, Taiwan, Japan, the Marshall Islands, South Africa and Tanzania. Specimens have been collected on the underside of coral rubble in shallow reefs; the species has not been observed to swim.

Etymology

From Latin brunneus (brown) and marginatus (margined / edged), referring to the dark-brown lines bordering the parapodia.

Remarks

Originally described as Gastropteron brunneomarginatum Carlson & Hoff, 1974 from Guam, and transferred to Siphopteron by a 1989 revision in his revision of the Gastropteridae, yielding the current combination Siphopteron brunneomarginatum (Carlson & Hoff, 1974).
The Japanese vernacular name "オレンジウミコチョウ" was later proposed, but the accompanying figure shows a clearly orange animal, suggesting it may not represent this taxon (whose ground colour is pale greenish yellow).

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References

• Gastropteron brunneomarginatum n.sp., Carlson C.H. & Hoff P.J. (1974). The Gastropteridae of Guam, with descriptions of four new species (Opisthobranchia: Cephalaspidea). Publications of the Seto Marine Biological Laboratory. 21(5/6): 345-363.
• Siphopteron brunneomarginatum (Carlson & Hoff, 1974) comb. nov., Gosliner T.M. (1989). Revision of the Gastropteridae (Opisthobranchia: Cephalaspidea) with descriptions of a new genus and six new species. The Veliger 32(4): 333-381.
• オレンジウミコチョウ (新称: orange海胡蝶), Fukuda H., 1994. Marine Gastropoda (Mollusca) of the Ogasawara Islands (Bonin) Islands. Part 2 : Neogastropoda, Heterobranchia and fossil species, with faunal accounts. Ogasawara Research 20, 1-126
• オレンジウミコチョウ, 小野篤司. (2004). 沖縄のウミウシ. ラトルズ.
• オレンジウミコチョウ, 中野理枝. (2004). 本州のウミウシ. ラトルズ.
• オレンジウミコチョウ, 中野理枝. (2018). 日本のウミウシ. 文一総合出版.
• Siphopteron brunneomarginatum, Terrence Gosliner, Ángel Valdés and David Behrens. (2018). Nudibranch and Sea Slug Identification Indo-Pacific 2nd Edition. New World Pubns Inc.