Gastropteron minutum Ong & Gosliner, 2017

Description

An extremely small gastropterid reaching up to 2–3 mm in length, the smallest known species of Gastropteron. The living animal has a ground colour of off-white, almost colourless, with opaque white and orange mottling speckling the body. Orange and off-white spots condense around the visceral hump to form low, rounded tubercles that cover the parapodia. Head shield and flagellum have fewer spots and are almost clear.
The head shield is narrow anteriorly, broadens centrally, and then narrows again posteriorly. Posteriorly, the head shield forms a simple rolled siphon without a medial crest. Parapodia cover the visceral hump when retracted. At the posterior end of the visceral hump, an elongate, acutely pointed flagellum extends posteriorly. The foot extends almost the entire length of the animal from head to end of visceral hump, narrowing at the posterior end into an elongate, acutely pointed posterior tip. The gill is situated on the right side of the body and consists of seven tiny lamellae.
The shell is thinly calcified, planispiral, tightly coiled, c. 300 µm in diameter. The protoconch is not clearly differentiated from the remainder of the shell. The radular formula is 20 × (4·1·0·1·4) in one specimen. Inner lateral teeth broad with single primary cusp. Masticatory margin of inner laterals containing 13–14 acutely pointed denticles. The penis is simple with slightly curved prostate. Penial papilla straight and devoid of penial armature.

Distribution

Type locality: Honokowai Beach Park, Maui, Hawaiian Islands. Known from Japan, the Hawaiian Islands, the Marshall Islands and the Philippines.

Etymology

Verbatim from the original description (Ong et al., 2017, p.761):

The name minutum is derived from the extremely small size of this species, reaching a maximum of 3 mm in length.

Remarks

Originally described as Gastropteron minutum Ong & Gosliner, 2017 in the Zoological Journal of the Linnean Society. This species corresponds to specimens previously figured as Gastropteron sp. 4 (Rudman, 2002) and Gastropteron sp. 5 (Gosliner, Valdés & Behrens, 2015).
The off-white body colour with opaque white and orange spots clearly differentiates this species from all other described species of Gastropteron. The presence of opaque white spots and rounded tubercles gives the entire animal a granular rather than smooth appearance, in contrast to the smooth body of most congeners. Phylogenetically, G. minutum is sister to Gastropteron rubrum (Rafinesque, 1814) from the Mediterranean and West Africa, and is morphologically and genetically quite distinct from G. bicornutum Baba & Tokioka, 1965, the only Pacific species with an elongate foot extension (COI divergence > 22 %).

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References

• Gastropteron sp. 4 (Rudman 2002, Sea Slug Forum), Rudman W.B. (1998-2010). Sea Slug Forum. Australian Museum, Sydney. http://www.seaslugforum.net/
• ヤマトウミコチョウ属の1種 2, 小野篤司. (2004). 沖縄のウミウシ. ラトルズ.
• Gastropteridae sp. 3, 中野理枝. (2004). 本州のウミウシ. ラトルズ.
• マメツブウミコチョウ（新称）, 中野理枝, 今川郁 & 今本淳. (2013). 南西諸島の後鰓類1. 頭楯亜目. ちりぼたん. 43(1-4): 1-22.
• Gastropteron sp. 5, Terrence Gosliner, Ángel Valdés and David Behrens. (2015). Nudibranch and Sea Slug Identification Indo-Pacific. New World Pubns Inc.
• Gastropteron minutum sp. nov., Ong E., Hallas J.M. & Gosliner T.M. (2017). Like a bat out of heaven: the phylogeny and diversity of the bat-winged slugs (Heterobranchia: Gastropteridae). Zoological Journal of the Linnean Society. 180(4): 755-789. https://doi.org/10.1093/zoolinnean/zlw018