Nakamigawaia spiralis Kuroda & T. Habe, 1961

Description

A head-shield slug of the family Aglajidae. The body is cylindrical, with strongly reduced parapodial lobes; the posterior end of the mantle-shield terminates in short, symmetrical caudal lobes. The eyes are sunken and obscured externally, and the head-shield is shaped for burrowing in sand-mud bottoms. In life the entire dorsal and ventral surface is a glossy, deep purplish black; only the inner surfaces of the parapodia are whitish. Living animals reach about 35 mm in length (preserved specimens around 25 mm).

A solid, well-calcified internal shell is fully enclosed in the posterior part of the mantle-shield. The shell is coiled in about three whorls with a slightly raised spire and is conspicuously coiled rather than bulloid in outline, unlike the shells of most other aglajids.

Anatomically, the oral tube lacks labial glands, the pharynx is small but strongly muscular and rigid, and a radula is entirely absent. A large crop follows the oesophagus, and the stomach is wrapped by the liver mass. The species also lacks the sensory mound otherwise characteristic of the Aglajidae and the bristled mound seen on either side of the mouth in some related genera (Baba, 1985).

Distribution

Type locality: Zushi, Sagami Bay, Japan. The species has been recorded along the Japanese coast from Sagami Bay (including Kasajima) through the Inland Sea of Seto (Mukaishima area) to western Kyushu (Amakusa), and from the Mariana Islands (Guam and Pagan). It burrows in sand-mud bottoms from shallow shores down to the subtidal zone.

Etymology

The specific epithet spiralis is Latin for "spiral-shelled, coiled". The Japanese vernacular name "Uzumaki-kisewata" carries the same meaning.

Remarks

The genus Nakamigawaia was erected by Kuroda & Habe in Habe 1961 as a monotypic genus for this species. Hellem & Malaquias 2021 revised the genus and described a second Western Pacific species, N. nakanoae, so the genus now comprises three species together with the Western Atlantic N. felis (Er. Marcus & Ev. Marcus, 1970). N. spiralis differs from N. nakanoae in lacking the open-dilated shell of the latter, and from N. felis by subtle features of the shell, male reproductive system and caudal lobes.

The anatomical redescription by Baba 1985 showed that, although the external morphology resembles that of Philine and other aglajids, the digestive system confirms placement of Nakamigawaia within the Aglajidae.

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References

• Nakamigawaia spiralis Kuroda & Habe in Habe, 1961, Baba K. (1985). Anatomical review of a cephalaspidean mollusk, Nakamigawaia spiralis Kuroda & Habe in Habe, 1961 (Aglajidae), from Japan. Special Publication of the Mukaishima Marine Biological Station 1985: 1-5.
• クロボウズ（仮称）,シモフリボウズ（仮称）, 小野篤司. (1999). ウミウシガイドブック. TBSブリタニカ.
• クロボウズ, 鈴木敬宇. (2000). ウミウシガイドブック〈2〉. TBSブリタニカ.
• クロボウズ, 小野篤司. (2004). 沖縄のウミウシ. ラトルズ.
• クロボウズ, 中野理枝. (2004). 本州のウミウシ. ラトルズ.
• Nakamigawaia spiralis, Terrence Gosliner, Ángel Valdés and David Behrens. (2015). Nudibranch and Sea Slug Identification Indo-Pacific. New World Pubns Inc.
• Andrea Zamora-Silva and Manuel António E. Malaquias Molecular phylogeny of the Aglajidae head-shield sea slugs (Heterobranchia: Cephalaspidea): new evolutionary lineages revealed and proposal of a new classification Zoological Journal of the Linnean Society, 2017
• クロボウズ, 中野理枝. (2018). 日本のウミウシ. 文一総合出版.
• Nakamigawaia spiralis, Hellem E. & Malaquias M.A.E. (2021). The taxonomic status of the headshield slug genus Nakamigawaia Kuroda and Habe, 1961 (Gastropoda: Cephalaspidea: Aglajidae), with the description of a new species from the Western Pacific. Journal of Natural History. 55(35-36): 2231-2244. https://doi.org/10.1080/00222933.2021.1986165