On the genus Armina (Gastropoda: Heterobranchia: Nudibranchia) in Thailand

Abstract

The marine sea slug heterobranch fauna from Thailand is composed of a total of 240 species, and Armina semperi is the only representative of the genus Armina recorded in the country. This species, described in the Philippines in the nineteenth century, has an uneasy taxonomic history and appears to be part of a complex of morphologically similar species, which has made its real identity difficult to determine. Comparison of samples of the genus Armina from Thailand with the original description of A. semperi showed that they were different. Furthermore, additional sampling allowed the discovery of a second species of the genus, similar to the so-called A. semperi and living in sympatry with it. In this paper, the taxonomic history of A. semperi is summarized and both species from Thailand are described as new to science. Armina scotti n. sp. is characterized by its black dorsum with pale yellowish notal ridges, widened forming three rounded blotches, not divergent posteriorly, not bifurcated; oral veil white, with a pale blue submarginal band and yellow edge; white rhinophores with a black subterminal band, which is pale blue on the inner-front sides, orange tips, and a black stripe running to the sides of the oral veil. The cutting edge of the jaws with up to four rows of denticles; radular formula 28 x 20.1.1.1.20; rachidian tooth with 5–6 strong denticles on each side of the median cusp; lateral teeth denticulate to smooth. Hermaphrodite duct very thin. Ampulla very long and wide; bursa copulatrix absent; prostate not differentiated; and penis proportionally very large and unarmed. Armina occulta n. sp. is characterized by its black dorsum with white notum narrowed forming two transverse dark bands, divergent and bifurcated posteriorly; orange mantle edge; black foot with a thick marginal band of deep blue; rhinophores deep blue with yellow tips, and a black subterminal band. The radular formula is 50 x 29.1.1.29; broad rachidian teeth, bearing 4–6 strong denticles only on the left side of the median cusp; a very thin hermaphrodite duct; very long and wide ampulla; undifferentiated prostate; large bursa copulatrix; and a penis bearing a stylet.