Thuridilla cf. gracilis (Risbec, 1928)

Mehrotra, Rahul, A. Caballer Gutierrez, Manuel, M. Scott, Chad, Arnold, Spencer, Monchanin, Coline, Viyakarn, Voranop & Chavanich, Suchana, 2021, An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters, ZooKeys 1042, pp. 73-188 : 73

Thuridilla cf. gracilis (Risbec, 1928) Figure 6C

Material examined.

1 specimen 18 mm, HWB; 2 specimens 8-15 mm, LB.

Ecology.

Found in coral reef habitats throughout the island. Depth 2-25 m.

Distribution.

Thuridilla gracilis sensu lato is known from Maldives, Seychelles ( Yonow 2012), India ( Apte 2009), Singapore ( Jensen 2009), Myanmar ( Sanpanich and Duangdee 2019), Indonesia ( Yonow and Jensen 2018), Vietnam ( Martynov and Korshunova 2012), Taiwan ( Huang et al. 2016), Japan ( Trowbridge et al. 2011), Australia ( Nimbs and Smith 2016), Madagascar, Malaysia, the Philippines, Papua New Guinea, Palau, New Caledonia, Guam, and Fiji ( Gosliner et al. 2008). Known from the Andaman and Gulf waters of Thailand ( Jensen 1992; Nabhitabhata 2009).

Remarks.

The taxonomic status of Thuridilla gracilis is at present unclear. Recent works ( Yonow and Jensen 2018; Papu et al. 2020) have highlighted the significant variability of specimens ascribed to the species and questioned the synonymisation of multiple white-striped species of Thuridilla ( Gosliner 1995; Rudman 2000b) under the single species T. gracilis . Indications suggest that the breadth of specimens currently considered as T. gracilis encompass a complex of species ( Händeler and Wägele 2007; Yonow and Jensen 2018; Papu et al. 2020) that requires a comprehensive analysis utilising both morphological and molecular means. Specimens from Koh Tao do not possess any blue markings, instead being closer to the original description and illustration of T. gracilis in possessing a thin orange-red marginal band along the parapodia that meets medially and having the white lines often extending to the tips of the rhinophores. Papu et al. (2020) provided photographs highlighting the external variability on the heads of specimens from Bangka, Indonesia suggesting possible diagnostic value. As such, we have emulated this and provided the range of variation visible in the heads of adult specimens from Koh Tao (Fig. 5 ).

Figures

Figure 5. Variation in the heads of Thuridilla cf. gracilis at Koh Tao. Specimens 14 mm (A), 18 mm (B), 9 mm (C), 16 mm (D).

Figure 6. A Elysia pusilla 7 mm B Elysia cf. tomentosa 37 mm C Thuridilla cf. gracilis 18 mm D Colinatys sp. 3 mm E Aliculastrum cylindricum 30 mm F Aliculastrum debilis 28 mm G A. debilis (shell 12 mm) captured alive and predated by Haptosquilla cf. nefanda H Roxaniella multistriata 9 mm I Atys semistriatus 8 mm J Atys sp. 11 mm K Diniatys dentifer 5 mm L Diniatys dubius 6 mm.