Vol. 63, 2024

Optical Properties of Body Mucus Secreted from Coral Reef Sea Slugs: Measurement of Refractive Indices and Relative Absorption Spectra

Ryogo Takano¹ and Euichi Hirose^1,*

¹Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan. *Correspondence: E-mail: euichi@sci.u-ryukyu.ac.jp (Hirose)
E-mail: ryo5.orange.813.ochoko@gmail.com (Takano)

(Received 13 March 2023 / Accepted 21 December 2023 / Published 15 March 2024)
Communicated by Ryuji Machida

Sea slugs are always covered in a mucus layer that has various functions including chemical defense that often involves aposematism and mimicry. Therefore, it is necessary for sea slugs to exhibit their body colors and patterns exactly, and the optical properties of mucus should support this requirement. We examined body mucus from heterobranch sea slugs collected in the Okinawan coral reefs. The refractive indices of mucus from 32 species ranged from 1.3371 to 1.3854 and were similar or slightly greater than the refractive index of seawater (ca. 1.34), indicating that light reflectance on the mucus layer is generally small. Moreover, dissolution of mucus into seawater would form a gradient of refractive indices and enhance the reduction of reflectance. We also obtained relative absorption spectra of the mucus from 32 species. In the range of visible light, absorption spectra of mucus suggest that the mucus layer is almost transparent and is not likely to interfere with the body colors. The presence of absorption peaks and/or shoulders in the UV (ultraviolet) range (280–400 nm) indicates that the mucus layer potentially serves as a sunscreen that absorbs UV radiation in 23 species, whereas prominent UV absorption was not found in the other 9 species. In a kleptoplasty sacoglossan Plakobranchus ocellatus, the refractive indices and presence or absence of UV-absorption showed that the optical properties of the mucus varied to some extent but did not show seasonal fluctuation. The UV-absorption in the mucus may also protect kleptoplasts in kleptoplasty sacoglossans. The present results support the importance of mucus as a functional optical layer for the shell-less life of sea slugs.

Key words: Mucus, Anti-reflection, UV protection, Sea slugs, Kleptoplasty

Citation: Takano R, Hirose E. 2024. Optical properties of body mucus secreted from the coral reef sea slugs: measurement of refractive indices and relative absorption spectra. Zool Stud 63:02. doi:10.6620/ZS.2024.63-02.

Supplementary materials: Fig. S1丨Fig. S2丨Fig. S3丨Fig. S4丨Table S1