Does the tunic nipple array serve to camouflage diurnal salps?

Abstract

The salp Thalia rhomboides is distributed in the shallow depths during the day, and is thus at increased risk of predation by visual predators and from the damaging effects of ultraviolet (UV) radiation compared with species distributed in deeper layers in the daytime. The integument (tunic) of T. rhomboides may have adaptive optical properties, but the absorption spectra of the unfixed tunic demonstrate that the tunic transmits UV as well as visible light, indicating that the tunic is not an effective barrier against UV radiation. Ultrastructural observation revealed that the surface of the tunic cuticle is covered in a nipple array consisting of hemispherical protuberances approximately 40 and 30 nm in diameter in solitary and aggregate zooids, respectively. Simulation of light reflection of a nipple array using rigorous coupled wave analysis (RCWA) indicated that the reflection is slightly lower for the nipple array than for a flat surface at high angles of incidence (θ > 80°). This result supports the idea that the nipple array serves to make the salp less visible. The simulation also indicated that the height and distribution of the salp nipple array do not have an optimal structure for causing an antireflection effect. A mechanical restriction might exist on the structures, and the nipple array could also serve another function. The size and distribution of nipples may be controlled by the need to meet the complex requirements of multiple essential functions.