Oregon Department of Fisheries, via Flickr

Tunicates are marine invertebrates and include sessile ascidians and planktonic salps. Most tunicates feed on suspended particles in the seawater via filtration. Since many tunicates have a notochord at some time in their life cycle, they belong to the phylum Chordata. Actually, tunicates are regarded as the closest sister group of vertebrates among extant animals on the earth.

Tunicates are the only metazoan group that can produce cellulose. In ascidians and salps, the cellulosic matrix forms an integumentary tissue called a ‘tunic’ that is on the outside the epidermis.

Most salps and some ascidian species have a transparent tunic, and as such these animals are barely visible underwater. This may be an adaptation for the avoidance of visual predators, such as fish. Although the transparent tunic is appropriately almost transparent in visible and ultraviolet light, careful divers can occasionally recognize transparent tunicates by light reflection from the tunic surface.

光反射发生在两个具有不同折射率不同的介质之间的边界，并且反射率取决于介质之间的光的入射角和折射率的差异。使用ABBE折射仪和椭圆计，我们从SALP中测量了上衣的折射率thetys阴道and the ascidianRhopalaeasp. The refractive index of the tunic for both species was estimated to be just slightly (0.03 or less) larger than seawater. The difference between the tunics and seawater is so small that reflectance on the tunic surface should be very small.

此外，在包括束缚物在内的各种动物的表面上发现了一系列乳头形突起（乳头阵列）。每个突起的高度仅为100 nm。乳头阵列最初是在飞蛾眼中发现的，这种结构众所周知可以降低反射率。所谓的“蛾眼效应”。

Nipple arrays were also found on the tunics of some salps that are distributed near the sea surface in the daytime, implying a function for invisibility under bright circumstances. According toour simulation of light reflection, reflectance on the nipple array is slight lower than that of a flat surface, but the height and distance between the nipples are not optimized for anti-reflection. Moreover, the difference of refractive indices between the tunic and seawater may be too small to allow the nipple array to have a functional effect.

As nipple arrays are known to be multifunctional structures, the tunicates’ nipple array possibly serves other function(s), such asbubble repellencyoranti-immune responses. To investigate these possible functions, we may need the help of fluid mechanic researchers.