Why are there Bright Colors in Sessile Marine Invertebrates?

Sponges, cnidarians and ascidians often have bright colors. These colors are common in species of shallow rocky areas world-wide, and appear not only in animals exposed to bright light but also in those living in dark areas where the colors are visible only with artificial illumination. Coloration can serve in visual predator-prey relationships, enabling a predator to learn to avoid unprofitable prey or inhibiting its ability to learn to recognize prey. Coloration can be incidental to pigments involved in physiological processes, including photosensitivity, photo protection, storage of food products, mechanical support, and aerobic metabolism. Pigments also can result from degradation of hormones, storage of waste products or digestion. Coloration also can be derived from photosynthetic pigments of symbionts. The intensity and pattern of pigmentation can depend on food supplies, depth, intensity of the ambient light, and geographic location. Coloration within a species of sponge, cnidarian or ascidian can be relatively constant, or consist of two to many morphs. The morphs may be under genetic control or can indicate cryptic species. In most sessile invertebrates, however, the causes of the color patterns are not known, and could include pleiotropic gene activity, cellular activity during early development, or random neutral mutations at the molecular level. A better understanding of the functions of the pigments and colors would involve experiments and observations in ecology, animal behavior, genetics, physiology and pigmentation chemistry.