Stealth Effect of Red Shell in Laqueus rubellus (Brachiopoda, Terebratulida) on the Sea Bottom: An Evolutionary Insight into the Prey-Predator Interaction

The selective advantage of empire red coloration in the shell of Laqueus rubellus (a terebratulid brachiopod) was examined in terms of prey-predator interactions. The study was based on a comparison of benthic suspension feeders living at a depth of about 130 m in Suruga Bay, Japan, with special reference to their visibility under visible and near-infrared light conditions. Almost all species exhibited red coloration under visible light, while only the shell of Laqueus was dark under infrared light, similar to rocks and bioclasts. Given the functional eyes of macropredators such as fishes and coleoids, which are specialized to detect light in the blue-to-green visible spectrum, and even the long-wavelength photoreceptors of malacosteids, Laqueus should avoid both visible and infrared detection by predators inhabiting the sublittoral bottom zone. This fact suggests that terebratulids have evolved the ability to remain essentially invisible even as the optic detection abilities of predators have improved. The present hypothesis leads to the possibility that the appearance of marine organisms is associated with the passive defensive strategy, making possible to provide a lower predation risk.