The distance chemosensory behavior of the sea urchin Lytechinus variegatus

Abstract

Many organisms that lack vision rely on chemical signals to glean information from their environment. Little is known, however, about the ability of sea urchins to detect and respond to such signals. This lack of understanding is especially surprising given the ecological impact of urchins in their respective communities. Regardless of geography, urchins exert strong top down control of plants, algae, and sedentary invertebrates, and these effects are especially evident when urchins, or urchin predators, are removed from an ecosystem. Facultative omnivorous species such as Lytechinus variegatus may greatly alter the abundances of other invertebrates in seagrass communities by preying on juvenile and adult bivalves as well as gastropod egg masses. These potential food resources, however, are patchily distributed within seagrass beds. To find such resources before other organisms can exploit them may require acute abilities to detect signals emanating from these patches. Experiments performed in this study demonstrated a consistent ability of L. variegatus to detect and orient to chemicals emanating from potential food resources over a distance of 1 m. Unlike what has been found in some other marine organisms, turbulent flow conditions did not negatively affect the ability of L. variegatus to find the source of this chemical cue. In fact, only the slowest flows hindered this ability; the bluff shape of the urchin formed a relatively large boundary layer at slow flows, preventing the delivery of chemical signals to the sensors. The relatively high success rates of L. variegatus in turbulent flows may allow it to effectively forage in areas where other organisms cannot. Thus, turbulence may provide a selective advantage for this animal, based on its comparative ability to detect and respond to signals in its environment.