Behavioral responses of the sea urchin Tetrapygus niger to predators and food

Abstract

To test the importance of predators and food availability in determining patterns of aggregation and movement in the urchin Tetrapygus niger, we studied the behavioral responses of animals with different body sizes under laboratory conditions. Experiments were carried out on a square platform in tanks with circulating water. Twelve sea urchins of three size ranges were placed following a uniform spatial distribution on the platform and maintained in the presence or absence of: (a) one predator (the fish Pinguipes chilensis or the asteroid Meyenaster gelatinosus), or (b) food (fronds of the brown kelp Lessonia nigrescens). The experiments were followed with a video camera. The movement patterns of the sea urchins, the number and size of aggregations, and the dispersion patterns were quantified. To determine the importance of aggregation as a defensive mechanism, artificial aggregations of large sea urchins were created on the platform in the presence or absence of predators. Behavioral responses were followed and analyzed as described above. Individuals of T. niger recognize the presence of predators, and respond with increased mobility (e.g., number of sea urchins moving or average speed of movement; i.e., escape responses) but do not increase the number or size of aggregations. Moreover, the increased mobility of sea urchins would account for the random spatial distribution of individuals at the end of the experiments with predators. The escape responses were specially marked in the presence of the starfish M. gelatinosus. In this case, the aggregations were unstable, suggesting that the aggregations of T. niger would not have a protective function against this starfish. By contrast, sea urchins aggregated in the presence of food. It is concluded that the generation of aggregations in this species would be more related to feeding patterns (or to other factors related to spatial heterogeneity; e.g., substrate topography) than to responses to predators. The escape behavior of sea urchins to starfish is also described.