AGONISM IN ASTEROIDS.

1. Intraspecific agonistic behavior, called bouts, which involves ray interactions between individual sea stars, is reported in species from all three living orders of asteroids and described for Patiria miniata and Pycnopodia helianthoides. This behavior indicates a certain sensitivity to conspecific contact on the aboral surface, in which each individual attempts to place a ray or rays on top of its opponent, an act which may initiate opponent withdrawal. Agonistic intraspecific bouts affect the distribution and feeding of both species although bouts may take place where food is not present. 2. Patiria bouts may last over two hours and sometimes terminate by individuals overlapping and sharing food. Evidence suggests that Patiria bouts are: (a) quite common; (b) not related to sexual behavior; (c) territorial; and (d) influenced as to outcome by relative body orientation; however, when territorial behavior is involved, relative body orientation does not affect bout outcomes. Pycnopodia bouts are of shorter duration (up to 10 minutes), terminating with the withdrawal of one or both animals, and sometimes resulting in an extensive pursuit. 3. Two forms of intraspecific Patiria bouts are noted: continuous-contact and intermittent-contact. Intermittent-contact bouts appear less intensive, permitting individuals to feed and to engage in bouts with more than one opponent at a time. It is speculated that intermittent-contact bouts are a type of ritualized activity of a fairly complex nature. 4. Interspecific bouts between specimens of Pycnopodia and two species of Pisaster (P. giganteus and P. brevispinus) have been observed only when the Pisaster sp. is in possession of food. Pycnopodia individuals approach and place rays on the aboral surface of the feeding sea star while attempting to obtain the food with other rays. The specimen of Pisaster sp. actively counterattacks using its pedicellariae.