Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 81

As noted above, medusae are radially symmetrical tentaculate predators that rely on contact to entangle their prey. They are not “sight‐hunters” or raptors (e.g. Greene 1985) capable of actively seeking a prey item and chasing it down. Yet despite their seeming inability to select prey, virtually all gelatinous predators for which electivity indices have been calculated show some degree of selectivity in their diet (Purcell 1997).

Most pelagic cnidarians feed mainly on copepods, the dominant metazoan zooplankton group in the vast majority of the world ocean (Purcell 1997). This is not surprising; a nonliving bit of “marine flypaper” cut into the shape of a medusa and floating with the ocean current would mainly snag copepods. It is the exceptions to the rule that are intriguing and lead one to wonder about how the selectivity is achieved. For example, Purcell (1997) observed that the hydromedusae Bougainvillia principis and Proboscidactyla fed mainly on barnacle larvae and molluscan veliger larvae, respectively. Are cnidarian dietary preferences the result of a limited prey field or actual selectivity? We will investigate cnidarian hunting from a theoretical perspective. Interactions with prey have two basic elements: the encounter and the capture (Purcell 1997). Factors influencing the encounter phase have been treated in a number of studies and may be divided into four subcategories: direct interception, encounter zone, water flow and swimming, and attraction between predator and prey.