Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 89

Madin (1988) proposed a general model to describe tentaculate predation incorporating the best elements of previous studies (Gerritsen and Strickler 1977; Mills 1981; Greene 1985; Greene et al. 1986; Larson 1987a, 1987b; Purcell and Mills 1988) and the author’s own observations. A successful predatory sequence consists of four parts: encounter, capture/selection, ingestion, and digestion.

Interaction with prey begins with the “encounter” phase, which is determined by the size and type of encounter zone (Figure 3.17a–d) and the swimming behavior of the medusa. It was assumed that sensory mechanisms were not used to target individual prey. Once a prey item is within its encounter zone, how effective a species will be at capturing it will be determined by a suite of characteristics: tentacle density and spacing, prey size and swimming behavior, and effectiveness of the nematocysts and venom.

Prey size is dictated to some degree by the morphology of the predator. Species with fine dense tentacles (A‐type in Figure 3.17) may be expected to capture small prey items, e.g. copepods, and those with more widely spaced robust tentacles (B and C‐type) to capture larger prey, e.g. ctenophores and other medusae. Many of the B‐ and C‐type predators increase their encounter probability with prey using swimming or hunting behavior. The effectiveness of the nematocysts in paralyzing prey and strength of the tentacles for retaining larger prey are also important in the successful capture of larger prey items. Species that prey on soft‐bodied species such as jellies and fishes have one type of nematocyst designed to harpoon and poison the prey, whereas those that prey on crustaceans have four to five types of nematocysts in specialized batteries designed to entangle and hold crustaceans. Clearly, those species (e.g. cubomedusae) that prey on strong swimmers such as fishes and larger shrimp that could potentially tear tentacles must have venom virulent enough to quickly paralyze their prey.

Once a prey item is captured, successful digestion will depend on whether the item can be conveyed to the mouth by the tentacles and whether it can be successfully introduced to the gastric cavity through the mouth and broken down into useful nutrients by the digestive apparatus.