Life in the Open Ocean. Joseph J. Torres

^b The calculated space within the encounter zone occupied by tentacles

      

Figure 3.17 Patterns of tentacle deployment seen in medusae, siphonophores, and ctenophores. (a) Tentacles radiate from the body, filling a disk‐shaped space; (b) tentacles are somewhere within a sphere around the body; (c) tentacles stream behind the body, filling a truncated cone; (d) tentacles are held ahead of the body in a cylinder or truncated cone; (e) tentacles radiate from a long stem, filling a cylindrical space; (f) tentacles or tentilla form a nearly flat curtain. Some siphonophores may have encounter zones of this shape.

      Source: Madin (1988), figure 1 (p. 416). Reproduced with the permission of the Bulletin of Marine Science.

       Leptomedusae

      Aequorea macrodactyla (Figure 3.17c) are large, lens‐shaped, medusae that swim nearly continuously, trailing the tentacles behind the bell to form a conical volume. Tentacles extend to about 50 times the bell diameter. Prey stick to the tentacles of the swimming medusa and are conveyed to the mouth. Like Stomotoca, the prey of Aequorea are largely gelatinous and include medusae, ctenophores, pteropods, and salps.

Laodicea undulata have a flat shape with large numbers of fine tentacles arranged radially around the bell, giving it an encounter volume like that shown in Figure 3.17a. It is believed to feed largely on small prey such as copepods.

      Dichotomia cannoides has a conical bell with about 50 tentacles arranged around the bell margin. Its tentacles are deployed in a disc (Figure 3.17a) or cone (Figure 3.17c) configuration. It is believed to feed on small prey.

       Trachymedusae

      Liriope tetraphylla has four tentacles that extend as much as 25 times the bell diameter and an unusually long gastric peduncle with a stomach at the end. Its hunting strategy is most like Stomotoca (Figure 3.17b), with a large spherical encounter volume and the tentacles only occupying a small fraction of it.

       Narcomedusae

      Solmundella bitentaculata (Figure 3.17d) has two tentacles that are held in front of the bell (aboral side) as it swims, forming a cylindrical or conical volume of about three bell diameters in front of the bell.

      Aeginopsis laurentii is another D‐type (Figure 3.17d) predator but with four tentacles that extend out in front of the bell in a cone. Like Solmundella, Aeginopsis is a mesopelagic species and little information is available on diet.

      Solmaris is a narcomedusan genus with several species living at epi‐ and mesopelagic depths. It conforms to the narcomedusan D‐type tentacle deployment; it has an array of several tentacles forming a larger cone.

       Scyphomedusae

      Pelagia noctiluca is a semeaostome scyphomedusa. As a scyphozoan, it is larger than its hydrozomedusan brethren with a bell diameter of up to 15 cm and tentacles extending outward up 30 times the bell diameter. It swims continuously as a C‐type predator feeding on a wide variety of prey types including salps, doliolids, pteropods, forams, amphipods, chaetognaths, and fishes.

      The Model

      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.

      Swimming and Hunting Behavior

Mills (1981) gives an example of four hydromedusan hunting behaviors (Figure 3.18) that correspond well with Madin’s general model. The first species, Proboscidactyla flavicirrata (Figure 3.18a) corresponds to the A‐type predator in Figure 3.18: neutrally buoyant with 40–80 tentacles radiating from a globular bell. P. flavicirrata employs a motionless ambush strategy, allowing small zooplankton to enter the “encounter zone” through their own swimming behavior. Its neutral buoyancy makes it an especially effective trap for its plankton prey.

      As a congener of Stomotoca pterophylla, Stomotoca atra is a B‐type (see Figure 3.18b) predator. By adding its swimming behavior, we gain a better understanding of how it uses its two long tentacles to hunt. It employs a hop–sink swim cycle to drag its long tentacles up and down through the water column. As it swims and sinks, the tentacles describe a sine curve about the width of the bell and 2 meters from top to bottom. It feeds on large prey (hydromedusae), and it greatly increases the probability of contacting a prey item by its method of interrogating the water column.

      The third species, Phialidium gregarium, is the primary prey of Stomotoca atra. P. gregarium