Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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very different one from that of S. atra. It swims upward, bell uppermost, then sinks down with the bell oriented downward and its tentacles trailing behind (Figure 3.18c). As it does so, vortices are created behind the bell that circulate small prey into the tentacles. P. gregarium would be classified as a C‐type predator in Madin’s Figure 3.17 model.

      The last example is Polyorchis pencillatus, a resident of shallow bays where it spends a great deal of its time on the bottom. Its hunting strategy on the bottom is to perch on its tentacles (Figure 3.18e) and use its manubrium to ingest prey from the surface sediments. At intervals it hops up off the bottom, stirring up the sediments, and then back down. Occasionally it swims up to the surface and drifts downward bell up (Figure 3.18d), becoming an A‐type predator in the Figure 3.17 model.

      Mills (1981) lists seven factors that contribute to feeding efficiency in medusae: (i) tentacle number and length; (ii) geometry of tentacle posture; (iii) velocity of tentacles moving through water; (iv) swimming pattern of medusa; (v) streamlining effects of the medusa bell on water flow; (vi) diameter of the prey; (vii) swimming pattern and velocity of prey. Together, Mill’s observations and Madin’s conceptual treatment provide a useful framework for examining the feeding strategies of medusae.

      Water Flow and Swimming

Schematic illustration of different hunting and feeding behaviors of medusae.

      Source: Adapted from Mills (1981).

      Attraction Between Predator and Prey

      High concentrations of medusae can be achieved by physical aggregating mechanisms or by rapid reproduction in place to form a true bloom (Graham et al. 2001). Physical cues that have been implicated in high concentrations of medusae include light‐mediated migrations such as diurnal vertical migration and aggregations associated with discontinuities in temperature, salinity, and density (pycnoclines) in the vertical plane. The reasons for accumulation of medusae at pycnoclines likely include higher concentrations of prey at the density discontinuities as well as passive mechanisms such as buoyancy at the cline.

      Wind, waves, and currents can also act to produce aggregations of medusae. Populations of medusae are often compressed along the shoreline, resulting in rafts of jellies on the beach during periods of onshore winds. Oceanic frontal systems may harbor increased densities of medusae relative to waters outside of the frontal zone, similar to increases in populations observed in fishes and other more mobile species at oceanic fronts. Interestingly, a unique, persistent aggregation of the medusa Chrysaora fuscescens may be found in Monterey Bay California, the result of upwelled water entrained by a coastal prominence in the northern part of the bay (Graham et al. 1992).

      Diets, Feeding Rates, and Impacts on Prey Populations

      Impacts of medusae vary considerably and depend largely on predator density. Purcell and Arai (2001) demonstrated that prey‐removal rate by the hydromedusa Aequorea victoria ranged from 0.1 to 73% of available herring larvae per day from coastal waters off Vancouver Island, British Columbia, depending upon predator concentrations. Clearly, medusan predation can have a profound influence on larval survivorship, particularly when wind and wave or reproductive activity act to concentrate weakly swimming prey and gelatinous predators in one location.

      The radial symmetry, stinging tentacles, and gelatinous character of medusae make them highly effective as predators, particularly as ambush predators. However, they also may find themselves as prey in the diets of other medusae. In particular, the semaeostome scyphomedusae often have hydromedusae in their diet when the smaller medusae are available in quantity, e.g. during early spring (Purcell 1991). At this time, no scyphozoan medusa is known to prey exclusively on other medusae, but it may be that the narcomedusae, the slow swimming hydromedusae important in the mesopelagic zone, specialize on other jellies (Purcell and Mills 1988).

      Source: Adapted from Purcell and Arai (2001).

Species Size Prey type (density) Prey eaten (no. • pred−1 • d−1) Clearance ratesa (no. • pred−1 • d−1) Prey consumed (% • d−1) References
Siphonophore
Physalia physalis na LarvaeСкачать книгу