Life in the Open Ocean. Joseph J. Torres
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
The specialized swimming behaviors described above for the hydromedusae increase their hunting efficiency considerably. However, even conventional “straight‐line” swimming creates flow fields around the swimming medusa that enhance prey capture. Analysis of movement in the disc‐like moon jelly Aurelia aurita shows that normal swimming motions, particularly the expansion of the swimming bell during the recovery stroke, entrain particles to within easy reach of the tentacles as the water rushes into the subumbrellar space (cf. Costello 1992).
Figure 3.18 Different hunting and feeding behaviors of medusae. Shaded areas indicate effective feeding spaces. (a) Motionless ambush strategy; (b) swimming‐sinking search pattern; (c) downward sinking with trailing tentacles creating vortices, which bring food particles within the feeding space; (d and e) two strategies employed by medusae such as Polyorchis penicillatus; part‐time swimming and fishing in the water column and part‐time resting on its tentacles on the bottom and capturing the prey directly with its manubrium.
Source: Adapted from Mills (1981).
Attraction Between Predator and Prey
Attraction between predators and their prey can take several forms. Siphonophores create “lures” with nematocyst batteries on the fishing tentacles to attract the prey (Purcell 1980). In a different form of attraction, medusae have been observed to swim toward areas of high prey concentration (Arai 1991) in the laboratory, as well as aggregating in areas where prey have recently been, suggesting a chemoreceptive mechanism of attraction resulting in higher densities of medusae.
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
Hydrozoan and scyphozoan medusae can have large impacts on local zooplankton prey fields, particularly when prey and predator densities, and therefore encounter probabilities, are high (Table 3.2). Most of the studies on diet and feeding in medusae have taken place in coastal systems with an eye toward describing interactions of medusae with the larvae of commercially important fish species. More general studies on field‐caught medusae (Table 3.3) reveal a varied diet that fluctuates with available prey (Mills 1995). It includes copepods, chaetognaths, fish eggs, fish larvae, larvaceans, other medusae, euphausiids, mysids, decapods, and ctenophores.
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).
Table 3.2 Predation rates, clearance rates, and predation effects from field observations of gelatinous predators feeding on fish eggs and larvae. Prey consumed percentages are estimated consumed daily in situ.
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 |
Larvae
|