Prey resource use by coexistent hydromedusae from Friday Harbor, Washington

Abstract

Prey selection patterns were quantified for a sympatric group of hydromedusae from Friday Harbor, WA. Selection patterns varied between species, but were largely replicable between sample dates and resembled dietary patterns found in similar studies from neighboring regions. Ambush-foraging medusae (Aglantha digitale, Sarsia tubulosa, and Proboscidactyla flavicirrata) fed primarily on crustacean and ciliated prey but the dietary niches of these hydromedusan species centered on different fractions of the available plankton. Consequently, little dietary overlap occurred between the ambush foraging hydromedusae. In contrast, the dietary niches of cruising predators (Aequorea victoria, Mitrocoma cellularia, and Phialidium gregarium) overlapped substantially because those species all fed on similar soft-bodied prey such as eggs and appendicularians. These results have two important implications for trophic patterns involving medusae. First, different mechanisms of prey encounter and capture used by hydromedusae (ambush vs. cruising patterns) result in important interspecific dietary differences and, hence, trophic roles of the medusae. Second, whereas cruising medusae may consume similar prey and hence form a feeding guild, ambush-foraging medusae may experience substantially less prey overlap and, for the community examined here, do not experience potentially strong feeding competition from other medusan species.Predation by hydromedusae can substantially affect prey populations of zooplankton (Larson 1987a; Matsakis and Conover 1991) and fish eggs or larvae (Purcell et al. 1987, Purcell andGrover 1990). Although hydromedusae prey on a wide spectrum of planktonic taxa, many hydromedusan species appear to selectively consume particular prey types (Larson 1987a;Purcell 1990;Mills 1995) via a variety of mechanisms (reviewed in Purcell 1997).Full understanding of the predatory impacts of hydromedusan predation requires information on the synergistic impacts of multiple species of coexisting hydromedusae. This is important because hydromedusan species often coexist (Mills 1981a), and the trophic impacts of sympatric species do not occur in isolation from one another. The use of similar, potentially limiting prey (Purcell 1991a) suggests that these species may experience competition for planktonic prey resources. Differential prey selection patterns by hydromedusae (Larson 1987a;Purcell 1990;Mills 1995) could help relieve intraguild competition among medusae, but the trophic niches of species comprising hydromedusan guilds have received little attention. We asked whether, first, prey resource partitioning occurred between sympatric hydromedusae and, second, what patterns were identifiable that would affect the dietary interactions of species comprising hydromedusan feeding guilds.We chose a group of sympatric hydromedusae ( Fig. 1) from waters surrounding Friday Harbor Laboratories for our 1 Corresponding author. AcknowledgementsFunding for this project was provided by NSF grants OCE 9218507 and OCE-9820172 to ...

In situ swimming and feeding behavior of eight co-occurring hydromedusae

“…In situ prey-selection data indicate that the different mechanisms of prey capture used by rowers versus jetters result in dietary differences and, therefore, different trophic roles (Costello & Colin 2002). We would predict, based upon the morphology, swimming performance and prey selection of the different medusan groups, that in order to optimize foraging success and trophic impact, rowing medusae should maximize and jetting medusae should minimize their time spent swimming.…”

“…Understanding determinates of the foraging behavior of hydromedusa can provide insight into factors determining their trophic impact. The modes of propulsion, bell morphology and prey selection vary among hydrozoan taxa and are thought to be directly linked to the foraging behavior of hydromedusae (Colin & Costello 2002, Costello & Colin 2002. Accordingly, Costello & Colin (1995) have suggested that propulsion and bell morphology can be used to predict the impact that different hydromedusae have upon zooplankton prey populations.…”

“…For these medusae, swimming and prey capture are simultaneous, interrelated events (Costello 1992, Costello & Colin 1994, Ford et al 1997. Colin & Costello (2002) have suggested that these medusae generally have oblate bell morphologies (fineness ratio generally < 0.5) and have termed them 'rowers' because, as they contract their bells during swimming, their bell margin acts as a paddle and propels them forward. Both the oblate bell shape and the rowing propulsive mode are favorable for these medusae because these traits maximize the flow generated during swimming, i.e.…”

“…In these medusae, it is suspected that swimming is decoupled from feeding. These taxa use jet propulsion to swim and, thus, are termed 'jetters' (Colin & Costello 2002). Jet propulsion enables ambush predators to reposition themselves more quickly but appears to be a more energetically expensive mode of propulsion (Colin & Costello 1996.…”

Functional characteristics of nematocysts found on the scyphomedusa Cyanea capillata

“…Predation by medusae plays an important role in the trophic ecology of many pelagic ecosystems (Purcell, 1989;Matsakis and Conover, 1991;Olesen, 1995;Costello and Colin, 2002). Despite its importance, many of the mechanisms that determine predation rates and prey selection are poorly defined.…”