Prey selection mechanism of ambush-foraging hydromedusae

Abstract

The widespread occurrence and frequent abundance of small hydromedusae suggests that they may play an important role in planktonic communities. However, rather than exhibiting dominant impacts on any specific planktonic group, field studies have demonstrated diverse dietary niches and only modest trophic impacts by small hydromedusae. To understand the functional bases for these patterns, we exposed 2 hydromedusae (Cladonema californicum and Leuckartiara sp.) to a variety of prey types (dinoflagellates, rotifers, barnacle nauplii, copepods and the hydromedusa Obelia sp.) while video-recording predation sequences (encounter, capture, ingestion). Both C. californicum and Leuckartiara sp. ambush prey and possess penetrating nematocysts (stenoteles and euryteles, respectively). Although similar prey selection patterns might be expected based on encounter models or nematocyst complements, the 2 species exhibited some markedly different ingestion patterns. For example, C. californicum positively selected copepod prey and negatively selected hydromedusae, whereas Leuckartiara sp. exhibited the opposite pattern. Quantification of predation sequences demonstrated that hydromedusan dietary variations resulted from speciesspecific differences in prey capture efficiencies as well as efficiencies in post-capture transfer to the gut. Species-specific prey selection patterns and limited ingestion capacities may explain the diverse prey selection patterns and limited trophic significance observed in field studies of ambush-foraging hydromedusae.

Behavior of Nemopsis bachei L. Agassiz, 1849 medusae in the presence of physical gradients and biological thin layers

“…Results of the present study indicate that prey densities were not high enough to saturate the feeding process (Purcell & Nemazie, 1992;Regula et al, 2009). Ingestion rates, as estimated from encounter rates and 100% ingestion efficiency, increased at higher prey densities, with no evidence of a decrease that would indicate saturation.…”

“…In experiments with biological thin layers, encounter rates for N. bachei reflected changes in the densities of copepod prey. These changes in encounter rates equate to changes in the mean number of hydromedusae feeding because N. bachei displayed 100% capture, transfer, and ingestion efficiencies (as defined by Regula et al, 2009). Increased feeding with increased prey densities also has been reported for other hydromedusae (Fulton & Wear, 1985;Daan, 1986;Matsakis & Nival, 1989), ctenophores (Reeve et al, 1978), scyphomedusae (Båmstedt et al, 1994), and siphonophores (Purcell, 1982;Purcell & Kremer, 1983).…”

Stealth predation and the predatory success of the invasive ctenophore Mnemiopsis leidyi

“…For successful capture of copepods, a large, slow-swimming predator such as M. leidyi must remain undetected until after the encounter with the prey. Other gelatinous predators accomplish this by foraging as "sit-and-wait" ambush predators (25,26). However, the mechanics of ambush foraging constrain their F max (25,26), greatly limiting their ecological impact (26,27).…”

Gill cell toxicity of northern boreal scyphomedusae Cyanea capillata and Aurelia aurita measured by an in vitro cell assay

“…paralysis. The development and distribution of certain types of cnidocytes and the injection of bioactive compounds are the crucial factors for prey capture and digestion by cnidarians (Kintner et al, 2005;Regula et al, 2009).…”