Nutritional ecology of gelatinous planktonic predators. Digestion rate in relation to type and amount of prey

In order to evaluate the stomach-content method in estimating predation rat es in the field, we systematically investigated how the digestion time of t hree gelatinous planktivorous predators were related to qualitative and qua ntitative characteristics of common prey items. The scyphomedusa Aurelia au rita showed a weak but statistically significant increase in digestion time with prey number. A meal of one or two Artemia nauplii was on average dige sted within 1.1-1.2 h, whereas meals of three to eight Artemia needed 1.6-1 .9 h for complete digestion. Meals of bigger prey, the copepod Calanus finm archicus, were digested in 2.3 (two prey) to 5.1 h (12 prey), and meals of one to four herring larvae were digested in 3.7 h. Five different prey spec ies gave average digestion times between 1.1 and 5.1 h, proportionally rela ted to the prey size. For a given prey there was none or a weak tendency of decreased digestion time with increased size of medusa. The scyphomedusa C yanea capillata digested two size classes of C. finmarchicus (5-30 prey per meal) in 1.5 and 2.0 h (average), respectively, and three species of gelat inous zooplankton (1 prey per meal) in 1.8 to 15.1 h, but with large intra- specific variability. The lobate ctenophore Bolinopsis infundibulum digeste d the same two size classes of C. finmarchicus (1-10 prey per meal) in 0.3 to 3.8 h, and herring larvae (1-8 prey per meal) in 0.7-3.5 h, with a weak positive relationship between digestion time and prey number for all three types of prey. Prey type was a significant factor for the digestion time of all three predators. C. capillata showed on average ca. 12 times longer di gestion time for A. aurita than for B. infundibulum, A. aurita digested her ring larvae 3.8 times slower than Artemia, and B. infundibulum digested her ring larvae 2.6 times slower than C. finmarchicus <1 mm, given that the sam e predator/prey size relationships were compared. The findings emphasize th e importance of using prey/predator specific digestion times in. field stud ies based on stomach contents. (C) 1999 Elsevier Science B.V. All rights re served.