Assessment of grazing by the freshwater copepod Diaptomus minutus using carotenoid pigments: a caution |
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Authors: | Descy J; Frost T; Hurley J |
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Institution: | Laboratory of Freshwater Ecology, Department of Biology, FUNDP, B-5000 Namur, Belgium; Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA; Water Chemistry Program, University of Wisconsin-Madison and Wisconsin Department of Natural Resources, Madison, WI 53706, USA |
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Abstract: | In order to assess feeding selectivity in freshwater zooplankton, we
conducted feeding trials using Diaptomus minutus
isolated from two Wisconsin lakes. Copepods were fed an algal assemblage
comprised of an equal biomass of a centric diatom, a cryptomonad and a
coccal green alga. The total amounts of photopigments were tracked using
high-performance liquid chromatography. The removals of carotenoids and
a-type phorbins (chlorophyll a
and phaeopigments) from feeding suspensions were compared with their
presence in the guts of animals, in fecal pellets, and in the final
suspension. Diaptomus minutus generally removed either
the diatoms primarily or all three algal cells equally. These removals were
not reflected in gut extracts of the animals, however, where alloxanthin
(marker of cryptomonads) was always present, but where fucoxanthin and
diadinoxanthin (markers of diatoms) were never observed. Pigment
disappearance was variable for total a-type phorbins,
but frequently >90% for carotenoids, particularly for fucoxanthin
and diadinoxanthin. Phaeophytin a was the major
a-type phorbin detected after gut passage. Our results
indicate that evaluations of zooplankton grazing which assume that algal
carotenoids remain detectable throughout gut passage must be made with
substantial caution and that differences in pigment processing are likely
to occur among zooplankton species. Furthermore, grazing experiments
designed to evaluate decreases of specific pigments in feeding suspension,
rather than their appearance in animals' guts, may prove a more valuable
approach to understanding the feeding selectivity of copepods.
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