Grazing interactions between pulmonate snails and epiphytic algae and bacteria

Abstract

1. The effects of grazing by Lymnaea peregra and Planorbis planorbis on Ceralophyllum demersum epiphyton were investigated under simulated natural conditions.2. Grazing resulted in significant reductions in algal cell densities, number of algal taxa and in diversity. Grazing did not influence equitability values.3. The use of Ivlev's Electivity index demonstrated positive and negative selection by feeding snails for large, pedunculate, and small, adnate algae, respectively. 4. Inoculation of artificial surfaces revealed that 61.1% and 62..**% of the algal taxa present in the diet of L. peregra and P. planorbis., respectively (mainly the smaller species), were able to survive passage through the gut, resettle and grow.5. The results are discussed with particular reference to the survival strategies available to the epiphytes, and the possible mutual benefits which algae and snails may derive from coexistence.

Altered complementary feeding strategies of the consumers Hydrobia ulvae and Idotea emarginata via passive selectivity

“…by taking up predominately lipids from their diets while excreting unnecessary components such as cell walls. Our data conWrm that the actual abundance of grazed algal cells did not automatically reXect the actual amount of digested material (see also Underwood and Thomas 1990;Brendelberger 1997a). Stable isotope data obtained for H. ulvae and I. emarginata in the single-grazer treatments showed very similar 13 C-and 15 N-uptakes.…”

“…Our data on 13 C-and 15 Nuptake as well as on the selectivity index Q support Wndings showing that the degrees to which microalgae are consumed by a grazer do not automatically reXect the actual amount of digested material (Underwood and Thomas 1990;Brendelberger 1997a). By using this isotope labelling approach we were able to provide evidence that consumer-resource systems not only depend on active foraging activities but that passive mechanisms such as assimilation and digestion can aVect a systems' community ecology considerably.…”

“…These results indicate that as long as there is no co-occurring grazer present, both invertebrates showed similar assimilation and digestion eYciencies to both microalgal species. Although it is known that microalgal cell sizes, structures and morphotypes can inXuence digestive pathways (Moore 1975;Underwood and Thomas 1990), such size-or morphotype-dependant eVects were not detected in the single-grazer treatments of our experiment. However, in the mixed-grazer treatments I. emarginata in general showed a higher uptake of the labelled algal material as H. ulvae.…”

“…This is not surprising since benthic microalgae are considered to be the major food source for benthic consumers in the euphotic zone (Sumner and McIntire 1982;Underwood and Thomas 1990;Hillebrand et al 2002) and thus, their abundance and species composition are crucial factors for studies in aquatic community ecology.…”

Is diatom size selection by harpacticoid copepods related to grazer body size?

“…larger ones) may not only depend on prey availability but also upon morphological constraints such as the size and functionality of mouthparts of the grazer. Furthermore, the degree of digestion and the survival of gut passage by some food particles are factors that can also influence food-grazer interactions, although this aspect of assimilation has been relatively neglected (Porter, 1973;Moore, 1975;Underwood and Thomas, 1990). In nature, however, the efficiency with which an organism utilizes a certain food source does not only depend on the ability of the consumer to digest the material but also on the specific food available and its abundance (Hicks and Coull, 1983;unpubl.…”