Science Serving Maryland's Coasts
George McManus, Ph.D.
Chesapeake Biological Laboratory
Effects of the Ctenophore Mnemiopsis leidyi on Ciliate Populations
For over sixty years studies have been done of the feeding of ctenophores, and the results have been used to establish the ctenophore's role in food webs and its role in regulating other populations. This study specifically observes the grazing of ctenophores in relation to effects on small ciliate populations (<40μm). Three grazing experiments were completed using natural zooplankton populations and one using cultured ciliates and copepods. Results show that the effect of ctenophore feeding can either reduce or increase ciliate populations. In the experiments with natural populations, when ctenophores less than approximately five grams wet weight were feeding, ciliate populations fell significantly. However, ciliate populations increased when larger ctenophores were feeding. Therefore, a positive correlation between ctenophore wet weight and the rate of growth of small ciliate populations exists. We hypothesize that the small ctenophores feed directly on the ciliates, while the larger ctenophores choose larger zooplankton prey (that themselves feed on ciliates). Thus, due to the larger zooplankton being reduced by the larger ctenophores, the ciliate populations are grazed less and can increase. A final experiment with cultured populations supports this theory since we found that ciliate populations kept with ctenophores increased less than ciliate populations placed with copepods and ctenophores but more than ciliates with copepods alone.
This study also reports a dry to wet ratio which is different from ratios used previously: dry weight =.013312 (±.000263) wet weight - .000235. At this time no known factor is documented as this cause, however salinity differences are suggested as a possibility.
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