Diaptomus vs. Net Phytoplankton: Effects of Algal Size and Morphology on Selectivity of a Behaviorally Flexible, Omnivorous Copepod
Abstract:Effects of algal size, colony form, and morphology on selection by Diaptomus sicilis and D. ashlandi were determined for certain net diatoms commonly found in the pelagic regions in lakes. Mechanisms of capture, observed by high-speed microcinematography, were correlated with selectivity results from traditional feeding experiments with mixtures of algae. The attribute of elongation (up to 365 µm) in one dimension possessed by Synedra spp. was not useful for avoiding grazing. In fact, at low concentrations, selectivities for Synedra were much higher than for Chlamydomonas of equal cell volume. This suggests a perceptual bias for capture of elongated algae. Films showed that D. sicilis could even bite off sections of 700-&µm-long Melosira colonies. However, long fragments of Synedra and Melosira were often left behind after attacks by Diaptomus. Elongation in two dimensions, an attribute possessed by the stellate colonies of Asterionella formosa was extremely effective for avoiding grazing once more than six to eight cells per colony was reached. This results may explain the abundance of the eight-cell form in nature. Selectivity of Diaptomus changed with concentration in mixtures of a 12-µm-diameter spherical green alga and a 240-µm-long Synedra. In these same experiments, the proportion of attacked Synedra that were only partially ingested—i.e., the proportion rejected after partial ingestion-increased linearly with attack rate on Synedra, and was not correlated with attack rate on Chlamydomonas or on the sum of both algal species. These and other data demonstrate that this concentration-variable selectivity is not an optimal-foraging strategy. We assert these observations can be properly viewed within the classical ethological framework of motivation and excitability of different motor patterns used to capture, handle, and ingest different kinds of algae.
Document Type: Research Article
Publication date: November 1, 1988
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