Mechanism and Adaptive Significance of Substrate Selection by a Sessile Rotifer

E-ISSN： 1939-9170|67|2|314-323

ISSN： 0012-9658

Source： Ecology, Vol.67, Iss.2, 1986-04, pp. : 314-323

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Abstract

We examined the nature and adaptive significance of substrate selection by larvae of the sessile rotifer Collotheca gracilipes. In a small artificial pond the preferred substrate was the anatomical undersurfaces (abaxial) of Elodea canadensis leaves, although four other macrophytes were present (Ceratophyllum demersum, Lemna minor, Myriophyllum spicatum, and Nymphaea odorata). Density of adults on Elodea at times reached >6 individuals/mm², with >98% attached to abaxial surfaces. However, larvae offered plants in pairwise combinations selected substrates in the following order; Lemma > Elodea > Myriophyllum > Nymphaea. No larval loyalty to parental substrate was exhibited. Larvae preferentially selected abaxial over adaxial surfaces of Elodea leaves (91% on abaxial) in continuous illumination experiments, but the larvae did not discriminate between the two surfaces in total darkness (48% on abaxial). In the absence of a plant substrate, cell debris from any one of four aquatic and two terrestrial plant species induced larval settlement on the bottom of plastic well—depressions. Activity could not be attributed to carbohydrate, lipid, nucleic acid, or protein moieties within these extracts. However, alpha—amylase induced rapid larval settlement in the depressions. In the presence of this calcium chelator, larvae did not distinguish between ad— and abaxial leaf surfaces to the same degree as without the enzyme. Phospholipase—C and EDTA induced similar effects. Larval preference for abaxial surfaces could be reduced slightly when PH was stabilized at <7.0. Because living Elodea in neutral—to—alkaline water can remove Ca⁺² from beneath its leaves and release it from adaxial surfaces while photosynthesizing, we hypothesized that larvae initiate substrate selection activities on any surface when they are in microhabitats having ambient Ca⁺² concentrations below a threshold concentration. Short—term, in vitro growth experiments showed that rotifers attached to abaxial surfaces of Elodea leaves grew significantly taller and produced more eggs per female than those which had been induced to settle on adaxial surfaces.