Mineral grains in caddisfly pupal cases and streambed sediments: Resource use and its limitation through conflicting resource requirements

Statzner, Bernhard, Sylvie Mérigoux, Maria Leichtfried

Limnol. Oceanogr., 50(2), 2005, 713-721 | DOI: 10.4319/lo.2005.50.2.0713

ABSTRACT: Sand and fine gravel in streambed sediments are considered to be an overabundant resource for caddisflies that build cases from it. However, larvae of Rhyacophila and Hydropsyche build pupal cases with material collected near the pupation site and typically attach these cases to cobbles in riffles, where the rapid flow facilitates respiration but decreases the availability of case-building material through erosion. Analyzing mineral grain sizes of the pupal cases and the streambed in samples taken from cobbles in a stream riffle, we found that the overall mass use in pupal cases of Rhyacophila and particularly Hydropsyche significantly increased with local mass availability of building material, indicating that mineral grains can be a limited resource for these taxa. In addition, the most abundant species (Hydropsyche siltalai Döhler) significantly changed the case architecture if the preferred grain fraction (2.5-3.15 mm) was a limited resource. Under resource limitation of the preferred fraction, H. siltalai used the 1.6-2-mm fraction instead, which could reduce the resistance of the cases to damage resulting from floods that move coarser bottom material. Our findings suggest that, in streams or near shores of lakes and oceans, water currents that modify grain availability can create conflicts in resource requirements for invertebrates, particularly if they need locally available fine grains for the building, coarse grains for attachment, and high oxygen renewal rates for metabolic needs.

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