Mineral grains in caddisfly pupal cases and streambed sediments: assessing resource use and its limitation across various river types^*

CNRS-Écologie des Hydrosystèmes Fluviaux, University of Lyon 1, 69622 Villeurbanne Cedex, France

^** Corresponding author : bernhard.statzner@univ-lyon1.fr

Received: 17 November 2010
Accepted: 4 February 2011

Abstract

Typically, lotic caddisflies attach their mineral pupal cases to cobbles in riffles, where rapid flows facilitate respiration but also decrease case-building material availability through erosion. Effects of local grain availability on grain quantities in and architecture of (per capita grain size use) pupal cases should be more important in Resident Construction Workers (RCWs, building immediately before pupation with minerals collected near the pupation location) than in Itinerant Construction Workers (ICWs, building months before pupation with minerals collected distantly from the pupation location). I tested these hypotheses analyzing mineral grain sizes in pupal cases and streambed sediments of cobble habitats in riffles of five running water types (headwater to large river in different regions) at baseflow or exceptional droughts. When pupae were abundant, the data supported both hypotheses at the local scale of samples, as grain size use by RCWs (as a group) but not by ICWs increased across all sites with local grain availability and abundant taxa among the former responded with four types of case-architecture modifications to grain size shortage. The data also supported the idea that at larger scales such as river or habitat types, mineral grains may be a limited resource for caddisflies building pupal cases with them. These findings suggest that water currents in streams or near shores of lakes and oceans that erode finer mineral grains can create conflicts in resource requirements for invertebrates that build with locally occurring finer mineral grains and simultaneously need high oxygen renewal rates and coarse grains for attachment.