Habitat and sediment preferences of Axarus festivus larvae

During fall draw-down of a reservoir in northeast Kansas it was observed that larvae of Axarus festivus were restricted to highly weathered Pennsylvanian Shale outcrops and surrounding coarse sediments with high-clay content derived from erosion of the shale outcroppings. Larvae constructed burrows into the outcrops and eroded coarse sediments, which they used to filter feed by setting up currents through the burrows. Burrows were widely distributed over the outcrops, with average densities ranging from 372-2,351 burrows m^-2. However, closer inspection revealed that burrows were more common at apices of individual shale strata, where weathering of the outcrop was most advanced. Here burrows were more uniformly distributed and densities ranged to 4,166 burrows m^-2. 73% of burrows contained larvae. Burrows were generally U-shaped, and averaged 1.8 mm in diameter and 42 mm in total length. Laboratory experiments revealed that 4th instar larvae removed from burrows could construct new burrows in weathered shale, but preferentially used old empty burrows if available. When given choices among alternative sediment combinations of sand vs. finely-ground shale, sand vs. coarsely-ground shale, and finely-ground shale vs. coarsely-ground shale, larvae exhibited statistically significant preferences for the finely-ground shales (P<0.001), coarsely-ground shales (P<0.001), and coarsely-ground shales (P<0.01), respectively. It is concluded that larvae (1) actively select shale or high-clay content sediments, (2) can differentiate among sediments with differing physical properties and (3) exhibit behavioral choices for sediment types that guide them toward shale outcrops.