| Larval net-spinning caddisflies construct silken catchnets to filter organic matter from streams. Net spinners often attain very high population densities, and competitive displacement into less desirable, lower flow velocity microhabitats could be an important reproductive isolating mechanism for them. Because less water and therefore less food would pass through the displaced caddisflies' nets, these individuals may mature more slowly than competitively superior individuals, thereby resulting in temporal reproductive isolation. I wanted to test this hypothetical isolating mechanism with Macrostemum carolina. In the Savannah River, M. carolina larvae construct three different kinds of retreats, each with a distinct water entrance hole: (1) at the end of a silken tube, (2) with a ∼180° silken backstop, and (3) flush with the top of the retreat. Although it is uncertain whether M. carolina retreat morphs partition available snag habitats based on flow velocity, genetic divergence between them may provide initial evidence for this isolating mechanism. Therefore, I compared the allele frequencies of three polymorphic nuclear loci and the mitochondrial DNA haplotype frequencies among individuals of each retreat morph. The morphs are not genetically differentiated (P > 0.05 for all analyzed loci), so they constitute a single panmictic population in the Savannah River. Therefore, the M. carolina retreat morphs are apparently not incipient species, and as such do not support my proposed reproductive isolating mechanism for net-spinning caddisflies. Nonetheless, I wanted to identify the proximate mechanism mediating this polymorphic retreat construction behavior, so I brought larvae of known phenotype into the laboratory and "forced" them to construct a second retreat. I found that: (1) larvae can construct more than one type of retreat, so this behavior is not under strict genetic control, (2) larvae do not preferentially re-construct their original retreat design, so this behavior is apparently not partially heritable, and (3) larvae primarily construct each retreat phenotype in a particular microhabitat. Therefore, the retreat polymorphism in M. carolina is phenotypically plastic and is controlled by microhabitat location, or a correlated environmental variable. Consequently, the ultimate mechanism shaping this behavioral polymorphism was probably competition for space with other net-spinning caddisflies. |
