Trophic and habitat requirements of two deposit-feeding stream invertebrates, Ptychoptera townesi (Diptera) and Paraleptophlebia spp. (Ephemeroptera)

Graduation date: 1986Ptychoptera townesi (false crane fly, Diptera) occur in high densities in an experimental stream section that has not been allowed to exceed bankfull flow for more than two decades, but are quite rare in areas both upstream and downstream from this section. By contrast, Paraleptophlebia spp. (mayfly, Epherneroptera) are relatively abundant throughout the channel. Paraleptophlebia spp. are able to feed and grow on a variety of resource types, whereas Ptychoptera townesi are restricted to feeding on fine particulate organic matter (FPOM) less than 250 [mu]m in diameter, particularly 0.45-53 [mu]m. Morphological and behavioral differences between these genera are indicative of differences in feeding capability. These genera also differ in response to refractory food resources. The alimentary tract of the dipteran is considerably more differentiated than the simple straight tube gut tract typical of the mayflies. In addition, Ptychoptera townesi consume relatively small amounts (per unit animal dry mass) of particulates (0.45 - 53 [mu]m) compared with the mayflies ingestion of leaf material. Ptychoptera townesi have longer gut content passage times (> 19 h) and higher efficiencies of conversion of ingested substrate to body substance, under conditions studied, than do Paraleptophlebia spp. (2-3 h) on\ud these respective food resources. Gut contents of Ptychoptera townesi become colonized by microbes typically found in the hindgut, and are compacted into pellets considerably larger in size than particles ingested. These animals lose mass when allowed to feed on this material. Ptychoptera townesi ingest and egest relatively sterile detrital material at a rate reduced from that on natural detritus. The pattern exhibited by Ptychoptera townesi involves slow handling of relatively small volumes of food, which probably pass once through an elaborate alimentary tract. That exhibited by Paraleptophlebia spp. involves rapid processing and partial recycling of large amounts of material through a relatively simple alimentary tract. Considered in a broader context, the results of this investigation have implications for the perceived role of deposit-feeding organisms in the organization and development of stream communities. Deposit-feeders are\ud capable of far more than, as generally claimed, selecting particles on the basis of size, independent of food quality, and digesting primarily the associated microbes for their nutrition