The effect of water flow and oxygen concentration on six species of caddis fly (Trichoptera) larvae

Abstract

Summary. The six species of caddis larvae selected for study were collected from the River Blyth, Northumberland, a small river with alternate shallow swift‐flowing and deeper slow‐flowing stretches. The species selected were Rhyacophila dorsalis, Wormaldia subnigra and Hydropsyche instabilis from swift water, Anabolia nervosa and Polycentropus flavomaculatus from slow‐flowing water and Stenophylax stellatus, a species which is generally distributed. Of these species, Anabolia nervosa and Stenophylax stellatus are case‐bearing larvae and Hydropsyche instabilis, Wormaldia subnigra and Polycentropus flavomaculatus spin nets or snares for food collection. Rhythmic undulatory current‐producing movements of the abdomen are made by all the larvae except Rhyacophila dorsalis and Wormaldia subnigra. Accurate identification entailed rearing the larvae to the adult state. Rhyacophila dorsalis, Wormaldia subnigra and Hydropsyche instabilis, species from swift waters which do not live long in still‐water aquaria, were successfully reared in small aquaria in which the water was continuously stirred by means of rotary stirrers. The larvae may be arranged in the following ascending order of ability to resist the flow of water when exposed to the full force of the current in an experimental channel:— Polycentropus flavomaculatus. Anabolia nervosa. Stenophylax stellatus. Wormaldia subnigra. Rhyacophila dorsalis, Hydropsyche instabilis. The behaviour of the net‐ and snare‐spinning larvae in aquaria showed that Hydropsyche instabilis and Wormaldia subnigra larvae only spin typical nets in flowing water and that the former will spin nets in slower‐flowing water than will the latter. Polycentropus flavomaculatus larvae will spin snares in still water. Oxygen determinations on samples of water from swift‐ and slow‐flowing areas of the River Blyth showed only small differences. Further, under certain circumstances water from swift‐flowing reaches may have a lower dissolved oxygen concentration than water from slow‐flowing reaches. The minimum oxygen concentration, i.e. that at which larvae become immobilized, was determined experimentally for each species in still water. This concentration was relatively high for the swift‐water species Rhyacophila dorsalis, Wormaldia subnigra and Hydropsyche instabilis, but in stirred water it was similar to that obtained for the other species in still water. In Hydropsyche instabilis, Polycentropus flavomaculatus, Stenophylax stellatus and Anabolia nervosa larvae decreasing oxygen concentration tended to increase the number of undulatory abdominal movements. In Hydropsyche instabilis and Polycentropus flavomaculatus larvae the number of movements was reduced by an increase in the rate of stirring of the water. In conclusion, the importance of water flow in determining the distribution of these larvae is stressed. Larvae unable to withstand the force of the current are unable to inhabit swift waters (Polycentropus flavomaculatus, Anabolia nervosa). Some larvae are dependent upon current for feeding (Hydropsyche instabilis, Wormaldia subnigra), and larvae readily immobilized at a relatively high oxygen concentration in still waters (Rhyacophila dorsalis, Wormaldia subnigra, Hydropsyche instabilis) can tolerate low oxygen concentrations in flowing water.