Behavior of Crayfish in Water Currents: Hydrodynamics of Eight Species with Reference to Their Distribution Patterns in Southern Ontario

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Abstract
When exposed to increases in current speed in a laboratory stream tank, crayfish altered their body posture to counteract the effects of drag, thus enabling them to maintain station at higher current speeds. Using a beam-type drag balance, direct drag measurements were made on live adults of eight Ontario crayfish species over a range of current speeds. The species were then ranked according to their relative streamlining efficiencies as indexed by these drag measurement data. The station-holding abilities of the species were compared by determining the mean slip speed of each species on a roughened Plexiglas substrate. There were significant differences in slip speed, the species falling into three groups. The first contained only Cambarus robustus, which had a mean slip speed of 49.7 cm∙s−1. The fast-water species, Orconectes rusticus, C. bartoni, and O. propinquus, all had mean slip speeds between 30 and 40 cm∙s−1. The remaining species, O. obscurus, O. virilis, C. fodiens, and O. immunis had mean slip speeds of less than 30 cm∙s−1. The crayfish species' substrate texture preferences were also tested. Cambarus bartoni was the only species which preferred a gravel to a mud substrate. Orconectes obscurus, O. virilis, C. fodiens, and O. immunis all preferred a mud substrate. We suggest that the superior streamlining capabilities of C. bartoni and O. rusticus may help to explain the occurrence of these species in fast-flowing rivers, as well as the latter's rapid and widespread expansion in Ontario since its introduction. On the basis of relative station-holding abilities, O. rusticus should continue to be more successful at expanding its range in southern Ontario via lotic waterways than the other introduced species, O. obscurus.Key words: crayfish, drag, station-holding ability, streamlining, drag coefficient, hydrodynamics, posture, slip speed, substrate, distribution