Resource Partitioning by Stream-dwelling Crayfish: The Influence of Body Size

Abstract

The crayfishes Orconectes luteus and 0. punctimanus are sympatric in over 200 km of the Jacks Fork and Current rivers in S-central Missouri. The combinations of substrate particle size, current velocity, depth and macrophyte cover used by youngof-the-year and adults of each species tended to be distinct. Orconectes punctimanus was more restricted in its use of habitats. Young-of-the-year were strongly associated with macrophyte cover and shallow water, and adults with larger substrate particles; both life stages preferred slow current velocities. Orconectes luteus was more evenly distributed over most of the habitats. Young-of-the-year used current velocities and substrate particle sizes somewhat more in proportion to their availability than did adults, which were concentrated within the higher ranges of both of these variables. Laboratory choice experiments involving combinations of species and size classes indicated that crayfish dominance was a function of body size and that habitat use by one size class could be modified when larger individuals of the same or a different species were present. The young of 0. punctimanus hatched earlier and grew faster than did those of 0. luteus. Orconectes punctimanus therefore maintained a distinct size advantage throughout its life cycle. Orconectes punctimanus probably inhabited its preferred set of microhabitat conditions and excluded 0. luteus from these areas. Because much of the remaining stream habitat was unsuitable for 0. punctimanus, competitive exclusion did not occur. Other factors, most notably fish predation, probably complement physical factors and species interactions in determining the distributional patterns of these crayfishes. INTRODUCTION The crayfishes of any particular aquatic habitat generally are limited to one or very few species, even if many species occur in the geographical region (e.g., Williams, 1954). Closely related species commonly exhibit disjunct distributions (see Bovbjerg, 1970, for a review) and an introduced species may rapidly replace one that occurred naturally (Capelli, 1982). The mechanisms involved in displacement or in maintaining species separation are not well understood, but crayfish behavior often gives insight into probable causes. It was well documented by Bovbjerg (1953, 1956), Penn and Fitzpatrick (1963) and Capelli (1982) that crayfish are aggressive, and Capelli and Munjal (1982) recorded interspecific encounters leading to attack in laboratory experiments. Intraspecific attacks are probably common either during the molting period, or by adults on juveniles after production of a maternal pheromone ceases (Little, 1976). Crayfish are mobile, and migrate (Momot, 1966; Flint, 1977), and disperse from areas of high density to areas of low density (Bovbjerg, 1964). Such behaviors have led several investigators to conclude that disjunct crayfish distributions were due to competitive exclusion (Aiken, 1965; Fitzpatrick, 1967; Crocker and Barr, 1968; Capelli and Magnuson, 1983). There are also situations where closely related species coexist in what appears to be a common habitat. Given their aggressive tendencies, different species existing in proximity is more surprising than is the exclusion of one species by another. The question why some crayfish are sympatric and others are not was the impetus for this study. In the Current and Jacks Fork rivers in S-central Missouri, Orconectes luteus (Creaser) and 0. punctimanus (Creaser) coexist in over 200 km of stream. The objectives 1 The Unit is sponsored by the U.S. Fish and Wildlife Service, the Missouri Department of Conservation and the University of Missouri.