2011 SOUTHEASTERN NATURALIST 10(1):121–132 Life-History Notes on Cambarus hubbsi Creaser (Hubbs Crayfish) from the South Fork Spring River, Arkansas Eric R. Larson1,* and Daniel D. Magoulick2 Abstract - Many crayfish species native to the southeastern United States are imperiled due to small range sizes and anthropogenic impacts such as habitat loss and introduction of non-native species. Furthermore, effective management of crayfish is limited by the scarcity of life-history and ecological data for many of these species. We report results of the first life-history study of the crayfish Cambarus hubbsi (Hubbs Crayfish). We collected 466 Hubbs Crayfish from the South Fork Spring River, AR throughout 2006 and recorded carapace lengths, wet weights, indicators of reproductive activity, and number of eggs on ovigerous females. Using length-frequency distributions, we identified four Hubbs Crayfish age classes and evaluated growth rates by plotting size by season (winter, spring, summer, autumn). Male Hubbs Crayfish were more common than females in all seasons except autumn, and males weighed more at equivalent lengths than females. Reproductive activity in Hubbs Crayfish peaked in late winter and spring, and ovigerous females were collected in March, April, and June. Ovigerous females were age II or III and carried few eggs relative to co-occurring crayfish of the genus Orconectes. Compared to these Orconectes species, Hubbs Crayfish is comparatively slow growing, long lived, with low reproductive potential, and as a result may be categorized as a K life-history strategist. Based on this species’ life-history strategy and previously documented habitat specificity and taxonomic distinctiveness, Hubbs Crayfish may require monitoring and management attention normally reserved for species with smaller ranges. Introduction North America is home to at least 382 species of crayfish, representing 60% of world crayfish diversity (Crandall and Buhay 2008). A recent conservation assessment found 48% of these crayfish are possibly extinct, endangered, threatened, or vulnerable (Taylor et al. 2007), and crayfish are anticipated to experience high extinction rates in the future due to this degree of imperilment (Ricciardi and Rasmussen 1999). Crayfish are predominantly threatened by small natural range sizes and related vulnerability to localized habitat loss or degradation, as well as impacts of and displacement by introduced crayfish (Lodge et al. 2000, Taylor et al. 2007). In addition, the management and conservation of crayfish is made difficult by the absence of basic ecological and life-history information for many of these species (Taylor et al. 2007). Here we report a life-history study on Cambarus hubbsi Creaser (Hubbs Crayfish). Hubbs Crayfish is endemic to, but widely distributed throughout, the Ozark 1School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98105. 2US Geological Survey, Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701. *Corresponding author - lars9570@u.washington.edu. 122 Southeastern Naturalist Vol. 10, No. 1 Plateau of northern Arkansas and southern Missouri (Pflieger 1996). Hubbs Crayfish is a stream- and river-dwelling crayfish that selects coarse substrates and fast-flowing riffles and runs, often occurring in lower densities than cooccurring species of the genus Orconectes (Flinders and Magoulick 2005, 2007). Although recognized by Taylor et al. (2007) as currently stable and not requiring conservation attention, Crandall (1998) recommended increasing conservation prioritization for Hubbs Crayfish due to the species’ restricted habitat specificity and contribution to regional taxonomic diversity. In addition, Hubbs Crayfish has disappeared from headwaters of the South Fork Spring River in Arkansas and Missouri, where the species was previously found (Pflieger 1996). The recently introduced Orconectes neglectus chaenodactylus Williams (Gap Ringed Crayfish) is suspected of displacing both Hubbs Crayfish and the rare endemic Orconectes eupunctus Williams (Coldwater Crayfish) from these streams through biotic mechanisms (Flinders and Magoulick 2005, Magoulick and DiStefano 2007). As part of ongoing studies evaluating the potential threat of the introduced Gap Ringed Crayfish to the South Fork Spring River native crayfish community (Larson and Magoulick 2008, Rabalais and Magoulick 2006), we collected life-history information on Hubbs Crayfish from a site in the lower South Fork Spring River, where the species remains common. Our objectives were to provide life-history values for this little-studied species that may be applied to its conservation and management, and contribute to the developing literature on life histories of crayfish native to the southeastern United States. Field-site Description The South Fork Spring River is a major tributary of the Spring River watershed, which drains 3926 km2 of the Salem Plateau physiographic region in north-central Arkansas and south-central Missouri. Agricultural pasture and forest dominate land cover in the Spring River drainage, with no substantial urban areas. The Spring River drainage contains a diverse stream-dwelling crayfish community consisting of six native species and the recently introduced Gap Ringed Crayfish, which remains restricted to the West Fork and upper South Fork Spring River (Flinders and Magoulick 2005). Life-history sampling for Hubbs Crayfish was conducted in the lower South Fork Spring River at a site (36º21'N, 91º37'W) immediately downstream of the Highway 289 bridge at Saddle, AR. The sample site included a large, fast-flowing riffle and downstream run, as well as an adjacent side channel with multiple smaller riffles and runs. High current velocities and large substrates at the site were consistent with previously documented habitat preferences of Hubbs Crayfish (Flinders and Magoulick 2005, 2007; Pflieger 1996). Methods Data collection Hubbs Crayfish were collected over 14 sampling occasions from January to December 2006 using a “kick seining” technique in which the substrate was 2011 E.R. Larson and D.D. Magoulick 123 physically disturbed and crayfish were flushed by the current into a downstream 3-mm mesh net. Kick seining was conducted for 3–4 hours per sampling occasion. For all Hubbs Crayfish collected, we measured wet weight with a portable field scale (to nearest 0.1 g) and total carapace length (TCL)—the distance from the anterior tip of the rostrum to the posterior edge of the carapace—with vernier calipers (to nearest 0.1 mm). We then recorded sex and reproductive activity as either gonopod form (I or II) for male crayfish or presence of visible glair gland secretions (hereafter referred to as glair) or eggs for female crayfish. Form I gonopods indicate maturity or seasonal reproductive activity in male crayfish of the family Cambaridae, while glair is visible under the abdomen of female crayfish and used to adhere eggs to pleopods (swimming legs). Egg counts were conducted non-destructively (without removal of eggs) in the field. All Hubbs Crayfish were released at the site of collection immediately following the recording of data. The thermal regime at the site was documented by an Onset Computer Corporation HOBO® temperature logger recording temperature (ºC) at hourly intervals from February to December 2006. Data analysis Life-history data for Hubbs Crayfish were combined by season for analysis, with seasons defined as winter (January–March), spring (April–June), summer (July–September), and autumn (October–December). We evaluated size structure, age structure, and growth rates for Hubbs Crayfish by creating lengthfrequency histograms by season, and then fitted normal distributions to these histograms with maximum likelihood estimation (MacDonald and Green 1988). Minimum size, maximum size, and means and standard deviations for each age class were plotted by season and fitted with spline lines to represent growth rates. Age classes turned over (e.g., Age 0 became Age I, etc.) between the winter and spring, and the spring age structure (0, I, II, and III) is used throughout the manuscript when discussing age-class attributes. Potential differences in lengthweight relationships between male and female crayfish were tested with analysis of covariance (ANCOVA) on log-transformed lengths and weights combined across all seasons. Sex ratios (M:F) were calculated for each season and tested for significance against the null hypothesis of an equal 1:1 sex ratio with chi-square (χ2) tests. The percent of Form I males and females with glair or eggs (ovigerous) by season were calculated, and mean sizes and standard deviations for these reproductive crayfish are provided. We plotted sex ratios and percent reproductive activity by sampling occasion for comparison to continuous stream water temperatures. We calculated water-temperature means and ranges by season, with coefficients of variation (CV) provided as a measure of seasonal water temperature variability. Statistical analyses were conducted in R (R Development Core Team 2008). Results We collected 466 Hubbs Crayfish: 81 individuals in winter, 159 in spring, 113 in summer, and 113 in autumn. Four age classes of Hubbs Crayfish were 124 Southeastern Naturalist Vol. 10, No. 1 identified from length-frequency histograms (Fig. 1). Age 0 crayfish recruited to the population in the spring, with the first juvenile detected at 5.2 mm TCL on 15 May at a water temperature of 16 ºC, and grew to a mean size of 11.7 (± 1.9 SD) mm TCL by autumn. Age I crayfish grew from a mean size of 13.2 (± 3.0 SD) mm TCL in winter to 19.5 (± 2.3 SD) mm TCL by autumn, and Age II crayfish grew from a mean size of 18.9 (± 0.6 SD) mm TCL in winter to 25.5 (± 1.2 SD) mm TCL by autumn (Table 1, Fig. 2). Age III crayfish were detected at mean sizes of 26.7 (± 2.1 SD) mm TCL in the winter and 29.2 (± 0.8 SD) mm TCL in the spring, but were absent or rare by summer (Table 1, Fig. 2). As a result, Age III may be the maximum age for Hubbs Crayfish in this population. The largest Hubbs Crayfish found during the study was a 30.7-mm-TCL individual collected on 11 January. Male Hubbs Crayfish weighed significantly more than female Hubbs Crayfish at equivalent lengths (F = 7.01, P < 0.01). Although sex ratios were slightly male-dominated in winter, spring, and summer and female-dominated in autumn, none were statistically significant from a 1:1 sex ratio (Table 1). The overall male-dominated sex ratio of 227:205 (1.11) was also not significantly different from a 1:1 sex ratio (χ2 = 1.12, P = 0.29). Reproductive activity for Hubbs Crayfish peaked in winter and spring and was lowest in summer (Table 1, Fig. 3). Fifty percent of male crayfish were Form I, and 29% of female crayfish had glair on 17 March at 13 ºC. During the summer months of August and September, no Form I male crayfish and no female crayfish with glair were collected. The first Form I male in autumn was found on 14 Figure 1. Length-frequency histograms by season for Hubbs Crayfish from the South Fork Spring River, AR, 2006. 2011 E.R. Larson and D.D. Magoulick 125 October at 15 ºC, and 23% of males were Form I by 22 November at 8 ºC. The first female with glair in autumn was found on 22 November, and 33% of females had glair by 16 December at 10 ºC. Three female crayfish with eggs were collected over the course of the study. A 27.4-mm-TCL female with 11 eggs was collected on 18 March at 12 ºC, a 24.4-mm- TCL female with 54 eggs was collected on 16 April at 22 ºC, and a 24.2-mm-TCL female with 33 eggs was collected on 12 June at 24 ºC. These crayfish were all at least Age II based on size. Similarly, the smallest Form I male was 18.7 mm TCL and collected on 31 May at 16 ºC, and the mean size of all Form I males was 24.0 mm TCL (± 3.6 SD). The smallest female collected with glair was 17.5 mm TCL and collected on 12 June, and mean size of all females with glair was 23.0 mm TCL (± 2.9 SD). These results indicate that Hubbs crayfish require at least one year, and more likely two years, to reach reproductive maturity. Discussion Cambarus is the second most diverse genera of crayfish in the world, with nearly 100 species found in North America (Crandall and Buhay 2008). Lifehistory information is rare for Cambarus crayfish, with exceptions including Table 1. Summary statistics by season for river temperature (ºC), Hubbs Crayfish size by age class (mm TCL), sex ratio (M:F), and proportion of the population reproductively active (%). Seasons are winter (January, February, March), spring (April, May, June), summer (July, August, September), and autumn (October, November, December). Coefficients of variation (CV) are reported for river temperature as a measure of consistency and variability within season. Age classes were determined and TCL means and standard deviations calculated by fitting normal distributions to length-frequency histograms (Fig. 1) with a maximum likelihood mixed model (MacDonald and Green 1988). Age classes turned over between winter and spring (e.g., Age 0 became Age I, etc.). Chi-square statistics (χ2) and P-values are provided to report if sex ratios deviate significantly from a null value of 1:1. Variable Winter Spring Summer Autumn Temperature Mean 11.7 21.0 20.8 12.1 Range 6.5–17.9 15.5–27.1 16.4–25.9 3.7–22.1 CV 18.5 13.8 7.8 31.8 Size TCL (SD) Age 0 13.2 (3.0) 5.8 (1.0) 10.1 (2.1) 11.7 (1.9) Age I 18.9 (0.6) 14.0 (2.0) 19.0 (2.1) 19.5 (2.3) Age II 26.7 (2.1) 20.2 (3.5) 24.8 (1.7) 25.5 (1.2) Age III - 29.2 (0.8) - - Sex Ratio Total M:F 38:34 73:62 61:51 55:58 Ratio 1.12 1.18 1.20 0.95 χ2 (P-value) 0.11 (0.74) 0.45 (0.50) 0.45 (0.50) 0.04 (0.84) Reproduction M Form I 26.3 13.7 1.6 12.7 F Glair 14.7 14.5 2.0 12.1 F Ovigerous 2.9 3.2 0.0 0.0 126 Southeastern Naturalist Vol. 10, No. 1 widespread and well-studied species such as Cambarus bartonii Fabricius (Appalachian Brook Crayfish), Cambarus diogenes Girard (Devil Crayfish), and Cambarus robustus Girard (Big Water Crayfish) (Guiasu 2002). Furthermore, the vast majority of Cambarus crayfish are distributed east of the Mississippi River, with the handful of Cambarus species found west of the Mississippi River either cave-dwelling or terrestrial burrowers (Guiasu 2002). Consequently, Hubbs Crayfish has a relatively unique combination of taxonomy, distribution, and habitat preference, comparable only to the closely related stream-dwelling Cambarus maculatus Hobbs and Pflieger (Freckled Crayfish) found in eastern Missouri’s Meramec River drainage (Crandall 1998, Pflieger 1996). This distinctiveness makes Hubbs Crayfish an interesting subject for ecological and life-history inquiries, and our findings may also have important implications for management and conservation of this species. Our results indicate Hubbs Crayfish is a K-life-history strategist due to its slow growth rates, late age to maturity, and low reproductive potential (Momot 1984), particularly in contrast to co-occurring Orconectes species found in the Figure 2. Seasonal size distributions for Hubbs Crayfish from the South Fork Spring River, AR, 2006. Means bounded by standard deviations are provided for each age class, with minimum (down triangles) and maximum (up triangles) sizes also reported. Age classes were determined, and means and standard deviations calculated by fitting normal distributions to length-frequency histograms (Fig. 1) with a maximum likelihood mixed model (MacDonald and Green 1988). Spline curves are plotted through age class means to represent the sigmoidal nature of crayfish growth rates by season. Age classes turned over (e.g., Age 0 became Age I, etc.) between the winter and spring. 2011 E.R. Larson and D.D. Magoulick 127 Spring River drainage (Flinders and Magoulick 2005, Larson and Magoulick 2008). This conclusion is supported by the limited life-history data available for Hubbs Crayfish in Pflieger (1996) and Flinders and Magoulick (2005). During state-wide surveys of Missouri crayfish, Pflieger (1996) only collected a single ovigerous Hubbs Crayfish female in the wild (6 May 1986), although two females produced eggs in the laboratory. These crayfish carried 57, 76, and 111 eggs at Figure 3. River temperatures (ºC), Hubbs Crayfish sex ratios (M:F), and proportion of population reproductively active (Form I for males, Glair and Ovigerous for females) by month and season. Data are from the South Fork Spring River, AR, 2006. 128 Southeastern Naturalist Vol. 10, No. 1 58.4, 61.0, and 66.0 mm total length (TL), respectively. Pflieger (1996) estimated Hubbs Crayfish at age 0 were 12.7 to 27.9 mm TL and age I individuals were 33.0 to 53.3 mm TL in November of each year. Assuming Pflieger’s (1996) values may be halved to approximate carapace lengths, they generally support our findings of slow growth rates and small size structure for age 0 and I Hubbs Crayfish, as well as late age to maturity in this species. Sampling small streams throughout the Spring River drainage from 16 March to 25 April 1999, Flinders and Magoulick (2005) reported only 12.5% of male Hubbs Crayfish were Form I and 0% of Hubbs Crayfish females were ovigerous. These were lower values than those found for four co-occurring Orconectes species. Additionally, Flinders and Magoulick (2005) found that reproductively active Hubbs Crayfish were large individuals (>22 mm TCL), also consistent with our results. Finally, both Pflieger (1996) and Flinders and Magoulick (2005) reported evidence that Hubbs Crayfish produces eggs later in the spring than co-occurring Orconectes species, a pattern that could contribute to smaller size structure and slower growth rates in juveniles of this species (Rabeni 1985). Larson and Magoulick (2008) documented life histories of both the native Coldwater Crayfish and introduced Gap Ringed Crayfish in the South Fork Spring River during 2006. Both of these species exhibited faster growth rates, earlier ages to maturity, higher fecundities, and greater reproductively active proportions of their populations than Hubbs Crayfish. Age 0 Coldwater and Gap Ringed Crayfish were 17–18 mm TCL by autumn, and most of these crayfish were reproductively active within their first year of life. The smallest reproductively active Coldwater and Gap Ringed Crayfish were 13–14 mm TCL. At 24 mm TCL, Coldwater and Gap Ringed Crayfish could be expected on average to carry between 80 and 120 eggs, in contrast to the generally lower values found for Hubbs Crayfish. Furthermore, at the peak of reproductive activity, virtually all Coldwater Crayfish males were Form I and 90% of Coldwater Crayfish females were ovigerous, while 60% of Gap Ringed Crayfish males were Form I and 30% of Gap Ringed Crayfish females were ovigerous. Momot (1984) interpreted r and K life-history strategies in crayfish as responses to latitudinal variation in seasonal food availability, arguing that r-selected crayfish occur at southern latitudes where food supply is constant and K-selected crayfish occur at northern latitudes where food supply is pulsed with summer. This interpretation cannot explain r and K life-history strategies among crayfish occurring at equivalent latitudes in the same streams and rivers. Ecologists have also historically characterized r-strategists as colonist or disturbance-adapted species and K-strategists as stable or competition-adapted species (MacArthur and Wilson 1967). Once again, this characterization does not adequately explain life-history differences between Hubbs Crayfish and Orconectes species such as the Coldwater Crayfish because these species occupy the same streams and consequently experience and are adapted to the same hydrological disturbance regimes (Lytle and Poff 2004). However, might these species vary in vulnerability to disturbance due to differences in habitat selection? Creaser (1931), Pflieger (1996), and Flinders and 2011 E.R. Larson and D.D. Magoulick 129 Magoulick (2005) observed that Hubbs Crayfish burrows under large cobble and boulders and is infrequently encountered in open habitats. Perhaps burrowing under large substrates provides Hubbs Crayfish with refuge from disturbances such as floods that Orconectes crayfish with more general habitat requirements may be vulnerable to (Clark et al. 2008)? Hubbs Crayfish may also avoid intense predation pressure from fish by remaining under large substrates, restricting access to food such as surface-growing periphyton but simultaneously lowering mortality rates. This hypothesis is supported by Dukat and Magoulick’s (1999) finding that mortality via predation increased significantly on Hubbs Crayfish relocated from riffles with high substrate diversity to pools with low substrate diversity. Life history has been found to strongly influence crayfish habitat selection, competitive ability, and predation vulnerability (Quinn and Janssen 1989, Rabeni 1985), and consequently the K-life-history strategy and high habitat specialization of Hubbs Crayfish are likely linked. Habitat selection by Hubbs Crayfish may also interact with sampling methodology to influence the life-history patterns we observed. Burrowing under large substrates may make Hubbs Crayfish difficult to collect in contrast to cooccurring Orconectes species with more general habitat preferences. This could be particularly problematic in detecting ovigerous crayfish, which become less active and select isolated habitats while bearing eggs (Mason 1970). Studies of sampling methodology for stream crayfish have primarily evaluated the influence of gear and habitat on estimates of abundance, density, or length-frequency (Price and Welch 2009, Rabeni et al. 1997), but have not evaluated the ability to detect rare individuals such as ovigerous females. Emerging statistical methods that account for imperfect detection of target organisms may be useful in future studies evaluating the presence or abundance of ovigerous crayfish or rare crayfish in general (MacKenzie et al. 2006). The introduced Gap Ringed Crayfish is suspected of displacing Hubbs Crayfish from a portion of its former range in the upper South Fork Spring River (Magoulick and DiStefano 2007, Pflieger 1996), and life-history traits such as higher fecundity or rapid juvenile growth rates have been implicated as mechanisms favoring introduced over native crayfish (Butler and Stein 1985, Quinn and Janssen 1989). However, the coexistence of Hubbs Crayfish across its native range with at least 11 Orconectes species with life histories similar to the Gap Ringed Crayfish makes this an unlikely mechanism for displacement. Other biotic (e.g., susceptibility to fish predation; Dukat and Magoulick 1999, Rabeni 1985) or abiotic (e.g., habitat or hydrological regime alteration; Larson et al. 2009) mechanisms may be responsible for Hubbs Crayfish range contraction in the South Fork Spring River. Isolating the causal mechanism or mechanisms will require additional studies on Hubbs Crayfish population dynamics, biotic interactions, and status and trends of habitat conditions in this and other drainages. Understanding the life history of Hubbs Crayfish has important implications for this species’ conservation. Crandall (1998) advocated increased conservation attention for Hubbs Crayfish due to the species’ unique taxonomy and narrow habitat specialization. This habitat specialization may make Hubbs Crayfish 130 Southeastern Naturalist Vol. 10, No. 1 vulnerable to both anthropogenic impacts and natural disturbances. Gravel mining of streams is a common practice throughout the range of Hubbs Crayfish, and has been found to disproportionately affect populations of riffle- and rundwelling species relative to co-occurring habitat generalists (Brown et al. 1998). In addition, the preferred habitats of Hubbs Crayfish can dry extensively due to seasonal and supraseasonal drought (Brown and Brussock 1991), and crayfish differ substantially in their tolerance to desiccation and stream drying (Larson et al. 2009). Although Hubbs Crayfish is known to occupy intermittent streams, it is not as common in these systems as some Orconectes species (Flinders and Magoulick 2003), and the resistance and resilience of Hubbs Crayfish to drought and stream drying is unknown. Our study complements past work on Hubbs Crayfish habitat specialization by demonstrating that this species may recover slowly from natural and anthropogenic disturbances due to its life-history strategy. Some common, widely distributed crayfish species have recently been found to be suffering dramatic population declines (Edwards et al. 2009), and consequently, assumptions of species’ security deserve scrutiny. 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