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Life Cycles of Allocapnia recta and Leuctra spp. (Plecoptera: Capniidae and Leuctridae) Across a Flow Gradient in a Central Kentucky Karst Headwater Stream
Scott A. Grubbs, Christopher M. Thomas, Benjamin T. Hutchins, and Jason M. Taylor

Southeastern Naturalist, Volume 5, Number 2 (2006): 321–332

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2006 SOUTHEASTERN NATURALIST 5(2):321–332 Life Cycles of Allocapnia recta and Leuctra spp. (Plecoptera: Capniidae and Leuctridae) Across a Flow Gradient in a Central Kentucky Karst Headwater Stream Scott A. Grubbs1,*, Christopher M. Thomas1, Benjamin T. Hutchins1, and Jason M. Taylor2 Abstract - Four species of stoneflies (Plecoptera) from an intermittent stream– perennial spring continuum were the subject of comparative life-cycle analyses. Allocapnia recta was common in each reach and exhibited univoltine-fast life cycles and determinate voltinism. Leuctra alta from the intermittent reach and a L. alta–L. sibleyi mix from the perennial reach each displayed an univoltine-fast cycle, implying that L. alta demonstrated determinate voltinism. Leuctra cf. tenuis was absent from the intermittent reach, and its growth followed a univoltine-slow cycle in the perennial reach. Introduction Water flow is widely recognized as an important variable regulating the distribution of aquatic biota in lotic systems (Power et al. 1988). Several researchers have demonstrated that permanent streams support different macroinvertebrate communities than those in intermittent streams (Bottorff and Knight 1988, Delucchi and Peckarsky 1989, Feminella 1996, Williams and Hynes 1976, Wright et al. 1984). While macroinvertebrates of springs and perennial coldwater streams are dependant on flow permanence and thermal stability (Erman and Erman 1995, Smith and Wood 2002, Williams and Hogg 1988), intermittent streams, in contrast, often support taxa with life-history characteristics for surviving channel drying (Delucchi 1988, Williams 1996). In general, macroinvertebrate assemblages along a flow gradient from permanent to temporary respond to greater variability of physical conditions (e.g., temperature, dissolved oxygen), species-specific tolerance to drought adaptation (e.g., diapausing eggs), and reduced predatory or competitive abilities during stream drying. There have been few studies comparing stonefly life cycles across environmental gradients (e.g., Harper 1973). Life cycles and life histories have been described for approximately 5% of the North American fauna (Stewart and Stark 2002), but most of the research has been conducted in wadable streams and rivers. Relatively little life-cycle work has focused on the fauna of springs and intermittent streams (Dobrin and Giberson 2003, Snellen and Stewart 1979). 1Department of Biology and Center for Biodiversity Studies, Western Kentucky University, Bowling Green, KY 42101. 2The Nature Conservancy, 6375 Riverside Drive, Suite 50, Dublin, OH 43017. *Corresponding author - 322 Southeastern Naturalist Vol. 5, No. 2 Voltinism, or number of generations per unit time (e.g., per year), is characterized either as determinate or indeterminate depending on whether populations exhibit (a) the same pattern across habitats regardless of climatological or hydrological conditions (determinate), or (b) a different pattern due to environmental differences (indeterminate) (Stewart and Stark 2002). For insect populations that have one generation per year (i.e., univoltine), voltinism can also be described as fast or slow (Hynes 1961). Populations that display univoltine-fast life cycles typically occur in an actively-growing larval or nymphal stage for only a short time period. The egg or immature stage remains for an extended period in some form of a delayed state (e.g., diapause). In contrast, univoltine-slow cycles have neither delayed egghatching nor egg, larval, or nymphal diapause. Eggs usually hatch soon after oviposition and larval or nymphal growth occurs for much of the year. In May 2002, we initiated a comparative life-cycle study on Leuctra sibleyi Claassen from a headwater intermittent-stream–karst (perennial)- spring continuum. During the first year, Allocapnia recta (Claassen) and L. alta James were collected from both reaches and added to the study. In addition, L. cf. tenuis (Pictet) was later collected as adults only along the perennial spring and was also added to the study. The contrasting hydrology of the study stream, the first reach distinctly intermittent and the second perennial, provided an opportunity to compare life cycles. The purpose of this study was to examine life-history attributes of A. recta, L. alta, L. sibleyi, and L. cf. tenuis in the intermittent vs. the perennial reaches to assess whether voltinism patterns were determinate or indeterminate across the two habitats. Site Description and Methods The study stream was located in Hart County, KY (elevation: 255 m a.s.l.), and within the Crawford-Mammoth Cave Upland Level IV Ecoregion (Interior Plateau Level III Ecoregion; Woods et al. 2002). This region is characterized by sandstone cliffs and limestone valleys. Subterranean streams, springs, and karst windows associated with the latter feature are common. The headwater study stream originates on a small sandstone ridge and flows 35 m as an intermittent reach before dropping 16 m over a hollowed cliff. The stream continues intermittently before two springlets contribute perennial surface flow for 18 m. Flow ceases downstream of the perennial reach flow and the channel becomes ephemeral. Both pH (6.73 vs. 5.68) and conductivity (90 vs. 40 μmhos) were slightly higher in the perennial reach. Mean width (0.7 m) and depth (4.9 cm) in the intermittent reach were similar to the perennial reach (1.1 m, 3.1 cm.). Depth was monitored at a single point for each reach (Fig. 1) during each site visit (n = 47). Water temperature was measured initially in each reach independently at 1-hr. intervals with a Ryan RL100 temperature recorder (Fig. 1), but only from the perennial reach after July 2003 due to non-significant differences (t = 1.32, p = 0.19, n = 116). 2006 S.A. Grubbs, C.M. Thomas, B.T. Hutchins, and J.M. Taylor 323 Each reach was partitioned longitudinally into transects 0.5 m apart. Monthly sampling for nymphal stoneflies took place between May 2002 and April 2004. Three transects were randomly selected per reach and sampling occurred at the midpoint of each channel. Sampling methods for the two reaches differed. The intermittent reach was sampled with a petite ponar dredge (0.023-m2 sampling area) from May–June 2002 and January–May Figure 1. Temperature (A) and stage data (B) for the period May 2002 through April 2004. The temperature graph is based on mean daily temperature recorded from perennial reach. 324 Southeastern Naturalist Vol. 5, No. 2 2003 and with an Ekman dredge (0.023-m2 sampling area) from December 2003–April 2004. No samples were taken from the intermittent reach from July–December 2002 and June–November 2003 because this channel was dry. Sampling in the perennial reach from May 2002–December 2003 was based on brushing cobble substrates into a bucket and scooping finer substrates with a 250-μm sieve. The Ekman dredge was used from January– April 2004. The three replicate samples per reach were composited in the field, rinsed through a 250-μm sieve, and preserved with 95% ethanol. Each composite sample was rinsed through a 250-μm sieve in the laboratory and sorted under a dissecting microscope. To compare nymphal growth between the intermittent and perennial reaches, head-capsule widths were measured for all individuals and used to construct size-frequency histograms. Nymphs of Leuctra are difficult to distinguish to species (Stewart and Stark 2002). Hence, all three species were represented on a single graph. Adults of all species were collected weekly during each species’ emergence period. Allocapnia recta adults were obtained from tree trunks and by beating tree branches into a beating sheet. Leuctra spp. adults were collected with the beating sheet and searching under streamside rocks. Adult flight records were superimposed onto each nymphal histogram. Results Small diapausing nymphs of Allocapnia recta were first encountered in both reaches in May 2002 (Fig. 2). Nymphs were obtained again in the perennial reach in July 2002, yet not prior to emergence in winter 2002– 2003. In contrast, mature nymphs were collected from the intermittent reach in January and February 2003 coincidental with emergence. Newly-hatched A. recta nymphs were present in both reaches in March 2003, suggesting a direct hatch of eggs, and were likewise present in the perennial reach through July 2003. Similar to 2002, diapausing nymphs were not collected during late summer and autumn from the perennial reach. Nymphs in both reaches grew rapidly during November and December 2003 prior to emergence in January and February 2004. Newly-hatched nymphs were collected from February–April 2004. Leuctra cf. tenuis was collected as adults only along the perennial reach in September (2002 and 2003) and October (2003) (Fig. 3). In contrast, L. alta and L. sibleyi were obtained as adults along both reaches in May–June 2002, April–May 2003, and April 2004 at ratios of L. alta:L. sibleyi of 44:21 (perennial reach) and 144:1 (intermittent reach). Because of the rarity of L. sibleyi and lack of L. cf. tenuis from the intermittent reach, subsequent life-cycle descriptions for the intermittent reach refer only to L. alta. Life cycles for the perennial reach referred individually both to L. cf. tenuis and a L. alta–L. sibleyi mix. Nymphs of L. alta and L. sibleyi could not be distinguished. Presumably, the only nymphs collected from the perennial reach from May–July and September 2002 were of L. cf. tenuis. Mature nymphs with 2006 S.A. Grubbs, C.M. Thomas, B.T. Hutchins, and J.M. Taylor 325 Figure 2. Size-frequency histograms of head-capsule width for Allocapnia recta from the intermittent and perennial reaches. Reaches are separated as intermittent (left side of kite diagram, solid boxes) and perennial (right side of kite diagram, striped boxes). n = 169 nymphs (intermittent reach) and 129 nymphs (perennial reach). Arrows refer to adult flight periods. 326 Southeastern Naturalist Vol. 5, No. 2 Figure 3. Size-frequency histograms of head-capsule width for Leuctra spp. from the intermittent and perennial reaches. Reaches are separated as intermittent (left side of kite diagram, solid boxes) and perennial (right side of kite diagram, striped boxes). n = 690 nymphs (intermittent reach) and 835 nymphs (perennial reach). The longer dashed lines separate nymphs of L. cf. tenuis. Arrows refer to adult flight periods of each species: solid arrow = L. alta and L. sibleyi, dashed arrow = L. cf. tenuis. 2006 S.A. Grubbs, C.M. Thomas, B.T. Hutchins, and J.M. Taylor 327 developed wingpads were absent during this period, strongly suggesting the emergence period for L. alta and L. sibleyi had ended. Tracking the timing of egg hatching of L. cf. tenuis, however, was problematic. The early-instar Leuctra nymphs collected in December 2002 in the intermittent reach were L. alta, but those in the perennial reach likely were a mix of all three species (Fig. 3). Early-instar nymphs of L. cf. tenuis were apparent by January 2003, and growth was steady through spring and summer 2003 prior to emergence in September and October. Early-instar nymphs L. cf. tenuis also likely appeared by February 2004. During both 2002 and 2003, early-instar nymphs of L. cf. tenuis were not collected in November, implying that a slight delay of egg hatch was evident. There were two distinct size classes of Leuctra nymphs in the perennial channel from January–March 2003, indicating that the smallest nymphs were of L. cf. tenuis and the larger nymphs were the L. alta–L. sibleyi mix. The L. alta–L. sibleyi nymphs from the perennial channel grew steadily from January through March 2003 prior to emergence in April and May. A similar pattern of two distinctive size classes in the perennial channel was seen in January–February 2004. In contrast, there was only a single size class present in the intermittent reach from January–March 2003 and 2004 that was L. alta. Early-instar L. alta–L. sibleyi (perennial reach) and L. alta (intermittent reach) nymphs did not reappear in both reaches until January 2004, suggesting an extended delay in egg hatching. Discussion The life cycles of four species of stoneflies (Allocapnia recta, Leuctra alta, L. sibleyi, L. cf. tenuis) are described, with each Leuctra species for the first time. In addition, this is the first attempt to describe the life cycles of multiple species of Leuctra from the same stream. The inability to discriminate among nymphs of multiple species has hampered previous efforts (Grubbs and Cummins 1996, Huryn and Wallace 1987). Both A. recta and L. alta were sufficiently abundant to allow for between- reach comparisons of voltinism. The dissimilar hydrologic regimes did not result in life-cycle flexibility for either species. Adults were free to disperse between reaches, implying that the stream as a whole supported genetically contiguous populations. Allocapnia recta and L. alta exhibited univoltine-fast cycles and determinate voltinism, yet their life-history strategies were distinctly different. Eggs of A. recta hatched soon after oviposition and early-instar nymphs entered into a diapause stage that lasted through most of autumn. Diapause was broken by late autumn and nymphal growth occurred rapidly prior to emergence in December through February. In contrast, the L. alta eggs remained in a delayed stage through summer and autumn. Early-instar nymphs were evident by early winter and growth proceeded rapidly prior to emergence in April and May. Only one L. sibleyi adult was obtained from the intermittent reach, and L. cf. tenuis was restricted to the perennial reach. That L. cf. tenuis was absent 328 Southeastern Naturalist Vol. 5, No. 2 from the intermittent reach was expected, because the emergence period for this species was during September and October when that channel had been dry since early summer. Nymphal growth in the perennial reach of L. cf. tenuis was steady during the summer months prior to emergence. The lack of summer and autumn flow clearly prevented establishment of this species in the intermittent channel. The life cycle displayed by L. cf. tenuis in the perennial reach closely resembled the univoltine-slow cycles displayed by L. tenuis (Pictet) in permanent streams in Ontario (Harper 1973) and Oklahoma (Ernst and Stewart 1985). This species is undescribed and closely allied to L. tenuis. While the Leuctra nymphs from the intermittent reach were of L. alta, the inability to distinguish between L. alta and L. sibleyi hampered a clearer life-cycle comparison of (a) these two species in the perennial reach, and (b) the former species between reaches. However, the two distinct size classes in the perennial reach from January–March (Fig. 3) indicated that both L. sibleyi (larger) and L. alta (smaller) were present. The adults of L. alta and L. sibleyi are morphologically similar (Grubbs 2005) but readily separated by size. The life cycle demonstrated by L. alta (intermittent reach) and the L. alta–L. sibleyi mix (perennial reach) mirrored the univoltine-fast cycles exhibited by L. duplicata Claassen from an intermittent stream in Quebec (Harper 1990) and L. iliberis Sanchez-Ortega and Alba-Tercador in Spain (Sanchez-Ortega and Alba-Tercador 1988). The lack of early-instar Leuctra nymphs in either reach by the time the intermittent channel had dried implied that eggs had not hatched and were developing very slowly. Harper (1973, 1990) discussed that both L. ferruginea and L. duplicata lacked diapausing eggs, even in intermittent habitats, but rather possessed eggs that developed directly albeit slowly. Egg hatching of L. alta in both reaches occurred after approximately the same embryonic development period and was likely induced by a common environmental signal. Hence, the rewetting of the intermittent stream channel in late autumn was not the hatching stimulus for L. alta in light of the perennially flowing condition in the perennial channel. Egg hatching of both L. alta and L. sibleyi was likely triggered by other environmental cues (e.g., decreasing temperature or day length). The L. alta eggs also possessed the mechanism to survive channel desiccation in the intermittent channel. Leuctridae exhibit broad life-cycle patterns in eastern North America, including semivoltine populations of Leuctra ferruginea (Walker) in warm-water Ontario streams (Harper 1973) and complex cycles of Zealeuctra claasseni (Frison) and Z. hitei Ricker and Ross in Texas (Snellen and Stewart 1979). Harper (1973) further revealed a univoltine population of L. ferruginea in the coldest stream he studied, demonstrating indeterminate voltinism due to a temperature gradient that influenced the number of generations per year. A similar pattern of indeterminate voltinism has been demonstrated in the European species L. nigra (Olivier) 2006 S.A. Grubbs, C.M. Thomas, B.T. Hutchins, and J.M. Taylor 329 (e.g., Hildrew et al. 1980, Iversen 1978). Zealeuctra claasseni and Z. hitei also exhibited indeterminate voltinism, yet this variation was within a single stream and induced by channel drying (Snellen and Stewart 1979). Egg hatching of univoltine populations was direct during wetter years, but delayed during drier years. Most studies examining stonefly life cycles have implied voltinism mainly on the presence of one (univoltine), two (semivoltine), or several (merovoltine) nymphal cohorts present per year. Several mechanisms that result in more complex life cycles, namely cohort splitting (Moriera and Peckarsky 1994), seed banking of eggs (Snellen and Stewart 1979, Zwick 1996), and cryptic semivoltinism (Taylor et al. 1999), have been demonstrated among stonefly taxa. There have been few investigations of the effect of stream drying on egg viability or time needed to induce hatching (e.g., Snellen and Stewart 1979) or of quantifying the length of the egg development period. Taylor et al. (1999) revealed that the perlodine stonefly Megarcys signata (Hagen) displayed a semivoltine life cycle because the eggs diapaused for up to 10 months. Prior research on M. signata examined only nymphal growth and adult emergence and assumed a univoltine cycle (Cather and Gaufin 1975). Perhaps L. alta eggs develop at different rates depending on dry versus wet channel conditions similar to Z. claasseni and Z. hitei. Many studies on Allocapnia have shown that eggs hatched soon after emergence, nymphs diapaused through summer and autumn, and life cycles were univoltine-fast (e.g., Harper and Hynes 1972, Harper et al. 1991). The A. recta adults in this study were collected mainly during January and February. Only the smallest size class nymphs were collected from spring through autumn (Fig. 2), indicating that A. recta eggs hatched soon after oviposition, and nymphs from the perennial reach were in the typical curled diapaused shape. Despite the dry intermittent reach during summer and autumn, late-instar post-diapause nymphs were present in January–February 2003 and again in December 2003–January 2004 immediately prior to and during the emergence period. Allocapnia recta nymphs must have survived in a hyporheic environment beneath the intermittent channel prior to rewetting in late autumn. Research that has compared macroinvertebrate communities across flow gradients have generally found distinct assemblages in perennial versus intermittent channels (e.g., Bottorff and Knight 1988, Feminella 1996, Wright et al. 1984). Similarly, aside from the species germane to this study, there were several taxa found exclusively in either the intermittent or perennial reach. Taxa found solely in the intermittent channel included Rhyacophila glaberrima Ulmer (Trichoptera), Ameletus sp., (Ephemeroptera) and Sphaerium sp. (Pelecypoda). 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