SEASONAL OCCURRENCE AND HABITAT CHARACTERISTICS OF SOME VERNAL POOL BRANCHIOPODA IN NORTHERN CALIFORNIA, U.S.A. Sean P. Gallagher

SEASONAL OCCURRENCE AND HABITAT CHARACTERISTICS OF SOME VERNAL POOL BRANCHIOPODA IN NORTHERN CALIFORNIA, U.S.A. Sean P. Gallagher A B S T R A C T New information on the life history and habitats of branchiopods in California vernal pools is presented. Pools were sampled biweekly during the 1992â 1993 season. Branchinecta lynchi had 3 distinct hatches in 1 season which were dependent on pools drying and refilling with cold temperature water. Linderiella occidentalis and Lepidurus packardi were not observed after pools first dried and then filled a second time in 1 season. Physical habitat factors, including pool depth, surface area, and volume, were examined in relation to species presence. Pool area and volume are not as important as depth for pools in which these species occurred. Habitat duration for B. lynchi was between 3 and 14 weeks. Most pools containing B. lynchi had a duration of 7 weeks and females with shelled cysts in their brood pouches were observed after 6 weeks early in the season and after 2 weeks later in the season when temperatures were higher. Linderiella occidentalis and Lepidurus packardi require deeper pools with durations greater than 7 weeks. Branchiopods in temporary pools, known as California vernal pools (Thorne, 1984), are poorly known and many species have yet to be described (King, Simovich, and Brusca, in preparation). However, publications on these branchiopods have recently appeared (Patton, 1984; Thiery and Fugate, 1994; Fugate, 1993). The 19 September 1994 listing under the U.S.A. Federal Endangered Species Act of four vernal-pool branchiopod species (Federal Register, 1994): Branchinecta conservatio Eng et al., 1990, B. lynchi Eng et al., 1990, B. longiantenna Eng et al., 1990 (Branchiopoda: Anostraca), and Lepidurus packardi Simon, 1886 (Branchiopoda: Notostraca), has greatly increased the need for information regarding these species. Balko and Ebert (1984) indicated that zooplankton distributions in the vernal pools which they studied near San Diego, California, were related to the seasonal frequency of pool drying and to life history characteristics. Eng et al. (1990) suggested that physical and chemical factors are important in influencing the distribution of 17 species of anostracans found in California. However, Syrdahl (1993) was not able to find a clear relationship between physical, chemical, or biotic factors and the presence or absence of some vernal pool macroinvertebrates in her one-season study of 14 pools in the northern Sacramento Valley. I examined the distribution and occurrence of Branchinecta lynchi, Linderiella occidentalis Dodds, 1923, and Lepidurus packardi in a large vernal-pool complex in Butte County, California, U.S.A., by sampling biweekly during the 1992-1993 pool season. These species are endemic to California (Eng et al., 1990; Ahl, 1991). The objectives of my study were to examine the seasonal occurrence and distribution of these species in relation to pool duration, depth, volume, temperature, and surface area, and to provide information on the life history of these three species. Habitat Description California vernal pools are temporary wetlands which flood from fall and winter rains and are dry in summer. The term vernal describes the distinctive flora associated with these pools as they dry during the spring (Thorne, 1984). I conducted my study in a 105 ha vernal-pool complex located northeast of the City of Chico, California (T 22 N, R 2 E, Sec 22), at an elevation of 60 m. The gently sloping grassland topography contains seasonal wetlands, including vernal pools and swales. Wetland delineation on the site identified 669 wetlands (Huffman, unpublished). The vernal pools on the site have a clay-silt and patchy cobble substrate underlain by an impervious hardpan. Vernal pools in the Chico area usually flood be-

Fig. 1. Climatological data for the 1992-1993 season (November-June) recorded at California State University Chico Farm, Chico, California (NOAA, 1992, 1993). A, Minimum and maximum air temperature. B, Daily rainfall. Dashed line is at 20 C. tween September and February and dry be- tween April and June (Gallagher, 1993). The climate is Mediterranean with most rainfall occurring between September and May. Dai- ly rainfall and maximum and minimum air temperatures for the 1992-1993 season are shown in Fig. 1. MATERIALS AND METHODS Field Collections.-All delineated wetlands on the site were examined every 2 weeks during the 1992-1993 season, beginning after pools filled on 10 December 1992 and ending when pools dried in mid-april 1993. Every delineated wetland was examined and I sampled those holding water during each visit. Sampling and collecting in each pool was conducted using the following qualitative method. I visually inspected each pool for invertebrates from a number of angles. Species of interest observed during visual inspection were captured with a dip net (700-lLm mesh). After the visual inspection, I used a plankton net (100-wm mesh) to capture organisms in the water column, and a dip net sweep was used to collect benthic and epibenthic organisms and capture invertebrates suspended in the water column. This technique was shown to be useful for collecting invertebrates in vernal pools by Ahl (1991) and Wolt (1972). I directly examined nets or emptied netted materials into enamel pans where the contents settled, and invertebrates of interest, freed from debris, were easier to detect. The distance, duration, and location of net sweeps and pulls varied. Larger and more turbid pools received more intensive effort unless organisms of interest were immediately detected. 1 collected specimens from representative pools when species could not be reliably identified in the field. These were preserved in 70% ethanol. Fairy shrimps were identified using the descriptions in Eng et al. (1990). Tadpole shrimp and other invertebrates were identified using Pennak (1989). When possible, all species were identified and sexed in the field and returned to the pool from which they were collected. Maximum and average depths were recorded for each pool during each visit. Daily rainfall and maximum and minimum air temperature data came from a recording station at California State University Farm located approximately 8 km from the study site (NOAA, 1992, 1993). Data Analysis.-Pool surface area, determined from

Fig. 2. Percentage of pools containing standing water (single line). Branchinecta lynchi (BI), Linderiella occidentali.r (Lo), Lepidurus packardi (Lp), and juvenile Anostraca (Juv.) during the 1992-1993 season. wetland delineations by Huffman (unpublished), was considered as the maximum area of inundation. Pool volume was calculated using surface area and depth measurements. For this study, wetlands which contained water during any 2 consecutive visits were considered vernal pools. Wetlands without standing water and areas with running water (e.g., drainages) were not included in further analysis. Analysis of variance (ANOVA) (Zar, 1984) was used to test if depth, volume, or surface area were important to species occurrences. Statistical significance was accepted at the 0.05 probability level. RESULTS Pools flooded during week 51 (week of 1 1 December 1992), some dried and refilled in March, and all were dry by 20 April 1993 (Fig. 2). On the initial visit of 22 De- cember 1992, 54 pools were directly ex- amined and 20 had juvenile fairy shrimp too small to be identified. Of the 669 wet- lands on the site, a total of 301 had standing water during any one visit and 191 held wa- ter during any two consecutive visits. These 191 were considered as vernal pools in the analysis. Most pools dried during week 11 I of 1993 (Fig. 2). Many pools refilled from rain during week 12 and remained full until week 14 (Figs. 1 B, 2). Two pools refilled following a period of rain in early June (Figs. 1, 2). The percentage of pools with water on each date is shown in Fig 2. A total of 91 pools had one or more of the three species considered in this study. I observed B. lynchi both alone and with Linderiella occidentalis and Lepidurus packardi in 40% of the pools examined (Fig. 2). I found Linderiella occidentalis in 14% of pools both alone (3 pools) and with Lepiduru.s packardi and B. lynchi (3 and 7 pools, respectively). I found Lepidurus packardi both with Linderiella occidentalis and B. lynchi in 8% of the pools examined, but did not find it by itself in any pool. I observed three hatches of Branchinecta lynchi during the 1992-1993 season (Fig. 2). Juvenile fairy shrimps were first observed two weeks after the pools filled (week 53, Fig. 2). Female B. lynchi had

Table I. Means, f-test, and P-values of surface area, depth, and volume of pools containing Branchinecta lynchi (B1), Linderiella occidentalis (Lo), Lepidurus packardi (Lp), pools with one or more of these species, and pools in which none of these species were observed during the 1992-1993 wet season. Numbers under means are standard errors. shelled cysts in their brood pouches by week three (six weeks after the pools flooded). During weeks seven and nine, most B. lynchi observed were males and in some pools intensive searching revealed only males. Male and female B. lynchi had mostly disappeared from the pools by week seven, even when the pools still contained water. They had completely dropped out by week 11 (Fig. 2). Air temperature did not rise significantly above 20 C (Fig. la) prior to the disappearance of B. lynchi. Thus some factor other than temperature was involved. By week 11, many pools were dry and had a cracked soil surface. During week 12, a storm with air temperatures below 20 C refilled many pools (Figs. 1, 2). I observed female B. lynchi with brood pouches containing shelled cysts two weeks after the second pool flooding. In one pool lacking anostracans, which was not completely dry by week 11, B. lynchi reappeared after the storm of week 12. A third cold storm (air temperatures below 20 C) during week 21 filled two pools which had been dry during week 20 (Figs. 1, 2). Female B. lynchi with shelled cysts in their brood pouches were observed in one of these pools at the end of week 23. I first observed female Linderiella occidentalis with brood pouches containing shelled cysts by week 5 (8 weeks after filling of the pools). This species persisted in pools until they dried (Fig. 2). I first observed female Lepidurus packardi with eggs (carapace length ~ 15 mm) during week seven and gravid females were present until pools dried. I observed Lepidurus packardi of variable sizes (5-20-mm carapace length) during each visit after week seven. Both these species persisted in pools which retained standing water (Fig. 2). Neither Lepidurus packardi nor Linderiella occidentalis were observed in pools which dried before week 12 and refilled after the storm of week 12 (Fig. 2). Populations of Linderiella occidentalis lasted from four to fourteen weeks, while those of Lepidurus packardi persisted from seven to sixteen weeks. Table 1 presents average surface area, depth, and volume for pools used during this study. Surface area is shown not to be as important as depth, volume, and duration of inundation for pools containing Branchinecta lynchi, Linderiella occidentalis, and Lepidurus packardi (Table 1, Fig 3). Pools with one or more species of these branchiopods are larger, deeper, and have more volume than those that did not contain these species (Table 1 ). Maximum depth was significantly different for pools containing only B. lynchi when compared to those containing B. lynchi, Linderiella occidentalis, and Lepidurus packardi (t = 4.28, P 0.001). Surface area and volume are shown to be less important than depth for pools containing these three species (Table 1). Pools with B. lynchi were generally smaller and shallower than those occupied by Linderiella occidentalis and Lepidurus packardi (Table 1 ). These smaller and shallower pools also had shorter duration of the aquatic phase (Fig. 3). Linderiella occidentalis and Lepidurus packardi need deeper, longer-lasting pools. However, pools containing Linderiella occidentalis were not significantly different in depth or volume from those containing Lepidurus packardi (Table 1).

Fig. 3. Habitat duration for vernal pools observed containing Branchinecta lynchi (B 1 ), Ginderiella occidentalis (Lo), and Lepidurus packardi (Lp) during the 1992-1993 season. *Second hatch in mid-march. DISCUSSION Pool depth and duration are important to the occurrence of Branchinecta lynchi, Linderiella occidentalis, and Lepidurus packardi. Of the habitat variables examined, volume, while significant because of the effect of depth on this calculation, was not as important as depth in making a pool function as habitat for the three species considered in this study. Thiery (1991) found that branchiopod species richness in temporary ponds in Morocco is related to depth. Balko and Ebert (1984) found habitat duration to be an important factor in the occurrence of branchiopods in vernal pools in the San Diego area. Duration of pool inundation was also an important factor in the occurrence of branchiopods in this study (Fig. 3). The duration of the aquatic phase for pools containing B. lynchi was from 3-7 weeks (Fig. 3). The B. lynchi observed in pools with a duration of only three weeks were in pools which refilled after week 12 (Figs. 2, 3). These late-season pools experienced higher temperatures (Fig. 1). Both Lepidurus packardi and Linderiella occidentalis demon- strated a requirement for longer lasting pools than did B. lynchi (Fig. 3). I found Linderiella occidentalis in pools with durations of 4-16 weeks. The majority of observations were in pools of seven weeks duration (Fig. 3). Lepidurus packardi requires pools that hold water for more than seven weeks (Fig. 3). When pools almost dried during week 1 1, Linderiella occidentalis and Lepidurus packardi survived in small depressions in the pool bottoms, indicating that these two branchiopods may be more tolerant of high temperature and low oxygen than B. lynchi. During weeks 15 and 17, I observed dehydrated Lepidurus packardi in depressions in dry pools. Female Branchinecta lynchi with shelled cysts in their brood pouches were first observed on 22 January 1993, six weeks after pools first filled. The reproductive period for B. lynchi in pools with seven weeks duration was one week. During the second pool flooding (after week 12, Fig. 2), females with shelled cysts in their brood pouches were observed after two weeks. In pools that dried and refilled a second time,

this species was able to reproduce for less than one week. Female B. lynchi in pools with a pond duration of 14 weeks were able to reproduce for four weeks. These pools were deeper and contained Lepidurus packardi and Linderiella occidentalis. In the two pools which dried and flooded a third time (weeks 21-23), females of B. lynchi with shelled cysts were observed in one pool, and juveniles, which did not live to reproduce, were in the other pool. The amount of time for reproduction in the one pool was only a few days. The other pool represented a false start. During the first ponding period of weeks 53-11, air temperatures were cooler, and were consecutively warmer during the following two pool-filling periods (Fig. 1 A). Sexual maturation for Branchinecta lynchi occurred faster later in the season when temperatures were higher. However, the length of time available for reproduction is not significantly different during the season, except in deeper, longer-lasting pools. Cyst-bearing female Linderiella occidentalis were first observed during week three and the majority of observations were during week five, indicating that ponding duration of seven weeks is needed for this species to reproduce. In one pool of fourweeks ponding duration, Linderiella occidentalis was not able to reproduce. In the five-week duration pool (Fig. 3), this species may have reproduced for a few days. In pools with ponding duration of seven weeks, reproduction lasted for a week. In pools of 14 and 16-weeks ponding duration (Fig. 3), Linderiella occidentalis reproduced for seven and nine weeks, respectively. However, only male Linderiella occidentalis were observed during week 16 and water temperatures above 20 C may limit adults of this species (Lanway, 1974; Patton, 1984; Eng et al., 1990). The reproductive period also may have been shortened in these pools, since higher temperatures during this period (Fig. 1A) may have caused females to disappear. The length of time needed by Lepidurus packardi to become reproductive is as short as four weeks (Ahl, 1991). This species was observed with eggs within five weeks during this study. In pools of seven-weeks duration, reproduction lasted for two weeks. In pools with 14 and 16-week duration, this species reproduced for seven and nine weeks, respectively. The observation of individuals with eggs in low spots, as pools dried, indicates that this species reproduces until the pools dry. If a pool dries before one of the three branchiopod species considered in this study can produce its first clutch, that pool is not a suitable habitat. A second hatch of Linderiella occidentalis and Lepidurus packardi did not occur after dry pools refilled late in the season. These branchiopods may have adaptations that inhibit hatching when pools are less likely to remain full long enough to complete their life cycles. On the other hand, these species may have had a second hatch following reflooding, but the larvae failed to reach identifiable sizes. However, this is unlikely, since Lepidurus packardi attains adult form within 48 h of hatching (Ahl, 1991) and would have been identifiable during the two weeks of observations. Ponding duration and pool depth are important factors influencing the suitability of vernal pools for these branchiopod species. ACKNOWLEDGEMENTS I thank Denton Belk and Nancy Kang for their comments on earlier drafts of this paper. I also thank Doug Alexander for encouraging me with this project. David Kelley Associates Inc., in Davis, California, provided financial assistance. LITERATURE CITED Ahl, J. S. 1991. Factors affecting contributions of the tadpole shrimp, Lepidurus packardi, to its oversummering egg reserves.-hydrobiologia 212: 137-143. Balko, M. L., and T. A. Ebert. 1984. Zooplankton distribution in vernal pools of Kearny Mesa, San Diego, California.â In: S. Jain and P. Moyle, eds., Vernal pools and intermittent streams. University of California Institute of Ecology, special publication 28: 76-88. Eng, L. L., D. Belk, and C. H. Eriksen. 1990. California Anostraca: distribution, habitat, and status.â Journal of Crustacean Biology 10: 247-277. Federal Register. 1994. Endangered and threatened plants; determination of endangered status for the conservancy fairy shrimp, longhorn fairy shrimp, and the vernal pool tadpole shrimp; and threatened status for the vernal pool fairy shrimp.â Federal Register 59: 48136â 48153. Fugate, M. 1993. Branchinecta sandiegonensis, a new species of fairy shrimp (Crustacea: Anostraca) from western North America.-Proceedings of the Biological Society of Washington 106: 296-304. Gallagher, S. P. 1993. Life history variation in the temporary pool snail, Fossaria sonomaensis, in the

northern Sacramento Valley.â Naturalist 130: 372-385. American Midland Lanway, C. S. 1974. Environmental factors affecting crustacean hatching in five ponds.-m.a. thesis, California State University, Chico, California. Pp. 1â 86. National Oceanic and Atmospheric Administration. 1992. Climatological data, California. September 1992 through December 1992. 96(9-12): 18. â â â â. 1993. Climatological data, California. December 1993 through June 1993. 97(1â 6): 18. Patton, S. 1984. The life history patterns and the distribution of two Anostraca, Linderiella occidentalis and Branchinecta sp.-m.a. thesis, California State University, Chico, California. Pp. 1â 50. Pennak, R. W. 1989. Freshwater invertebrates of the United States, third edition. Protozoa to Mollusca.â John Wiley and Sons, Inc. New York, New York. Pp. 1-628. Syrdahl, R. L. 1993. Distribution patterns of some key macroinvertebrates in a series of vernal pools at Vina Plains Preserve, Tehama County, California.â M.S. thesis, California State University, Chico, California. Pp. 1-83. Thià ry, A. 1991. Multispecies coexistence of branchiopods (Anostraca, Notostraca Spinicaudata) in temporary ponds of Chaouia plain (western Morocco): sympatry or syntopy between usually allopatric species.-hydrobiologia 212: 117-136., and M. Fugate. 1994. A new American fairy shrimp, Linderiella santarosae (Crustacea: Anostraca: Linderiellidae), from vernal pools of California, U.S.A.-Proceedings of the Biological Society of Washington 107: 641-656. Thorne, R. F. 1984. Are California's vernal pools unique?â In: S. Jain and P. Moyle, eds., Vernal pools and intermittent streams. University of California Institute of Ecology special publication 28: 1-8. Wolt. T. B. 1972. Cyzicus mexicanus (Crustacea: Conchostraca): life history and a comparison of growth, maturity, and egg production in five ponds.-m.a. thesis, California State University, Chico, California. Pp. 1-46. Zar, J. H. 1984. Biostatistical analysis.-prentice-hall Inc., Englewood Cliffs, New Jersey. Pp. 1-620. RECEIVED: 5 May 1995. ACCEPTED: 17 July 1995. Address: United States Fish and Wildlife Service, 3310 El Camino Ave., Suite 130, Sacramento, California 95821, U.S.A. (e-mail: Sean-P-Gailagher(a) mail.fws.gov).