Limnol. Oceanogr., 44(3), 1999, 487±493 q 1999, by the American Society of Limnology and Oceanography, Inc. Dispersal of the fairy shrimp Branchinecta coloradensis (Anostraca): Effects of hydroperiod and salamanders Andrew J. Bohonak1 Section of Ecology and Systematics, Corson Hall, Cornell University, Ithaca, New York 14853; Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, Colorado 81224 Howard H. Whiteman Department of Biological Sciences, Murray State University, Murray, Kentucky 42071-0009; Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, Colorado 81224 Abstract The absence of fairy shrimp (Anostraca) from permanent ponds and lakes is hypothesized to be the result of vertebrate predation. However, hatching cues for anostracan diapausing eggs include factors associated with the ®lling of temporary pond basins, and desiccation often increases the fraction of eggs that hatch. Thus, it is possible that in some species, eggs dispersed to permanent habitats never hatch, and vertebrate predation is not the proximate factor limiting distributions. We experimentally transplanted live egg-bearing females of the fairy shrimp Branchi- necta coloradensis into permanent and temporary ponds in small chambers and allowed the chambers to overwinter in situ. There was no discernible effect of pond drying on hatching success (mean success 5 50.9%). We also determined whether metamorphic salamanders (Ambystoma tigrinum nebulosum) could disperse viable fairy shrimp eggs by feeding on B. coloradensis in one pond and defecating in another. Hatching success for a salamander ``ingested'' treatment was estimated as 0.9%. Results of a third experimental treatment suggested that the eggs being carried by females were not fully mature, so that ingestion resistance might vary throughout the reproductive period of B. coloradensis. By combining these results with data on salamander movement and diet, we estimated salamander-mediated dispersal rates for fairy shrimp eggs. Metamorphic A. tigrinum nebulosum sp. are likely to be dispersing thousands of B. coloradensis eggs among ponds annually. Because our results demonstrate that dispersal between ponds can occur in large numbers, they support the prevailing hypothesis that vertebrate predators limited to permanent ponds are the proximate mechanism preventing the invasion of fairy shrimp. Differences in community composition between tempo- tacean order Anostraca exempli®es this pattern, and the fac- rary and permanent bodies of water can be striking, even tors that determine its distribution have been under investi- when those communities are only a few meters apart. Large- gation for at least the past 100 yr (e.g., Hay and Hay 1889; bodied invertebrates commonly found in temporary ponds Avery 1939; Gallagher 1996). are conspicuously absent from neighboring bodies of water Studies focusing on the distribution and ecology of tem- that do not dry (e.g., Williams 1987 and references therein; porary pond invertebrates have progressed in two general Batzer and Wissinger 1996; Wellborn et al. 1996). The crus- directions. First, it has become widely recognized that size- selective predation by vertebrates is an important determi- 1 To whom correspondence should be addressed. Present address: nant of community composition in freshwater systems Center for Conservation Research and Training, 3050 Maile Way, (Brooks and Dodson 1965; Dodson 1974; Northcote 1988). Gilmore 409, University of Hawaii, Honolulu, Hawaii 96822. The extreme rarity of Anostraca (fairy shrimp), Spinicaudata Acknowledgments (clam shrimp), and Notostraca (tadpole shrimp) in permanent We thank Carla CaÂceres, Nelson Hairston, Jr., and two anony- ponds and lakes is consistent with this hypothesis because mous reviewers for comments. Scott Wissinger and Wendy Brown predatory vertebrates are nearly ubiquitous in these systems, assisted in ®eldwork and provided invaluable intellectual input. and the introduction of ®sh has been directly observed to Steve Horn, Randy Moorman, John Gutrich, and Sara Townsend cause the extinction of large zooplankton populations (Nils- assisted with salamander surveys. We are grateful to the Rocky Mountain Biological Laboratory son 1972; Parker et al. 1996). and The Nature Conservancy for use of the Mexican Cut Nature In contrast to this recent focus on size-selective predation, Preserve. A.J.B. was funded by TNC and the Edna Bailey Sussman many earlier investigators concentrated on the role of abiotic Foundation. H.H.W. received funding from Purdue University, the factors in preventing large zooplankton from actively invad- Helen Gaige Fund (American Society of Ichthyologists and Her- ing permanent ponds and lakes. For example, some research- petologists), the Colorado Division of Wildlife, a CISR grant from ers hypothesized that diapausing fairy shrimp eggs might Murray State University, and by contract DE-AC09-76ROO-819 be- need to dry or freeze in order to hatch the following year tween the U.S. Department of Energy and the University of Geor- (e.g., Hay and Hay 1889; Mozley 1932; Weaver 1943). If gia's Savannah River Ecology Laboratory. Both A.J.B. and H.H.W. received funding from the Theodore Roosevelt Memorial Fund this were true, the presence of predatory vertebrates would (American Museum of Natural History), the Snyder Memorial Fund not be solely responsible for the distribution of these zoo- of RMBL, and the National Science Foundation (DEB-9423603 and plankton species. However, laboratory experiments on egg DEB-922981). hatching in a number of species failed to provide any such 487 488 Bohonak and Whiteman generalizations, even though an osmotic change is necessary net, samples were taken with a 2.2-liter Van Dorn sampling for the termination of diapause in most species (Avery 1939; bottle mounted on a 1-m pole. Because fairy shrimp adults Moore 1951; Brendonck 1996). were occasionally observed to avoid the plankton net, visual The most obvious approach to separating predatory and censuses were made for the presence or absence of B. co- embryological factors is correlative, and in fact, Anostraca loradensis adults weekly throughout the summer. In 1992, a and other large Branchiopoda are occasionally found in per- plankton net with 20-mm Nitex mesh was also used to col- manent bodies of water where vertebrates are very rare or lect adults, and it appeared that B. coloradensis adults were absent (Kerfoot and Lynch 1987 and references therein). unable to avoid this net. Because precise estimates of abun- However, habitat permanence and vertebrate predation co- dance were not essential for determining dispersal rates for vary so strongly as to preclude the separation of these factors B. coloradensis (see below), we present the range of adult from surveys alone in most regions. Here, we investigated densities (per liter) observed between 1990 and 1992 without the limitations to anostracan distributions in such a system. regard to methodological biases. We investigated vertebrate predation and invertebrate dis- We found that fairy shrimp distributions at the Mexican tributions at the Mexican Cut Nature Preserve, 17 km north Cut Nature Preserve were associated with the presence or of the Rocky Mountain Biological Laboratory in the Elk absence of branchiate (gilled) forms of A. tigrinum (x2 5 Mountains of Colorado (39.08N, 107.18W). The preserve 10.89, 1 df, P 5 0.001; Fisher's Exact Test P 5 0.003), contains closely spaced ponds on two plateaus on Galena which covaries with pond permanence. Eight out of nine Mountain at altitudes of approximately 3,400 and 3,450 m permanent ponds at the Lower Cut of the Mexican Cut Pre- (the ``Lower Cut'' and the ``Upper Cut,'' respectively). Pond serve contained paedomorphic salamanders and/or juveniles, sizes range between 15 and 4,600 m2, with maximum depths and except for pond 9, fairy shrimp nauplii and adults were of 0.3±2.5 m. Invertebrate distributions change markedly not collected in any of these ponds between 1990 and 1992 along hydroperiod gradients that covary with vertebrate pre- (Table 1). Fairy shrimp have also been absent from the same dation (Dodson 1974; Wissinger et al. in press). More com- ponds in more recent samples (Wissinger and Bohonak pers. plete descriptions of the biota, hydroperiod, and chemistry comm.). of the ponds have been reported elsewhere (Sprules 1972; Wissinger and Whiteman 1992; Wissinger et al. in press). Salamander life history and population censusesÐSize- Motivated by the variety of results obtained in laboratory selective predation in the Mexican Cut ponds is complicated hatching studies with anostracan eggs, we tested the impact by the life history and mobility of the predators. Branchiate of hydroperiod on B. coloradensis diapausing eggs in this forms of the tiger salamander A. tigrinum nebulosum (larvae ®eld setting. Although size-selective predation has the po- and obligately aquatic paedomorphic adults) are limited to tential to limit fairy shrimp distributions at this site (Sprules permanent ponds, where they have the potential to affect 1972), it is plausible that this potential is rarely realized be- zooplankton community composition (Sprules 1972; Dodson cause fairy shrimp eggs dispersed into permanent bodies of 1974). Metamorphic A. tigrinum nebulosum adults coexist water do not hatch. with paedomorphs in this population, and feed actively in A second goal of the study was to quantify the dominant both permanent and temporary ponds. Analyses of gut con- vector for fairy shrimp movement