|Figure 1. Very small pothole on the Colorado Plateau.||Figure 2. Hog lake, a large vernal lake in Tehama County, Calif.|
By definition ephemeral pools dry up periodically, typically holding water for only a few days to months, yet these may represent one of the most permanent kinds of aquatic environments on Earth, judging from the age of some species that inhabit them (Fryer 1996). For example, lower Triassic tadpole shrimp fossils were assigned to the living species Triops cancriformis (Kerfoot and Lynch 1987), and Walossek (1993) recently reported finding branchiopod fossils in Cambrian rock. Despite the age of some of these groups, many branchiopods appear to be closely adapted to current climatic conditions in their pools. Cues for hatching, time to maturity, temperature tolerances, and other aspects of their ecologies are all relatively closely matched to current conditions within pools each species inhabits.
Figure 3. Aerial view of potholes in Navajo sandstone, Grand County, Utah.
Seasonal precipitation patterns and temperature regimes, the
predictability of precipitation (intra- and inter-annual variation),
chemical and physical properties of the substrate, and whether there
is overland flow of precipitation before accumulating in the pools all
affect the temporary pool environment. Unlike permanent bodies of
water, there is no capacity to dampen out climatic fluctuations,
e.g., storing water from wetter than average years that would allow
continued survival of aquatic species through drier periods. Any
large shift in filling and drying patterns will result in a different
system, probably with retention of some species, but significant
changes in community and ecosystem properties are also likely.
|Figure 4. Adult fairy shrimp, Branchinecta packardi.|
While ephemeral pool communities have a fairly simple structure,
species composition of communities varies significantly. Most pools
are populated with widespread species, but some species are endemic to
particular geographic regions, or pool conditions. Much of the
diversity in ephemeral pools may still be undocumented, as evidenced
by a recent survey of California Central Valley pools, where over 70%
of the invertebrates encountered were reported from California for the
first time, and over 50% may be new species (King et al. 1996).
|Figure 5. Adult tadpole shrimp, Triops longicaudatus.||Figure 6. Adult clam shrimp, Leptestheria compleximanus.|
Ephemeral pool organisms are faced with eventual elimination of the environment that supports them, resulting in certain death for any aquatic forms still in the pool when it dries up. Organisms that are well adapted to these habitats have some way to survive the dry phase of ephemeral pools. Usually these adaptations apply to only one stage of the life cycle, so it is important to complete their life cycle as quickly as possible so the dormant phase can be reached before the pool dries up. There is generally an emphasis on reaching sexual maturity, mating, and producing viable eggs as quickly as possible. However, predictability of the environment has affected life history characteristics of obligate ephemeral pool species, thus pools that typically last a long time have slower maturing species than those occupying smaller, shorter-lived pools.
Figure 7. Mosquito and chironomid midge larvae.
Figure 8. Spadefoot toad (Spea intermontanus) metamorph.
Cryptobiosis was first documented in 1702 by Anton van Leeuwenhoek,
when he observed tiny "animalcules" in the sediment collected on house
roofs. He dried them out then added water and found the animals began
moving about again. The animals van Leeuwenhoek studied were probably
free-living nematodes, which along with rotifers and tardigrades are
some of the best known cryptobiotic animals. These groups are able to
dry up at any stage, and typically live in terrestrial environments
such as soil, moss, and tree bark. In ephemeral pools, cryptobiosis
is usually limited to a single stage of an animal's life history,
often the egg or cyst. Animals must complete their life cycle, from
tolerant stage to tolerant stage before the pool dries up if the
species is to survive.
Figure 9. Aquanothrus sp., an oribatid mite living in small potholes.
Branchiopod crustaceans are among the better known cryptobiotic species. They have cryptobiotic eggs that pass the dry phase in the pool sediment that are extremely tolerant of all kinds of adverse conditions. Planel et al. (1980) glued brine shrimp (Artemia) cysts to the outside of a spacecraft, retrieved them after the space flight and hatched viable shrimp from the cysts.
|Figure 10. Hatching tadpole shrimp cyst.
||Figure 11. Fairy shrimp nauplius larva (about 7 hours old).|
Most branchiopods have extremely rapid early development. After the
eggs are fertilized, the embryo undergoes additional development to
the nauplius or metanauplius stage before entering diapause. The
cryptobiotic stage is really a cyst, not an egg, in branchiopods.
Under the appropriate conditions, cysts hatch (Figure 10) and the
larvae begin growing and changing very fast. Figures 11 and 12 show
fairy shrimp and tadpole shrimp nauplii about 7 hours after
hydration. The rapid growth rate requires numerous molts (Figures 13
and 14), and quickly take on the adult form, generally by 24 hours in
warm water pools (Figures 14-16). The time required to reach maturity
and start producing the next generation of viable cysts (Figures 4-6)
varies greatly among species, and even within the same pool, depending
on genetic controls and environmental influences on these control
|Figure 12. Tadpole shrimp nauplius larva (about 7 hours old).
||Figure 13. Tadpole shrimp nauplius larva beginning first molt (<10 hours old).
||Figure 14. Tadpole shrimp nauplius larva beginning later molt (about 15 hours old).
||Figure 15. Tadpole shrimp adult (between 24 and 36 hours old).|
Climatic variability occurs at a number of temporal scales, and while branchiopods are adapted to a particular range of climatic conditions at an evolutionary scale, there is intra- and interannual variation in conditions (at an ecological scale) that can also affect long term survival of a branchiopod population in a pool. For instance, if all cysts hatched the first time they got wet, a very small rain event would cause the cysts to hatch, but the pool would probably dry before the hatchlings matured and laid more eggs. Eventually, the population would be eliminated from the pool. Branchiopods deal with this inherent variability in climatic parameters by producing eggs with different diapause characteristics in each clutch. Some hatch after drying and getting wet again. Others go through more than one dry/wet cycle before they hatch. Hildrew (1985) took cysts through 9 cycles and still did not get all the cysts to hatch. It is not known what other cues operate to break dormancy in conjunction with wetting the cysts, but water temperature, changes in oxygen tension, solute concentrations, or perhaps changes in pH as the sediment is inundated may be involved for different species.
Figure 16. Adult fairy shrimp.
How species deal with existing climatic variability, and potential impacts of climate change in the future are discussed for 2 types of pools: potholes of the Colorado Plateau and vernal pools of California's Mediterranean climate.
Potholes are not erosional features of Holocene asphaltic deposits.
Also known as weathering pits, they are depressions in bedrock, often
with little or no watershed (Figure 17). They range in size from
very small depressions holding less than a litre of water, to enormous
pits over 15 m deep (Netoff et al. 1995). On the Colorado Plateau
they occur primarily in sandstone, and appear to be formed by water
dissolving the cement, and wind removing the loose sand from the pit
(Netoff et al. 1995).
Figure 17. Aerial view of potholes in Navajo sandstone, Grand County, Utah.
On the Colorado Plateau, some species (e.g., Branchinecta packardi,
Figure 4) have wide tolerance ranges (Belk 1977), and will hatch
almost anytime water fills the potholes (T. Graham pers. observ.).
Others are active only under warm temperatures and are usually found
only following summer monsoon rains. In rare years when rain falls in
May or June, these summer species will hatch much earlier than usual.
The most conspicuous of these organisms are the tadpole shrimp
Triops longicaudatus (Figure 5), the clam shrimp
Leptestheria compleximanus (Figure 6), and the fairy shrimp
Streptocephalus texanus. Under scenarios of climate change that
result in an eastern shift in monsoonal rain patterns in North America,
the Colorado Plateau could lose its summer precipitation and conceivably
the characteristic summer pothole ecosystem if rain doesn't fall
predictably when temperatures are high enough to trigger hatching.
The current variability in precipitation affects pothole ecosystems in a number of ways. For potholes near permanent water, in years of high precipitation, aquatic insects become very abundant in potholes, dispersing from streams and reproducing in potholes. This has been the situation near Moab, Utah over the past 3-4 years, where winter and spring precipitation filled potholes so full they did not evaporate during May and June before the summer monsoons hit in July. Predatory insects were able to build and maintain relatively large populations, which effectively eliminated all branchiopod crustaceans that hatched in spring or summer. Pools that usually were teeming with branchiopods in August contained only insects and ostracods. Prior to that, the area experienced years of drought during which summer rains were very low in both frequency and quantity. Branchiopods hatched, but few pools lasted long enough for the shrimp to complete their life cycles and replenish the sediment cyst bank. The combination of these weather patterns could reduce populations significantly, especially if cysts were uncommon, and/or had short lives in sediment. Fortunately, these climatic changes have not become landscape scale patterns, but they do indicate how quickly a species or group of species could be lost from these systems if precipitation and temperatures patterns change on a longer time scale.
Because of the vulnerability of branchiopods to predation, branchiopods are at risk whether precipitation increases or decreases with global climate change. The reduced precipitation (especially in summer) during the late '80's depleted the sediment cyst bank, reducing the potential size of future hatching populations by some unknown amount. The increased precipitation in the early '90's, induced more dispersal of predaceous insects, and provided long- lasting habitat that allowed their populations to build up. These insects are very efficient predators, and quickly ate any branchiopods that hatched before they could mature and produce new eggs, successful reproduction of branchiopods was again probably very limited during these years.
|Figure 18. SEM photo, dorsal view, of Aquanothrus sp., an undescribed species of mite in Colorado Plateau pans.
||Figure 19. SEM photo, ventral view, of Aquanothrus sp.|
There is an intriguing miniature ecosystem on the Colorado Plateau found in very small weathering pits that may hold water for only a few hours to a couple days (Figure 1). In 1988, I discovered a small mite crawling in these pools (Figures 18 and 19). Dr. Roy Norton, an expert on oribatid mites, informed me it is a new species, one of only two species known in the genus Aquanothrus (Norton et al. 1997). The other species, A. montanus, is found in larger ephemeral pools in South Africa. The little pools on the Colorado Plateau also contain rotifers and other cryptobiotic organisms, but the mite is not desiccation-tolerant. It appears to resist drying out instead (T. Graham pers. observ.). As stated above, drought resisters are not as hardy as drought tolerators, and this is definitely the case with the mite. Sediment with mites held in jars for over a year yield very few if any live mites (T. Graham, pers. observ.), but rotifers in the same sediment are still alive and become active within a few minutes after getting wet. Climatic shifts that increase the time this species must be quiescent, either from reduced precipitation or increased evaporation, could affect the ability of Aquanothrus to survive on the Colorado Plateau.
Figure 20. Ephemeral snowmelt pool in the Sierra Nevada, Toulomne County, Calif.
Precipitation in a Mediterranean climate is relatively predictable in
terms of when it will occur (between November and March), but
quantities vary immensely between years, and across relatively small
distances. This climate, coupled with topographic variation (even at
small scales, Figure 24) gives rise to a number of kinds of vernal
pools, and the fairy shrimp of California at least have partitioned
the vernal pool habitat, resulting in the proliferation of species,
each adapted to slightly different conditions. In the Central Valley,
five species of Branchinecta and Linderiella occidentalis
can be found, but rarely is more than one species found in a given pool
(Eng et al. 1990, D. Belk and B. Helm pers. comm.).
|Figure 21. Volcanic mudflow vernal pool with clay and cobble bottom, eastern Tehama County, Calif.
||Figure 22. Vernal pool with clay hardpan bottom, Vina Plains Nature Conservancy Preserve, Calif.|
Figure 23. Vernal pool flowers, with different species occurring in bands related to soil moisture and temperature gradients formed as the pool dries out. Sacramento National Wildlife Refuge, Calif.
The quality of vernal pool environments is intimately tied to timing
and amount of precipitation, and water temperature. The temperature
itself is important in determining which species might hatch in a
pool, but the pool must last longer, on average, than the time needed
for a species to reach maturity and produce viable eggs if it is to
maintain a population. Relatively small changes in precipitation
timing or amount, with or without changes in temperature regime, could
alter this balance. A small change in average temperature may not
directly affect a species, but it could alter pool longevity enough to
reduce the chances of a particular species being able to reproduce in
that pool. Likewise, a shift in when rains begin to fall, even if the
average amount doesn't change, could affect a species' viability
simply because the water is too warm to induce hatching when it
falls. Given the sensitive nature of the vernal pool fauna to
environmental conditions, and the wide array of species that are
adapted to slightly different conditions, vernal pools may be a very
good indicator system of how global climate change is affecting
Figure 24. Aerial view of vernal pool complex showing spatial arrangement and variety of pool sizes, Dales Lake vernal pool complex, eastern Tehama County, Calif.
I would like to thank John Gardner (Brigham Young University) for the
scanning electron micrographs, Roy Norton (State University of New York,
Syracuse) for the light micrographs of mites, and Todd Keeler-Wolf
(California Dept. of Fish and Game) for the vernal pool pictures.
Ashcroft, G.L., D. T. Jensen, and J. L. Brown. 1992. Utah Climate. Utah Climate Center, Utah State University, Logan, UT 84322-4825.
Eng, L.L., D. Belk, and C.H. Eriksen. 1990. Californian anostraca: distribution, habitat, and status. Journal of Crustacean Biology 10:247-277.
Fryer, G. 1996. Diapause, a potent force in the evolution of fresh-water crustaceans. Hydrobiologia 320:1-14.
Hildrew, A. G. 1985. A quantitative study of the life history of a fairy shrimp (Branchiopoda: Anostraca) in relation to the temporary nature of its habitat, a Kenyan rainpool. Journal of Animal Ecology 54:99-110.
Kerfoot, W.C. and M. Lynch. 1987. Branchiopod communities: associations with planktivorous fish in space and time. pp. 367-378. in W.C. Kerfoot and A. Sih (eds.) Predation: direct and indirect impacts on aquatic communities. University Press of New England, Hanover, NH.
King, J.L., M. Simovich, and R. Brusca. 1996.
Kuller, Z. And A. Gasith. 1996. Comparison of the hatching process of the tadpole shrimps Triops cancriformis and Lepidurus apus lubbocki (Notostraca) and its relation to their distribution in rain-pools in Israel.
Maeda-Martinez, A. and D. Belk. in press. Phyllopod assemblages common to Mexico and the United States. Hydrobiologia.
Netoff, D.I., B.J. Cooper, and R.R. Stroba. 1995. Giant sandstone weathering pits near Cookie Jar Butte, southeastern Utah. pp. 25-53. in C. van Riper III (ed.) Proceedings of the second biennial conference on research in Colorado Plateau National Parks. NPS/NRNAU/NRTP-95/11. National Park Service.
Norton, R.A., T.B. Graham, and G. Alberti. 1997. A rotifer-eating ameronothroid (Acari:Ameronothridae) mite from ephemeral pools on the Colorado Plateau. pp. 539-542. in R. Mitchell, D.J. Horn, G.R. Needham, and W.C. Welbourn (eds.) Acarology IX, Proceedings (IXth International Congress on Acarology. Ohio Biological Survey, Columbus.
Planel, H., Y. Gaubin, R. Kaiser, and B. Pianezzi. 1980. Effects of space environment on Artemia eggs. pp. 189-198. in G. Personne, P. Sargeloos, O. Roels, and E. Jaspers (eds.) The brine shrimp Artemia, Vol. I. Wettern Universa Press.
Simovich, M. and S. Hathaway. in press. Diversified bet-hedging as a reproductive strategy of some ephemeral pool anostracans (Branchiopoda). Crustacean Biology.
Walossek, D. 1993. The Upper Cambrian Rehbachiella and the phylogeny of the Branchiopoda and Crustacea. Fossils and strata 32:1-202.