Alabama Cave Shrimp

Palaemonias alabamae

Description

Alabama cave shrimp (Palaemonias alabamae ) is a small decapod crustacean that grows to 1.2 in (3 cm) in length. It is similar in outward form to a common ocean shrimp, but its carapace (outer shell) is colorless and largely transparent. It differs from the only other species of the genus, the endangered Kentucky cave shrimp (P. ganteri ), by its smaller size, shorter rostrum (flattened frontal portion of the head), and fewer dorsal spines.

Behavior

Alabama cave shrimp feeds on detritus and plant matter that is washed into the caves. It is thought to have a low reproductive potential, bearing one-half to one-third fewer eggs than its closest relative, the Kentucky cave shrimp.

The species probably has a slow larval development. Throughout this period, many larvae die due to adult female death and genetic or embryonic developmental problems. As a result, the number of embryos emerging from the eggs are reduced by 50-66%. During the first summer, larval growth is rapid, but sexual maturity is not reached until the second summer.

In 1975 the first observation of gravid shrimp (females full of eggs) was made during every month between July and January. The number of eggs carried ranged from four to 30. It is believed that the eggs mature during the autumn months and are ready to hatch in the winter.

The sex ratios of Alabama cave shrimp seem to be equal. Longevity is unknown; however, results from aquaria studies on the closely related Kentucky cave shrimp indicated an estimated life span of 10-15 years.

Habitat

This albino shrimp is adapted to underground pools and streams eroded into the Warsaw Limestone formation of Alabama's Interior Low Plateau. Water levels in the five inhabited caves fluctuate seasonally; some portions of the caves dry out completely during the summer.

Shelta Cave, located in an urbanized area of Huntsville, consists of three large rooms with smaller alcoves. Water is present in all of the cave areas during wet periods; however, water levels fluctuate as much as 22 ft (6.7 m), leaving some areas of the cave seasonally dry. Miller Hall, the westernmost chamber, contains the only permanent body of water, West Lake, and the only permanent stream, West Creek. West Creek is shallow—6-8 in (15.2-20.3 cm) deep during low flow—has few riffles, flows to the southeast, and sinks 197 ft (60 m) its source. High flows in the cave occur during winter and spring months of heavy precipitation. The recharge area (source of water) surrounding Shelta Cave encompasses approximately 34 sq mi (88.1 sq km) and is privately owned. Land use in the recharge area is urban or industrial/suburban.

Bobcat Cave is found on the Redstone Arsenal, a U. S. Army installation, and access is restricted. The cave consists of one large room with a low ceiling and several alcoves and passages. Water levels fluctuate dramatically throughout the year and at high levels may block the entrance passage. During summer and fall low water levels, the cave pool retreats through the cave floor to the aquifer below. Initial recharge surveys of Bobcat Cave have been completed. On the Redstone Arsenal, the land immediately surrounding the cave is in pasture and is leased for cattle grazing. Redstone Arsenal airfield is located about 1 mi (1.6 km) east of Bobcat Cave. Surrounding land is suburban, forested, pastured, or in agriculture. The suburban/urban areas are expanding and becoming more densely populated.

Hering Cave has a large tunnel-like stream passage and a large boulder-strewn outflow channel that exits from the cave entrance. Stream depth varies from about 3 in (7.6 cm) to approximately 8 ft (2.4 m) deep, with the cave flooding during rainstorms.

Glover Cave contains a tunnel stream passage with very few side passages and three standing pools of deep water. Water flowing from Hering Cave enters Glover Cave through an entrance and a sinkhole. Glover Cave can contain swiftly flowing water during the wet winter and spring seasons.

Brazelton Cave is a solution tunnel with numerous permanent pools of water and seasonal stream flow. This stream is hydrologically connected to the stream that flows in Glover Cave. Brazelton Cave floods completely during intense rainstorms and seasonally in the winter and spring.

Distribution

A search of more than 200 caves in northern Alabama has failed to locate Alabama cave shrimp anywhere but in the five aforementioned localities (Shelta, Bobcat, Hering, Glover, and Brazelton Caves) in Madison County, Alabama, in the Huntsville Spring Branch and Indian Creek drainages. Shelta Cave is within the northwest limits of Huntsville, Alabama. Bobcat Cave is located approximately 8 mi (12.9 km) southwest of Shelta Cave on Redstone Arsenal. Brazelton, Glover, and Hering caves are located approximately 12 mi (19.3 km) southeast of Huntsville and are privately owned. (These three caves are hydrologically connected and are considered one system.)

A sighting of three cave shrimp was reported in December 1993 from a cave in western Jackson County about 15 miles northeast of Hering Cave, but this sighting had not been verified as of September 1997.

The occurrence of Alabama cave shrimp in Shelta Cave is seasonal; no individuals have been observed during the months of March through June, and only a single specimen was observed during February. Typically, the winter and spring months receive more precipitation than other months. Difficulties in finding shrimp appear to coincide with high water levels as aquatic habitat expands and disperses the shrimp. During low water, the aquatic habitat shrinks and forces shrimp into the remaining available habitat, thereby increasing the chances of finding them. The greatest number of shrimp observed occurred in November (24 shrimp) and December (25 shrimp) 1968.

During December 1988, a six-day search of Shelta Cave failed to locate the Alabama cave shrimp, and shrimp surveys conducted there from 1990 to 1993 were also unsuccessful in relocating the species. No Alabama cave shrimp have been observed in Shelta Cave since 1973, and surveys indicate that the shrimp has apparently been extirpated. At least ten gravid females were observed in Bobcat Cave on the following dates: one in May 1992, an undetermined number in July 1991, three in August 1991, and four in October 1991. From a total of 128 shrimp observed in Bobcat Cave, only five gravid female cave shrimp were noted.

Not much information is available on the sparse populations of Alabama cave shrimp discovered in the hydrologically connected Glover, Hering, and Brazelton cave system.

Threats

The cave systems containing Alabama cave shrimp are found in karst formations. Karst is a limestone region characterized by solution features such as sinks, springs, underground streams, tunnels, and caverns. The susceptibility of karst to groundwater pollution has been well-documented. Surface pollutants can easily and rapidly enter the subsurface aquifer, particularly during storm events.

Urbanization of areas surrounding Shelta and Bobcat Caves and development in the recharge area of the Glover, Hering, Brazelton system may cause contamination of the aquifers containing Alabama cave shrimp. Groundwater contamination may result from sewage leakage, industrial contaminants, road and highway runoff, toxic spills, pesticides, and siltation. Urbanization has also increased water demand in Huntsville, Alabama. Huntsville experienced severe water shortages in the 1990s due to increased demand and drought. In response, the city drilled the Drake well, which is capable of pumping up to 2,000 gal per min (7,570.8 l per min). This well is located less than 0.5 mi (0.8 km) from Bobcat Cave. Increased water consumption has the potential to affect Bobcat and Shelta Cave aquifers by lowering groundwater levels and reducing the amount of available Alabama cave shrimp habitat.

Habitat degradation has occurred in Shelta Cave from unknown causes. Water samples taken in 1987 indicated that the aquifer had become contaminated by cadmium (almost five times the drinking water standards), heptachlor epoxide, and dieldrin.

Suburbanization is affecting areas near Glover, Hering, and Brazelton Caves. Forested land is being cleared for new homes on Keel Mountain. Septic tank systems are needed for each new home since no sewer system in place. The shrimp found in Glover, Hering, and Brazelton Caves will no doubt be in danger of surface water and groundwater contamination from sewage leakage, lawn fertilizers, pesticides, and increased surface runoff from residential development.

Predation may also have an impact on cave shrimp populations. Southern cavefish, (Typhlichthys subterraneus ) is know to eat cave shrimp in Shelta Cave. Other potential predators in this cave include the Tennessee cave salamander and two troglobitic crayfishes. Potential predators that have been observed in Bobcat, Brazelton, Glover, and Hering Caves include bullfrogs, the southern cavefish, troglobitic crayfish, unidentified salamanders, and the Tennessee cave salamander. Predation by naturally occurring predators is a normal aspect of the population dynamics of a species. However, the effect of predation on a declining troglobitic species with an apparently low reproductive potential would be more significant than if the population were stable.

Conservation and Recovery

Until the early 1970s a maternity colony of the endangered gray bat (Myotis grisescens ) provided energy in the form of guano (dung) to the aquatic system of Shelta Cave. The bat colony abandoned the site, possibly as a result of the installation of an entrance gate, development around the cave, or a combination of these and other factors. The entrance gate was modified in 1981 in an attempt to accommodate gray bats but based on more recent studies, additional modifications or a different style gate are needed to promote recolonization. Loss of the Shelta Cave gray bat colony caused a decrease in the organic input to the aquatic community of the cave, and may have resulted in, or contributed to, a corresponding decrease in the populations of other cave species. No bat colonies are known to have occurred in Bobcat, Brazelton, Glover, or Hering Caves, but individual bats have been seen hibernating or flying in Bobcat, Glover, and Hering Caves.

The entrances to Shelta Cave are owned by the National Speleological Society (NSS), which has erected gates to control unauthorized access to the caves. (Speleology is the scientific study of caves.) The NSS has also produced a management plan for Shelta Cave with the purpose of protecting and recovering the biological resources of the cave. The Environmental Protection Agency has restricted the use of heptachlor epoxide and has banned all uses of dieldrin. The Geological Survey of Alabama, Department of Army, and the U. S. Fish and Wildlife Service (FWS) have conducted hydrogeologic studies of Shelta and Bobcat Cave aquifers. A Huntsville schoolteacher and his students, along with members of the Huntsville Grotto, conducted water quality measurements and monitored cave species, including bats in Shelta Cave and fauna of caves in Madison, Marshall, Morgan, and Jackson Counties. The University of Alabama in Huntsville has completed hydrologic modeling for Bobcat Cave and is developing a pollution model for the cave. Water quality and risk assessment studies of Bobcat Cave are also being developed. In a separate effort not directly related to the recovery of Alabama cave shrimp, the Alabama Department of Environmental Management initiated a groundwater protection education project for Madison County.

The FWS and the U. S. Army have developed a habitat management plan to protect Bobcat Cave against potentially damaging groundwater contamination. The FWS is also working closely with the NSS to develop regulation for recreational spelunkers (cave explorers) who use Shelta Cave. In the past there was evidence that spelunkers collected the cave shrimp, and, because of its low rate of reproduction, collection may have caused the shrimp's decline.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
(404) 679-4000
http://southeast.fws.gov/

References

Bouchard, R. W. 1976. "Crayfishes and Shrimps." In H. Boschung, ed., Endangered and Threatened Plants and Animals of Alabama. Bulletin No. 2. Alabama Museum of Natural History, Birmingham.

Environmental Protection Agency. 1986. "Report on the Remedial Action to Isolate DDT from People and the Environment in the Huntsville Spring Branch-Indian Creek System, Wheeler Reservoir, Alabama." Environmental Protection Agency, Atlanta.

Smalley, A. E. 1961. "A New Cave Shrimp from Southeastern United States." Crustaceana 3 (2): 127-130.

U. S. Fish and Wildlife Service. 1997. "Recovery Plan for the Alabama Cave Shrimp." U. S. Fish and Wildlife Service, Atlanta.