Bruneau Hot Springsnail

Pyrgulopsis bruneauensis

Description

The Bruneau hot springsnail, Pyrgulopsis bruneauensis, have a small, globose to low-conic shell reaching a length of 0.22 in (0.5 cm) with 3.75 to 4.25 whorls. Fresh shells are thin, transparent, and white-clear, although appearing black due to pigmentation. In addition to its small size, less than 0.11 in (0.3 cm) shell height, distinguishing features include a verge (penis) with a small lobe bearing a single distal glandular ridge and elongate, muscular filament.

Behavior

The Bruneau hot springsnail appears to be an opportunistic grazer that feeds upon algae and other periphyton in proportions similar to those found in its habitat. However, snail densities are lowest in areas of bright green algal mats and highest where periphyton communities are dominated by diatoms, which may provide a more nutritious food source than other food types, thus contributing to this greater density. It logically follows from this that Bruneau hot springsnail may make food selections based on nutritional richness rather than just choosing "preferred" individual food items. Fluctuations in Bruneau hot springsnail abundance correspond with changes in food quality based on chlorophyll content.

Sexual maturity can occur within two months, with a sex ratio approximating 1:1. Reproduction occurs at temperatures between 75.2 and 95°F (24 and 35°C); this occurs throughout the year except when inhibited by high or low temperatures. At sites affected by high ambient temperatures during summer and early fall months, recruitment corresponds with cooler periods. Sites with cooler ambient temperatures also exhibit recruitment during the summer months. Springs with cooler minimum temperatures most likely get warmer than 68°F (20°C) in the summer, providing the species opportunities for increased growth and reproduction. The Bruneau hot springsnail, whose individuals are dioecious, deposit its single round to oval eggs on hard surfaces such as rock substrates or other snail shells when suitable substrates are unavailable.

Biologists believe that some natural transfer of the Bruneau hot springsnail may occur among sites. The mechanisms for dispersal possibly include waterfowl passively carrying the Bruneau hot springsnail up or down the river corridor and spates, sudden overflows of water resulting from a downpour of rain or melting of snow, in the Bruneau River that would carry this taxon into other warm spring areas downstream. These mechanisms of dispersal would favor upstream to downstream genetic exchange.

Habitat

The hot spring and seep habitats of this snail are hydraulic outflows from the confined, regional geothermal aquifer that underlies Bruneau, Little, and Sugar valleys in north-central Owyhee County, an area of approximately 600 sq mi (155,400 hectares). This water flows through natural faults and fractures in the deep-lying volcanic and subsurface sedimentary rocks until it discharges at the surface through artesian vents, where the ground-level elevation is lower than the potentiometric or hydraulic head of the geothermal aquifer. The vast majority of the groundwater in this aquifer originates as natural recharge from precipitation in and around the Jarbidge and Owyhee mountains south of the Bruneau area. Ground water flows northward from volcanic rocks to sedimentary rocks where it is discharged as either natural springflow, well withdrawals, or leaves the area as underflow.

There also exists a shallow, unconfined cold-water aquifer within the upper layer of sedimentary rock. This second aquifer system is recharged from the infiltration of precipitation, streamflow, and applied irrigation water. Some scientists also believe that there may be recharge from upward-moving geothermal water into the cold-water aquifer. There also may be additional shallow-water aquifer recharge occurring through leaks in irrigation wells.

The Bruneau hot springsnail occurs in flowing thermal springs and seeps along an 5-mi (8-km) reach of the Bruneau River in water temperatures ranging from 60.3 to 98.4°F (15.7 to 36.9°C) This species has not been located outside the thermal plumes of hot springs entering the Bruneau River. The Indian Bathtub spring, the type locality, occurs at an elevation of 2,672 ft (814.4 m); the other thermal springs where this snail is found are at comparable elevations. The highest snail densities occur at temperatures ranging from 73 to 98°F (22.7 to 36.7°C). Some of the Bruneau hot springsnail colonies are separated by distances of less than 3.3 ft (1 m).

The Bruneau hot springsnail occurs in these habitats on the exposed surfaces of rocks, gravel, sand, mud, algal film and the underside of the water surface; however, during the winter period of cold ambient temperatures and icing, snails are most often located on the undersides of outflow substrates that are least exposed to cold temperatures. In madicolous habitats, those with thin sheets of water flowing over rock faces, the species has been found in water less than 0.39 in (1 cm) deep. Current velocity is not considered a significant factor limiting the distribution of this snail, since they have been observed to inhabit nearly 100% of the available current regimes. In a September 1989 survey of 10 thermal springs in the vicinity of the Hot Creek-Bruneau River confluence, the total number of Bruneau hot springsnails per spring ranged from one to 17,319. The species abundance fluctuates seasonally but is generally stable under persistent springflow conditions. Although on-site conditions are important, snail abundance is influenced primarily by temperature, spring discharge, and chlorophyll ratios.

Common aquatic community associates of the Bruneau hot springsnail include the molluscs Physella gyrina, Fossaria exigua, and Gyraulus vermicularis ; the creeping water bug (Ambrysus mormon minor ); and the skiff beetle (Hydroscapha natans ). In addition, Hot Creek and several of the thermal springs along the Bruneau River support populations of Poecilia reticulata and Tilapia sp. These are exotic guppies that were apparently released into upper Hot Creek at the Indian Bathtub, from which they spread downstream and into nearby thermal springs and seeps along the Bruneau River.

Distribution

The most recent habitat survey in 1996 found Bruneau Hot Springsnails in 116 of 204 flowing thermal springs and seeps in their 5-mi (8-km) historical range along the Bruneau River. Eighty-six of these occupied springs are located upstream of the confluence of Hot Creek with the Bruneau River, 10 are at the confluence of Hot Creek, and 20 are downstream of the confluence of Hot Creek with the Bruneau River. Surveys conducted since 1991 indicate a moderate but significant decrease in suitable habitat and occupied pools. Since 1991, the total number of thermal springs in the Bruneau River has declined from 214 to 204, the number of springs occupied by Bruneau hot springsnails has declined from 130 to 116, and the population densities of occupied areas have declined from about 55 to 47 individuals per square foot (1 sq ft=0.09 sq m). Total site area, including all thermal springs and seeps whether occupied or unoccupied by Bruneau hot springsnails, increased by 4.3% from 1991 to 1996. Most of this increase was due to lower flows at one unoccupied spring site, resulting in more exposure of thermal outflow area below Buckaroo Dam, downstream of the majority of the occupied springs.

The Indian Bathtub area and most of the thermal springs along the Bruneau River upstream of Hot Creek are on lands administered by the Bureau of Land Management, while most Bruneau Hot Springsnail habitats downstream of the Indian Bathtub and Hot Creek confluence are on private land.

Threats

The primary threat to the Bruneau hot spring-snail is a major reduction in its free-flowing thermal spring and seep habitats caused by agricultural-related ground water withdrawal and pumping. This activity has depleted and contiues to deplete the regional geothermal aquifer upon which snail habitat depends. Some scientist are convinced that leaks from uncased or poorly cased wells are also reducing water levels in the geothermal aquifer. The species and its habitat are also vulnerable to habitat modification from the sediments deposited by flash floods. In summary, the cumulative effects of water withdrawal continue to threaten the increasingly fragmented populations of the Bruneau hot springsnail and their thermal habitats.

Ground water withdrawals from wells for domestic and agricultural purposes began in the area of the geothermal aquifer in the late 1890s. By the mid-1960s the decline in discharge from the Indian Bathtub spring became very noticeable, coinciding with the accelerated increase in ground water withdrawal to provide irrigation for croplands newly put into production.

The two most apparent effects of pumping stress are declines in hydraulic head and declines in spring discharge. Changes in discharge from thermal springs correlate with changes in hydraulic head, which fluctuate seasonally and are substantially less during late summer than in the spring.

Discharge fluctuations, which occur at most occupied springs, very frequently correspond with ground-water withdrawal rates; there are lower flows in the late spring to early fall when the need for pumping is greatest, and higher flows during late fall to spring when the need for pumping is lowest. Discharge from many of the thermal springs along Hot Creek and the Bruneau River has decreased or has been lost in the last 25 years, thus further restricting habitat for this taxon. The Hot Creek/Indian Bathtub spring site lost more than 90% of both its habitat and snail population during the period from 1954 to 1981. Rapidly dwindling spring flows were instrumental in this precipitous decline.

Spring discharge at the Indian Bathtub in 1964 was approximately 2,400 gal (9,085 l) per minute; by 1978, it had dropped to between 130-162 gal (492-613 l) per minute; and by the summer of 1990, discharge fell to zero through the early fall water withdrawal season. Visible spring discharge at the Indian Bathtub continues to be seasonal, intermittent most years, and quite low.

Snail population at the Indian Bathtub spring occur on vertical rockfaces protected from flash floods. In 1991, a flash flood sent huge amounts of sediment into the Hot Creek drainage, resulting in a 50% reduction in the size of the Indian Bathtub, a portion of which is now covered by approximately 10 ft (3 m) of sediment. Rockface habitat in the immediate vicinity of Indian Bathtub was also severely reduced and covered with sediment during this and other recent flash floods.

Ongoing population monitoring studies indicate a lack of movement or recruitment of Bruneau hot springsnails back to the Hot Creek/Indian Bathtub sites. Several factors have been cited as contributing to this situation, including silty substrate that lacks available rockface surfaces for reproduction, weak migration abilities, fish predation, and a lack of an upstream colonization that may have prevented the Bruneau hot springsnail from returning to the upper Hot Creek and Indian Bathtub sites.

Ground water withdrawals have generally declined over the past 15-20 years, primarily due to cropland retired from production through a crop-land reclamation program. However, the volume of water pumped may increase significantly in the next few years as croplands will again be put into production. If present water management practices continue, if a substantial proportion of the crop-lands are returned to production, and if drier spring and summer climatic conditions return—all of which affect pumping rates and duration—water levels in the aquifer will either continue to decline or will eventually stabilize at a lower level, resulting in the further loss of Bruneau hot springsnail habitat.

While huge springflow declines have been documented at Indian Bathtub spring and several other springs, springflow data has not been collected in all the remaining 116 springs containing Bruneau hot springsnails. Some scientists believe that prior to the recent decline in water levels in the aquifer and the consequent fragmentation of remaining populations all of the springs and seeps supporting snails were connected, which allowed the natural dispersal and transfer of individuals. Studies done in the early 1990s indicate a general decline in the total number of thermal springs along the Bruneau River, the number of springs occupied by Bruneau hot springsnails, and the densities per unit area of Bruneau hot springsnails in occcupied pools. In 1993, dead Bruneau hot springsnails were found at one previously occupied spring site where flows had recently diminished and nine spring sites showed noticeable reductions in discharge. At this time there is no information available indicating how much lower water levels can continue to decline before all thermal springs along the Bruneau River are lost. As potentiometric surfaces in the geothermal aquifer continue to decline, additional spring discharges will be reduced or lost, resulting in the continued loss of Bruneau hot springsnail habitat.

Cattle grazing has damaged Bruneau hot springsnail habitats and directly eliminated snails, especially along Hot Creek. Cattle have destroyed and displaced snails through trampling instream substrates, and their browsing removes heat-moderating riparian vegetation, allowing water temperatures to climb to levels that first damage reproduction and then can kill Bruneau hot springsnails. Livestock grazing in the watershed adjacent to Hot Creek, combined with ongoing drought conditions, contributed to an increase in sedimentation of that creek which eliminated Bruneau hot springsnail seep and spring habitats for almost 500 ft (152 m) in the Indian Bathtub/Hot Creek drainage. The Bureau of Land Management is going to control livestock grazing by installing fencing on the north end of Hot Creek drainage and the west side of the Bruneau River. The Bureau of Land Management also plans to install additional fencing along the east side of the Bruneau River. Both fencing projects, if properly maintained, will protect remaining snail habitat from the effects of livestock.

There are no current commercial uses for this species, although certain mollusc species have subsequently become vulnerable to illegal collection for scientific purposes after their rarity was widely publicized. Collection could now become a threat to this taxon because the distribution of the Bruneau hot springsnail is restricted and generally well-known.

There are no known diseases that affect Bruneau hot springsnails, but juvenile snails smaller are vulnerable to a variety of predators. Damselflies and dragonflies have been observed feeding upon Bruneau hot springsnails in the wild. The presence of wild guppie populations in Hot Creek and several of the other small thermal springs downstream along the west bank of the Bruneau River are a potential threat to this species, as they have been observed feeding upon these snails in the laboratory. In addition to guppies, a species of Tilapia has ascended into and reproduced in Hot Creek. The presence of this new potential exotic predator may constitute a threat to the Bruneau hot springsnail by restricting repopulation of the snail into Hot Creek and at other thermal spring sites that may be available to both species. The guppy and Tilapia are each capable of summer migration, when water temperatures are suitable, into the Bruneau River corridor, both upstream and downstream of Hot Creek. Movement of these exotic fish species into other thermal springs occupied by the Bruneau hot springsnail might affect their continued survival within individual spring sites. It should be noted that madicolous habitats support neither of these two exotic fishes or dragonflies, but do harbor numerous damselflies.

Sedimentation of Bruneau hot springsnail habitats is a significant threat to this species. Substantial sediments deposited by periodic flash floods cannot be flushed away by the remaining weak and declining springflows. Measures which could protect Bruneau hot springsnail habitats in the Indian Bathtub and Hot Creek areas from the effects of flash flooding have not been implemented. These measures include the construction of small retention dams in the Hot Creek water-shed to trap runoff sediment while maintaining thermal seep habitats. Flooding and sedimentation therefore continue to threaten Bruneau hot springsnail habitat.

Conservation and Recovery

The Bureau of Land Management manages the public lands containing Bruneau hot springsnails and their habitats along Hot Creek and the Bruneau River. The Bureau of Land Management issues permits for livestock grazing on these lands and grants authorizations that could lead to the drilling of new wells and increased ground water use on Bureau of Land Management lands. In the past, the Bureau of Land Management has shown an interest in conserving this snail species by soliciting input from the U.S. Fish and Wildlife Service (FWS) regarding impacts that may result from any proposed activities. As discussed earlier, the Bureau of Land Management has implemented fencing to protect Bruneau hot springsnail habitats from grazing impacts.

The FWS entered into a short-term conservation agreement with Owen Ranches, Inc., the landowners of the Bruneau hot springsnail's habitat in Indian Bathtub spring. This conservation agreement included fencing, through funds provided by the FWS, to regulate livestock use and improve stream conditions. Although the agreement expired in October 1992, the current landowner has honored the terms of the agreement and voluntarily excludes livestock grazing from the Indian Bathtub spring.

The cropland reclamation program is a voluntary program that offers annual rental payments, incentive payments for certain activities, and cost-share assistance to establish approved cover on eligible cropland. This program encourages farmers to plant long-term resource-conserving covers to improve soil, water, and wildlife resources. The duration of the contracts are between 10 and 15 years; all of the current lands in the program have expired. It is unlikely that all those eligible for the new agreements will participate due to a dramatic drop in the rental rates currently offered through the program, declining from about US$50 per acre to about US$20 per acre (1 acre=0.4 hectare). Area landowners have indicated that this drop in rental fees will not provide the necessary incentive to continue participation.

After the Bruneau hot springsnail was list as endangered in January 1993, a joint lawsuit was filed by various Idaho plaintiffs to challenge that listing. In December 1993, the district court issued a ruling in favor of the Plaintiffs and set aside the final listing rule for the Bruneau hot springsnail. The district court decision was appealed by two intervening Idaho conservation groups and in 1995, the appellate court overturned the district court decision and reinstated the Bruneau hot springsnail to the endangered species list.

Contacts

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

Snake River Basin Office
U.S. Fish and Wildlife Service
1387 S. Vinnell Way, Room 386
Boise, Idaho 83709
(208) 378-5243

Reference

U.S. Fish and Wildlife Service. 17 June 1998. "Notice of Determination To Retain Endangered Status for the Bruneau Hot Springsnail in Southwestern Idaho Under the Endangered Species Act." Federal Register 63 (116): 32981-32996.