Bone Cave Harvestman

Texella reyesi

Description

The Bone Cave harvestman is a troglobite, which is a species which spends its entire life in openings underground usually with small or absent eyes, attenuated appendages, and other adaptations to its subsurface dwelling.

This species is a long-legged, blind, pale orange harvestman. It measures 0.06-0.11 in (1.41-2.67 mm) in body length. Scute length is 0.05-0.07 in (1.26-1.69 mm). The leg II length is 0.24-0.46 in (6.1-11.79 mm). The leg II to scute length is 0.17-0.34 (4.3-8.68 mm). The exoskeleton is rough. A few small tubercles can be observed on the eye mound, which is broadly conical; the retina is absent; the cornea variable. The penis possesses a ventral plate prong and is round apically. There are two dorsal, 17 lateral, and four ventral setae. The apical spine of this species is bent and apically pointed; its length is 0.002 in (0.05 mm). The glans have a basal knob which appears narrow and conical in shape. The middle lobe is long and the parastylar lobes are claw-shaped. The stylus of the Bone Cave harvestman is long, curved and ventrally carinate; apically spatulate. The basal fold is well-developed.

Juveniles are white to yellow in color, while adults are pale orange. This species displays geographical polymorphism. Northern populations have longer legs, a smoother exoskeleton, and reduced or absent corneas.

Behavior

This species spends its entire life underground. It is endemic to the karst (limestone) formations. These formations include caves, sinkholes, and other subterranean voids.

The Bone Cave harvestman is sensitive to less-than-saturated humidities. Most individuals are found only under large rocks, but are occasionally seen walking on moist floors. In Temples of Thor Cave, individuals are typically found on a rough slope about 32.8 yds (30 m) from the entrance in absolute darkness. Humidity is high in this particular area.

Habitat

This species is dependent on outside moisture and nutrient inputs generated from the subsurface. This species inhabits areas of the cave where temperature and humidity are constant. The surface vegetation ranges from pasture land to mature oak-juniper woodland.

Karst is formed by the slow dissolution of calcium carbonate from limestone bedrock by mildly acidic ground water. This process results in subterranean voids resembling a honeycomb. The water enters the subsurface through cracks, crevices, and other openings, dissolving soluble beds of rock.

Nutrients to this ecosystem are provided from the outside surface washed in. These nutritional sources include plant material, feces, eggs, and carrion. Cave crickets are believed to provide an important component to the nutritional balance of this cave ecosystem. These crickets introduce nutrients through eggs, feces, nymphs, and dead body parts on which many invertebrates are known to feed.

Raccoon feces provide a rich medium for fungi growth which, in turn, is a haven for collembolans (small insects).

Distribution

The Bone Cave harvestman was originally discovered in 1989 in Bone Cave, Williamson County, Texas.

As this species has only been distinguished as a separate species of Texella recently, past distribution is difficult to ascertain. This species currently inhabits 54 caves, 46 known occurrences, and eight possible locations from northern Travis to northern Williamson Counties, Texas.

Threats

This species and seven other invertebrates of the karst (limestone) formations are threatened by land development, pollution, vandalism, and/or fire ants.

The primary threat to Bone Cave harvestman is habitat loss due to urban development activities. Continued urban expansion such as residential subdivisions, schools, golf courses, roads, commercial and industrial facilities, etc. poses a threat in the form of cave filling or collapse, water diversion, vegetation/fauna alteration, and increased pollution.

Some caves have already been filled as a result of road construction and building site preparation. Development directly above caves could result in the collapse of cave ceilings.

Ranchers may have also filled some caves. Justification is placed in reducing hiding places for predators of cattle and goats as well as preventing these animals from falling into the formations.

Troglobites rely upon and in fact require a controlled environment of high humidity and constant temperature. If water drainage paths are altered, this balance is no longer on an even keel. Water diversion away from the caves could lead to the direct mortality of this species. Increased water infiltration could lead to flooding and loss of air space.

As the karst ecosystem relies on the infiltration of nutrients from the surface, a fluctuation in the vegetation or fauna would alter nutrient supplies. During development, native vegetation may be replaced with non-native species, as well as cause the introduction of exotic animal species, such as fire ants. An overall nutrient depletion would result. The removal of vegetation could also lead to temperature fluctuations, a change in moisture regime and potential for contamination and increased sedimentation from soil erosion.

Conservation and Recovery

A Recovery Plan was published for the Bone Cave harvestman in 1994. The conservation of this rare insect requires the strict protection of its cave habitats from disturbances and other changes associated with residential, agricultural, or commercial development. Other necessary actions include studies to monitor the abundance of the Bone Cave harvestman and research into its biology and habitat needs. There should also be a public education campaign to develop a broad base of support for the protection of rare cave habitats.

Contact

U.S. Fish and Wildlife Service
Regional Office
P. O. Box 1306
Albuquerque, New Mexico 87103
http://southwest.fws.gov/

References

U.S. Fish and Wildlife Service. 1993. "Endangered and Threatened Wildlife and Plants; Coffin Cave Mold Beetle (Batrisodes texanus ) and the Bone Cave Harvestman (Texella reyesi ) Determined to Be Endangered." Federal Register 58(158): 43818-43819.

U.S. Fish and Wildlife Service. 1994. Recovery Plan for Endangered Karst Invertebrates in Travis and Williamson Counties, Texas. Albuquerque, New Mexico.