Spruce-fir Moss Spider

Microhexura montivaga

Description

The spruce-fir moss spider, Microhexura montivaga, was originally described in 1925 from collections made two years earlier in the mountains of western North Carolina. Only a few specimens were taken, and little was known about the species until its rediscovery approximately 50 years later. M. montivaga is one of only two species belonging to the genus Microhexura, the northernmost representative of the family Dipluridae. M. idahoana, the other species in the genus, occurs only in the Pacific Northwest. Diplurids belong in the primitive sub-order Mygalomorphae, which are often popularly referred to as "tarantulas."

The genus Microhexura is one of the smallest of the mygalomorph spiders, with adults measuring only 0.10-0.15 in (0.25-0.38 cm). Coloration of the spruce-fir moss spider ranges from light brown to a darker reddish brown, and there are no markings on the abdomen. The carapace is generally yellowish brown. The most reliable field identification characteristics for the spruce-fir moss spider are chelicerae that project forward well beyond the anterior edge of the carapace, a pair of very long posterior spinnerets, and the presence of a second pair of book lungs, which appear as light patches posterior to the genital furrow.

Behavior

There is no record of prey having been found in the webs of the spruce-fir moss spider, and so its food sources are unknown.

The egg sac, thin-walled and nearly transparent, may contain seven to nine eggs, laid in June. The female remains with the egg sac and, if disturbed, will carry the egg sac with her fangs. Spiderlings emerge in September. The means of dispersal of the spiderlings from the parental moss mat is not known. Ballooning, a process by which the spiders use a sheet of silk played out into the wind to carry them into the air, has been suggested as a possible means of long-range dispersal, but this species's high sensitivity to desiccation would likely preclude this dispersal method. The life span of the species is also unknown, although experts in the field have estimated that it may take three years for the species to reach maturity.

Habitat

The typical habitat of the spruce-fir moss spider is found in damp but well-drained moss and liverwort mats growing on thoroughly shaded rocks or boulders in mature, high-elevation Fraser fir (Abies fraseri ) and red spruce (Picea rubens ) forests. The forest stands at the sites where the species has been observed are composed primarily of Fraser fir intermingled with only scattered spruce. The moss mats found to contain the spider have all been found under fir trees. The spider cannot tolerate extremes of moisture; although very sensitive to desiccation, it can also be harmed by large drops of water. This is why the damp moss mats the spider inhabits can be neither too dry nor too wet. The spider constructs its tube-shaped webs in the interface between the moss mat and rock surface, although occasionally the web extends into the interior of the moss mat. The tubes are thin-walled and typically broad and flattened with short side branches.

Distribution

Status surveys conducted from 1989 through 1992 for the spruce-fir moss spider at its five historic locations and in other seemingly suitable habitat found the spider at only three sites in western North Carolina and one in eastern Tennessee. The historic North Carolina occurrence in Yancy County appears to have been extirpated, and only the population located along the Avery County and Caldwell County line in North Carolina seems relatively stable. The two other North Carolina populations are located in Swain County.

Both of the Swain County populations are extremely small with only one spruce-fir moss spider having been found at each of these two sites in recent years, probably because the forests at these locations are rapidly declining. The Tennessee population is located in Sevier County.

This population was considered relatively healthy at the time of the 1989 survey; however, revisits to this site in 1992 found dwindling spider numbers, apparently in conjunction with a rapid decline of Fraser fir at the site and associated desiccation of moss-mat habitat.

The most recent monitoring of this occurrence in 1994 indicates that it will likely be extirpated within the next several years.

Threats

The primary threat to the survival of the spruce-fir moss spider is the rapid deterioration of its damp, high-elevation forest habitat. This habitat alteration and loss has been primarily brought about by the infestation of an exotic insect, air pollution precipitated out as acid rain, and past land use history. The spider is also threatened by factors that have not yet been fully identified, as well as by its very small population base that increases its vulnerability to stochastic extinction.

The spruce-fir moss spider, very sensitive to lack of moisture, requires situations of high and constant humidity, a microclimate which is regulated and preserved by the forest canopy. The dominant canopy species in the forest stands where the spider lives is the Fraser fir, and the decline and death of Fraser firs begins a cycle inimical to this species. As the canopy dwindles more unfiltered light reaches the forest floor, promoting evaporation in previously shaded damp areas and the eventual desiccation of the moss mats often found there without which this spider cannot survive. Dr. Frederick Coyle, a leading expert on the spruce-fir moss spider, provides some anecdotal commentary on this problem in a 1991 letter sent to Keith Langdon. Dr. Coyle writes that the spider, common at one of the Swain County sites as late as 1983, had become extremely rare only five years later. He states that many of the moss mats at this site had become dry and loose, due largely, in his opinion, to deterioration of the forest canopy at the site.

Fraser fir at all four extant spider occurrences have suffered extensive mortality, believed to be primarily due to infestation by the balsam wooly adelgid (Adelges piceae ), an alien insect pest believed to have been introduced around 1900 into the United States from Europe. The adelgid was first detected in North Carolina on Mount Mitchell in 1957, although it was likely established at that site as early as 1940, and from Mount Mitchell it spread to Fraser fir communities throughout the southern Appalachians. Most mature Fraser fir are easily killed by the adelgid, with death occurring within two to seven years of the initial infestation.

While the loss of the Fraser fir appears to be the most significant threat to the remaining spruce-fir moss spider populations, the combined effects of several other factors seem to be highly taxing to its forest habitats and have contributed to the decline of the high-elevation spruce-fir forest stands. In 1988 it was estimated that trees 45-85 years of age at the summit of Mount Mitchell, the site in Yancy County where the species appears to be extirpated, showed an average defoliation of 75%-90%, and that all the trees exhibited some form of growth reduction. He hypothesized that atmospheric pollution was a possible factor in the decline. Regional scale air pollution in combination with other stress factors is believed to have played a significant role in the deterioration of the health of high-elevation red spruce in the East. The past land-use history in the southern Appalachians, especially former logging and burning practices, may well have contributed to present spider-site deteriorations. Winter tree injury has also been identified as a possible contributing factor to these declines. The death and thinning of the canopy trees within these stands also cause the remaining trees to be more susceptible to wind and related storm damage, which has become a major concern at the site in Sevier County, Tennessee.

The spruce-fir forest at the Avery County/Caldwell County location has not experienced the degree of decline that has occurred and is still occurring at the other sites known to support or to have supported populations of the spider. This habitat and spider occurrence is, nonetheless, also threatened by the same factors that are believed to have resulted in the decline of the spruce-fir forest and the associated loss of suitable moss-mat habitat at these other locations.

The spruce-fir moss spider is not currently known to be commercially valuable; however, because of its extreme rarity and uniqueness, it is conceivable that it could be sought by collectors. This spider is one of only two members of the genus Microhexura, is the only representative of the primitive family Dipluridae in eastern North America, and is one of the smallest of the world's "tarantulas." While collecting or other intentional take is not presently identified as a factor contributing to the decline of this species, the low numbers, slow reproductive rate, and extremely restricted range of the spruce-fir moss spider make it unlikely that the species could withstand even moderate collecting pressure.

It is presently unknown whether disease and predation have played roles in the decline of the spruce-fir moss spider, and further research is much needed in this area. While predation is not thought to be a significant threat to a healthy population of the spruce-fir moss spider, it could limit the recovery of the species or contribute to the local extirpation of populations already depleted by other factors. Possible predators of the spruce-fir moss spider include pseudoscorpions, centipedes, and other spiders.

In summary, the spruce-fir moss spider has been greatly reduced in number throughout its historic range; survives in only four locations, two of which have only one individual each; and has only one occurrence that might be described as currently stable, although this population is also threatened by many of the same factors that are believed to have resulted in the extirpation or decline of the other historically known populations. The four remaining populations are geographically isolated from one another, and three of them are so small as to have very little genetic variability left. All these circumstances make the spruce fir-moss spider endangered.

Conservation and Recovery

The U.S. Fish and Wildlife Service (FWS) believes that a detailed characterization of the spruce-fir moss spider's habitat, better knowledge of threats to this habitat, and additional information concerning its biology will be necessary in order to properly manage and implement protection and recovery measures. These, as well as other research needs and activities necessary to ensure the long-term survival of the species, will be addressed by the FWS in the development and implementation of a recovery plan for the spruce-fir moss spider.

Contacts

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

U.S. Fish and Wildlife Service
Asheville Field Office
330 Ridgefield Court Asheville, North Carolina 28806
(704) 665-1195 ext. 225

References

Coyle, F. A. 1985. Observations on the Mating Behavior of the Tiny Mygalomorph Spider, Micro-hexura montivaga Crosby & Bishop (Araneae, Dipluridae). Bulletin. British Arachnological Society 6 (8): 328-330.

Eagar, C. 1984. "Review of the Biology and Ecology of the Balsam Woolly Aphid in Southern Appalachian Spruce-fir Forests." In: P. S. White (ed.), The Southern Appalachian Spruce-fir Ecosystem: Its Biology and Threats. Research/Resources Management Report SER-71. U.S. Dept. of Interior, National Park Service.

Harp, J. M. 1991. "Status of the Spruce-fir Moss Spider, Microhexura montivaga Crosby and Bishop, in the Great Smoky Mountains National Park." Unpubl. report to the National Park Service, U.S. Department of the Interior. 12 pp. plus appendix.

Harp, J. M. 1992. "A Status Survey of the Spruce-firMoss Spider, Microhexura montivaga Crosby and Bishop (Araneae, Dipluridae)." Unpubl. report to the North Carolina Wildlife Resources Commission, Nongame and Endangered Wildlife Program, and the U.S. Fish and Wildlife Service, Asheville, North Carolina. 30 pp.

U.S. Fish and Wildlife Service. February 6, 1995. "Spruce-Fir Moss Spider Determined To Be Endangered." Federal Register 60 (24): 6968-6974.