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Socorro isopod

The Socorro isopod (Thermosphaeroma thermophilum) is one of only seven freshwater species in what is otherwise primarily a marine family [1]. Its historical distribution likely include Cook, Socorro, and Sedillo warm springs which together fed a marsh extending for a half mile to the east of Cook Spring. Cook and Soccoro Spring were capped and their water diverted to the city of Socorro, New Mexico. The marsh no longer exists. However, the species has only been observed in Sedillo Spring which in the late 1970s was diverted to a hotspring spa, confining the isopod to 50 meters of habitat containing two small concrete pools and a narrow stream below the pools.

The Sedillo Spring population remained stable between 1978, when it was listed as an endangered species, and August 1988 when all isopods in the pool were nearly extirpated by invasive root growth which blocked the spring outflow [2]. Flows were restored a month later, perhaps flushing a small number of native isopods from the underground plumbing into the pool. These were successfully augmented with isopods that had been housed at the Biology Department of the University of New Mexico. In response to the near extinction, the Socorro Isopod Propagation Facility was established in 1990. The facility consists of two separate systems of four artificial pools connected by pipes. Six hundred isopods (75/pool) were introduced in 1990. By 1995, the south branch of the propagation facility was extirpated while the north branch had stabilized. In 1999, the north branch was extirpated due to two accidents. 400 isopods were introduced to the south branch (100/pool) in 1999 and continued to exist through at least 2003. A third captive population was established at the Albuquerque Biological Park in 1998 [3]. The two facilities have increased the total size of the population, the total extent of available habitat, and the number of independent populations [2].

Ongoing threats to the species include disruption of thermal groundwater discharge from surface and sub-surface explosive tests on Department of Defense lands immediately west of the natural spring, vandalism, and other human-caused modification of spring flows [3]. Between 1995 and 2002 vandals removed a valve and a protective culvert controlling flow to the spring, diverted and occluded the surface flow, removed the concrete wall lining the spring pool, dumped a junk car immediately adjacent to the spring, and removed vegetation from the propagation facility pools [3, 4]. Captive populations at the Socorro Isopod Propagation Facility have diverged morphologically and genetically from the native population, probably in response to different habitat structures which altered the intensity of (1) natural selection caused by adult cannibalism on juveniles and adults, and (2) sexual selection caused by increased male body size and changes in female spatial distributions[5].

[1] U.S. Fish and Wildlife Service. 1982. Socorro isopod (Thermosphaeroma thermophilum) Recovery plan. U.S. Fish and Wildlife Service, Albuquerque, NM.
[2] Lang, B.K., D.A. Kelt and S.M. Shuster. In press. The role of controlled propagation on an endangered species: demographic effects of habitat heterogeneity among captive and native populations of the Socorro isopod (Crustacea: Flabellifera). Biodiversity and Conservation, in press.
[3] New Mexico Department of Game and Fish. 2004. Threatened and Endangered Species, 2004 Biennial Review, Final Draft Recommedations. New Mexico Department of Game and Fish, Santa Fe, NM.
[4] New Mexico Department of Game and Fish. 2002. Threatened and Endangered Species, 2002 Biennial Review. New Mexico Department of Game and Fish, Santa Fe, NM.
[5] Shuster, S.. M., M. P. Miller, B. K. Lang, N. Zorich, L. Huynh and P. Keim. 2005. The effects of controlled propagation on an endangered species: Genetic differentiation and divergence in body size among native and captive populations of the Socorro Isopod (Crustacea: Flabellifera). Conserv. Genetics. 6: 355-368.

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