Socorro Isopod

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Socorro Isopod

Thermosphaeroma thermophilus

ListedMarch 27, 1978
FamilySphaeromatidae (Isopod)
DescriptionTiny crustacean with a flattened, oblong to egg-shaped body.
HabitatWarm springs and algae-covered pools.
FoodAlgae, detritus.
ReproductionThree to 57 eggs laid every two months.
ThreatsRestricted habitat.
RangeNew Mexico


The Socorro isopod is a tiny aquatic crustacean with an average length of 0.32 in (8 mm). It has a flattened oblong to egg-shaped body with as many as eight mid-region (thoracic) segments. The abdomen is formed of two distinct segments. It has seven pairs of legs, antennae on the head, and oar-like extensions (uropods) on the last segment. The body is smooth and colored grayish-brown with small black spots and lines forming a band through each of the thoracic segments. Exposed edges of the body are tinged with bright orange. Various species of isopods are called pill bugs, sow bugs, or wood lice. The Socorro isopod was previously classified as Exosphaeroma thermophilus.


Females produce broods every two months with April being the peak reproductive period. Brood sizes range from three to 57 eggs, and gestation is about 30 days. Isopods feed on algae, detritus, dragonfly larvae, and are occasionally cannibalistic.


The Socorro isopod habitat consists of two small pools and two runs with relatively stable physical characteristics. Water temperatures range between 88 and 90°F (31 and 32°C). Algae covers most of the pool surfaces.


The Socorro isopod is found naturally only in Socorro County, New Mexico, in three inter-connected warm springs.

The surviving population is confined to the water system of an abandoned bathhouse known as the Evergreen about 1.8 mi (3 km) west of the city of Socorro. This water system is supplied by thermal outflows from Cedillo Springs and consists of an animal watering tank, a smaller pool, and about 132 ft (40 m) of irrigation pipe. Captive populations have been established at the University of New Mexico in Albuquerque and at the Dexter National Fish Hatchery in Dexter, New Mexico. The wild population is relatively stable at about 2,500.


The Socorro isopod is threatened by the limited size of its existing habitat. Its native warm springs were long ago capped and the water diverted to the city of Socorro's municipal water supply. The amount of water in the isopod's present pool is so small that any interruption of flow jeopardizes its survival. In 1987 the plumbing broke down, the water system dried up, and the isopod ceased to exist in the wild. Captive isopod populations at the University of New Mexico and the Dexter National Fish Hatchery were used to restock the repaired pool, and population levels have nearly returned to normal.

Conservation and Recovery

In 1988 the state of New Mexico received a grant from the federal government to construct a larger, more natural, and more stable habitat for the Socorro isopod. This habitat consists of a series of connected pools supplied by a natural water flow. Cooperation among the state, the city of Socorro (which owns the water rights), the private landowner, and the U. S. Fish and Wildlife Service on behalf of the isopod has been exceptional. In fact, this little crustacean has attracted such favorable local attention that a nearby school's soccer team has been named the Socorro Isopods.


Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P.O. Box 1306
Albuquerque, New Mexico 87103


Cole, G. A., and C. A. Bane. 1978. " Thermosphaeroma subequalum (Crustacea: Isopoda) from Big Bend National Park, Texas." Hydrobiologia 59(3):23-28.

U.S. Fish and Wildlife Service. 1982. "Socorro Isopod (Thermosphaeroma thermophilus ) Recovery Plan." U.S. Fish and Wildlife Service, Albuquerque.