LIFE HISTORY CHARACTERISTICS OF THERMOSPHAEROMA THERMOPHILUM, THE SOCORRO ISOPOD (CRUSTACEA: PERACARIDA)

¹ Department of Biology, The University of New Mexico, Albuquerque, NM 87131

The endangered species (Federal endangered species list), Thermosphaeroma thermophilum, or "Socorro Isopod," is endemic to a small spring near Socorro, New Mexico, that is thermally stable year-round. Isopods were observed in the field, and monthly samples were collected between March 1978 and February 1979 for laboratory examination. Males were larger than females, and sex ratio in the habitat was consistently biased toward males, particularly when sexually receptive females were abundant. Reproduction occurred primarily, although not exclusively, in spring and fall. Food seemed scarce, and intense predation by the omnivorous isopods appeared to exclude most invertebrate species, including predaceous aquatic insects, from the habitat. Fish do not inhabit the spring and avian predation is minimal or non-existent, perhaps permitting isopods to reach high densities and decreasing risks associated with male mate-searching behavior. Field and laboratory data indicated that isopods live less than 1 year. Males grow and reach sexual maturity faster than females. No evidence of hermaphroditism or sex-change was observed. Females were iteroparous and brood size increased with a female's age, but females were small and varied little in size, suggesting selection for an optimal female size.