Sensitivity of life history parameters in the earthworm Aporrectodea caliginosa to small changes in soil water potential Martin Holmstrup * National Environmental Research Institute, Department of Terrestrial Ecology, Vejlsùvej 25, PO Box 314, DK-8600, Silkeborg, Denmark Received in revised form 22 November 2000; accepted 12 December 2000 Abstract The effects of decreasing soil water potential on cocoon production, cocoon development and growth of juveniles were investigated in the earthworm Aporrectodea caliginosa. The earthworms were subjected to water potentials between 22 and ca. 2300 kPa, using two soil types with various wetness. The duration of the drought exposure was 14 days, whereafter the earthworms were returned to ideal moisture conditions. Cocoon production was negatively affected when the water potential was lower than 212 kPa, and below 240 kPa cocoon production was completely arrested. The negative effect on cocoon production persisted for at least 2 weeks after the worms were moved back to soil with optimal moisture conditions. For those worms that were exposed to the most severe drought level, 2330 kPa, cocoon production was still signi®cantly impaired two months after drought exposure. Juvenile growth was sensitive to decreasing water potential. Even at 26 kPa the growth of juveniles was signi®cantly lower than in controls and at ca. 220 kPa there was no growth at all. Cocoon development was less sensitive to water potential than growth and cocoon production. Thus, development seemed not to be affected by a 14 day exposure at water potentials above ca. 2700 kPa, whereas at higher levels there was an increasing effect. When cocoons were exposed to ca. 23 MPa the development was immediately arrested. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cocoon production; Development; Diapause; Drought; Growth; Soil water potential 1. Introduction Although earthworms are adapted to the terrestrial environment, they have many similarities to freshwater animals with regard to their excretory and respiratory physiology (Ramsay, 1949; Oglesby, 1969). They produce large amounts of hypotonic urine and need to keep their body surface moist to facilitate oxygen uptake, across the integument. Earthworms have only very moderate morphological or physiological means for reducing water transport through the cuticle (Carley, 1978). There- fore, soil moisture is the most important environmental factor for the performance and distribution of earthworms (Lee, 1985). It is well-known that it is not the soil wetness, but rather the soil water potential that controls the response of earthworms to changing moisture condi- tions (Collis-George, 1959; Gerard, 1967; Kretzschmar and Bruchou, 1991). To understand earthworm distribution it is important to know the range of soil water potentials within which earth- worms are able to remain active and continue development and reproduction. Several ®eld studies have studied this issue, but most have focussed on the activity and vertical movements of earthworms. These reports suggest that the activity of European lumbricids is limited to soil water potentials not lower than 2100 to 2200 kPa (Gerard, 1967; Nordstro Èm, 1975; Rundgren, 1975; Kretzschmar, 1991; Baker et al., 1992). This limit has been identi®ed on the basis of activity indices such as surface casting, and the induction of quiescence or diapause. Little is known of the relationship between soil water potential and life history parameters such as growth and reproduction. The aim of this study was to describe the effect of low soil water potential on important life history parameters in the earthworm Aporrectodea caliginosa (var. tuber- culata) Savigny. In a series of laboratory experiments adults, juveniles and cocoons of this species were subjected to a simulated summer drought by manipulat- ing the wetness of two different agricultural soils. In addition to immediate effects on growth, reproduction and cocoon development, the recovery of cocoon produc- tion after a drought period was also studied to evaluate the long-term effects of drought. Soil Biology & Biochemistry 33 (2001) 1217±1223 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(01)00026-8 www.elsevier.com/locate/soilbio * Tel.: 145-8920-1573; fax: 145-8920-1414. E-mail address: mho@dmu.dk (M. Holmstrup).