Abstract
Drought tolerance of juvenile and adult life stages is relatively well understood, but very little is known about the tolerance of eggs to drought in this group of animals. The aim of the present study was therefore to investigate the water relations and drought sensitivity of eggs of the hygrophilic springtail, Folsomia candida Willem, 1902 (Isotomidae), exposed to a range of soil water potentials above and below the permanent wilting point of plants (-1.5 MPa). Under saturated conditions, eggs absorbed water during development and increased water content from 1.1 to 2.9 mg mg-1 dry weight. By increasing drought conditions, water absorption was gradually reduced and was nullified approximately at the soil water potential equivalent to the osmolality of egg fluids (630 mOsm corresponding to -1.53 MPa). Eggs had a lower permeability for water (68 ± 13 µg water cm-2 h-1 mm Hg-1) than adults (about 400 µg water cm-2 h-1 mm Hg-1), but eggs were much more sensitive to drought than adults. Eggs did not survive exposure to -1.5 MPa, whereas adults readily survive this level of drought by absorbing water vapour. In conclusion, eggs of F. candida are sensitive to drought and would perish if soil water potential in the field approaches the wilting point of plants, which is often reached during summer droughts. The persistence of this species depends on the survival of post-embryonic life stages.
Highlights
Springtails (Collembola) are geographically widespread soil arthropods that can occur at high densities approaching 105 individuals m–2 or more (Petersen & Luxton, 1982)
Embryonic development has been studied in a number of species of springtails including F. candida (Tieg, 1942; Marshall & Kevan, 1962), no studies have attempted to quantify the water content as done here
Water content had almost tripled (2.9 mg mg–1 dry weight (DW)) by the end of the experiment and there was a close correspondence between water content and egg volume (Fig. 1B)
Summary
Springtails (Collembola) are geographically widespread soil arthropods that can occur at high densities approaching 105 individuals m–2 or more (Petersen & Luxton, 1982). Adult and juvenile life stages of soil living springtails are not very resistant to desiccation compared to, e.g., insects (i.e. they are unable to reduce evaporative water loss) since their skin is permeable to water vapour (Harrison et al, 1991; Kærsgaard et al, 2004) Despite their permeable skin, many species can tolerate drought conditions far below the wilting point of plants (soil water potential of –1.5 MPa) by absorbing water vapour made possible by the accumulation of compatible osmolytes such as myoinositol, trehalose and alanine (Bayley & Holmstrup, 1999; Sjursen et al, 2001; Holmstrup et al, 2015). In addition to these physiological defence mechanisms, juvenile and adult springtails can migrate to moister soil layers or microsites and improve their chances of survival during drought by behavioral means (Hågvar, 1983; Verhoef & Van Selm, 1983; Detsis, 2000; Tsiafouli et al, 2005)
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