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Effects of low ph and high temperature on hatching and survival of the water mite Unionicola foili (Acari: Unionicolidae).

Article Excerpt
ABSTRACT. Adult females, larvae, and eggs of the water mite Unionicola foili were removed from their host mussel Utterbackia imbecillis, and their tolerance to varying pH (4.1, 5.2, 7.0, and 7.8) and temperature (25 [degrees], 33 [degrees], and 38 [degrees] C) was examined. Longevity of adult U. foili was significantly reduced at pH 4.1, whereas survival of larvae was significantly reduced at pH 5.2. Hatching of mite eggs was unaffected by exposure to low pH. Survival of adult mites was significantly reduced when exposed to increasing temperature treatments. Larval mites experienced a significant decrease in survivorship at 33 [degrees] C, but exposure to higher temperature yielded no further changes in longevity. Egg hatching was not affected by exposure to increasing temperature. Although eggs of U. foili were comparatively more resistant to low pH and elevated temperature than adults or larvae, the reason for the observed differences remains to be tested. Overall, U. foili were fairly sensitive to low pH and elevated temperature. Active stages of U. foili appear to be more vulnerable to pH changes than their adult host mussels, making them useful biomonitors of acute exposures to acid-contaminated waters.

Keywords: Unionicola foili, water mite, hatching, survival, Temperature, pH

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Water mites of the genus Unionicola (Acari: Unionicolidae) are common symbionts of freshwater mussels of the family Unionidae. Their life cycle is complex and includes larvae that leave a host mussel and undergo a brief parasitic phase with chironomid dipterans. Following this association, larvae re-invade a host mussel and undergo developmental transformation that is typical of acariformes, eventually becoming sexually mature adults. The extent to which members of the genus depend on host mussels is variable. Some species are free-living as nymphs and adults, using mussels only as a site for oviposition and post-larval developmental transformations. Other species are obligate symbionts of their bivalve hosts.

Although there is a substantial amount of information pertaining to the tolerance of their insect (Thornton & Wilhm 1974; Jernelov et al. 1981; Pascoe et al. 1989) and mussel hosts (Holwerda & Veenhof 1984; Pynnonen 1990; Keller & Zam 1991) to a variety of environmental conditions, there is very little known about the effects of environmental stress on unionicolid water mites. With the exception of a few biological monitoring studies (Scullion & Edwards 1980; Kowalik & Biesiadka 1981; Rundle 1990; Biesiadka & Kowalik 1991; Cicolani & Di Sabatino 1991) and a laboratory study by Rousch et al. (1997), the physiological ecology of water mites as a group is limited. The lack of information on the tolerance of water mites to changing environmental conditions is disconcerting because field studies suggest that water mites are sensitive to acid stress (Rundle 1990) and organic pollutants (Cicolani & Di Sabatino 1991) and thus may be useful as indicators of pollution.

The present study addresses the tolerance of various developmental stages of the water mite Unionicola foili to changes in pH and thermal stress. Adults of this species are common symbionts of freshwater mussels of the genus Utterbackia (Unionoida: Unionidae). Larval U. foili typically...