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.G. Gallardo et al. J. Exp. Mar. Biol. Ecol. 240 1999 179 –191
1. Introduction
Environmental stress such as food limitation and high free ammonia concentration is common in rotifer mass cultures when population density increases, resulting in
decreased reproduction and survival. To mitigate the stress of high population densities, hormonal treatment may be effective. In an earlier paper Gallardo et al., 1997, we
reported that in batch cultures of the rotifer Brachionus plicatilis, population growth of
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rotifers treated with growth hormone GH, 0.0025 and 0.025 I.U. ml or g-amino-
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butyric acid GABA, 50 mg ml was 1.7 and 2 times higher than that of controls,
respectively. It was surprising to find that a vertebrate hormone is effective on rotifers, but vertebrate growth hormone has also been found to be effective on other invertebrates
such as oysters Paynter and Chen, 1991 and abalone Taylor et al., 1996. Also, GABA has been found to be effective for inducing metamorphosis in the red abalone
Haliotis rufescens Searcy-Bernal et al., 1992.
To confirm our batch culture results Gallardo et al., 1997 and examine the effect on subsequent generations of hormone-treated rotifers, we conducted individual culture
experiments on isolated females carried through the third generation Parent, F , F . It
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is possible that significant effects would be observed in subsequent generations even if these were not treated with hormones.
Results of our individual culture experiment aimed at confirming the batch culture result showed the absence of a positive effect of GABA in contrast to our batch culture
results. This led us to hypothesize that GABA may be effective in enhancing rotifer reproduction when culture conditions are not optimum. Therefore, in our succeeding
experiments, the rotifers were exposed to low food and high free ammonia levels to determine whether GABA and possibly GH also could mitigate the effects of these
stressors. In the batch cultures where GH and GABA were effective, food became limited and water quality deteriorated as a result of increasing population density. This
paper reports the effect of GH and GABA on rotifer reproduction in optimal and limited environments or in the presence of toxic levels of free ammonia.
2. Materials and methods
