Aquatic Toxicology 74 (2005) 285–293 Unexpected effects of zinc pyrithione and imidacloprid on Japanese medaka fish (Oryzias latipes) Francisco S´ anchez-Bayo a,∗ , Kouichi Goka b a Laboratory of Toxicology, Faculty of Horticulture, Chiba University, 648 Matsudo, Matsudo-shi, Chiba 271-8510, Japan b National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan Received 25 January 2005; received in revised form 25 May 2005; accepted 10 June 2005 Abstract Biological effects of the biocide zinc pyrithione (Zpt), used in anti-dandruff shampoos and antifouling paints and the agricultural insecticide imidacloprid on Japanese medaka fish (Oryzias latipes) were assessed in experimental rice fields. Both chemicals are toxic to medaka, in particular Zpt, which also causes teratogenic effects such as spinal cord deformities in embryos at very low, sublethal concentrations. Rates of malformation in medaka fry from paddies treated twice a week with anti-dandruff shampoo (0.18–0.37 L/L each time) over a period of 4 months were within the natural background, perhaps due to the quick dissipation rate of this chemical in the environment. Both Zpt and imidacloprid caused stress syndrome in juvenile medaka, with fish from Zpt-shampoo fields having a significantly lower weight to body length ratio than those from control fields. As it often happens with stressed fish, a massive infestation by a Trichodina ectoparasite was observed in medaka from imidacloprid fields. However, despite their high stress levels, fish from the Zpt fields did not suffer such infestation, supposedly because the disinfectant action of this biocide. © 2005 Elsevier B.V. All rights reserved. Keywords: Rice; Biocides; Chronic exposure; Physiological stress; Fish parasites 1. Introduction Rice cultivation requires the flooding of fields between planting of seedlings and fructification, a period usually lasting 3–4 months. During that time, several fish species may inhabit the paddy and adjacent drains (Heckman, 1979), and be subjected to the effects ∗ Corresponding author. Tel.: +81 47 308 8831. E-mail address: sanchezbayo@faculty.chiba-u.jp (F. S´ anchez-Bayo). of pesticides applied routinely to the crop. Apart from the acute toxicity of some insecticides that may result in direct mortality, fish are very sensitive to sublethal concentrations of such toxicants and their accompa- nying surfactants. Fish may be stressed (Sancho et al., 1997), their hatching and development may be delayed (Gormley and Teather, 2003), they may show gill damage (Hofer et al., 1995), swimming impair- ment, changes in temperature or salinity tolerances (Heath et al., 1997) and malformations (Luckenbach et al., 2001) of several kinds. 0166-445X/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2005.06.003