Environmental Biology of Fishes 67: 107–116, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Parental responses to changes in costs and benefits along an environmental gradient Rebecca E. Hale a , Colette M. St. Mary a & Kai Lindstr ¨ om b a Department of Zoology, University of Florida, 223 Bartram Hall, Gainesville FL 32611-8525, U.S.A. (e-mail: hale@zoo.ufl.edu) b Department of Ecology and Systematics, Biocenter 3 (Viikinkaari 1), P.O. Box 65, 00014, University of Helsinki, Finland Received 18 October 2001 Accepted 12 November 2002 Key words: fanning, dissolved oxygen, hypoxia, aquatic surface respiration, development, Jordanella floridae, fish Synopsis We evaluated the effects of dissolved oxygen on offspring survival, parental costs, and the pattern of parental care in Florida flagfish, Jordanella floridae (Cyprinodontidae). Specifically, we quantified (1) embryonic development and survival in the absence of parental care, (2) behavior of non-reproductive adults, and (3) behavior of parental males across a gradient in dissolved oxygen. Embryo developmental rates and survivorship increased with dissolved oxygen, with a relatively sharp increase in survival between medium and high oxygen treatments. Non-reproductive adults increased their frequency of aquatic surface respiration, reduced overall activity, and increased opercular beat rate as oxygen declined, suggesting increased costs of activity with reduced oxygen. Taking these cost measures together, costs appear to increase slowly as oxygen starts to decline and then increase sharply as conditions approach hypoxia. In contrast, parental effort increased gradually with dissolved oxygen. We conclude that the increase in care from low to medium oxygen primarily results from a sharp decline in physiological costs, whereas the continued increase in care from medium to high oxygen primarily results from an increase in offspring value. In addition, our results highlight that the benefits of fanning for offspring are not well understood and that they may increase with oxygen, contrary to what has been previously assumed. Introduction Considerable work has evaluated how variation in parental costs (Drent & Daan 1980), the value of off- spring to parental fitness (reviewed in Clutton-Brock 1991), and the extent to which care can increase offspring survival (Dale et al. 1996, Listøen et al. 2000) should affect parental investment. Consistent with predictions of parental care models (Williams 1966, Montgomerie & Weatherhead 1988, Sargent & Gross 1993), intraspecific variation in parental care can be attributed to variation in costs and benefits. Specifi- cally, experimental studies have demonstrated that the amount of care provided decreases as the costs of care increase (e.g., Sabat 1994, Smith & Wootton 1994, 1995), and that it increases with offspring value (e.g., Andersson et al. 1980, Onnebrink & Curio 1991) and with the benefits parents receive from providing care (Amat 1995, Listøen et al. 2000). Physical environmen- tal variables are likely to affect multiple factors in the parental investment tradeoff and, thus, the amount of care parents should provide. For aquatic organisms, such as fishes, a gradient in dissolved oxygen almost certainly affects the costs and benefits of parental care. Both adults and developing embryos are subject to the effects of low dissolved oxygen (hypoxia). Adult fish reduce activity (Hughes 1973, Chapman et al. 1995) and increase frequency of air breathing and/or aquatic surface respiration (ASR) (e.g., Gee et al. 1978, Kramer & McClure 1982) under reduced oxygen conditions. Because parental care increases parental activity, costs of care may be high in hypoxia due to respiratory stress and increased oxy- gen debt. Indeed, Jones and Reynolds (1999) observed