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The interaction between predator and prey is an evolutionary arms race, for which early detection by either party is often the key to success. In aquatic ecosystems, olfaction is an essential source of information for many prey and predators and a number of cues have been shown to play a key role in trait-mediated indirect interactions in aquatic communities. Here, we review the nature and role of predator kairomones, chemical alarm cues, disturbance cues, and diet cues on the behaviour, morphology, life history, and survival of aquatic prey, focusing primarily on the discoveries from the last decade. Many advances in the field have been accomplished: testing the survival value of those chemicals, providing field validation of laboratory results, understanding the extent to which chemically mediated learning may benefit the prey, understanding the role of these chemicals in mediating morphological and life-history adaptations, and most importantly, the selection pressures leading to the evolution of chemical alarm cues. Although considerable advances have been made, several key questions remain, the most urgent of which is to understand the chemistry behind these interactions.

This study examines the effect of parent size, fry growth rate, and habitat-related brood predation pressure on two measures of reproductive success (percent brood survival and fry number at independence from their parents) of free-ranging convict cichlids (Cichlasoma nigrofasciatum). Four populations in Costa Rican streams were studied, two in "pool" habitat and two in "stream" habitat. The number of fry that emerged from the spawning site (as free-swimming young) was positively correlated with female standard length (SL) for sites with a relatively wide size range of breeding females. SL of males had no effect on fry number at emergence, nor did size of either parent affect the probability of a brood reaching fry independence. Large parents reared more fry to independence than small parents in stream habitat but not in pool habitat. There was a significant difference among sites in fry number at emergence but not at independence (mean = 27 fry), suggesting a maximum number of fry that two parents can economically defend. Fry growth rate had no effect on either measure of reproductive success. Reproductive success was largely determined by the effect of habitat. Adult Cichlasoma dovii in pool habitat may have reduced mean brood survival from 47% (n = 126 broods) in stream habitat to 14.9% (n = 141 broods) in pool habitat. Predators such as juvenile C. dovii may be more effective in the dim ambient light in pool habitat, overwhelming the effect of parent size on brood defence.

Convict cichlids (Cichlasoma nigrofasciatum) exhibit extended biparental care of their young. Parents will adopt unrelated (foreign) conspecific young of similar size to or smaller than their own but reject larger foreign young. Adoption of smaller foreign young may benefit the parents by reducing loss of their own young to predators by the dilution effect, which may be enhanced by differential predation on the smaller young. Another factor influencing adoption is that larger foreign young may pose a direct predatory threat to the host parents' young. Measures of aggression among free-swimming young of different sizes showed that there was at least a 1 mm (standard length) size difference before larger young attacked smaller, a 3 mm difference before injury occurred, and a 4–5 mm difference before predation occurred. Parents were initially more discriminating than their young with respect to the size of foreign young accepted. As their young grew and became stronger swimmers, parents less actively rejected larger foreign young; however, they continued to reject them before they were large enough to pose a direct predatory threat. Parental rejection of relatively large foreign young is therefore based more on protecting their own young from differential predation than from aggression by larger adoptees.

Parental care theory predicts that investment in current reproduction should be increased when the prospects of success in current reproductive effort are relatively high, and reduced when they are poor relative to expected success from future reproductive effort. A number of studies have shown that levels of postspawning parental investment (brood defence, parents' willingness to risk predation) increase when the brood is augmented. However, few studies have shown a change in pre-spawning (gonadal) investment in response to indirect indicators of reproductive success, such as nest site quality. Convict cichlids (Cichlasoma nigrofasciatum) are small freshwater fish that have biparental care of their young. Pairs of convict cichlids were required to spawn in either "risky" or "secure" spawning caves in the presence of potential egg predators. As predicted by parental care theory, females laid significantly more eggs in secure spawning caves than in risky spawning caves. The ability of this iteroparous fish to adjust gonadal investment in this manner would serve to optimize the use of its resources under varying environmental conditions and ultimately allow it to realize maximize lifetime reproductive success.