Embryo Ecology of the Pacific Surf Smelt, Hypomesus pretiosus (Pisces: Osmeridae)

A study of the ecology of developing embryos of the Pacific surf smelt, Hypomesus pretiosus, was conducted. Embryos were maintained in the laboratory at 7.6, 12.1, and 17.6°C and the time to specific embryonic stages determined. Embryos held at 7.6°C developed to stage 24, 18 days after collection; those held at 12.1°C hatched after 13 days; at 17.6°C hatching occurred 8.5 days after collection. Embryos maintained at 15°C and salinities of 20, 25, and 30%0 averaged 84% survival. There was no significant difference in survival between the groups (ANOVA, P = 0.53). Field observations indicated that embryos are spawned in patches in the upper intertidal zone near the time of high tide. They are attached to gravel substrates by the zona radiata membrane which ruptures and quickly turns inside out at the time embryos are fertilized. After several days of development, stage 18 to 22 embryos detach from the original spawning substrates and are washed seaward and down into the gravel substrate in the intertidal zone. However, there was no significant difference (ANOVA, P >2: 0.09) in the number of eggs found at each of 4 depth strata in the upper, middle, and lower intertidal zones

Development of predator defences in fishes

A variety of development characteristics, morphological, behavioural, and experiential, contribute to the extreme vulnerability of young fishes to predation. The influence of these characteristics is complicated by the fact that the larval period is one of substantial and rapid change. Yet survival is the ultimate goal;-it is only by reaching maturity that individual fish have the opportunity to reproduce. With such high stakes it is not surprising that predator defences are of major importance during all phases of life. Developmental constraints may limit the defensive options for young fishes. Avoidance behaviours, which reduce the likelihood of encountering a predator or of being attacked by it, are particulaly evident in the youngest stages. Here size, coloration and dispersal are used to help elude the predator's attention. As fishes grow and acquire greater morphological and behavioural sophistication, there is more scope for predator evasion when avoidance fails. Older fishes are increasingly able to respond to external stimuli and can detect and react to predators or join conspecifics in common defence (schooling). Behavioural development is not simply a consequence of growth and the concomitant physical alterations of the body; it is also mediated by experience that comes through interaction with the physical and biotic environment. Predispositions to respond to experience may be a product of evolutionary history. Although mortality rates decline markedly with development and maturity, changes in size or behaviour can render fishes vulnerable to new suites of predators. Effective predator avoidance can compromise other activities, such as foraging, and individuals may be forced to reconcile conflicting demands. Developmental niche shifts that occur, for example, when certain size classes take refuge in less profitable feeding habitats, represent one such trade-off. Niche shifts may also be mediated by the influence of the programme for morphological development on sensory or behavioural capabilities. In addition to all of these developmental consderations, natural variations in environmental conditions - such as temperature, photoperiod, predator density and variety, and presence of alternative prey - represent additional challenges to predator defences during the rite of passage from birth to reproduction.</p