Environmental cues control or synchronize the reproductive cycle of many marine invertebrates. Of these environmental cues, photoperiod and temperature have been shown to moderate reproduction either individually or in combination. In addition, they may act directly or, in the case of photoperiod, set circannual clock mechanisms. These environmental cues may affect a number of reproductive parameters, including sex determination, gametogenesis and spawning. Gonadotrophic and spawning hormones appear to act as the transducers between the environment and the gamete, and limited evidence indicates that temperature and photoperiod can alter levels of these. Such processes occur in a range of estuarine invertebrates that constitute important components of the diets of overwintering birds. Global warming is likely to uncouple and alter the phase relationship between temperature and photoperiod and this is likely to have significant consequences for animals that develop gametes during the winter and spawn in the spring in temperate northern latitudes. Species that cue reproduction to photoperiod are likely to be particularly vulnerable. Although this is unlikely to lead to extinctions, it may cause local extirpations. However, this will depend on speed of adaptation to changing climate in relation to speed of climate change and the degree of mixing between populations across the range of the species. More likely will be significant impacts on fecundity, spawning success and recruitment, and this may have significant implications for overwintering birds of national and international importance, and, ultimately, on the conservation status of estuaries such as the Humber in the UK.

Nereidae are vital to the functioning of estuarine ecosystems and are major components in the diets of over-wintering birds and commercial fish. They use environmental cues to synchronize reproduction. Photoperiod is the proximate cue, initiating vitellogenesis in a temperature-compensated process. The prevailing paradigm in Nereidae is of a single ‘juvenile’ hormone controlling growth and reproduction. However, a new multi-hormone model is presented here that integrates the environmental and endocrine control of reproduction. This is supported by evidence from in vitro bioassays. The juvenile hormone is shown to be heat stable and cross reactive between species. In addition, a second neuro-hormone, identified here as a gonadotrophic hormone, is shown to be present in mature females and is found to promote oocyte growth. Furthermore, dopamine and melatonin appear to switch off the juvenile hormone while serotonin and oxytocin promote oocyte growth. Global warming is likely to uncouple the phase relationship between temperature and photoperiod, with significant consequences for Nereidae that use photoperiod to cue reproduction during the winter in northern latitudes. Genotypic adaptation of the photoperiodic response may be possible, but significant impacts on fecundity, spawning success and recruitment are likely in response to short-term extreme events. Endocrine-disrupting chemicals may also impact on putative steroid hormone pathways in Nereidae with similar consequences. These impacts may have significant implications for the functional role of Nereidae and highlight the importance of comparative endocrinology studies in these and other invertebrates.