Effects Of Alarm Cues On Foraging,Feeding And Reproduction Of The Sea Urchin Mesocentrotus Nudus

Kelp beds play an important role in marine ecosystems,in which are composed of macroalage and other marine organisms on hard bottom of the cold temperate continental shelf of the world.The kelp bed ecosystem is considered as one of the most dynamic ecosystems.In recent years,kelp beds have been severely damaged worldwide from productive kelp beds to barrens with a great decline in primary productivity and habitat structural complexity.This changes the nutrient structure of the marine ecosystems and ultimately affects the structure of marine community,biodiversity and ecosystem stability.In addition,sea urchins as the typical herbivores consume large amounts of macroalgae in order to survive and reproduce in the kelp beds.Overgrazing by sea urchins is the most important reason for the phase shift from kelp beds to barrens.Therefore,an effective approach to regulating foraging and feeding behaviors of sea urchins is essential for the management of kelp bed ecosystem.Sea urchins can sense chemical cues released by crushed sea urchins,which are called alarm cues.In this study,we intended to use the sea urchin Mesocentrotus nudus as the experimental object to study the effects of alarm cues on foraging,feeding behavior and reproduction of sea urchins.The present study can provide new insights into the management of kelp bed ecosystem.1.Effects of alarm cues on foraging and feeding behaviors of M.nudusA cost-effective approach to controlling foraging and feeding behaviors of sea urchins is essential for the management of kelp beds.Laboratory experiments were designed to investigate whether alarm cues from crushed conspecific urchins can effectively prevent the foraging and feeding behaviors of the sea urchin M.nudus under the static seawater condition.The present study found that the number of M.nudus that foraged successfully was significantly lower when alarm cues were placed between the kelp and the sea urchins.This result indicates that alarm cues could play an important role in critical kelp-bed areas.It probably prevents sea urchins from foraging by acting as a barrier.However,there was no significant difference of Aristotle’s lantern reflex between the groups with and without alarm cues.These results indicate that alarm cues greatly affect foraging behavior,but not Aristotle’s lantern reflex of M.nudus.All together,the present study suggests that alarm cues have an application potential in the management of the kelp beds as green engineering.Future studies are essential to further investigate the chemical basis of the alarm cues of sea urchins for the application in large-scale.2.Foraging behavior of M.nudus exposed to alarm cues in various conditionsConspecific alarm cues crushed from M.nudus prevent sea urchins from foraging the kelp,but do not repel them far away from the kelp.However,it remains largely unknown of whether this phenomenon was affected by alarm cues or by the attraction of the kelp.The present study found no significant difference in the duration in the danger area with or without the kelp around alarm cues.This suggests that the phenomenon is the strategy of sea urchins but not by the attraction of kelp.We found that alarm cues appearing between the kelp and sea urchins significantly affected foraging behavior of sea urchins fasted for 21 days.This indicates that alarm cues can effectively prevent fasted sea urchins from foraging the kelp.Further,there was no correlation between foraging velocity and the duration in the danger area.Pearson correlation analysis revealed no significant correlation between foraging velocity and the duration in the safety area close to different amounts of alarm cues,suggesting that alarm cues prevent sea urchins with strong foraging ability to forage.Collectively,the present results indicate that alarm cues as highly available biological barriers are cost-effective approaches to preventing overgrazing of sea urchins in the protection of kelp beds ecosystems.Notably,the present study is a short-term laboratory investigation that does not consider the complexity of natural conditions.Future studies are essential to test the present findings in the field.3.Effects of alarm cues on the reproduction of M.nudusSea urchins sense alarm cues extracted from crushed conspecifics and perform anti-predation behaviors in exposure to alarm cues.This indicates of the fitness benefits of alarm cues in sea urchins.The present study investigated whether fitness costs of alarm cues exist in fertilization,hatchability,deformity and larval size of the sea urchin M.nudus.In the present study,we found that the fertilization and hatching rates was significantly lower in the group in the presence of alarm cues than those in the group not in the presence of alarm cues,indicating that fitness costs of alarm cues exist in sea urchins.However,there was no significant difference in deformity rate,larval length,stomach length and stomach width of M.nudus either in or not in the presence of alarm cues.The group in the presence of alarm cues showed significantly shorter larval width than the group not in the presence of alarm cues.This indicates that smaller larvae of sea urchins more probably survive in the environment of alarm cues.The present study revealed that fitness costs of alarm cues exist in sea urchins and shed light on the ecological roles of alarm cues in the kelp ecosystems.