Urchin mediated positive interactions

There is increasing interest in how positive direct and indirect interactions involving two or more species, maintain community structure. This project aims to investigate the important aspects of three urchin-mediated positive interactions. First we investigated whether the positive relationship between the kelp Ecklonia radiata and the herbivorous gastropod Phasianotrochus eximius is mediated by a third species, the sea urchin Holopnuestes purpurascens, via a facilitation cascade. Both the sea urchin and gastropod are found enmeshed within the laminae of the common kelp Ecklonia radiata, which both species consume. Sampling and experiments carried out both in the laboratory and at four sites within Sydney, Australia revealed that both H. purpurascens and P. eximius were more abundant on kelp than other algal species, and that the gastropod was more abundant on kelp with than without the urchin. When the presence/absence of H. purpurascens on kelp plants was manipulated experimentally in the field, P. eximius recolonised kelp plants at higher rates when sea urchins were present. Individual P. eximius on plants inhabited by H. purpurascens were larger than those on plants without the urchin. Adult P. eximius used visual cues to locate its two associated species. Gastropods were tethered to plants for 24 hours to test mortality of individuals with as opposed to without sea urchins. Survival of individuals on plants with H. purpurascens was greater than on plants without H. purpurascens suggesting that the H. purpurascens and the refuge it shares with the P. eximius, may be protecting the gastropod from predation and/or adverse microclimates. In addition, P. eximius was found to not only consume the kelp and epiphytes in which it inhabits but also the excrement of the urchin. Carbon and nitrogen measurements for these three food sources showed a higher nitrogen value for urchin excrement compared to the other food sources, suggesting that there may be a nutritional benefit for the gastropod in associating with the sea urchin. This study provides one of the first demonstrations of a facilitation cascade in a subtidal marine environment. The second part of this project examined the relationship between the isopod Exosphaeroma echinensis and the sea urchin Evechinus chloroticus in northern New Zealand. A series of field and laboratory experiments showed that the isopod preferentially inhabits the spines of the sea urchin over other potential habitats. This relationship appears to be commensal in that the sea urchin spines are likely to provide shelter to the isopod. The isopod uses visual cues as opposed to olfactory cues to locate its host and is efficient in host recognition in the field. The final part of this thesis examined the methods of host recognition by the shrimp Gnathophylloides mineri, which is symbiotically associated with the sea urchin Tripneustes gratilla. G. mineri locates its host via olfactory cues emitted into the water column by its host. Within each of the urchin-mediated interactions investigated within this thesis, the sea urchin appears to play a positive role by providing refuge to its associated species. Thus, this study has highlighted the important role that sea urchins can play in mediating positive associations and adds to the growing evidence that positive interactions (both direct and indirect) are a common mechanism by which community structure is maintained. A major focus in ecological literature on sea urchins to date has been on their negative effects, such as overgrazing resulting from reduced predator abundances. This thesis highlights some of the positive effects that some sea urchins can have by providing food and shelter to associated species.