Blöcher, Nina (2008) Ökologische und biologische Aspekte des Polydora ciliata- Aufwuchses auf Littorina littorea. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 60 pp.

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Abstract

The periwinkle Littorina littorea is an important member of the benthic community in the western Baltic. It is infested with the boring polychaete Polydora ciliata, which is also known to inhabit the shells of other molluscs such as blue mussels and oysters. In these species an infection can lead to a decrease of shell stability, flesh weight and reproductive efficiency. Furthermore, for infested periwinkles in the Korth Sea an increase in predation pressure exerted by a typical predator, the shore crab Carcinus mae nas, has been observed. In the present study the interaction of Littorina littorea and Polydora ciliata in the western Baltic has been examined. The aim was to find out whether this boring polychaete has any influence on the snail and if so, of what nature this influence is. Three questions were discussed:
1. What kind of epibionts colonize Littorina and what is the extend of this Aufwuchs? For this purpose epibionts and shell condition in relation to the infestation rate were examined. Further on, an experiment was designed to assess the role of the periostracum as a repellent against settling polydora worms.
2. How does Polydora infestation influence the relative fitness of Littorina? The relative fitness of infested and non infested Littorina was compared by measuring reproduction efficiency, flesh weight and survival rate under hypoxic stress.
3. How does Polydora infestation influence the interaction of Littorina with other organisms? By carrying out two predation experiments it was tested whether infested snails were crushed more easily by Carcinus maenas and whether this would alter the size refuge for Littorina (no choice experiment). The second experiment (choice experiment) examined whether the prey choice behavior of Carcinus differed in reference to present or absent infestation of the prey. In addition, shell stability was measured experimentally in relation to the degree of infestation.
The results of this study show that infestation with Polydora ciliata is a common phenomenon of periwinkles in Kiel Fjord. Furthermore, all tested parameters, with exception of shell stability, display negative effects of infestation for the snail. Nonetheless these findings do not support the characterization of Polydora as a parasite since there is no evidence of a trophic dependency of Polydora on Littorina. Also, there is no evidence of a direct defense reaction of the snail against the presence of Polydora in its shell. This leads to the conclusion that Polydora is an epibiont with exclusively negative influence on the periwinkle. Due to increasing environmental stress an increase of the influence of Polydora on the snail population can be expected. As Littorina plays an important role in the benthic community this may have far reaching consequences for the ecosystem at large.

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