SELECTION OF SUBSTRATA BY JUVENILE CHOROMYTILUS CHORUS (MYTILIDAE) - ARE CHEMICAL CUES IMPORTANT

The importance of filamentous algae, and filamentous structures in gen eral, to patterns of recruitment in mytilids is well documented. Surpr isingly, the relative importance of physical and chemical cues in medi ating this interaction has rarely been examined for mytilid larvae or plantigrades. Here we examine the distribution and abundance of three mytilid species on three common species of algae on the mid to low int ertidal zone of southern Chile and then test the responses of plantigr ades of one of these mytilid species to extracts from the surface of t hese algae and from conspecifics. We found that mytilids were always a ssociated with the robust filamentous red alga Gymnogongrus furcellatu s, but were present on less than 30% of the fronds of Iridaea laminari oides and Ulva rigida. In addition, average densities of mytilids on G . furcellatus exceeded 500 individuals per frond, more than an order o f magnitude higher than on the other algal species. This alga forms an important refuge from gastropod predation for one of the mytilids exa mined, Choromytilus chorus (Molina), which only recruits successfully to the rocky substratum in the presence of this alga. In laboratory tr ials, we tested the response of juvenile (<30 mm) C. chorus to surface extracts from three species of algae which co-occur with this mytilid and extracts from the byssal threads and periostracum of conspecifics . We predicted that juveniles of this mytilid would show a strong pref erence for extract from G. furcellatus. Surprisingly, extract from thi s alga was the only extract rejected by these juvenile mussels; they p roduced significantly fewer byssal threads in response to treatments a s compared to controls. We found no evidence that juveniles of C. chor us are responding to positive cues, but they emphatically rejected a n egative cue (antifouling paint). We concluded that chemical cues are u nlikely to be important in the formation of aggregations of C. chorus.