Macroalgae as a refuge from predation for recruits of the mussel Choromytilus chorus (Molina, 1782) in Southern Chile

Abstract

Settlement of the mytilid, Choromytilus chorus, in the Marine Reserve of Mehuin, Chile, occurs on three different types of filamentous structures: (1) the filamentous alga Gymnogongrus furcellatus; (2) the byssus of adult Choromytilus; and (3) the byssus of the small mytilid Semimytilus algosus. Observations made in permanent plots suggest that only the individuals that recruited on the red alga, G. furcellatus, had a high probability of survival. The main agent of post-settlement mortality was the predatory whelk Nucella crassilabrum. Laboratory feeding trials show that the whelk prefers the small sizes of this mussel (1 to 3 cm in length) and above this size the mussel is largely free from whelk predation. Field observations confirm that mussel recruits remain attached to this alga for approximately a year and then they become fixed to the rocky substratum. A field experiment was done to test whether the alga did indeed provide a refuge from 'whelk predation. Survival of mussel recruits that had settled naturally on artificial collector was monitored. An artificial collector ( Gymnogongrus imitation made with plastic nets) that was attached at only a single point to the substratum, restricted access of Nucella, and produced a correspondingly high level of survival among the mussel recruits. Alternately, an artificial collector fixed to the substratum at several point, allowed free access to Nucella and mussel survival was significantly lower. These result indicate that Gymnogongrus provides Choromytilus recruits with a short term refuge from predation. By restricting access of the whelk Nucella, Gymnogongrus allows Choromytilus recruits to reach a size at which Nucella is no longer a threat, ensuring the successful attachment of Choromytilus to the primary substratum. This algal refuge has important implications for the structure and dynamics of the Choromytilus rocky shore community.