Evaluating the importance of mussel recruit associations with biological and non-biological substrates across a tidal stress gradient

Background/Question/Methods

The mussel Mytilus californianus forms dense beds that provide a key source of food and habitat for a diverse biological community. Mussels are broadcast spawners and rely on oceanic larval input to replenish populations. Recruiting mussels are considered poor settlers of bare rock and are thought to require associations with habitat-forming species to facilitate growth and survival. For this reason, mussel beds can take upwards of 20 years to recover when damaged. Little data exist to identify substrates with which recruits associate, despite their likely importance to the persistence and recovery of mussel beds. It is also unclear if the importance of recruit associations varies along environmental stress gradients, although theory predicts they decrease in importance with decreasing stress.

To identify substrate associations and evaluate their importance across a tidal stress gradient, I conducted field surveys of mussel recruits and available substrate in the upper and lower tidal range of the mussel zone at four central California sites. I used a chi-square analysis to determine if the frequency of recruits settled with a substrate was significantly different from what was expected based on that substrate’s availability. I then used standardized percent differences to determine the directionality of each substrate association.

 Results/Conclusions

At all sites, mussel recruits had strong positive and negative associations with available substrate but only in the upper portion of the mussel zone (p<0.002). In the lower portion of the mussel zone the frequency of mussel recruits on any given substrate was what would be expected based on substrate availability (p>0.150). This result may reflect the role of dessication and heat stress in driving the importance of positive associations between mussel recruits and settlement substrate. In the upper portion of the mussel zone, mussel recruits had particularly strong negative associations with bare rock, algal crusts, and acorn barnacles. Strong positive associations were seen between mussel recruits and fucoid algal canopies, articulated coralline algae, some turf algae, and with acorn barnacle shells but only when in combination with algal canopies. Many of these species associations were unexpected. The current scientific consensus is that mussel recruits require substrate that is byssal-like, however, these results indicate that the presence of an algal canopy is more important.