The morphology of macroalgal substrates can help predict the attachment of juvenile mussels

Understanding and addressing substrate availability has been shown to be critical for facilitating recruitment in shellfish reef restoration initiatives globally. For many mussel species, macroalgae is vital for the settlement and establishment of juveniles on reefs, but the factors which determine a macroalga’s effectiveness as an attachment substrate are poorly understood. This study aimed to; 1) develop an index that can be used by restoration practitioners to score the morphological features of macroalgae for their potential to support juvenile mussel recruitment and 2) test the accuracy of this index on the macroalgal substrate attachments of different juvenile size classes of the green-lipped mussel Perna canaliculus on two remnant mussel reefs in northeast New Zealand. Eight morphological features of macroalgae, identified from published studies, were used to create the Macroalgal Morphology Index (MMI). The index scoring criteria for each of these features were able to predict the likelihood of P. canaliculus attachments to macroalgae for juveniles < 10 mm in shell length with 75% accuracy but with only 40–60% accuracy for juveniles 10 – <30 mm. Holdfast complexity and canopy cover were the two most useful features of macroalgae for predicting the attachment of all sizes of juvenile mussels. Meanwhile, the four features of macroalgae that describe their branching morphology were only strong predictors of attachment for juveniles < 10 mm. Overall, these findings suggest that the MMI can aid restoration practitioners in the selection of suitable macroalgal substrates for facilitating juvenile recruitment at mussel reef restoration sites in New Zealand and potentially elsewhere.