Macroalgae and conspecifics serve as critical attachment substrates for juvenile green-lipped mussels (Perna canaliculus) during different stages of development

In recent decades, there has been a global push to restore shellfish reefs and the array of ecosystem services they provide, but these efforts are met with many challenges, the most common of which are substrate limitation due to habitat degradation and a lack of larval recruitment due to population declines. The restoration of green-lipped mussel (Perna canaliculus) reefs in New Zealand is similarly impeded by a lack of natural recruitment into populations of cultured adults that have been transplanted to the seafloor. To obtain a more complete understanding of the substrates needed to support the attachment of P. canaliculus juveniles throughout their development on mussel reefs, this study characterised the presence of different size classes of juvenile mussels on various substrate types within different microhabitats and intertidal zones on a remnant, intertidal mussel reef in the Hauraki Gulf. Across the reef, early juvenile P. canaliculus <5 mm in shell length (SL) were present on conspecifics (37 %) and macroalgae (33 %) with similar frequency, but juveniles 5 – <10 mm SL were present on conspecifics (64 %) almost twice as often as on macroalgae (34 %). Meanwhile, juveniles in all larger size classes ≥10 mm SL were present less often on macroalgae (average 4 %) and most often on conspecifics (average 27 %), followed by bare rock and shell for juveniles ≥20 mm SL (average 11 %). These findings indicate that macroalgae serves as a critical attachment substrate for juvenile P. canaliculus during early development, while conspecific substrate supports the establishment of juveniles throughout later development, alongside rock and shell in a lesser capacity. Additionally, for early juveniles <5 mm SL in emergent rock microhabitats, their presence was 31 % higher on macroalgae than on conspecifics in the upper intertidal but 28 % higher on conspecifics compared to macroalgae in the lower intertidal, indicating that juvenile substrate associations vary between locations within the reef that experience different levels of hydrodynamic force, intraspecific competition, and predation. This study offers valuable insights into the early life history of P. canaliculus that can inform mussel reef restoration practitioners of the types of substrates that; 1) may be limited at restoration sites, 2) are likely to be most effective for facilitating recruitment at restoration sites if applied in close proximity to allow juveniles to transition between substrates during ontogeny, and 3) can be used as a selection criteria for future sites, alongside a set of environmental parameters.