Is attachment substrate a prerequisite for mussels to establish on soft-sediment substrate?

•The benefits of attachment substrate to establishing adult and juvenile mussels was examined experimentally.•The persistence of establishing adult mussels was not improved by the presence of shell material on soft-sediment substrates.•Juvenile mussels preferred live adult mussels and mussel shells for attachment when establishing on soft-sediment.•Juvenile mussels exhibit higher survival from sea star predation when on live adult mussels as attachment substrate.•Live adult mussels provide a superior attachment substrate for both establishing adult and juvenile mussels on soft-sediment.

It is unknown whether the presence of hard substrate is a necessary prerequisite for the attachment and establishment of mussels, especially on soft-sediment habitats where hard substrates are scarce. Therefore, we examined the importance of natural attachment substrates in the form of mussel shell and adult conspecifics for the establishment of juvenile and adult green-lipped mussels (Perna canaliculus) on soft-sediment. In field experiments where shell material was added as substrate to soft-sediment it made no difference to the subsequent retention of adult mussels. Laboratory experiments showed that juvenile mussels preferentially sought out, and attached to adult mussels compared to remaining on unmodified soft-sediment. Furthermore, juvenile mussels attached to live adult mussels had higher survival in the presence of a common sea star predator compared to juveniles on unmodified soft-sediment or attached to mussel shell. The results suggest that establishment of mussel beds on soft-sediment requires only adult mussels, which receive sufficient anchorage through attachment to neighbouring adults and in so doing, providing a stable, complex substrate suitable for improving the survival of establishing juvenile mussels by protecting them from sea star predators.