Unveiling commonalities in understudied habitats of boulder-reefs: life-history traits of the widespread invertebrate and algal inhabitants

Compared to stable reef habitats, dynamic boulder-reefs (commonly called boulder-fields when intertidal) host many habitat specialist species. Most occur underneath boulders where they are largely hidden from view; only limited research has assessed their life-histories despite their widespread importance for biological diversity. But some abundant under-boulder species likely structuring this system are habitat generalists widely researched elsewhere. Here we review this research, focusing on three widespread under-boulder sessile taxa: spirorbids, serpulids (tubeworms) and nongeniculate coralline algae, and three mobile taxa: sea urchins, chitons and crabs. Spirorbids have extensive reproductive/colonization capabilities but are readily out-competed. We thus characterize spirorbids as mostly early-successional, while serpulids often have greater competitiveness. Nongeniculate corallines occur underneath boulders where light reaches, although they can withstand low levels of that and most other resources. Such traits characterize nongeniculate corallines as late-successional. Thus, succession underneath boulders may shift deterministically from early tubeworms to late nongeniculate corallines. Habitat generalist sea urchin and chiton species often have strong inter-specific interactions in exposed habitats. Future experiments may find that under-boulder aggregations of these taxa, and also crabs, are impacting algal and invertebrate assemblages. These experiments will be required if dynamic boulder-reefs are to be as thoroughly understood as other benthic systems.