Abstract Microhabitat associations in sessile invertebrates are largely determined by larval choices and early differential post-settlement mortality. Either process can have a broad community-wide impact when it regulates a cascade of foundation species, each facilitating multiple dependent taxa. On mixed sediments in shallow subtidal of Onega Bay (the White Sea, 65° N) cockles, barnacles and ascidians act as foundation species forming a multi-level facilitation cascade. Barnacles Balanus crenatus monopolize empty shells of the Greenland cockle Serripes groenlandicus , whereas ascidians (mainly Styela rustica ) attach almost exclusively to barnacles and conspecifics. Field observations and experiments evidence that barnacles facilitate ascidians and suggest that individual patches shift from barnacle to ascidian dominance over time. Ascidian recruits are found on barnacles and almost never on Serripes shells, a pattern that may result either from substrate-specific larval settlement or from differential post-settlement mortality. Here, we experimentally tested whether larval substrate preferences generate this pattern. In a laboratory experiment, fertilized Styela eggs were added to aerated seawater tanks containing Serripes shells with and without barnacles, and the distribution of ascidian juveniles was quantified after 10 days. Contrary to our hypothesis, settlement density on Serripes shells exceeded that on barnacles. This result rules out larval substrate selection as the primary mechanism and instead implicates substrate-specific post-settlement mortality of a secondary foundation species as the most likely driver of facilitation.
Varfolomeeva et al. (Thu,) studied this question.