Mobile ‘reefs’ in the northeastern Gulf of California: aggregations of black murex snails Hexaplex nigritus as habitat for invertebrates

Abstract

We documented positive interactions created by aggregations of black murex snail Hexaplex nigritus that support an epifaunal community annually. We examined abundance, species richness, and species composition of epifauna associated with aggregations, sampling snails from 12 aggregations and collecting epifauna from snails. We estimated 49 100 organisms (±7400) within a 25 m 2 area of an aggregation, many of which were juvenile invertebrates, and 193 species (95% CI 178-224) representing at least 7 trophic guilds. Epifauna occur on H. nigritus because shell structure provides benthic habitat heterogeneity, although biological characteristics of this foundation species may influence the associated composition of epifauna. To test this hypothesis, we anchored artificial reefs constructed of black murex shells and compared epifaunal communities that developed on them to those on aggregations. Abundance of epifauna was not different between aggregations and artificial reefs (p = 0.4); however, species composition was different (p = 0.005, R = 0.7). Epifaunal communities of aggregations had higher numbers of filter feeders and grazers than artificial reefs. Black murex snails illustrate that mobile benthic organisms can function as a foundation species and that biological characteristics of a foundation species may influence the associated composition of species. This system is ephemeral in nature, and epifauna that occur each spring and summer on snails may vary considerably. Loss of large H. nigritus aggregations has been documented due to overfishing, and declines in black murex populations result in the loss of a benthic substrate.