Dataset: Owl limpet growth rates and mussel cover in Northern and Central California collected from eight intertidal sites from Jun 2021 to May 2023

This dataset contains growth measurements of tagged Lottia gigantea individuals collected as part of a long-term monitoring effort. Growth rates were calculated from repeated shell measurements collected during annual resurveys. The dataset includes mussel cover measured at the time of the surveys. The related dataset (found in the Related Dataset section) includes the 2023–2024 survey period separately and includes co-occurring grazer species counts and densities.

Variation in biotic communities:

We surveyed L. gigantea habitats to test whether the biotic environment differed among the four range core sites and four expanding edge sites, and whether variation in L. gigantea performance was associated with the biotic environment. We considered Dillon Beach, Bodega Marine Reserve, Fort Ross, and Kruse Ranch to be in the expanded range (Sanford et al., 2019), and Vandenberg, Seal Beach, Soberanes Point, and Hopkins Marine Station to be in the range core. Given that L. gigantea have a patchy distribution (Fenberg & Rivadeneira, 2011), we haphazardly established 50-100 1m² plots with limpets present at each site, across approximately 300 m of intertidal habitat. Each plot was marked with marine epoxy (Zspar splash zone compound) in its center. Within each plot, we tagged a single focal L. gigantea using a small (< 6 mm) Floy Tag fixed to the anterior end of the shell using a small dab of marine epoxy (Zspar splash zone compound). For each tagged focal limpet, we recorded growth rate using calipers to relate performance to variation in environmental factors.

Within each plot, we recorded co-occurring grazing species and sessile space occupying species (see Related Datasets for co-occurring grazer species counts and densities). In summer 2021, we surveyed conspecific density and mussel cover in each plot. To record conspecific density, we counted all L. gigantea individuals within each 1m² plot. To determine mussel cover, we photographed each plot using a digital camera (Olympus Touch), and measured mussel cover for the full plot in imageJ (Schneider, 2012). 

To quantify performance across the range in response to biotic drivers, we recorded organismal growth rate as a function the environmental data collected within each 1m² plot. In L. gigantea growth rate is a strong metric for performance and fitness, as shell length scales with gonad mass (Kido & Murray, 2003). We measured shell length for all tagged limpets from the anterior to posterior end of the shell using calipers. Each focal limpet was measured annually between 2021 and 2023, and because limpet starting size differed, we calculated relative growth rate (RGR) as ln(SL_Final/SL_Initial/Δt), where SL is shell length and t is time (Paine et al., 2012).