Dataset: Owl limpet growth rates and density in Northern and Central California collected from eight intertidal sites from May 2023 to Nov 2024

This dataset represents a subset of a longer-term monitoring effort of tagged Lottia gigantea individuals conducted across California intertidal sites. The dataset includes annual measurements collected between 2023 and 2024 and associated survey-derived habitat and grazer community metrics. The related dataset (found in the Related Dataset section) contains growth measurements from earlier survey years, but does not include community sampling data.

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.

For each plot in summer 2023, we sampled the community of competing species within the territories of the focal limpet from 8 sites. Owl limpets are sympatric with several other intertidal grazing species across their range, including limpets (L. scabra, L. digitalis, L. pelta), chitons (Nutallina spp.), and snails (Tegula funebralis) (Stimson, 1973). L. gigantea territories are clearly recognizable as darker areas on the substrate (Fenberg, 2013), so we counted all individuals of the above mentioned grazing species within the territories of the tagged focal limpets and measured the territory area on imageJ to record density (Schneider et al., 2012). In cases which the territory boundary was uncertain, or territories overlapped, we counted all individuals within a 0.25 m² quadrat, placed with the focal owl limpet at its center. We did not count grazers < 5 mm as Lottia congeners are often indistinguishable at that size, cryptic, and difficult to count accurately.

Quantifying performance:

To quantify performance across the range in response to biotic drivers, we recorded organismal growth rate as a function of 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 (see Related Dataset for additional data), 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).