Movement, Abundance Patterns, and Foraging Ecology of the California Two Spot Octopus, Octopus bimaculatus
- Author(s): Hofmeister, Jennifer Krista Kaulalani
- Advisor(s): Caldwell, Roy L
- et al.
Understanding the reciprocal interactions between animal behavior and other inter-related systems such as physiology, morphology, ecology, and evolution has been called a “Grand Challenge” for organismal biology. Behavior offers two unique contributions to our understanding of organism response to environmental change: 1) behavior is a rapid and reversible response, and 2) organisms can directly influence their surrounding environment, and thus the stimuli their physiological and morphological systems are exposed to, by “choosing” their environment.
My dissertation utilizes the temperate octopus Octopus bimaculatus to understand the reciprocal interactions between behavior and ecology in a human-altered landscape. I use a combination of animal population surveys, dietary analysis, mathematical models, and acoustic telemetry to understand the feedback loops been behavior and ecology. Octopuses play a key predatory role in shaping communities that is unmatched by any other invertebrate. Additionally, their “live fast, die young” life history strategy means their populations respond quickly and dramatically to changes in the environment, which suggests they could be an indicator of ecosystem change. Specifically, I addressed three fundamental questions about octopus behavior and ecology on Santa Catalina Island, CA: 1) What are key environmental variables influence octopus environmental choice?; 2) Where and when do octopuses move?; and 3) Does octopus diet reflect differences in predatory behavior within a human-altered habitat?
The first question addressed a key gap in current knowledge of the environmental variables that influence octopus abundance and distribution in a rocky reef ecosystem by combining intensive population surveys with imperfect detection modeling. Binomial mixture modeling is used when the likelihood of false non-detection is high, and is ideal for studying highly cryptic species like octopuses. Abundance and detection probability was modeled with site and survey-level covariates such as abiotic variables, octopus predator and prey abundances, and habitat structural characteristics. No single abundance covariate explained the data for either year, but detection in 2013 was best modeled with Julian date. Model-averaged estimates of abundance had high ranges of possibility, and all correlations with estimated octopus abundance and environmental variables had weak support. These results highlight the high variability in patterns of octopus abundance and the necessity for integration of multiple environmental factors to elucidate drivers of octopus abundance and small-scale distribution.
The second question quantified movement and habitat use of individual octopuses. This study pioneered acoustic telemetry research on small octopus species and was the first of its kind in the continental United States. I collected and tagged O. bimaculatus and recorded their position over the course of approximate 2 weeks and measured movement continuously over one day for each animal. I found that O. bimaculatus is highly mobile and, combined with my survey data, concluded that they do not stay in a single den for more than a few days. Additionally, movement distances and diurnal movement patterns are highly variable, suggesting a behavioral response to avoid predation.
The third question aimed to identify the diet of O. bimaculatus using Stable Isotope Analysis (SIA) and assess if octopus diet differed between Marine Protected Area (MPA) sites and non-protected sites. SIA allows for the indirect assessment of diet because the isotopic composition of prey is incorporated with reliable fidelity into the predators’ tissues. I analyzed the carbon and nitrogen stable isotope ratios in octopuses and their prey collected inside and outside the Blue Cavern MPA. I found that octopus diet differs between MPA and non-MPA sites and octopuses located within the MPA have a more varied diet than those located outside. This difference indicates a change in the predator-prey relationships with the establishment of the MPA and suggests larger changes in the community structure.
Together, these findings yield novel insights into what influences the populations and behavior of octopuses in a rocky reef environment, and lay the groundwork for directly testing how octopuses will respond to and influence the changes in their surrounding community.