Fall 2016

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Organisms adapt to responses in their environment, and over time if that adaptation is successful in increasing survival within the population, the species will evolve accordingly. An example of induced evolution is through predator-­prey interactions. As a predator hunts its prey, the prey must find ways to avoid predation in order to survive. Evolved traits or behaviors to avoid predation include escape through size refuge, various physical or chemical defensive strategies, vertical migration through a water column, inducible morphological shifts, unpalatability, mimicry, or camouflage. To camouflage oneself, organisms have been observed to use coloration or change their shape—morphological changes to the body—to look similar to their environment. However, some organisms are constrained by their morphology and have evolved other methods of camouflage, such as attaching items to oneself to blend into its surroundings.

The superfamily of spider crabs, Majoidea, has hooked setae on their appendages and carapaces, allowing for the attachment of items onto the exoskeleton. Approximately 75% of majoid crabs decorate themselves with materials from their environment to their bodies for camouflage. Decoration differs intra-­ and interspecifically: amount of setal cover on the exoskeleton as well as the life stage and sex of the individual crab are known to influence the behavior of decoration. Behavioral changes exhibited by some decorator crabs have been vii categorized as “sexually dimorphic ontogenetic shifts,” that is, decoration behavior changes as a crab ages, but the behavioral shift is different for males and females. Additionally, females and juveniles tend to be more cryptic and have more decorative cover than males, in other words, crabs decorate less with increased size. Three strategies of specialized decoration have been observed in decorator crabs: mimicking the environment to camouflage and avoid predation, placing preferred dietary items on the body as decoration and for food storage, or choosing materials containing toxic chemical deterrents to decrease predation.

Local to the Eastern Pacific coast, the masking crab, Loxorhynchus crispatus, may behave similarly to other decorator crabs by utilizing one of these three decoration strategies. Despite variations in its local habitat, generally the same taxonomic groups are found on L. crispatus throughout its habitat range (Wicksten 1978). This trend may suggest that the crabs are preferentially choosing these taxa for decoration, yet this selectivity remains unexplored in previous studies. Additionally, L. crispatus has been placed into the category of experiencing the sexually dimorphic ontogenetic shift in decorating behavior, hence potentially juveniles, males, and females choose different decorative items, however this has also not yet been explored. Thus, this study aimed to answer the following questions: (1) is there evidence that the crabs are targeting their decorative organisms in a non-­random way?, and (2) how does selection of decorative items change both ontogenetically and by sex?

Decoration composition was observed using in situ observational surveys and decoration preference was determined by laboratory decoration choice experiments. For observational surveys, SCUBA divers searched for crabs at three sites, Breakwater and McAbee in Monterey Bay, and Stillwater in Carmel Bay. Photographs were taken of crabs found in each site to obtain carapace decoration composition, which was compared to benthic viii habitat assemblages measured through Random Point Contact. Three categorical groups— sessile invertebrates, articulated coralline algae, and fleshy algae—containing particular decorative species were given to crabs (n=27) in tanks for decoration choice experiments. Crabs were monitored for a week, and photographs of decoration were taken once every hour for the first eight hours and then once a day for the next week. For observational surveys, crabs and habitats were significantly different at all sites, and juveniles, females, and males decorated significantly differently from each other, but this depended on the site. While juveniles chose sessile invertebrates as decoration cover, males used fleshy algae, and females used both invertebrates and algae. Additionally, while males decorated the least, juveniles had the highest amount of decoration cover. Preference experiments showed the same trends with what items juveniles, females, and males chose to decorate themselves. However, while juveniles had significantly different decoration assemblages than adults, males and females were not different. These results indicate that while camouflage may be the main strategy used by L. crispatus, other factors may also be driving the behavior and differences seen between crabs and habitats, as well as between juveniles and adults. Such factors may include passive decoration—by invertebrate larvae or algal sporophytes onto limited bare space that happens to be a crab’s carapace, or an increase in decoration due to a flexible antipredatory response. Additionally, more natural history, such as home range and migratory patterns, preferred diet, and potential pressure from predators, is necessary to better understand decorating behavior in L. crispatus. In conclusion, this study determined that crabs are selecting decorative items non-­randomly and different items are chosen by juveniles, females, and males.