Feeding electivity of Pugettia gracilis, the graceful kelp crab (Decapoda: Epialtidae), and its potential importance to nearshore kelp forests

Abstract

Kelp forests are an integral part of complex marine food webs, and it is important to be aware of the roles of the varied consumers in kelp forests to understand the complexityofthefoodwebsinsuchanecosystem. Thetraditionalecologicalparadigm in regards to kelp bed food webs is the top down control by sea otters, which has been studied in great detail in Alaska. However, an experimental urchin removal study in the San Juan Channel showed that neither a monthly complete harvest of sea urchins (simulating sea otter predation), nor an annual size-selective harvest of sea urchins (simulating commercial urchin harvest), significantly increased the density of perennial or annual (incl. Nereocystis luetkeana) species of macroalgae after 2 years. These results suggest that other factors, such as grazing by other invertebrates, may play a large role in influencing community structure in the San Juan Channel. Invertebrate herbivores have been shown to play a crucial role in kelp bed destruction in N. luetkeana systems and it is known that crabs are important trophic links in kelp-dominated habitats and can influence food web dynamics by acting as consumers. I explored the potential role of Pugettia gracilis as kelp consumers using controlled choice and no-choice feeding experiments. I hypothesized that P. gracilis would show a feeding preference for N. luetkeana over two other local abundant kelp species. I performed my feeding trials with ten P. gracilis individuals, and three kelp species: Alaria marginata, Nereocystis luetkeana, and Saccharina latissima. The results of my trials showed that P. gracilis shows a feeding preference for N. luetkeana over A. marginata and S. latissima. These results show that despite the small size of P. gracilis, in dense population abundances it may be an important part of current trophic dynamics 2 and population changes of N. luetkeana forests in the San Juan Islands and elsewhere in the Salish Sea.