The Trophic Ecology of Porcelain Crabs Petrolisthes Spp. on Oyster Reefs

Porcelain crabs are found in dense populations in tropical and subtropical estuaries worldwide and are important components of oyster reef ecosystems. They are thought to primarily consume phytoplankton, but in recent laboratory studies porcelain crabs also readily consumed zooplankton which provided as much as 200 times more energy than a mixed microalgae diet. There is little known about the feeding behavior of porcelain crabs in their natural environment. Stable isotopes of carbon and nitrogen were used in this work to identify porcelain crab prey and the crab’s trophic level in Copano Bay, Texas. Muscle tissue from porcelain crabs taken from three sites in the bay were analyzed for their carbon and nitrogen isotopic ratios during the summer and fall. Porcelain crab muscle tissue summer δ¹³C mean value was -16.4 ± 0.3‰, while the δ¹⁵N mean was 8.0 ± 0.1‰. During the fall, the δ¹³C mean value was -21.5 ± 0.3‰ and the δ¹⁵N nitrogen mean was 10.1 ± 0.8‰. There was no variation in porcelain crab isotopic composition within and among sites in the bay. However, there were seasonal differences within locations in Copano Bay. Summer carbon ratios were similar to those of benthic microalgae (-16‰), while fall values were comparable to phytoplankton (-22‰) measured in other locations, which may suggest a diet of primarily diatoms in the summer and phytoplankton in the fall. Nitrogen isotopic values of porcelain crabs will be compared to possible prey items in Copano Bay. These crabs may serve as a connection between producers and higher trophic levels.