How pH and Temperature Impact the Growth, Molting and Exoskeleton of the Tuna Crab, Pleuroncodes planipes

Tuna crabs, Pleuroncodes planipes, are transported into Southern California waters periodically, enhancing the diet of fish, whales and other organisms. They are unique crustaceans because adults go through both a partially pelagic phase, then a benthic phase, which occurs once they reach a certain size. This phase transition depends on growth and potentially morphological changes, like increased exoskeleton calcification. For some crustaceans, growth and calcification are sensitive to ocean pH and temperature conditions associated with climate change. Such sensitivity may be impactful for tuna crabs that get transported over long distances, into habitats that range in these conditions. The main objective of this study was to determine how pH and temperature can affect the growth and exoskeleton of P. planipes, and potentially their phase transition. A 10 month multi-stressor experiment was conducted on 60 adult animals (24-28mm carapace length) using a full factorial combination of local ambient pH (8.0), reduced pH (7.5), ambient temperature (12C), and increased temperature (18C) conditions. Molting, growth and water parameters were monitored throughout the experiment and exoskeleton morphology and composition were analyzed at the end using SEM and EDX. Intermolt duration was shorter in warm temperature treatments, regardless of pH, but molt increment (carapace length) was greater in the cooler treatments. EDX analysis of the exoskeleton revealed no differences in %wt Ca or Mg among treatments. These results suggest that tuna crabs may be robust to different pH conditions, but ocean temperature (warming) can alter growth and may alter the timing of benthic settlement, thereby impacting the duration of the transient pelagic phase upon which so many organisms depend on for food.