The role of carapace spines in the swimming behavior of porcelain crab zoeae (Crustacea: Decapoda: Porcellanidae)

- Porcelain crab larvae have greatly elongated spines relative to other crab zoeae.
- Porcellanids are strong lateral swimmers, over twice as fast as other crab larvae.
- Removal of anterior spine caused inability to maintain constant depth.
- Removal of posterior spines prevented backward swimming and directional control.
- Swimming ability may explain limited offshore dispersal of porcelain crab larvae.

Porcelain crab zoeae display vastly different morphological and swimming characteristics compared to other crab zoeae. Porcellanids have highly elongated anterior and posterior carapace spines and swim with smooth lateral directionality. While long spines have been shown to aid in predator deterrence in crab larvae, there is no empirical evidence regarding their effect on swimming behavior. To investigate this possible association, the carapace spines of 40 zoeae of the porcelain crab, Petrolisthes cinctipes (Randall, 1840), were experimentally altered and changes in swimming behavior, spatial orientation and speed were recorded. When anterior, posterior, or both sets of spines were removed, the larvae swam with significantly reduced lateral speed. Ablation of only the anterior (rostral) spine resulted in a loss of depth control and relegated forward progress to erratic, ineffective lurches. Removal of the posterior spines resulted in an inability to swim backward, and reduced the ability to maintain a straight trajectory while swimming forward. There was no visible pitching of the zoeae in any of the treatment groups, which suggests that balance is not the primary function of the elongated spines. This represents the first experimental evidence linking the unusual elongated carapace spines of porcellanid zoeae to their ability to swim laterally, and may help explain their ability to remain in nearshore coastal waters throughout their development.