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ThesisLarval ecology and synchronous reproduction of two crustacean species : Semibalanus balanoides in New England, USA and Gecarcinus quadratus in Veraguas, Panama(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2011-02) Gyory, JoannaThe environmental cues for synchronous reproduction were investigated for two highly abundant, ecologically important crustacean species: the temperate acorn barnacle, Semibalanus balanoides, and the tropical terrestrial crab, Gecarcinus quadratus. Larval ecology of these two species was also studied to determine potential sources of larval mortality and recruitment success. High-frequency observations revealed that early-stage larval abundance of S. balanoides was related to storms, and possibly turbidity. Field observations and experiments studied the effect of turbidity and phytoplankton on larval release response. Release coincided with increased turbidity at three sites along the northeast coast of the United States. A three-year time series of phytoplankton and zooplankton data showed that larval release was not consistently related to phytoplankton abundance (total or single species). When gravid barnacles were exposed to phytoplankton or synthetic beads, they released in response to both, suggesting that presence of particles is more important than identity of particles. Feeding experiments showed that adult cannibalism on newly released larvae is lower in highly turbid conditions. It is suggested here that S. balanoides synchronizes its reproduction with the onset of phytoplankton blooms, but turbidity may fine-tune the timing if it provides predation refuge for larvae. Adult G. quadratus females undertake synchronized breeding migrations to the ocean after the first rains of the rainy season, presumably when the risk of desiccation is lowest. They wait for darkness and an ebbing tide before releasing their eggs into the water. First-stage zoeas have dark pigmentation, long dorsal and rostral spines, and a pair of lateral spines. Hatching in darkness may help zoeas avoid predation from planktivorous diurnal fish, and the zoeal spines may deter predation from planktivorous nocturnal fish. In the laboratory, a G. quadratus zoea reached the megalopa stage in 21 days. A mass migration of megalopae and juveniles out of the water was observed 30 days after adult females released their eggs. Plankton pump samples taken near the island suggest that zoea abundance and distribution may be related to the phase of the internal tide. Synchronous reproduction in these two species appears to be the result of predator avoidance behaviors.