Starczak, Victoria R.
Levine, Hazel E.
Barnacles are often abundant on roots and branches of mangrove trees in tidal channels and coastal lagoons of the Pacific coast of Panama. Yet, in some coastal lagoons, barnacles are absent. We investigated pre- and post-settlement factors that affect barnacle distributions in two adjacent coastal lagoons in Bahía Honda, Panama, one with moderate to large barnacle populations, and the other with nearly non-existent populations. Although mean barnacle recruitment was higher on mangrove root segments during the dry season (December-April) than in the wet season (May-November), it was not significantly different between the two coastal lagoons. The coastal lagoon with fewer barnacles is considered an estuary, with high freshwater flow and low salinities (0.1) during the wet season that were lethal to barnacle nauplii and cyprids. Furthermore, coastal water was not observed to enter the lagoon, even during flood tides. In contrast, more barnacles were found in the lagoon with higher salinities (8.5). During the dry season, freshwater flow was greatly reduced in both lagoons, resulting in a similar salinity range (22-33). We conclude that the lack of barnacles in the estuarine coastal lagoon is largely due to high flushing rates and low salinities that reduce larval concentrations during the wet season. Moreover, low adult abundance in the lagoon's interior may further reduce larval supply and settlement.
Solow, Andrew R.
Gravid adults of the common intertidal barnacle Semibalanus balanoides (L.) brood fully developed larvae until individuals perceive some cue from the environment that triggers synchronous larval release. The prevailing hypothesis has been that phytoplankton blooms trigger release because they provide a food source for nauplius larvae. Through observations and field experiments, we tested the hypothesis that turbidity from any source, not just phytoplankton blooms, can trigger release. We documented five larval release events at three sites in the northeastern United States. Two events coincided with chlorophyll increases, and all five coincided with turbidity increases. In experiments, the larval release response was equivalent when adults were exposed to diatoms or inert synthetic beads, and it was significantly higher than under exposure to filtered seawater. We also tested the hypothesis that turbidity can decrease the risk of cannibalism for newly-released nauplii. Under experimentally manipulated conditions, adults consumed significantly fewer nauplii in a high-turbidity environment. We suggest that reproduction in this species may have evolved to coincide roughly with the local onset of winter/spring phytoplankton blooms, but the timing of larval release may have been fine-tuned further by cannibalism and predation pressures. The potential for turbid conditions to serve as a refuge for planktonic larvae of other marine organisms merits further investigation.
The 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.