Larval mortality during export to the sea in the fiddler crab Uca minax
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Dense populations of the fiddler crab Uca minax (Le Conte 1855) are common along tidally influenced freshwater rivers and streams >50 km from the sea. Adults do not migrate from inland sites to release larvae, but instead release them directly into an environment where the zoeae cannot survive. Laboratory salinity tolerance experiments were used to determine how long larvae from the inland-most population of U. minax along the Pee Dee River, South Carolina, USA can survive zero salinity compared to larvae from a brackish water population (salinity 5) near the mouth of Winyah Bay in the same estuary. Larvae from the brackish water population were also exposed to a salinity of 5 and their survival tracked. These experiments were conducted from May to August 2004 and 2005. To determine if inland larvae suffered significant mortality in transit due to salinity stress, current profiles were measured in the field and used to model the time taken by a larva using ebb-tide transport to travel to permissive salinities. The laboratory tolerance experiments showed that larvae from the inland freshwater population had LT50’s of 4–5 days at 0 salinity, which were significantly longer than those of the brackish water zoeae (2–3 days). Zoeae from the brackish water population survived for at least one larval molt at a salinity of 5 with LT 50’s of ∼12 days. Estimated travel times to reach permissive salinities from the inland-most population based on current profiles were 3–5 days for larvae using night-time only ebb-tide transport and 1.5–2.5 days for those using ebb-tide transport both day and night. Previously published field data indicate that U. minax larvae do use both day- and night-time ebb-tide transport, and are found in high densities in the water column during the day. These results lead to the conclusion that U. minax stage I zoeae do not undergo significant salinity-induced mortality during their 50+ km trip to the sea.
KeywordsTidal Cycle Acoustic Doppler Current Profiler Zoeal Stage Discharge Velocity Freshwater Population
We thank the staff at the Baruch Marine field Laboratory, especially Paul Kenny, for valuable assistance with field logistics. We also thank the West’s for allowing us to establish a field site at the Bates Hill Plantation. The Geoprocessing laboratory at Mount Holyoke College assisted us with our field site map. We thank Alan Kohn and two anonymous reviewers for valuable comments and suggestions on drafts of this work. This work was supported by the National Science foundation (NSF IBN-0237484) and the Office of Naval Research (N00014-02-1-0972). This is manuscript no. 1455 for the Baruch Marine Field Laboratory.
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