LARVAL RELEASE RHYTHMS AND TIDAL EXCHANGE IN THE ESTUARINE MUDPRAWN, UPOGEBIA-AFRICANA

The mudprawn, Upogebia africana is common in intertidal regions of man y South African estuaries. The life cycle is complex, incorporating a marine phase of development during the larval stages. Breeding peaks a re in summer and first-stage larvae are released into the plankton at night. Maximum release activity and export to the marine environment f ollow a semi-lunar cycle synchronized to the time when high water in t he estuary is crepuscular. This occurs after peak spring tidal amplitu de. Estuarine reinvasion by postlarvae is also nocturnal, and maximum return occurs after neap's when low water at sea occurs around sunset. Rhythmic cycles of larval export and postlarval estuarine reinvasion are therefore asynchronous during the lunar cycle and are best explain ed by the timing of the change in light intensity relative to high and low water respectively. If maximum activity rhythms of Stage 1 and po stlarvae are independent of tidal amplitude, then timing of maximum re lease and reinvasion during the lunar cycle would alter as the time of sunset shifts between solstices. Much of southern Africa experiences a semi-arid type climate and most estuaries close off from the sea for varying periods owing to sandbar development across tidal inlets. Lar vae do not metamorphose if trapped in estuaries and recruitment ceases . Thus, mudprawn populations are directly affected by tidal inlet dyna mics. In extreme cases populations become locally extinct if inlets re main closed for extended periods.