Amphidromy And Migrations Of Freshwater Shrimps. II. Delivery Of Hatching Larvae To The Sea, Return Juvenile Upstream Migration, And Human Impacts

Abstract

Hatching (Stage-1) larvae of amphidromous shrimps do not feed and must reach salt water within a few days to molt to Stage 2, the first feeding instar. Stage-1 larvae are transported from to the sea after upstream hatching by drifting in stream flow or are carried to estuaries for hatching by females migrating downstream. Hatching usually occurs during seasons or periods of high stream flow. After development in the sea, newly metamorphosed benthic postlarvae (juveniles) must find stream mouths and migrate upstream to the adult freshwater habitat. Such migrations are striking, occurring during periods of low but continuous flow, with many juveniles walking or swimming alongside the shore at night. The migratory behavior is a positive rheotaxis, with downstream river flow the directional cue. Juveniles are capable of climbing over or around low obstacles in their path provided that there is some downstream flow. Both larval drift and juvenile migrations are blocked by high dams without passageways and by the reservoirs behind them. Water extraction from streams is a significant source of larval mortality. Human impacts can be mitigated by appropriate conservation measures, e.g., restriction of water extraction during periods of larval abundance, and construction of passageways up and around dams and reservoirs to allow juvenile migration to upstream habitats.

Amphidromy in shrimps: a life cycle between rivers and the sea

“…After passing through several larval stages in an estuary or the open sea, the planktonic larva becomes benthic as it metamorphoses to the more shrimp-like postlarvae, a transitory stage little different from the subsequent juvenile stages (Anger, 2001;Bauer, 2011b). In M. rosenbergii, the small juvenile rapidly undergoes further molts and growth, and within 1-2 weeks shows signs of migratory behavior.…”

“…Lesser variation in light levels, e.g., by cloud cover or moonlight, seem to have little effect on juvenile migrations, as shown by Kikkert et al (2009) in three species from different families of amphi-dromous shrimps. Bauer (2011b) suggested, based largely on a lack of observation of movement during the day, that migrating juveniles are quiescent in protected habitat along the riverbank, resting, feeding, and molting. Support for this hypothesis was given by the increase in size (growth) with increasing distance upstream from the sea observed in migrating juveniles of various amphidromous species (Hartmann, 1958;Bauer & Delahoussaye, 2008;Kikkert et al, 2009).…”

“…As a result, juveniles often move upward in short jumps as the immediate microflow quickly waxes and wanes. The opportunistic crawling and climbing ability of these juveniles can be utilized to get them above artificial man-made obstacles (dams, weirs), using shallow inclined "shrimp ramps" equipped with a slow flow (see review in Bauer, 2011b).…”

“…The purpose of this manuscript, stimulated by a presentation at the decapod crustacean session of the "Primer Congreso Latinoamericano sobre Macroinvertebrados de Agua Dulce" (February 2012, San José, Costa Rica), is to amplify and update previous short reviews on amphidromy (Bauer, 2011a(Bauer, , 2011b. The objectives of this paper are to review, in caridean shrimps, the evolutionary costs and benefits, the evolutionary origins, and the migrations associated with amphidromy, and to give suggestions for future research on amphidromy in Latin America.…”

“…The objectives of this paper are to review, in caridean shrimps, the evolutionary costs and benefits, the evolutionary origins, and the migrations associated with amphidromy, and to give suggestions for future research on amphidromy in Latin America. The impact of human activities on amphidromy has been recently reviewed (Bauer, 2011b) and will not herein be treated extensively.…”