Estuarine Larval Development and Upstream Post-Larval Migration of Freshwater Shrimps in Two Tropical Rivers of Puerto Rico1

MIGRATORY FRESHWATER SHRIMPS REPRESENT IMPORTANT LINKS between the headwaters and estuaries of many tropical rivers. These species exhibit amphidromous life cycles in which larvae are released by females in upper reaches of rivers; first stage (i.e., newly hatched) larvae drift passively to coastal environments where they develop and metamorphose into postlarvae that subsequently migrate back upstream to adult (freshwater) habitat. Recent studies in rivers draining northeastern Puerto Rico and Costa Rica’s Caribbean slope have been the first to document migratory drift of first stage larval freshwater shrimps (March et al. 1998, Pringle & Ramı́rez 1998, Benstead et al. 1999). While we now have information about the migratory drift of first stage shrimp larvae and the effects of damming and water abstraction on larval migration, no information exists on the location of larval development (i.e., whether estuarine, fully marine, or both). We also lack data on subsequent recruitment of metamorphosed postlarvae to upstream populations of adults and the factors that control upstream migration. In 1996, we conducted short-term estuarine sampling to determine if developing (post-first stage) freshwater shrimp larvae were present in the estuaries of two rivers (Espı́ritu Santo and Mameyes) that drain the Luquillo Experimental Forest (LEF) in northeastern Puerto Rico. We also used simple traps to examine upstream migration of post-larval freshwater shrimps and its relationship to river discharge in the same two rivers. The Espı́ritu Santo and Mameyes are similar in size and mean discharge, and both support diverse assemblages of amphidromous freshwater shrimps. Ten species are found in the two study watersheds: Atya lanipes, A. innocous, A. scabra, Micratya poeyi, Xiphocaris elongata, Macrobrachium carcinus, M. heterochirus, M. crenulatum, M. faustinum, and M. acanthurus (Covich & McDowell 1996). The rivers differ with respect to hydrological modification. At the time of this study, the Rı́o Mameyes was the last free-flowing river draining the LEF (i.e., no dams or major water abstraction). The Espı́ritu Santo has a low-head dam and water intake (constructed in 1984 for municipal water supply) located on its main stem (fifth order), ca 4 km from the coast at 5 m elevation. For sampling of developing larvae, seven stations were chosen in each estuary between the upper tidal limit (distinguished by salinity 1 ppt above streambed at high tide) and the river mouth. Stations were ca 100 and 200 m apart in the Mameyes and Espı́ritu Santo, respectively. The Mameyes estuary was sampled for larvae on two occasions (5 and 6 September 1996); the Espı́ritu Santo estuary was sampled once (4 September 1996). All sampling took place during the day. At each of the seven stations, we took one vertical plankton tow (net diameter 0.5 m; mesh 363 m) through the entire water column at both the river margin (2–3 m from the bank) and the center of the channel. Depth was recorded at each sampling site. Salinity was measured at 0.5-m depth intervals. Samples were preserved immediately in 70 percent ethanol and transported to the laboratory where larvae were counted, measured (carapace length), and sorted into 1-mm size classes using a stereomicroscope. We used published larval descriptions (e.g., Choudhury 1971, Hunte 1979) to check that larvae were derived from freshwater species. We sampled upstream migration of metamorphosed larvae using a trapping method adapted for postlarval shrimps (A. Burky, University of Dayton, pers. comm.; design available from the first author). Traps (150 150 80 mm) were constructed from transparent 2-mm Plexiglass sheets with 1-mm wire mesh at the upstream end; paired oblique baffles formed a 7 mm wide, vertical entrance at the opposite end. The trapping site in the undammed Mameyes was situated ca 500 m above the upper tidal limit; in the Espı́ritu Santo traps were placed ca 750 m upstream of the low-head dam (to incorporate potential effects of the dam on upstream migration). We set traps (N 4–6) in both rivers simultaneously on 28 occasions between 16 July and 6 September 1996. Traps were tethered to stakes in shallow riffle areas close to the bank with the mesh end facing upstream. Traps were set at dusk and collected the following morning; postlarvae were transported to the laboratory in the traps, identified to genus (Chace & Hobbs 1969; M. poeyi was grouped with Atya spp.), and counted.