DISTRIBUTION OF RIVER SHRIMP IN THE GUADALUPE AND SAN MARCOS RIVERS OF CENTRAL TEXAS, U.S.A. (DECAPODA, CARIDEA)

BY

FRANCIS HORNE and STEVE BEISSER 1) Biology Department, Southwest Texas State University, San Marcos, Texas, 78666, U.S.A.

Of the decapod crustaceans, river shrimp are probably one of the least well- known taxa in the United States. As primarily tropical species that belong to the genus Macl'obrachium and the family Palaemonidae, adult prawns occur in rivers and estuaries along the coast of the Gulf of Mexico and the lower Atlantic coast to North Carolina (Hedgpeth, 1949). Of the 28 species reported from North and South America, six species of Macrobrachium are found in the United States and four are known from Texas (Holthuis, 1952 ; Holthuis & Provenzano, 1970; this paper). The current short note reports the extension of the range of Macrobrachium in Texas and presents a discussion of the status of river shrimp in the Guadalupe and San Marcos Rivers of Central Texas. To establish the distributional pattern of MacrobracJJiaYn, cylindrical minnow traps with an opening at one end were baited with dead fish and set for varying periods from 1971 to 1974. Traps were usually checked after four or five days. Site No. 1 was located 3 km from the mouth of Guadalupe River, Site No. 2 was at the Du Pont Chemical Plant on the Guadalupe River, Site No. 3 was just below the first dam on the Guadalupe River, and Site No. 4 was at the headwaters of the San Marcos River. Collection data are presented in Table I and fig. 1. From trapping data dams appear to obstruct migration upstream of M. acan- thurus (Wiegmann, 1836), M. ohione (Smith, 1874), and M. ol f er.rii (Wieg- mann, 1836), but not of M, carcinus (Linnaeus, 1758) (fig. 1). Only the large prawn, M. carcinus was trapped all the way from the mouth of the Guadalupe River to the headwaters of the San Marcos River. Migration of the other three river shrimp upstream was blocked by the first mainstream dam on the river (site No. 3 ) . To reach the headwaters of the San Marcos River six mainstream dams would have to be traversed. The shrimp would have to leave the water and walk around them. From our many observations in the field and laboratory, only M. carcinus readily leaves the water to walk on land. Hedgpeth (1949) reported M. ohione from nearby Austin, Texas, over 416 km up the Colorado River, which has no large mainstream dams. This species of river

1) Supported by an Organized Research Grant from Southwest Texas State University. 57

TABLE I Macrobrachiuna collections from Central Texas, U.S.A.

* Ovigerous females. ** Tivoli - Collection site 1. Victoria - Collection site 2. Cuero Dam - Collection site 3. shrimp moves much further into fresh water than any of the other species, and is well-known from the upper reaches of the Mississippi and Ohio Rivers (Hedgpeth, 1949; Holthuis, 1952). If the life cycles of all of the river shrimp are similar to that of M. carcinuJ (cf. Lewis et. al., 1966) and M. acanthuru.r (see Choudhury, 1971), then the sensitivity of the larval instars to low salinity would limit breeding success to estuarine situations. That is, the eggs must hatch close enough to the coast so that the larvae will drift downstream into brackish water in a few days. Larvae die after only a few days in fresh water (Lewis & Ward, 1965; Choudhury, 1971 ) . After several years of extensive trapping, the smallest M. carcinus collected from the headwaters of the San Marcos River were (1) 12.3 cm total body length and 26.4 g, and (2) 13.0 cm total body length and 30.5 g. From growth rates of M, c?z?°cinu.r( cf . Lewis et al., 1966), these two shrimp were estimated to be about two years old. Apparently, there is no breeding success in the upper reaches of the