Population ecology and life history of <i>Macrobrachium carcinus</i> Linnaeus, 1758 (Decapoda: Caridea: Palaemonidae) in a spring-fed, headwaters river of central Texas, USA

Abstract The upper San Marcos River, a pristine and thermally stable headwater river in central Texas, provided a unique opportunity to study the ecology and life history of the amphidromous Macrobrachium carcinus (Linnaeus, 1758). The goals were to further the understanding of seasonal shifts in population structure related to reproductive strategy, and to assess length-weight relationships and growth. M. carcinus were collected by divers from the uppermost 2.2 km of the San Marcos River, approximately 400 river km inland, using hand-nets from February 2017 through March 2018. Of the 162 individuals captured and tagged, 83 were identified as male, 57 as female, and 22 were unidentified. Recapturing 38 tagged individuals provided growth measurements and movement patterns. Males and females exhibited allometric length-weight growth curves but differed with males reaching larger sizes than females. Thirteen ovigerous females were captured between June and November, three of which were recaptured later, having released their eggs. Sex-ratios showed no difference between reproductive months and non-productive months suggesting females are not moving downstream for egg release. This study provides a better understanding of the population structure and reproductive strategy of an intermittently present population of M. carcinus in a headwaters system and highlights the need for additional research on coastal connectivity and barriers to movement.