The Effects of Habitat Complexity and Predation on the Distributionof Grass Shrimp (Decapoda: Palaemonetes) in the Lower St. Johns River Basin, Florida

Few studies have evaluated the abundance and distribution of grass shrimp (Decapoda: Palaemonetes) in the oligohaline and tidal freshwater portions of an estuary. In order to address this knowledge gap, I examined the seasonal patterns of habitat use by grass shrimp along the estuarine gradient in the oligohaline and tidal freshwater portions of the Lower St. Johns River Basin, Florida. Four species of grass shrimp (P. intermedius, P. paludosus, P. pugio and P. vulgaris) were found in this portion of the estuary; their absolute and relative abundances varied considerably along the estuarine gradient. In addition, all four species of grass shrimp were consistently more abundant in tapegrass (Vallisneria americana) than in adjacent sand flats. To determine the possible mechanism responsible for this pattern of habitat use, I performed laboratory experiments on habitat use by grass shrimp under the risk of predation. In the absence of predatory largemouth bass (Micropterus salmoides), grass shrimp used the vegetated and unvegetated portions of the experimental tanks equally. However, with the addition of largemouth bass, grass shrimp significantly increased their use of the vegetated portion of the tank. Sub-optimal and/or fluctuating environmental conditions (e.g. salinity) are hypothesized to contribute to the variation observed in lifehistory characteristics, such as ovigerous female size, ovary size and clutch size. I examined the seasonal and spatial patterns of ovigerous grass shrimp abundance in the Lower St. Johns River Basin. Ovigerous females were detected during three of the four sampling periods, possibly indicating that grass shrimp in the St. Johns River are not continuous spawners. Abundances of ovigerous females were greatest at two oligohaline stations (Buckman and Moccasin Slough). However, lifehistory characteristics (female length and mass, mean number of ova, mean individual ova mass, total ovary mass) differed significantly between these two locations. Field and laboratory studies indicate that grass shrimp assemblage structure and reproduction are influenced by both abiotic and biotic factors in the Lower St. Johns River Basin