Vol. 8: 15–28, 2009 AQUATIC BIOLOGY Published online December 29 doi: 10.3354/ab00203 Aquat Biol OPENPEN ACCESSCCESS Life history variation along a salinity gradient in coastal marshes Shannon B. Martin1, 2,*, Alan T. Hitch1, 3, Kevin M. Purcell1, 4, Paul L. Klerks1, Paul L. Leberg1 1Department of Biology, University of Louisiana at Lafayette, Lafayette, Louisiana 70504, USA 2Present address: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, Apalachicola Field Laboratory, Eastpoint, Florida 32328, USA 3Present address: School of Forestry and Wildlife Sciences, Auburn University, Auburn, Alabama 36849, USA 4Present address: Department of Biological Sciences, North Dakota State University, Fargo, North Dakota 58105, USA ABSTRACT: Coastal habitats are susceptible to changes in the environment associated with alter- ations in salinity. A field study was conducted on natural populations of the sailfin molly Poecilia latipinna, the western mosquitofish Gambusia affinis and the least killifish Heterandria formosa col- lected from coastal marsh sites along a salinity gradient, to investigate the influence of salinity on body condition and reproductive life history traits. In brackish marsh sites male P. latipinna had the best body conditions, while females had similar body conditions across all 3 marsh types. Female P. latipinna had greater reproductive allotment and fecundity in brackish marshes, where this fish was most abundant. Specimens of G. affinis collected from fresh marsh sites had less favorable body condition, and females had lower reproductive allotment and fecundity than those collected from higher salinities. While G. affinis was more abundant in freshwater marshes than in higher salinity marshes, this higher abundance did not correspond with a better body condition or higher reproduc- tive effort, suggesting that G. affinis may be stressed in freshwater. There was no difference in the conditions between (both male and female) H. formosa from fresh and intermediate marshes. Female H. formosa from fresh marshes had a similar reproductive allotment and lower fecundity than those from intermediate marshes. The 3 species exhibited different life history patterns along the salinity gradient, and some of these patterns conflicted with expectations based on species abundances. When assessing habitat quality along an environmental gradient, measures of abundance should be accompanied by more sensitive indicators of environmental stress. KEY WORDS: Sea level rise · Saltwater intrusion · Environmental stress · Environmental stability hypothesis · Density dependence · Gambusia affinis · Heterandria formosa · Poecilia latipinna Resale or republication not permitted without written consent of the publisher INTRODUCTION ing winds, and rising sea levels can drive saltwater inland into lower-salinity coastal areas (Titus & Rich- Coastal habitats are continually changing because man 2001, Nicholls et al. 2007). This saltwater intrusion they lie at the interface between land and sea. This is expected to become more prevalent as climates dynamic interaction makes coastal systems highly vul- warm and sea levels rise, especially in areas such as nerable to natural disturbances and climate change. the northern Gulf of Mexico that have a shallow Coastal environments are affected by storms, waves, coastal slope (Titus & Richman 2001). Louisiana is ex- and rainwater runoff, and changes in water tempera- periencing some of the highest rates of relative sea ture, currents, winds, and solar radiance, as well as sea level rise (i.e. eustatic sea level coupled with subsi- level (Bindoff et al. 2007, Nicholls et al. 2007). Salinity dence) globally (1.0 to 1.2 cm yr–2) (Penland & Ramsey levels may also change, since tropical storms, prevail- 1990, Bindoff et al. 2007). Tidally influenced coastal *Email: [email protected] © Inter-Research 2009 · www.int-res.com 16 Aquat Biol 8: 15–28, 2009 marshes are delineated along an environmental gradi- though these 3 species occur along a salinity gradient ent, with salinity levels decreasing farther inland of coastal marshes in Louisiana, their relative abun- (Odum 1988, Mitsch & Gosselink 2000) and are sus- dances differ with salinity: H. formosa and G. affinis ceptible to saltwater intrusion. occur at higher densities at fresh marshes, while P. Changes in salinity can stress aquatic organisms latipinna occurs at higher densities in intermediate residing in coastal marshes. It may be possible to and brackish marshes (A. T. Hitch et al. unpubl. data). detect salinity stress before it reaches lethal levels by These salinity preferences may not, however, be static, examining variation in key life history traits of resident as Purcell et al. (2008) reported that G. affinis from organisms along a salinity gradient. Salinity changes brackish marshes were locally adapted to elevated may result in stress due to the demands of maintaining salinity. Poeciliids may serve as a good indicator spe- osmotic balance (Nordlie 2006). The energetic costs of cies of environmental change given their opportunistic dealing with stress and concurrently maintaining life history strategy (Winemiller & Rose 1992) through homeostasis may constitute a substantial portion of the their plastic and quantifiable life history traits. organism’s energy budget and leave less energy avail- Some teleosts can only successfully reproduce within able for other functions (i.e. locomotion, growth, and a more narrow range of salinities in which they occur reproduction) (Sibly & Calow 1989). As much as 25% (Griffith 1974). Because accelerated sea level rise and of the energy remaining after basic maintenance consequent saltwater intrusion might negatively affect requirements are met may be required to cope with a coastal marsh fish populations, our objective was to mild or brief disturbance (Wedemeyer et al. 1990). Life examine the variation in body condition and reproduc- history theory predicts that energy should then be tive life history traits in natural populations of poecili- reallocated to physiological maintenance of an organ- ids that reside along a salinity gradient of coastal ism at the expense of growth and reproduction (Sibly & marshes. If the local salinity creates chronic sublethal Calow 1989, Stearns 1992). This response to stress stress, then the energy allocated to additional osmo- could be detected by observing tactical changes in key regulation and the response to stress could reduce life history traits such as age and size at maturity, the resources allocated to reproduction. The diverted reproductive allotment, and number and size of off- energy allocated towards maintenance could affect life spring (Roff 1992, Stearns 1992). Even population history traits such as adult size, body condition, repro- declines as observed in fishes (Adams et al. 2003) and ductive allotment, fecundity, as well as the timing and amphibians (Blaustein & Kiesecker 2002) may be due level of offspring provisioning (i.e. lecithotrophy or to exposure to sublethal stress affecting the survival matrotrophy). The 3 species in the present study differ and reproduction of a population’s members. The in various characteristics: (1) salinity tolerance: Heter- stress may reduce an individual’s health and perfor- andria formosa < Gambusia affinis < Poecilia latipinna, mance, which directly influences its fitness by way of (2) maternal provisioning: G. affinis < P. latipinna < reduction in reproduction and/or survival through in- H. formosa (Reznick & Miles 1989), and (3) body size: creased susceptibility to disease or predation (Wede- H. formosa < G. affinis < P. latipinna. On the basis of meyer et al. 1990). Tradeoffs between responses to these differences, and assuming that salinity stress is salinity stress and biotic factors such as competition or the major factor affecting differences in interspecific predation can influence the occurrence of species and energy allocation along a salinity gradient, we pre- thus community structure (Dunson & Travis 1991). dicted that H. formosa and G. affinis should be in Some of the most common resident fishes inhabiting better body condition, show higher reproductive allot- the vegetated margins of fresh and brackish marshes ment and produce more and larger offspring in fresh- of the northern Gulf of Mexico are the sailfin molly water marshes, while P. latipinna should perform Poecilia latipinna, the western mosquitofish Gambusia better in habitats with higher salinity levels such as affinis, and the least killifish Heterandria formosa brackish marshes. In addition to our results, we will (Martin 1980, Gelwick et al. 2001, Lorenz & Serafy discuss how factors varying along the gradient in asso- 2006). These 3 species of livebearing fishes belong to ciation with salinity might confound these predictions. the family Poeciliidae (Rosen & Bailey 1963). After a lengthy step-wise acclimation process, all 3 species can tolerate a relatively broad range of salinity levels, MATERIALS AND METHODS at least for a short period of time (Meffe & Snelson 1989). H. formosa is the least tolerant of elevated salin- Field collections. Individuals of Gambusia affinis, ity levels, with a tolerance range of 0 to 20 ppt (S. Mar- Heterandria formosa, and Poecilia latipinna were col- tin pers. obs.), followed by G. affinis, with a range of 0 lected from May to August 2005 from populations along to 58.5 ppt (Chervinski 1983), and P. latipinna is the a coastal salinity gradient from fresh (0 to 1 ppt),