Energetic adaptations to larval export within the brackish living palaemonine shrimp, Palaemonetes varians

Decapod crustaceans have repeatedly colonised brackish, freshwater, and terrestrial environments. Many decapods, which inhabit brackish and freshwater habitats, export larvae into estuarine and coastal areas where conditions for larval development may be better. In this study, we assess the starvation resistance, biochemical composition and respiration rate during larval development, and the effects of temperature on these factors within the brackish living palaemonine shrimp, Palaemonetes varians. Our results demonstrate that P. varians is highly resistant to starvation and may be considered facultative lecithotrophic in its first and second larval instars and planktotrophic from its third instar. This high starvation resistance is associated with a relatively large size, high carbon content (~45%) and C:N ratio (~4.2), and visible yolk reserves at hatching. These energy reserves are interpreted as an adaptation to the exportation of larvae from peripheral adult environments into mid and lower estuarine waters. Respiration rates varied with the moult cycle and were similar between fed and unfed larvae, suggesting that starved larvae do not suppress their metabolism as an energy saving strategy. Despite higher respiration rates at higher temperatures, energy loss throughout development (estimated from respiration rates) increased with decreasing temperature, whilst larval growth and development rates decreased with increasing temperature. High energy reserves at hatching, as within Palaemonetes varians, is an important life history adaptation in the colonisation of brackish and freshwaters, initially enabling the exportation of larvae from adult environments and eventually enabling lecithotrophy and direct development.