| Summary: | Within the marine environment, the diverse development modes among marine invertebrate taxa follow a macro-ecological pattern across latitude. Generally, across the latitudinal gradient from the tropics to the poles, larval development becomes increasingly independent of external food resources. Low latitude species are fecund and produce relatively poorly provisioned eggs, which develop into swimming and feeding larvae. This mode of development becomes increasingly scarce with increasing latitude, whilst development from large eggs (produced in low numbers) into larvae, which are non-feeding, becomes increasingly prevalent. From the tropics to the poles, there is an increasing mismatch between the longer periods required for larval development (resulting from increasing cold) and the shorter periods of food availability (resulting from increasing seasonality). This macro-ecological trend in development modes results from the convergent evolution of diverse taxa, which have adapted to high latitude environments in much the same way: through producing larvae which develop independently of external food resources. Developmental plasticity during the larval phase is a mechanism by which larvae are able to cope better with unfavourable conditions or variations in their environment. Critically, the interaction between per offspring investment (POI; the quantity and quality of resources allocated to offspring) and developmental plasticity, and the role of this interaction in the evolution of larval development modes, is little considered or studied. Experiments comprising this thesis assess the potential role of this interaction in the evolution of abbreviated and lecithotrophic development within decapod crustaceans, and the establishing of the macro-ecological trend in development outlined above. Experiments used the palaemonine shrimp, Palaemonetes varians, which inhabits temperate, salt marsh, and peripheral brackish water, as a study species. Palaemonine Abstract ii shrimp originated from a tropical marine clade and extant species are found in marine, brackish, and fresh water environments. The evolutionary transition from marine to fresh water has involved life history adaptations in development mode within this group. As species occupy differing habitats along this environmental gradient, this group has been used to study evolutionary adaptations along the environmental gradient from marine to fresh water. Sampling of a wild population of P. varians from Lymington salt marsh (Hampshire, UK) revealed the highly variable environment that this species inhabits. POI within this population varied inter- and intra-annually, though these variations could not be correlated with variations in environmental temperatures. Larvae hatch with significant yolk reserves and can be considered facultative lecithotrophic in the first and second larval instar, and planktotrophic from the third larval instar. Larval development was successful at temperatures between 15 and 30 °C and temperature-mediated developmental plasticity was observed; at higher temperatures, larvae increasingly developed through fewer larval instars. Development through fewer larval instars resulted in more rapid development, but development to a lesser juvenile dry weight at settlement. Consequently, this developmental plasticity may have ecological implications. Developmental plasticity was also influenced by the energy content of larvae at hatching (as proxy for POI). Larvae with greater energy content developed through fewer larval instars at all temperatures, indicating that higher POI buffers larvae against poor conditions during development. Greater energy content also enabled larvae to tolerate starvation for longer and to develop to more advanced larval stages in the absence of food. Developmental plasticity within decapod crustaceans enables larvae to tolerate unfavourable conditions during development. Interestingly, it maximises the potential fitness benefits provided by POI by enabling larvae to settle as juveniles earlier, though at a smaller size. The interaction between POI and developmental plasticity forms a ‘pre-adaptation’ for the evolution of abbreviated development. The results presented in this thesis indicate that, indeed, the evolutionary transition to abbreviated development and lecithotrophy is based on selection for increasing POI. The abbreviation of development associated with increasing POI arises through developmental plasticity in larval instar number.
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