Responses of early-life stages of coastal marine invertebrates to different environmental variables

Abstract

Quantifying species responses to the effects of changing environmental conditions is critical for a better understanding of how climate change affects invasion, expansion, and contraction of marine coastal species. Climate change is leading to modifications in the marine coastal environment, to conditions not experienced before; climate change results in that marine organisms experience simultaneous changes in several environmental variables (=drivers: e.g. temperature, salinity, food). How simultaneous changes in multiple drivers are experienced depend on species-specific traits (e.g. physiological tolerance, developmental time); for instance, co-occurring native and non-native species may experience and respond to climate change in different ways. In addition, within species, responses to multiple drivers may vary across populations and environmental gradients. The general objective of this thesis was to quantify the effects of environmental drivers (temperature, salinity and food limitation) on performance of native and non-native species with focus on larval stages and using crabs as model systems. There were two main objectives, first to compare native and non-native species in the responses to multiple environmental drivers and to quantify larval responses to temperature across their distribution range. I focused on larvae because they play a critical role in population dynamics: larvae are important for the dispersion and connectivity of populations, and are more sensitive to changes in environmental conditions than adults. I used three ecologically relevant species of coastal areas of the North Sea and North Atlantic Ocean as models: Hemigrapsus sanguineus, Carcinus maenas and Hemigrapsus takanoi. C. maenas is native to Europe; Hemigrapsus spp. are both non-native species in the European coast, where they coexist with C. maenas as juveniles and adults in the benthos. I used factorial experiments rearing larvae from hatching to megalopae at different combinations of temperature and other environmental drivers (salinity, food limitation). Larval performance was quantified as survival, duration of development, and growth. The first series of result show that both non-native (Hemigrapsus spp) species had higher performance (high survival, shorter duration of development and high growth rates) than the native C. maenas at higher temperatures and at moderately low salinities (18 – 24 °C, 20 – 25 ‰). These results are comparable to another non-native species in Europe, the Chinese mitten crab Eriocheir sinensis. In H. sanguineus, larvae show moderate level of tolerance to limited access to food at high temperature, which contrasted to the low tolerance shown in native C. maenas. Experiments and modelling show that the nature of the multiple driver response depends strongly on the metric used to measure time, where my emphasis is on biological time (time to metamorphosis). The results from the populations comparisons showed species and gradient-specific responses. For H. takanoi, distributed over a salinity gradient (North Sea -Baltic Sea), larvae from the North Sea populations always showed higher survival and faster development compared with those from the Baltic Sea. The population near the limit of the distribution showed very low survival, suggesting that subsidies or complex ontogenetic migration patterns are needed for population persistence. Results did not show genetic differentiation among the studied populations in the mitochondrial cytochrome c oxidase subunit one gene (COI) suggesting that there is high connectivity among populations. For C. maenas distributed across a latitudinal gradient (South: Vigo, Spain; North: Bergen and Trondheim, Norway) and reared under different temperatures (range 6 to 27 °C in steps of 3 °C), there was little variation in survival and growth among populations. However, larvae from the Norwegian populations had a slightly shorter duration of development at low temperatures than those from Vigo, this response has an adaptive value in that it could sustain survival in scenarios of reduced temperature, by shortening the larval phase, when mortality rates are high. Besides, results from this experiment (as well as for the mentioned above) showed high intrapopulation variability in larval performance which has a potential to affect range expansion of the above-mentioned species. Variation in the responses of larval stages to the effects of different environmental drivers highlights the importance of using physiological descriptors to quantify the performance of marine invertebrates to changing environments. Larval responses vary in rates of survival but also in the duration of time to achieve metamorphosis, as well as the rate at which the organisms grow, with concomitant effects on post-metamorphic success, which in seasonal habitats may strongly depend on temperature. The results from the thesis highlight the importance of quantifying the responses of marine invertebrates to changing environmental conditions, considering different species and species distributed across different gradients as well as variations among and within species.