BackgroundThe life history stages of brachyuran crustaceans include pelagic larvae of the Zoea type which grow by a series of moults from one instar to the next. Zoeae actively feed and possess a wide range of organ systems necessary for autonomously developing in the plankton. They also display a rich behavioural repertoire that allows for responses to variations in environmental key factors such as light, hydrostatic pressure, tidal currents, and temperature. Brachyuran larvae have served as distinguished models in the field of Ecological Developmental Biology fostering our understanding of diverse ecophysiological aspects such as phenotypic plasticity, carry-over effects on life-history traits, and adaptive mechanisms that enhance tolerance to fluctuations in environmental abiotic factors. In order to link such studies to the level of tissues and organs, this report analyses the internal anatomy of laboratory-reared larvae of the European shore crab Carcinus maenas. This species has a native distribution extending across most European waters and has attracted attention because it has invaded five temperate geographic regions outside of its native range and therefore can serve as a model to analyse thermal tolerance of species affected by rising sea temperatures as an effect of climate change.ResultsHere, we used X-ray micro-computed tomography combined with 3D reconstruction to describe organogenesis in brachyuran larvae. We provide a detailed atlas of the larval internal organization to complement existing descriptions of its external morphology. In a multimethodological approach, we also used cuticular autofluorescence and classical histology to analyse the anatomy of selected organ systems.ConclusionsMuch of our fascination for the anatomy of brachyuran larvae stems from the opportunity to observe a complex organism on a single microscopic slide and the realization that the entire decapod crustacean bauplan unfolds from organ anlagen compressed into a miniature organism in the sub-millimetre range. The combination of imaging techniques used in the present study provides novel insights into the bewildering diversity of organ systems that brachyuran larvae possess. Our analysis may serve as a basis for future studies bridging the fields of evolutionary developmental biology and ecological developmental biology.

Understanding organismal responses to environmental drivers is relevant to predict species capacities to respond to climate change. However, the scarce information available on intraspecific variation in the responses oversimplifies our view of the actual species capacities. We studied intraspecific variation in survival and larval development of a marine coastal invertebrate (shore crab Carcinus maenas) in response to two key environmental drivers (temperature and salinity) characterising coastal habitats. On average, survival of early larval stages (up to zoea IV) exhibited an antagonistic response by which negative effects of low salinity were mitigated at increased temperatures. Such response would be adaptive for species inhabiting coastal regions of freshwater influence under summer conditions and moderate warming. Average responses of developmental time were also antagonistic and may be categorised as a form of thermal mitigation of osmotic stress. The capacity for thermal mitigation of low salinity stress varied among larvae produced by different females. For survival in particular, deviations did not only consist of variations in the magnitude of the mitigation effect; instead, the range of responses varied from strong effects to no effects of salinity across the thermal range tested. Quantifying intraspecific variation of such capacity is a critical step in understanding responses to climate change: it points towards either an important potential for selection or a critical role of environmental change, operating in the parental environment and leading to stress responses in larvae.

The importance of salinity experienced during embryonic development and initial larval biomass on larval growth was studied in the South American estuarine crab Chasmagnathus granulata. Ovigerous females were maintained at three salinities (15, 20, and 32&) from egg laying to hatching of zoea 1. Larvae from all treatments were reared under constant conditions of photoperiod (12:12), temperature (18°C), and salinity (first instar at 20&, subsequent instars at 32&). Biomass was measured as dry weight, carbon, and nitrogen content per individual at egg laying, hatching of zoea 1, premoult zoea 1, and zoea 4, and in 8-day-old megalopa. From hatching to premoult zoea 4, biomass was higher for larvae from prehatching salinities of 15 and 32&. There was a significant positive correlation between biomass at hatching and at premoult zoea 1 and zoea 4. Accumulated biomass during zoeal stages tended to be higher for larvae from broods with higher biomass at hatching, although this trend was not always significant. Zoea 4 either directly metamorphosed to megalopa or moulted to zoea 5, following, respectively, a short or long developmental pathway. The proportion of zoea 4 that followed the long pathway was negatively correlated with biomass of zoeal stages. Biomass at hatching was correlated with biomass of megalopae developed through the short pathway, although it was not correlated with the accumulated biomass at this stage. Megalopae developed through the long pathway (i.e. metamorphosed from zoeae 5) had higher biomass than those from the short pathway.The present results suggest that prehatching salinity and initial egg and larval biomass can be very important for larval growth.

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Effects of reduced salinities on dry weight (DW) and biochemical composition (total lipid and protein contents) of zoea 1 larvae were evaluated in four decapod crustacean species differing in salinity tolerance (Cancer pagurus, Homarus gammarus, Carcinus maenas, Chasmagnathus granulata). The larvae were exposed to two different reduced salinities (15xand 25xin C. granulata, 20x and 25x in the other species) for a long (ca. 50% of the zoea 1 moulting cycle) or a short period (16 h, starting at ca. 40% of the moulting cycle), while a control group was continually maintained in seawater (32x ).In general, the increments in dry weight, lipid and protein content were lower at the reduced salinities than in the control groups. In the zoea 1 of H. gammarus (stenohaline) and C. pagurus (most probably also stenohaline), the lipid and protein contents varied greatly among treatments: larvae exposed to low salinities exhibited very low lipid and protein contents at the end of the experiments compared to the controls. In some cases, there were negative growth increments, i.e. the larvae had, after the experimental exposure, lower lipid and protein contents than at the beginning of the experiment. C. maenas (moderately euryhaline) showed a lower variation in protein and lipid content than the above species. The zoea 1 of C. granulata (fairly euryhaline) showed the lowest variability in dry weight, protein and lipid content. Since salinity tolerance (eury-v. stenohalinity) is associated with the osmoregulatory capacity, our results suggest a relationship between the capability for osmoregulation and the degree of change in the biochemical composition of larvae exposed to variable salinities. Besides larval growth of these species should be affected by natural reductions of salinity occurring in coastal areas at different time scales. These effects may be potentially important for population dynamics since they should influence the number and quality of larvae reaching metamorphosis. D