Spatial variation in the gastrointestinal microbiome, diet, and nutritional condition of a juvenile flatfish among coastal habitats

Gut microbiota are important for the health, fitness and development of animal hosts, but little is known about these assemblages in wild populations of fish. Such knowledge is particularly important for juvenile life stages where nutritional intake critically determines early development, growth, and ultimately recruitment. We characterise the microbiome inhabiting the gut of young-of-the-year European plaice ('YOY plaice') on sandy beaches, their key juvenile habitat, and examine how these microbial communities vary spatially in relation to diet and nutritional condition of their plaice hosts. Body size, diet (stomach fullness and eukaryotic 18S ribosomal sequencing), nutritional condition (RNA:DNA) and gut microbiota (16S prokaryotic ribosomal sequencing) were compared in fish at two spatial scales: between beaches separated by 10s of kilometres and between sites at different depths on the same beach, separated by 10s of metres. The main microbial phyla in YOY plaice guts were Proteobacteria, Spirochaetes, Tenericutes and Verrucomicrobiae. Within the Proteobacteria there was an unusual dominance of Alphaproteobacteria. Differences in body size, diet and nutritional condition of YOY plaice between beaches were accompanied by differences in gut microbial assemblage structure. Notably, substantially reduced nutritional condition and size at one of the beaches was associated with lower stomach fullness, reduced consumption of annelids and differences in the abundance and presence of specific microbial taxa. Differences were also detected in microbial assemblages, body size, and diet between depths within the same nursery beach, although stomach fullness and nutritional condition did not vary significantly. The functional links between the environment, gut microbiota, and their hosts are potentially important mediators of the development of young fish through critical life stages. Our study indicates that these links need to be addressed at 10 km and even 10 m scales to capture the variability observed in wild populations of juvenile fish.

Using multiple natural tags provides evidence for extensive larval dispersal across space and through time in summer flounder

Dispersal sets the fundamental scales of ecological and evolutionary dynamics and has important implications for population persistence. Patterns of marine dispersal remain poorly understood, partly because dispersal may vary through time and often homogenizes allele frequencies. However, combining multiple types of natural tags can provide more precise dispersal estimates, and biological collections can help to reconstruct dispersal patterns through time. We used single nucleotide polymorphism genotypes and otolith core microchemistry from archived collections of larval summer flounder (Paralichthys dentatus, n = 411) captured between 1989 and 2012 at five locations along the US East coast to reconstruct dispersal patterns through time. Neither genotypes nor otolith microchemistry alone were sufficient to identify the source of larval fish. However, microchemistry identified clusters of larvae (n = 3-33 larvae per cluster) that originated in the same location, and genetic assignment of clusters could be made with substantially more confidence. We found that most larvae probably originated near a biogeographical break (Cape Hatteras) and that larvae were transported in both directions across this break. Larval sources did not shift north through time, despite the northward shift of adult populations in recent decades. Our novel approach demonstrates that summer flounder dispersal is widespread throughout their range, on both intra- and intergenerational timescales, and may be a particularly important process for synchronizing population dynamics and maintaining genetic diversity during an era of rapid environmental change. Broadly, our results reveal the value of archived collections and of combining multiple natural tags to understand the magnitude and directionality of dispersal in species with extensive gene flow.

Temporal dynamics in growth and white skeletal muscle composition of the mummichog Fundulus heteroclitus during chronic hypoxia and hyperoxia

Specific growth rate (G(S) ) and white skeletal muscle composition were measured in the mummichog Fundulus heteroclitus over a period of 28 days at four levels of dissolved oxygen (DO): severe hypoxia (c. 1.2 mg O(2) l(-1) ), moderate hypoxia (3.0 mg O(2) l(-1) ), normoxia (7.1 mg O(2) l(-1) ) and hyperoxia (10.6 mg O(2) l(-1) ). The G(S) was calculated over 0-8, 0-14, 0-28 and 14-28 days, and muscle protein, lactate dehydrogenase (LDH), DNA, RNA and water were measured at 0, 8, 14 and 28 days. Exposure of fish to severe hypoxia was associated with significantly reduced G(S) , lower muscle protein content and lower RNA:DNA compared with other DO treatments. When calculated over the first and second half of the 28 day exposure, however, G(S) of fish in severe hypoxia increased significantly during the second two-week interval, to the same rate as that of normoxic fish. Muscle LDH activity and water content were not significantly affected by DO level. Neither moderate hypoxia nor hyperoxia significantly affected G(S) or any biochemical variable. The results demonstrate that F. heteroclitus can tolerate wide variation in ambient oxygen concentration and, during prolonged exposure to severe hypoxia, shows significant compensation for the initial negative effects on growth. The capacity of F. heteroclitus to grow over a wide range of DO probably contributes to its ability to exploit habitats characterized by marked variation in oxygen availability.

Development, validation and field application of an RNA-based growth index in juvenile plaice Pleuronectes platessa

A general mechanism relating RNA concentration and growth rate is derived from four physiological assumptions and developed into a growth index for juvenile plaice Pleuronectes platessa. The index describing instantaneous growth rates (G, day⁻¹) in the laboratory with the lowest Akaike information criterion with small-sample bias adjustment was a function of RNA concentration (R, g(RNA)g⁻¹(wet mass)), temperature (T, ° K), body mass (M, g) and DNA concentration (D, g(DNA)g⁻¹(wet mass)): G = β₀ + β(R) R + β(T)T + β(T2)T² + β(M)M + β(D)D + β(RT)RT. RNA concentration began to respond to changes in feeding conditions within 8 days, suggesting that the index reflects growth rate in the short-term. Furthermore, the index distinguished between rapid growth and negative growth of juvenile P. platessa measured directly in laboratory and field enclosures, respectively. An application of the RNA-based growth index at two beaches on the west coast of Scotland suggested that the growth of juvenile P. platessa varies considerably in space and time and is submaximum in late summer.

Characterizing the escape response of juvenile summer flounder Paralichthys dentatus to diel-cycling hypoxia

Swimming speed, angular correlation and expected displacement were measured in juvenile summer flounder Paralichthys dentatus acclimated to either oxygen saturation (c. 7.8 mg O(2) l(-1); saturation-acclimated fish) or diel-cycling hypoxia (cycling between 11.0 and 2.0 mg O(2) l(-1)) for 10 days and subsequently exposed to more severe diel-cycling hypoxia (cycling between 7.0 and 0.4 mg O(2) l(-1)). Saturation-acclimated P. dentatus exhibited an active response to declining dissolved oxygen (DO) by increasing swimming speed, angular correlation and expected displacement to peak levels at 1.4 mg O(2) l(-1) that were 3.5, 5.5 and 4.2 fold, respectively, greater than those at DO saturation. Diel-cycling hypoxia-acclimated P. dentatus also exhibited an active response to declining DO, although it was relatively less pronounced. Diel-cycling hypoxia-acclimated P. dentatus swimming speed, however, still doubled as DO decreased from 7.0 to 2.8 mg O(2) l(-1). Diel-cycling hypoxia-acclimated P. dentatus did not recover as well from low DO exposure as did saturation-acclimated fish. This was reflected in their relatively more random swimming (low angular correlation between successive moves) and poor maintenance of rank order between individuals during the recovery phase. Even saturation-acclimated P. dentatus did not resume swimming at speeds observed at saturation until DO was 4.2 mg O(2) l(-1). Paralichthys dentatus were very sensitive to decreasing DO, even at DO levels that were not lethal or growth limiting. This sensitivity and their poor recovery may preclude juvenile P. dentatus from using highly productive nursery habitats affected by diel-cycling hypoxia.