The second life of terrestrial and plastic carbon as nutritionally valuable food for aquatic consumers.

Primary production is the basis for energy and biomolecule flow in food webs. Nutritional importance of terrestrial and plastic carbon via mixotrophic algae to upper trophic level is poorly studied. We explored this question by analysing the contribution of osmo- and phagomixotrophic species in boreal lakes and used 13 C-labelled materials and compound-specific isotopes to determine biochemical fate of carbon backbone of leaves, lignin-hemicellulose and polystyrene at four-trophic level experiment. Microbes prepared similar amounts of amino acids from leaves and lignin, but four times more membrane lipids from lignin than leaves, and much less from polystyrene. Mixotrophic algae (Cryptomonas sp.) upgraded simple fatty acids to essential omega-3 and omega-6 polyunsaturated fatty acids. Labelled amino and fatty acids became integral parts of cell membranes of zooplankton (Daphnia magna) and fish (Danio rerio). These results show that terrestrial and plastic carbon can provide backbones for essential biomolecules of mixotrophic algae and consumers at higher trophic levels.

Body length rather than routine metabolic rate and body condition correlates with activity and risk-taking in juvenile zebrafish Danio rerio.

In this study, the following hypotheses were explored using zebrafish Danio rerio: (1) individuals from the same cohort differ consistently in activity and risk-taking and (2) variation in activity and risk-taking is linked to individual differences in metabolic rate, body length and body condition. To examine these hypotheses, juvenile D. rerio were tested for routine metabolic rate and subsequently exposed to an open field test. Strong evidence was found for consistent among-individual differences in activity and risk-taking, which were overall negatively correlated with body length, i.e. larger D. rerio were found to be less active in a potentially dangerous open field and a similar trend was found with respect to a more direct measure of their risk-taking tendency. In contrast, routine metabolic rate and body condition were uncorrelated with both activity and risk-taking of juvenile D. rerio. These findings suggest that body length is associated with risk-related behaviours in juvenile D. rerio for which larger, rather than smaller, individuals may have a higher risk of predation, while the role for routine metabolic rate is relatively limited or non-existent, at least under the conditions of the present study.

Experimental assessment of the probabilistic maturation reaction norm: condition matters.

The probabilistic maturation reaction norm (PMRN) describes an individual's probability of maturing at a given age as a function of size and other relevant phenotypic traits. Population-level shifts in the PMRN are often interpreted to indicate genetic as opposed to phenotypic changes in maturation in fish. Inferences derived from trends in the PMRN have been challenged, warranting an experimental assessment of the method. This was accomplished in a laboratory experiment using zebrafish (Danio rerio). Fish were reared under different food levels to induce variation in growth and maturation. Plasticity in maturation was not entirely captured by the demographic age- and length-based PMRN. Adding condition to the PMRN captured a greater amount of environmental variation in maturation probability. Nevertheless, significant differences in the PMRNs among the food levels remained after accounting for the influences of age, size and condition on maturation probability indicating plasticity of the PMRN. This was particularly pronounced between fish held on low food levels as compared with fish experiencing abundant resources, with the latter experiencing higher size-specific maturation probabilities. Our analysis emphasizes the need for incorporating salient physiological traits influencing maturation, such as condition, to make accurate inferences about documented shifts observed in the position of PMRNs on maturation trends in wild fish stocks.

Size-dependent reproductive success of wild zebrafish Danio rerio in the laboratory.

Size-dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size-selective harvesting of the F(1)-offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small-sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L(S))-at-hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area-at-hatch and greater yolk-sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L(S) between days 60 and 90 post-fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large- and small-sized spawners by disentangling size-dependent maternal and paternal effects on reproductive variables in D. rerio.