Patterns of feeding strategy change throughout ontogeny according to size-specific abilities and requirements. Characterizing the extent and potential repeated occurrence of dietary differences within the population can improve understanding of the intraspecific predation regime and population dynamics. Here, we investigated size-related feeding habits and trophic niche partitioning of an iconic pelagic generalist predator, the jumbo squid Dosidicus gigas, by a combined analysis of morphologic indicators of feeding capability (fin and feeding apparatuses), trophic biochemical tracers (stable isotopes and fatty acids), and stomach contents. Results showed significant variation in prey composition, isotopic values, and fatty acid profiles with size. D. gigas exhibited a shift in diet at approximately 250 mm mantle length (ML). The trophic niche of small (ML ≤ 250 mm) and large (ML > 250 mm) squid revealed low potential for resource overlap, suggesting segregation in terms of spatial and food resources of different size groups. Large individuals tend to feed deeper and on smaller prey to optimize food capture costs and energy benefits. This feeding strategy is likely related to variability in the development of feeding capacity, increasing swimming ability, and metabolic demand as squid grow and may reduce intraspecific competition to improve survival. These results highlight the dietary flexibility of D. gigas and demonstrate that niche differentiation acts as a major factor in a cohort, which may have important implications for their population dynamics and management. In addition, this study demonstrates that using multiple diet tracers can highlight subtle differentiations in diet correlated to growth in a pelagic generalist predator.

The starter diet for Japanese eel (Anguilla japonica) has always been a difficult problem for the realization of total artificial reproduction. Therefore, this research analyzed the nutritional composition of artificially fertilized eggs, and transcriptome of samples from early hatchlings of fry to better understand nutrients requirements. The composition of crude lipid and crude protein in fertilized eggs was 7.24% and 10.56%, respectively. Seven kinds of essential amino acids (EAA) were detected but took a comparable lower content (3.19%) than other marine fish eggs. We randomly assembled 265.74 million clean reads and identified 1751 differentially expressed genes (DEGs) (P < 0.01) from pre-leptocephalus larvae. A total of 23 KEGG pathways related to the digestive and metabolic system were detected. Genes related to the secretion pathway of saliva, pancreatic juice and other digestive juices were significantly changed. The genes of carbohydrate metabolism, glycerolipid metabolism and glycerophospholipid metabolism were up-regulated significantly with the growth of the larvae (day 0 to 12). This study will facilitate future studies on the nutrition of eel larvae and other biological traits to reproductive research.