The food source of Sargasso Sea leptocephali
The mysterious food source of anguilliform leptocephali has been difficult to understand, so this review evaluates potential interrelationships among recent discoveries on this subject. There are typically few identifiable gut-content objects in lepto-cephalus intestines, which usually contain amorphous materials. Gut content observation studies and stable isotope research have suggested that marine snow detrital-type particles are a food source, but this was difficult to validate. Recent gut-content DNA-sequence analyses indicated that small 4–25 mm Sargasso Sea European eel larvae, Anguilla anguilla, frequently ingest calycophoran siphonophore tissues as well as other taxa not likely to be ingested individually. A high-magnification photographic study of Sargasso Sea leptocephalus gut contents recently detected possible hydrozoan tentacles and apparent fatty acid-rich single-celled, heterotrophic thraustochytrid protists (class Labyrinthulomycetes), which have been found in marine snow in previous studies, but are not amplified by some DNA primers. Calycophoran siphonophores are abundant in the Sargasso Sea and have extensive tentacle arrays and short-lived eudoxid reproductive stages that might be appropri-ate sizes to be eaten directly or contribute to marine snow aggregates. The two groups may be interrelated because thraus-tochytrids are ubiquitously present decomposers that colonize detrital materials in oceanic and coastal ecosystems, so both siphonophore tissues and thraustochytrids may be present in marine snow consumed by European eel and other leptocephali. Therefore, future research on what leptocephali consume as food should be approached from a size-scaling perspective using systematic direct gut-content observations in combination with appropriate primers for next-generation DNA sequencing.