Morphology and gut contents of anguillid and marine eel larvae in the Sargasso Sea
The head morphology and body shapes of anguilliform larvae (leptocephali) vary widely by having fewer, much longer teeth in the small larvae and more numerous, but relatively smaller teeth in larger developmental stages. The feeding ecology of leptocephali has been difficult to understand because they do not appear to feed on typical zooplankton like other fish larvae but are mostly found with amorphous organic material in the gut. Observations of the basic morphology of mainly the head and gut contents of anguillid larvae and several families of larger marine eel larvae were made using photographs taken during three recent sampling surveys for leptocephali in the Sargasso Sea spawning area of the European, Anguilla anguilla, and American, Anguilla rostrata, eels. The gut contents of leptocephali consisted of amorphous material that sometimes flowed out of the intestine, and like in the Indo-Pacific, appendicularian houses and fecal pellets and other visible objects were sometimes present. High-magnification microscope images showed the presence of many spherical objects, amorphous and other materials that were likely related to bacteria, protists, fungi, or other organisms. The presence of filter structures confirmed that large oval objects were appendicularian houses, and possible hydrozoan objects were seen in Eurypharynx pelecanoides and Avocettina infans gut contents. The gut contents of A. anguilla, E. pelecanoides, and Kaupichthys hyoproroides leptocephali appeared to contain round ~,40 ~km heterotrophic thraustochytrid protists (class Labyrinthulomycetes) that likely colonized marine snow materials consumed by the larvae. These observations support the hypothesis that leptocephali, whose teeth structure and relative teeth sizes change with growth, primarily target overlapping size ranges of marine snow particles as a food source in the Sargasso Sea, and the particles contain a wide range of components that aggregate from the food-web.