Leptocephali are the unusually large, transparent larvae of five orders of bony fishes. They are found at warm temperate and tropical latitudes throughout the world ocean and during peak periods of reproduction form an important component of the blue water micronekton/macrozooplankton community. However, their importance is not one of numbers and biomass, it is in the fact that they are (polar oceans excepted) a globally distributed group of unique trophic specialists. For the 230 years between their discovery and 1993, a phase I leptocephalus was never observed with food in its gut. However, recent histological evidence suggests that phase I leptocephali may ingest larvacean houses and zooplankton fecal pellets. Further evidence suggests that particulate organic material (POM) and dissolved organic matter (DOM) provide an important source of nutrition. Both particulate and dissolved organic material comprise the major carbon reservoir in the world ocean, yet their significance to pelagic metazoan life remains a cipher. To describe the leptocephalus trophic niche, larvacean houses and the larvae will be examined and the proteoglycan structure of both will be determined; gut morphology will be examined (including gut contents, if present) in four different species. Sites of nutrient uptake will be determined via radiolabelled amino acids and autoradiography. Daily metabolic demands will be quantified by determining diel consumption rates of individual larvae. Growth of the larvae will be determined by microincremental analysis of otoliths and compared to growth indices determined by RNA DNA analysis. Proximate and elemental composition will provide values for the calories and carbon stored in the living populations and their daily production. The leptocephalus larval form may be the only example of a vertebrate that obtains its nutrition from DOM and POM. If this is the case, leptocephali would comprise a unique trophic link in the pelagic food web. Our research will answer the riddle of how leptocephali acquire and utilize energy and describe the leptocephalus larva's position in the trophic hierarchy of the pelagial.
