Abstract
The American eel, Anguilla rostrata, is a catadromous species that undergoes extensive morphological and physiological changes as it transitions from a benthic lifestyle to that of a pelagic migratory species in preparation for its reproductive migration to the Sargasso Sea (Tesch, 2003). One major change during this metamorphosis (silvering) is the reduction of the gastrointestinal (GI) tract (Pankhurst and Sorensen, 1984). This study examined silver-phase (maturing) American eels, collected at the start of their seaward migration, to histologically observe the loss of intestinal tissue over time and determine the mechanism of reduction. Histological sections of the mid-intestine sampled from 6 groups of silver-phase eels (Freshwater, Seawater, Freshwater Fed, Seawater Fed, Freshwater Matured, and Seawater Matured) were measured to identify changes in tissue area over time and to explore how factors, such as feeding, environment, and maturation state, may have affected the rate of intestinal reduction. Total area, muscle area, and villi area of intestinal cross-sections significantly declined in the Freshwater group. These reductions were gradual and most apparent at later stages of the metamorphosis. Gastrointestinal weight in proportion to body weight was significantly lower in silver-phase eels than yellow-phase (immature) eels. Apoptosis was identified in paraffin embedded tissue sections through nonisotopic (TUNEL assay) and isotopic (caspase-3 activity assay) approaches. This study is the first to directly investigate atrophy as the mechanism of intestinal reduction during the adult metamorphosis. Results support this hypothesis, as no significant differences in the density of apoptotic cells of intestinal muscle were observed between yellow- and silver- phase eels, and decreased cross sectional area resulted from cell shrinkage. Maturation appeared to encourage atrophy of intestinal muscle and villi. These finding add to our knowledge of the American eel metamorphosis and provide insight into the potential effects of delayed migration due to anthropogenic activities, such as dams.