Using stable isotopes and C:N ratios to examine the life-history strategies and nutritional sources of larval lampreys

Natural abundance stable-isotope analysis (C-13 and N-15) and C:N ratios were used to study the ammocoete phase of two common non-parasitic lamprey species (least brook lamprey Lampetra aepyptera and American brook lamprey Lethenteron appendix) in two tributaries of the Ohio River (U.S.A.). The C:N ratios suggest that each species employs different lipid accumulation strategies to support its metamorphosis and recruitment into an adult animal. Ammocoete C-13 values generally increased with increasing C:N values. In contrast to C-13, ammocoete N-15 values were weakly related to the total length (L-T) in L. aepyptera, but positively correlated to both L-T and C:N ratios in L. appendix. In L. appendix, C:N also correlated positively with L-T, and presumably age. A Bayesian mixing model using C-13 and N-15 was used to estimate nutritional subsidies of different potential food resources to ammocoetes at each site. The models suggested that although nutritional subsidies to ammocoetes varied as a function of site, ammocoetes were generally reliant on large contributions (42-62% at three sites) from aquatic plants. Contributions from aquatic sediment organic matter were also important at all sites (32-63%) for ammocoetes, with terrestrially derived plant materials contributing smaller amounts (4-33%). These findings provide important insights into the feeding ecology and nutrition of two species of lampreys. They also suggest that similar and other quantitative approaches are required to (1) fully understand how the observed stable-isotopes ratios are established in ammocoetes and (2) better assess ammocoete nutritional subsidies in different natal streams.