Assessing Food and Nutritional Resources of Native and Invasive Lamprey Larvae Using Natural Abundance Isotopes

Lampreys (Family: Petromyzontidae) are primitive jawless fishes that have been identified in the fossil record from at least 360 million years ago.However, human impacts have frequently restricted native lamprey populations, reduced their numbers and contracted their ranges. In contrast, in the Laurentian Great Lakes invasion by the parasitic lamprey species Petromyzon marinus (sea lamprey) caused large scale ecosystem changes until human control efforts significantly reduced P. marinus numbers. Although lampreys have played a vital role in numerous ecosystems, there is currently limited information on the sources of food and nutrition during the longest stage of their life cycle (i.e., the ammocoete stage). If conservation and restoration strategies for native lamprey, and control strategies for invasive sea lamprey, are to be optimized and cost-effective, a fundamental understanding of the factors contributing to the growth, nutrition and survival of ammocoetes is critically needed. Prior conclusions and interpretations on lamprey ammocoete diet have focused on gut-content analysis studies, and our understanding of ammocoete diet and nutrition are therefore limited by older and largely non-quantitative techniques.

Natural abundance isotopes represent a potentially more robust and quantitative approach than gut content analysis for assessing the types of organic materials supporting an organism’s somatic growth and energetic maintenance. In the present study we used natural abundance stable isotopes (δ¹³C, δ¹⁵N, and δ²H) to estimate the major sources of autochthonous and allochthonous organic matter (OM) to the diets and nutrition of three species of ammocoetes (i.e., two native species, Lampetra aepyptera, Lethenteron appendix, and one invasive species, Petromyzon marinus) collected in 2010 from rivers in Michigan (USA) and Ohio (USA).

For all three species of lamprey, ammocoete δ¹³C increased from ~-32‰ to ~-22‰ with increasing animal length and was strongly positively correlated with animal C:N. Ammocoete δ¹⁵N suggested that ammocoetes are primary consumers of primary producers and detrial materials. Ammocoete δ²H values showed a strong negative relationship with both animal length and C:N. The observed size-dependent shifts in isotopic values suggest that ammocoete diet changes during growth over the extended ammocoete phase of the lamprey life cycle. However, the possibility cannot be ruled out that some part of the ontogenetic shift in ammocoete isotopic values may be under the influence of non-trophic (i.e., internal) mechanisms that obscure food source signatures.

Contributions of different potential dietary and nutritional sources to ammocoetes were estimated using Bayesian modeling of animal and potential food source isotopic signatures. The model findings indicate significant subsidization of the animals’ nutrition by allochthonous terrestrial sources of fresh plant and soil OM (20-90%), and that aquatic microalgae was the dominant autochthonous (aquatic) subsidy to ammocoete nutrition (i.e., up to 80%), especially in larger and presumably older ammocoetes. Aquatic macrophytes and diagenetically altered sediment OM also contributed to ammocoete diet (2-70%), but contributions were highly variable and influenced by site. The more accurate identification and quantification of allochthonous and autochthonous food and nutritional subsidies to the ammocoete stage provide important information on potential key factors affecting the overall life history of both native and non-native lamprey, and support the growing body of evidence of the significant role of allochthonous terrestrial OM to a range of lotic and aquatic consumers.