Differential changes in body size and stoichiometry in genetic lineages of the earthworm Eisenia nordenskioldi with elevation

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Exploring species and their functional diversity is of fundamental importance in biology and ecology. Many earthworm species comprise a wide range of genetic lineages/cryptic species, but it remains unclear whether these lineages/cryptic species represent distinct ecological entities that differ in traits. We barcoded 280 individuals of the earthworm species Eisenia nordenskioldi from 42 forest plots across seven elevations (800–1700 m) on Changbai Mountain, northeast China, and found them to comprise two genetic lineages with a p-distance in the COI gene of 16.7%. We evaluated whether the two distinct lineages differ in morphological (body length, body with, body mass and length:width ratio) and stoichiometric traits (C, N and P concentrations and C:N, C:P and N:P ratios), and whether these traits vary with elevation. At elevations where lineages co-existed, we further evaluated their ecological niche overlap. When co-existing, the two lineages of E. nordenskioldi significantly differed in body length, body mass, length:width ratio, and C and N concentrations as well as C:N and N:P ratios. With increasing elevation, body length, body mass and the length:width ratio of the two lineages linearly increased, whereas body width and C:N ratio linearly decreased; while C and N concentrations as well as C:P and N:P ratios showed a hump-shaped relationship with elevation. Across elevations trait distribution overlapped by 29%–100%, but the overlap in body mass, body length and length:width ratio decreased to 0%–15% when the two lineages co-existed. The significant differences in multiple traits indicate that the two lineages have different physiologies, supporting their status as cryptic species. The documented variations in morphological and stoichiometric traits of the two lineages/cryptic species call for integration of genetics and ecology for understanding biodiversity and ecological functioning of soil animal species.