Decryption of Amphipod Cryptic Species: Ecological Differentiation of Syntopic Lineages of a <i>Gammarus</i> Species Complex

ABSTRACT Cryptic species represent an ever‐increasing proportion of discovered biological diversity. Thus, the number of observations of cryptic species co‐occurring in syntopy also increases, raising the question of mechanisms facilitating this phenomenon. Gammarus fossarum is a widespread, ecologically important, and diversified cryptic species complex with dozens of deeply divergent lineages of Miocene origin, some of which have been observed in syntopy in its European range. We used a multifaceted approach to study the ecology of two syntopic, genetically differentiated G. fossarum lineages in the stable environment of a calcareous spring fen. We investigated the effects of spatial distribution of sampling sites and local environmental factors (water chemistry, substrate composition) on the fine‐scale distribution of the two lineages, together with their trophic ecology (δ 13 C, δ 15 N and gut content analysis). Moreover, we assessed functional morphological differentiation of the lineages, and compared identification based on mitochondrial DNA with variation at three nuclear markers to confirm their reproductive isolation. Small‐scale syntopy of both lineages prevailed across the studied locality, having been observed at 14 out of 15 sampling spots. Lineage ratios varied within the spring fen; they were significantly affected by spatial proximity between the sampling spots, but not by local environmental conditions. The two lineages significantly differed in body size as well as trophic niche. However, they were essentially morphologically indistinguishable, although significant shifts were observed in a few phenotypic traits mainly related to feeding. The lack of nuclear allele sharing confirmed the existence of a strong reproductive barrier between them. The two studied G. fossarum lineages are reproductively isolated species that significantly differ in trophic ecology and body size in syntopy. Their local coexistence may thus be facilitated by trophic niche differentiation due to alternative resource use. Our results add to the growing body of evidence that morphologically cryptic species may not be functionally fully equivalent, indicating that cryptic species complexes may be more ecologically differentiated than currently known.