The great pipefish Syngnathus acus is one of the most representative European syngnathid, being highly associated with seagrass and macroalgal beds. Surprisingly, the ecology of this ovoviviparous marine fish has received scanty attention. The population inhabiting three sites on Cíes Archipelago (Atlantic Islands National Park, NW Spain) was monitored in 2017-2018 for spatial and temporal changes in abundances, reproduction traits, trophic niche occupancy and dietary regimes across reproduction states through an isotopic (δ13C and δ15N) approach. Abundances were highly variable across seasons and sites, decreasing significantly from mid-autumn. The population consisted almost exclusively of large adults that migrate by the end of the breeding season, which extended from mid-spring to summer. Operational sex ratios suggest that the species is sex-role reversed. S. acus is a secondary consumer (Trophic position= 3.36 ± 0.05) preferring amphipods but displaying annual and seasonal dietary plasticity. Mature fish were less selective than immatures (especially females) with a higher preference for amphipods (36-68%) in the former. The second most preferred prey were carideans, copepods or isopods, depending on the year and reproduction state. Overall, the wider trophic niches in females and immature specimens compared to males and mature fish would indicate a higher variability in both the use of prey resources and/or their origin. The present study highlights the trophic plasticity and unique features of S. acus population in Cíes Archipelago, especially regarding the outstanding size of the fish and the exceptional occurrence of breeders.

Pipefishes (Family Syngnathidae) are representative fishes in many European coastal areas asso-ciated with vegetal assemblages and other shallow habitats. The polyandrous pipefishes Syngnathus abaster and S. acus co-occur in Metruk Tuzla (Bargilya Wetland, Aegean Sea, Turkey), a protected but threatened wetland due to anthropogenic disturbances. Both species were sam-pled during the breeding season (April-2018 and June-2019) for the assessment of growth and trophic features revealed from stable isotope profiles (δ13C and δ15N) and gut content. Both species showed positive allometric growth, with similar length-weight relationships. The isotope approach indicated that both species occupy similar trophic niches, with mean δ13C and δ15N values of -17.0 ± 1.2 ‰ for δ13C and 13.2 ± 0.6 ‰ for δ15N in S. abaster, and -17.3 ± 1.2 ‰ for δ13C and 13.3 ± 0.7 ‰ for δ15N in S. acus. The niche area for all sex types (undifferentiated specimens, males and females) in S. acus was slightly broader than in S. abaster. The simulation analysis of the trophic position (TP) in both species agreed with the isotopic assessment (TP = 3.67 ± 0.61 in S. abaster and 3.46 ± 0.66 in S. acus). Despite the high trophic resemblance between S. abaster and S. acus, the gut content assessment revealed the large contribution of copepods (especially Calanipeda aquaedulcis) to the dietary regime in both pipefishes. However, while the diet in S. abaster seemed to be almost monospecific, cladocerans were also an important resource in S. acus. Hence, we suggest that potential inter-specific competition for dietary resources would be minimized likely due to the occupancy of different microhabitats, a hypothesis that needs to be confirmed by further research.