The trophic structure of a pipefish community (Pisces: Syngnathidae) from a western Mediterranean seagrass meadow based on stable isotope analysis

Syngnathus abaster and S. typhle (Pisces. Syngnathidae) from a dense Cymodocea nodosa meadow in the western Mediterranean Stagnone di Marsala (Italy) were studied using δ13C and δ15N analysis. Because of the presence of these two species in the same habitat and the specialized parental care by the male, the effect of species and sex on the isotopic composition was also studied to investigate the different feeding strategies between and within species. S. abaster and S. typhle exhibited enriched 13C and 15N values throughout the sampling period (δ13C mean ± s.e. = -10.5 ± 1.8‰ and -10.8 ± 2.0‰, δ15N mean ± s.e. = 11.9 ± 0.7‰ and 10.6 ± 1.0‰ respectively), suggesting that these species receive their energy mainly from mixed sources, particularly from sedimentary and particulate organic matter and the seagrass C, nodosa as ultimate organic matter sources. ANOVA results demonstrated that the interaction between season, species and sex significantly affected the isotopic composition of the pipefish (P<0.001 for both δ13C and δ15N). Differences between species and sex, although significant, were smaller than the values generally reported for trophic level differences (~1‰ and ~3.5‰ for δ13C and δ15N respectively) and change in foraging habitat. Slight isotopic differences may mirror small differences in resource exploitation and resource partitioning. Evidence from stomach content analysis from the literature coupled with stable isotope measurements, while disagreeing somewhat, provide additional knowledge of pipefish feeding strategies.