Global change effects have favoured the introduction of new species in marine ecosystems in recent years. Gracilaria vermiculophylla, a red seaweed native from the north-eastern Pacific, has successfully colonised large regions in the Northern Hemisphere. In this research, we implemented species distribution models (SDMs) to (i) examine which were the most important environmental factors defining the presence of G. vermiculophylla at a global scale, and (ii) determine the potential current and future distribution of G. vermiculophylla based on two climate scenarios (representative concentration pathways (RCP 2.6 and RCP 8.5)). Our results suggest that temperature and salinity were the most important variables explaining the distribution of the target species. Additionally, the SDMs for present climate settings showed a potential wider distribution than is recorded to date. In addition, a subtle habitat expansion of 2.9° into higher latitudes was reported under the RCP 2.6 scenario by the end of this century. The high-carbon-emission scenario (RCP 8.5) delivered a potential large habitat expansion (6.0°), even reaching arctic latitudes, and a remarkable habitat loss of 11° in its southern distribution range. SMDs also forecasted suitable areas for this species in the Southern Hemisphere, pointing toward a potential global expansion in the coming decades.

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.

Pipefishes (family: Syngnathidae) are representative fishes in many European coastal areas associated with vegetal assemblages and other shallow habitats. The polyandrous pipefishes Syngnathus abaster and Syngnathus acus co-occur in Metruk Tuzla (Bargilya Tuzla coastal Wetland - BTW, Aegean Sea, Türkiye ‒a protected but threatened wetland due to anthropogenic disturbances. Both species were sampled to assess the growth and temporal changes on their trophic features using stable isotope (δ13C and δ15N) and gut content analyses. Both species exhibited positive allometric growth with similar length–weight relationships. The isotopic approach revealed that both species occupied similar trophic niches, as evidenced by comparable δ13C and δ15N values (-17.0‰ ± 1.2‰ and 13.2‰ ± 0.6‰, respectively in S. abaster; -17.3‰ ± 1.2‰ and 13.3‰ ± 0.7‰ respectively in S. acus). The niche area for all sex types (undifferentiated specimens, males and females) in S. acus was slightly broader than that in S. abaster. Simulation analysis of the trophic position in both species agreed with the isotopic assessment (trophic position = 3.67 ± 0.61 in S. abaster and 3.46 ± 0.66 in S. acus). Although the trophic positions were consistent with the isotopic assessment, gut content analysis disclosed a notable contribution of copepods, especially Calanipeda aquaedulcis, to the dietary regime in both species. Notably, the diet in S. abaster was nearly monospecific, whereas cladocerans were crucial in S. acus. This study suggests the plausible occurrence of interspecific competition regarding dietary resources. However, we hypothesise that such competition could be mitigated by the occupation of discrete microhabitats in both species. To validate this hypothesis, rigorous investigation and empirical analysis are necessary.