The introduction of non-native seaweed species poses significant threats to coastal ecosystems, but many introductions remain undetected due to their small size and the morphological similarity among related species. This study aims to identify specimens resembling seaweeds in the genera Deltalsia, Symphyocladiella, and Xiphosiphonia collected from the Eastern Scheldt in The Netherlands during a general sampling survey in March 2022. Molecular analysis using the rbcL gene revealed these specimens to be Symphyocladiella gracilis, with only 0.13% genetic divergence from sequences from Pacific North America. Morphological observations confirmed this identification, with specimens matching descriptions from the Pacific. The combination of first-order branches with both determinate and indeterminate growth, lateral branches coalescing over 1–1.3 segments, and a recessed apical cell, is unique among species found in Europe, allowing its morphological identification. This is the first record of S. gracilis on European coasts, representing the fifth species of the tribe Pollexfenieae introduced in this region. The species' detection near extensive Pacific oyster cultivation sites in the Eastern Scheldt, points to aquaculture as the most likely introduction pathway. These findings highlight the importance of molecular tools and taxonomic expertise for detecting cryptic introductions in small, morphologically challenging seaweed species.
Allelopathic interactions may influence non-native plant establishment, yet their role in freshwater invasions remains poorly understood, particularly under naturally occurring concentrations of allelopathic substances. The Homeland Security Hypothesis predicts that secondary metabolites released by native plants may inhibit non-native species, contributing to biotic resistance. We experimentally tested this by evaluating germination responses of the non-native macrophyte Iris pseudacorus to three naturally occurring concentrations of phenolic compounds released by the native Ludwigia peploides, a dominant species in the Middle Paraná River floodplain. Germination was evaluated using a Gompertz model. Phenolic exudates did not produce a general inhibitory effect on I. pseudacorus germination but affected its components differently across concentrations. Total germination (Gmax) showed a stimulatory response, increasing along the phenolic gradient, while germination rate (k) and inflection time (t0) showed a non-monotonic response: low and medium concentrations reduced germination rate and delayed germination, whereas the highest concentration increased germination rate and accelerated it, indicating concentration-dependent shifts in the temporal pattern of germination. Overall, our findings do not support the Homeland Security Hypothesis. Instead, chemically mediated interactions between native and non-native macrophytes may therefore influence early invasion stages in different ways, depending on exudate concentration and the germination trait considered. Future studies should assess whether these patterns occur in the field and determine whether inhibitory thresholds emerge at higher phenolic concentrations.
The euryhaline seagrass, Ruppia maritima, has emerged as a promising restoration species in coastal and estuarine systems because of its high seed production and durable seed coats, yet its physiological tolerances remain understudied. We experimentally evaluated the effects of six temperatures (16–36°C) and six salinities (0–30PSU) on seed germination (%), germination timing (day of experiment), and early seedling growth (cm/d) using fully crossed, replicated laboratory trials. In this study, germination failure at 25 and 30 PSU confirmed previously established upper salinity tolerance thresholds for R. maritima. Remodeling the data on salinity ranges that support R. maritima germination (0–15PSU), we found a significant temperature-salinity interaction, with highest germination at low salinities (0–5PSU) and moderate temperatures (16–24°C), where germination success ranged from 34% to 69%. Germination onset occurred earlier at warmer temperatures (28–32°C; ~2–4 days) and low salinity (0–5PSU) but was delayed under cooler temperatures (16–20°C; ~6–8 days) and higher salinities (10–15PSU), with no significant temperature-salinity interaction (p = 0.06). Early seedling growth was primarily temperature-driven, increasing from 16°C to a maximum between 28–32°C before declining at 36°C. Maximum predicted growth occurred at 28°C and 5PSU, whereas high salinity (≥25PSU) reduced growth across temperature levels. These findings show stage-specific responses to abiotic stress. This information highlights critical environmental sensitivities and seasonal windows that may enhance the effectiveness of seed-based restoration efforts in variable estuarine environments, while also informing nursery propagation practices to optimize seedling growth using key physiological characteristics identified in this study.
