Halophila Stipulacea Research Articles

Reduced transfer of metals and metalloids from pelagic Sargassum spp. accumulated in artificial floating barrier

Since 2011, holopelagic Sargassum have been massively stranding in the coastal environments of the Caribbean Islands inducing damages to coastal ecosystems, public health and the economy. To limit the risks associated with Sargassum stranding, floating barriers with nets can be placed in front of sensitive areas, to divert Sargassum away from the coast. To evaluate the potential transfer of metallic trace element (MTE) from Sargassum to adjacent marine life, seagrasses (Halophila stipulacea, Thalassia testidinum) and urchin (Lytechinus variegatus) were sampled, both close (0 m) and far (200 m) from barriers installed during 4 years in two bays: Baie Cayol (BC) and Cap Est (CE) in Martinique (FWI). A bay without barriers Baie-Tresor (BT) was also sampled in order to compare the effects of Sargassum accumulated in a natural environment versus an environment with floating barriers. The short-term effects of barriers were evaluated by measuring the evolution of MTE after four days, in the algae (Dictyota spp.), located close to Sargassum accumulations. All sampling was realized during two periods of active (July 2021) and reduced (January 2022) Sargassum stranding. The measured concentrations of 19 metal(loid)s trace elements revealed that the proximity of Sargassum to the barriers did not increase MTE concentration. The absence of increase in MTE was observed all sites (BT, BC and CE) and during periods of limited and important Sargassum stranding. Similarly, translocations of Dictyota close to Sargassum accumulations did not reveal any increase in MTE concentrations in the algae after 4 days. The present study suggests that the use of barriers to manage Sargassum stranding would not constitute an important threat of MTE contamination of marine environments.

Movements, foraging and habitat selection of southern stingrays (Hypanus americanus) in a tropical ecosystem altered by an invasive seagrass

This study provides some of the first evidence of how a common Caribbean fish species that relies on seagrass and sand habitats interacts with an invasive seagrass. The invasive seagrass Halophila stipulacea, first documented in the Caribbean in 2002, has rapidly expanded its range, displacing native seagrasses, and overgrowing bare sand. The southern stingray (Hypanus americanus) uses shallow seagrass and sand habitats for foraging. This paper examined the impacts of the invasive seagrass, H. stipulacea, on southern stingray behavior, foraging and movement patterns using acoustic telemetry and visual observations. From 2015 to 2018, 15 southern stingrays (14 juveniles of unknown sex and 1 female) were tagged with acoustic transmitters and passively monitored within an acoustic array in Brewers and Perseverance Bays, St. Thomas, United States Virgin Islands. The residence time, rates of movement and activity spaces for 50% and 95% utilization distributions (UD) were calculated for day and night periods. Tagged southern stingrays were detected within the array on average for 260 d (range 11–801 d) and residency for all individuals averaged 53% over the monitoring period (1079 d). Mean rates of movement per individual ranged from 40 to 150 m h−1, was lowest during the nighttime and were significantly higher during crepuscular periods. Activity spaces during day and night periods were nearly identical and averaged 0.05 km2 and 0.25 km2 for 50% and 95% UD, respectively. Distances between day and night 50% UD activity spaces was 148 m (range 8–409 m) and mean percent overlap was 38%. In addition to acoustic monitoring, benthic cover of available habitats and visual surveys of stingray behavior and habitat preferences were conducted along fixed transects. Monotypic stands of the invasive H. stipulacea seagrass represented 42% of benthic cover, followed by bare sand (27%), mixed patches of native and invasive seagrasses (23%) and the native Syringodium filiforme (7%). A habitat selection index (HSI) comparing the percent cover of available habitats and habitats occupied by stingrays found a strong preference for the native seagrass S. filiforme (HSI = 2.30) and bare sand (HSI = 1.79) compared to H. stipulacea (HSI = 0.70). Visual surveys also revealed that 50% of foraging stingrays were in S. filiforme, while 28% and 17% were foraging in H. stipulacea and bare sand, respectively. Our results are an important baseline for examining how further expansion of H. stipulacea may affect southern stingray movement patterns and foraging preferences.

First record of the seagrass Halophila stipulacea (Forsskal) Ascherson in the waters of the continental United States (Key Biscayne, Florida)

The first record of Halophila stipulacea is reported for the continental waters of the United States. In August 2024, a small meadow was identified inside Crandon Marina on Key Biscayne, Florida, USA. Following surveys have revealed that H. stipulacea has spread to adjacent areas immediately outside of the marina, often growing either in close proximity to, or interspersed with, the native seagrasses Thalassia testudinum, Syringodium filiforme, and Halodule wrightii. This serves as an initial report and extends the geographic scope of this introduced species in the Western Atlantic basin.

Halophila stipulacea: A Comprehensive Review of Its Phytochemical Composition and Pharmacological Activities.

Halophila stipulacea (Forsskål and Niebuhr) Ascherson is a small marine seagrass that belongs to the Hydrocharitaceae family. It is native to the Red Sea, Persian Gulf, and Indian Ocean and has successfully invaded the Mediterranean and Caribbean Seas. This article summarizes the pharmacological activities and phytochemical content of H. stipulacea, along with its botanical and ecological characteristics. Studies have shown that H. stipulacea is rich in polyphenols and terpenoids. Additionally, it is rich in proteins, lipids, and carbohydrates, contributing to its nutritional value. Several biological activities are reported by this plant, including antimicrobial, antioxidant, anticancer, anti-inflammatory, anti-metabolic disorders, and anti-osteoclastogenic activities. Further research is needed to validate the efficacy and safety of this plant and to investigate the mechanisms of action underlying the observed effects.

Limited potexvirus diversity in eastern Gulf of Mexico seagrass meadows.

Turtlegrass virus X, which infects the seagrass Thalassia testudinum, is the only potexvirus known to infect marine flowering plants. We investigated potexvirus distribution in seagrasses using a degenerate reverse transcription polymerase chain reaction (RT-PCR) assay originally designed to capture potexvirus diversity in terrestrial plants. The assay, which implements Potex-5 and Potex-2RC primers, successfully amplified a 584 nt RNA-dependent RNA polymerase (RdRp) fragment from TVX-infected seagrasses. Following validation, we screened 74 opportunistically collected, apparently healthy seagrass samples for potexviruses using this RT-PCR assay. The survey examined the host species T. testudinum, Halodule wrightii, Halophila stipulacea, Syringodium filiforme, Ruppia maritima, and Zostera marina. Potexvirus PCR products were successfully generated only from T. testudinum samples and phylogenetic analysis of sequenced PCR products revealed five distinct TVX sequence variants. Although the RT-PCR assay revealed limited potexvirus diversity in seagrasses, the expanded geographic distribution of TVX shown here emphasizes the importance of future studies to investigate T. testudinum populations across its native range and understand how the observed fine-scale genetic diversity affects host-virus interactions.

Sediment carbon storage in subtidal beds of the invasive seagrass Halophila stipulacea along an extreme water depth gradient, St. Thomas, U.S. Virgin Islands

Blue carbon ecosystems such as mangroves, salt marshes, and seagrass beds are found globally and are fundamental to fisheries production, storm surge protection, and carbon sequestration. The contribution of seagrass ecosystems to global carbon stocks is still not well understood, including in the United States Virgin Islands (USVI). No study has been published to-date assessing the sediment carbon density (SCD) in seagrass beds in the USVI. This study focused on the carbon storage ability of the invasive species, Halophila stipulacea, which is compact in size compared to common native seagrasses and has spread rapidly to become a dominant seagrass in the USVI. This species forms dense mats across a wide depth range (<1 m to 50 m) typically uninhabitable to its native counterparts (Syringodium filiforme and Thalassia testudinum). Several biotic and abiotic factors influence the carbon storage ability of seagrass, yet little is known about carbon storage sequestration along a depth gradient for H. stipulacea. This study provides the first assessment of the biological characteristics (shoot density, leaf area, leaf height, and percent cover) and carbon storage ability of H. stipulacea across a depth gradient (shallow: 5–10 m; medium: 15–20 m; deep: 25–30 m) at two sites in St. Thomas, USVI. Mean sediment carbon density (SCD) values per core reported for H. stipulacea in this study ranged from 3.88 to 15.67mgC/cm3; these were comparable to regional and global seagrass studies. Biological characteristics were not an accurate predictor of SCD. A significant interaction between water depth and site was found to affect mean SCD of H. stipulacea beds. It is likely that site-specific factors most likely account for variations seen within the data. Although carbon values in this study compared to values reported in the literature, other factors such as land use, proximity to carbon sources, sediment microbial community, and water current patterns may be driving SCD values. These findings highlight the need for site and species-specific carbon storage assessments on local to regional scales to accurately estimate current and forecasted blue carbon stocks.

A matter of choice: Understanding the interactions between epiphytic foraminifera and their seagrass host Halophila stipulacea

In sub/tropical waters, benthic foraminifera are among the most abundant epiphytic organisms inhabiting seagrass meadows. This study explored the nature of the association between foraminifera and the tropical seagrass species H. stipulacea, aiming to determine whether these interactions are facilitative or random. For this, we performed a "choice" experiment, where foraminifera could colonize H. stipulacea plants or plastic "seagrasses" plants. At the end of the experiment, a microbiome analysis was performed to identify possible variances in the microbial community and diversity of the substrates. Results show that foraminifera prefer to colonize H. stipulacea, which had a higher abundance and diversity of foraminifera than plastic seagrass plants, which increased over time and with shoot age. Moreover, H. stipulacea leaves have higher epiphytic microbial community abundance and diversity. These results demonstrate that seagrass meadows are important hosts of the foraminifera community and suggest the potential facilitative effect of H. stipulacea on epiphytic foraminifera, which might be attributed to a greater diversity of the microbial community inhabiting H. stipulacea.

Impact of invasive seagrass Halophila stipulacea on life history characteristics of juvenile yellowtail snapper (Ocyurus chrysurus)

Seagrass ecosystems provide essential nursery habitat to numerous coral reef fishes. Native Caribbean seagrasses Syringodium filiforme and Thalassia testudinum provide several juvenile fish species with foraging habitat and protection during this vulnerable stage of life. In 2002, an invasive seagrass Halophila stipulacea was discovered off the coast of Grenada and has since rapidly spread throughout the Caribbean. Halophila stipulacea has been shown to displace native seagrass species and may pose additional threats to juvenile reef fish populations that depend on native seagrass habitats as nurseries. The purpose of this study was to determine the effects of H. stipulacea on the early life history of yellowtail snapper (Ocyurus chrysurus). Settlement, mortality, and condition of juvenile yellowtail snapper were compared among native and invasive seagrass habitats around southern St. Thomas, United States Virgin Islands. From May 2020 to June 2021, monthly surveys within fixed plots during new moon periods provided information on settlement rates and survivorship among seagrass types. Length and weight of 260 juvenile yellowtail snapper were measured to derive condition factor among seagrass species. Significantly higher settlement and similar trends in mortality were seen in H. stipulacea when compared to native seagrasses. No significant difference was observed in condition factor of juvenile yellowtail snapper among seagrass species, although trends demonstrated a higher condition in individuals from native seagrass species. This study is the first of its kind to identify the direct effects of H. stipulacea during the early life history stages of a commercially important fish species in the Caribbean.

Shifts in biodiversity and physical structure of seagrass beds across 5 decades at Carriacou, Grenadines.

Caribbean seagrass beds are facing increasing anthropogenic stress, yet comprehensive ground-level monitoring programs that capture the structure of seagrass communities before the 1980s are rare. We measured the distribution of seagrass beds and species composition and abundance of seagrass and associated macroalgae and macroinvertebrates in 3 years over a 47-year period (1969, 1994, 2016) at Carriacou, Granada, an area not heavily impacted by local human activity. Seagrass cover and physical parameters of fringing beds were measured in transects at high (HWE) and low wave energy (LWE) sites; frequency of occurrence of all species, and biomass and morphology of seagrasses, were measured at 100 m2 stations around the island. Losses in nearshore seagrass cover occurred at HWE but not LWE sites between 1969 and 2016 and were associated with increases in the seagrass-free inshore zone (SFI) and erosional scarps within beds. Total biomass did not vary across years although there were progressive changes in seagrass composition: a decline in the dominant Thalassia testudinum and concomitant increase in Syringodium filiforme, and establishment of invasive Halophila stipulacea in 2016 at LWE sites. Species richness and diversity of the seagrass community were highest in 1994, when 94% of macroalgae (excluding Caulerpa) were most abundant, and sea urchins were least abundant, compared to 1969 and 2016. Multivariate statistical analyses showed differences in community composition across the 3 years that were consistent with trends in urchin abundance. Increases in SFI and scarp number in seagrass beds at HWE sites occurred mainly after 1994 and likely were related to increased wave forcing following degradation of offshore coral reefs between 1994 and 2016. Our observations suggest that landward migration of seagrass beds with rapidly rising sea level in future will not be realized in reef-protected seagrass beds at Carriacou barring reversal in the processes that have caused reef flattening.

Population genomics unveils the century-old invasion of the Seagrass Halophila stipulacea in the Mediterranean Sea

The tropical seagrass Halophila stipulacea invaded the Eastern Mediterranean Sea in the late nineteenth century and progressively spread throughout the basin ever since. Its spread is expected to continue north-westward as the Mediterranean Sea becomes warmer, potentially changing the seagrass biogeography of the basin. Given the power of genomics to assess invasion dynamics in non-model species, we report the first ddRAD-seq study of H. stipulacea and small-scale population genomic analysis addressing its century-old Mediterranean invasion. Based on 868 SNPs and 35 genotyped native (Red Sea) and exotic (from Cyprus, Greece, and Italy) samples, results suggest that genetic structure was high, especially between major geographic discontinuities, and that exotic populations maintain comparably lower genetic diversity than native populations, despite 130 years of invasion. The evidence of high heterozygosity excess, coupled with previously reported male-dominated and rare flowering records in the exotic range, suggests that clonal propagation likely played a pivotal role in the successful colonization and spread of H. stipulacea in the Mediterranean. This shift in reproductive strategy, particularly evident in the Italian populations located closest to the western boundary and representing more recent establishments, underscores the importance of this cost-effective mode of reproduction, especially during the initial stages of invasion, raising questions about the species future expansion trajectory. Our findings serve as a catalyst for future research into the species’ invasion dynamics, including deciphering the intricate roles of acclimatization and rapid adaptation, important for a comprehensive assessment of invasion risks and improving management strategies aimed at conserving seagrass ecosystems.

Sediment carbon storage differs in native and non-native Caribbean seagrass beds

Non-native species are expanding globally and can alter ecosystem functions, including food web dynamics, community structure and carbon storage. Seagrass are foundation species that contribute a variety of ecosystem services in near-shore coastal ecosystems, including a significant sink of carbon. In the Caribbean, the rapidly expanding non-native Halophila stipulacea has unknown impacts on carbon storage. To investigate the impacts on carbon storage, we quantified organic carbon (Corg) content in sediment and seagrass tissues from monotypic H. stipulacea beds, mixed native seagrass beds dominated by Thalassia testudinum and Syringodium filiforme, and unvegetated substrate in St. John, USVI. We found native seagrass-vegetated sediment contained 1.3 times more Corg than sediment covered by H. stipulacea, and 1.6 times more Corg than unvegetated areas on average. Whereas, H. stipulacea-dominated substrate stored 1.2 times more Corg than unvegetated substrate. Likewise, native species contained 2.2 times more aboveground biomass and 6.0 times more belowground biomass than H. stipulacea. Since seagrasses are critical sources of carbon sequestration, our results suggest that invading H. stipulacea is associated with lower carbon stocks which has potential implications for conservation activities and climate change mitigation.

Climate change and the presence of invasive species will threaten the persistence of the Mediterranean seagrass community

The Mediterranean Sea has been experiencing rapid increases in temperature and salinity triggering its tropicalization. Additionally, its connection with the Red Sea has been favouring the establishment of non-native species. In this study, we investigated the effects of predicted climate change and the introduction of invasive seagrass species (Halophila stipulacea) on the native Mediterranean seagrass community (Posidonia oceanica and Cymodocea nodosa) by applying a novel ecological and spatial model with different configurations and parameter settings based on a Cellular Automata (CA). The proposed models use a discrete (stepwise) representation of space and time by executing deterministic and probabilistic rules that develop complex dynamic processes. Model applications were run under two climate scenarios (RCP 2.6 and RCP 8.5) projected from 2020 to 2100 in four different regions within the Mediterranean. Results indicate that the slow-growing P. oceanica will be highly vulnerable to climate change, suffering vast declines in its abundance. However, the results also show that western and colder areas of the Mediterranean Sea might represent refuge areas for this species. Cymodocea nodosa has been reported to exhibit resilience to predicted climate scenarios; however, it has shown habitat regression in the warmest predicted regions in the easternmost part of the basin. Our models indicate that H. stipulacea will thrive under projected climate scenarios, facilitating its spread across the basin. Also, H. stipulacea grew at the expense of C. nodosa, limiting the distribution of the latter, and eventually displacing this native species. Additionally, simulations demonstrated that areas from which P. oceanica meadows disappear would be partially covered by C. nodosa and H. stipulacea. These outcomes project that the Mediterranean seagrass community will experience a transition from long-lived, large and slow-growing species to small and fast-growing species as climate change progresses.

The effect of anaerobic remineralization of the seagrass Halophila stipulacea on porewater biogeochemistry in the Gulf of Aqaba

IntroductionSeagrasses form oxidizing microenvironments around their roots, creating complex and strong redox gradients, thus affecting the rates of microbial carbon mineralization in their surrounding sediments. Since seagrasses are continuously being lost worldwide, a deeper understanding of the changes that occur within different seagrass sediments following the disappearance of the plants is of ecological and global importance.MethodsWe conducted a slurry experiment with sediments that have different characteristics from the northern tip of Gulf of Aqaba; the different sediments included different compartments of the tropical seagrass Halophila stipulacea (old and young leaves, rhizomes, or roots). We measured the changes over time in dissolved inorganic carbon (DIC), alkalinity, ferrous iron (Fe2+), hydrogen sulfide (H2S), sulfate (SO42-), and sulphur isotope ratios in sulfate within water. These measurements were used to calculate the rate of remineralization of each seagrass compartment, allowing us to predict the potential effects of the disappearance of different H. stipulacea compartments on key microbial processes in the surrounding environment.ResultsWe show that H. stipulacea’s rhizomes had the fastest decomposition rates, followed by the young leaves, roots, and old leaves (which also indicates the preservation potential of old leaves).DiscussionHigh concentrations of hydrogen sulfide were detected only in the slurries containing rhizomes and young leaves. High sulfide concentrations can lead to seagrass mortality and cause a positive feedback loop where the loss of seagrass due to sulfide generates further sulfide accumulation. This positive feedback loop can also be further reinforced by the loss of burrowing fauna in the sediment. This emphasizes the importance of understanding the extent of different pathways of seagrass disappearance on the surrounding environment and other geochemical feedbacks.

Variation in nutritional quality of an invasive seagrass does not explain its low palatability to two key herbivores in a Caribbean Bay

The seagrass Halophila stipulacea continues to spread rapidly through the Caribbean. Documenting native herbivore use of this invasive plant is important for understanding its impacts on marine communities and the mechanisms favoring its expansion. This study used observational and experimental data to determine if juvenile green turtles (Chelonia mydas) and long-spine urchins (Diadema antillarum) from the US Virgin Islands consumed H. stipulacea in the presence of native seagrass. Juvenile turtles associated significantly more with beds of native seagrass than mixed (native and invasive), and monospecific H. stipulacea, beds or sand bottoms. When individuals were followed, turtles foraged significantly more within mixed beds, but selectively fed on native species within them and were never observed feeding on monospecific H. stipulacea stands. When offered the native Thalassia testudinum and Syringodium filiforme, along with H. stipulacea, sea urchins significantly preferred S. filiforme over the other two choices. Measurements of fourteen nutritional and chemical parameters in these three seagrasses showed strong interspecific differences, with fewer and less predictable seasonal variations. Expressing these nutritional data by wet mass, rather than dry mass, also decreased the number of significant seasonal contrasts. Despite these differences, no parameters explained low herbivore preference for the invasive seagrass satisfactorily because nutrient values of H. stipulacea were often similar to those of the preferred natives. Our results conflict with recent studies pointing at low nutritional quality aiding against herbivory and suggest other mechanisms, such as herbivore learning and recognition of a novel resource, could be more important than currently appreciated.

Identification of potential invasive alien species in Spain through horizon scanning

Invasive alien species have widespread impacts on native biodiversity and ecosystem services. Since the number of introductions worldwide is continuously rising, it is essential to prevent the entry, establishment and spread of new alien species through a systematic examination of future potential threats. Applying a three-step horizon scanning consensus method, we evaluated non-established alien species that could potentially arrive, establish and cause major ecological impact in Spain within the next 10 years. Overall, we identified 47 species with a very high risk (e.g. Oreochromis niloticus, Popillia japonica, Hemidactylus frenatus, Crassula helmsii or Halophila stipulacea), 61 with high risk, 93 with moderate risk, and 732 species with low risk. Many of the species categorized as very high or high risk to Spanish biodiversity are either already present in Europe and neighbouring countries or have a long invasive history elsewhere. This study provides an updated list of potential invasive alien species useful for prioritizing efforts and resources against their introduction. Compared to previous horizon scanning exercises in Spain, the current study screens potential invaders from a wider range of terrestrial, freshwater, and marine organisms, and can serve as a basis for more comprehensive risk analyses to improve management and increase the efficiency of the early warning and rapid response framework for invasive alien species. We also stress the usefulness of measuring agreement and consistency as two different properties of the reliability of expert scores, in order to more easily elaborate consensus ranked lists of potential invasive alien species.

Influence of environmental variables on the abundance of Synapta maculata (Holothuroidea: Synaptidae) in a multi-species seagrass meadow in the southern Red Sea of Saudi Arabia

The Red Sea is a harsh environment characterized by high salinity and temperature, and how benthic organisms respond to the environment therein is still relatively unexplored. Here, we looked at the intertidal population of the snake sea cucumber, Synapta maculata, found in a species-rich seagrass meadow on the southern coast of the Red Sea in Saudi Arabia. The objectives of the study were (1) to examine the changes in S. maculata abundance from spring to fall in a multi-species seagrass meadow (Halodule pinifolia, Halodule univervis, Halophila ovalis, and Halophila stipulacea) and (2) to determine the relationship between the abundance of S. maculata with the selected abiotic environmental variables (depth, salinity, temperature, pH, dissolved oxygen, wind speed, wind direction, air temperature, relative humidity, and barometric pressure). The abundance of S. maculata was assessed using three permanent 50 × 4 m belt transects. Results showed that the abundance of S. maculata was present from March to June (approximately 4 individuals per 200 m2). We observed a drastic decline in the following months that coincided with the die-off of the seagrass meadow. The abundance of S. maculata differed significantly among sampling months. Correlation analysis revealed a significant positive relationship between the abundance of S. maculata and dissolved oxygen, wind speed, and barometric pressure. At the same time, salinity, water temperature, and air temperature showed a significant negative relationship. The generalized linear model suggested salinity, pH, and dissolved oxygen were the main environmental factors that influence the population of S. maculata. Overall, the population dynamics of S. maculata in this area was driven by the combination of extreme abiotic environmental factors and the presence of seagrass meadows.

The Effects of Non-Indigenous Macrophytes on Native Biodiversity: Case Studies from Sicily

Biological invasions are widely recognized as a major threat to native biodiversity, ecosystem functioning and services. Non-indigenous species (NIS) may in time become invasive (invasive alien species (IAS)), determining significant environmental, socioeconomic and human health impacts such as biodiversity loss and ecosystem service degradation. The Mediterranean islands, particularly Sicily and the circum-Sicilian islands (northwestern Mediterranean Sea), which are important hotspots of biodiversity, are notably vulnerable to anthropogenic pressures such as biological invasions. Therefore, monitoring NIS distribution as well as understanding their effects on native biodiversity is critical in these areas for planning effective conservation strategies. Here, we report four different case studies from Sicily that highlight how NIS may affect native biodiversity and habitats. The first three case studies were carried out within Marine Protected Areas (MPAs) and highlight (1) the ability of Caulerpa cylindracea to promote the establishment of other NIS, including biofouling worms belonging to the genus Branchiomma; (2) how the shift in habitat from the native Ericaria brachycarpa to the invasive Asparagopsis taxiformis may drastically erode the primary producer biomass and associated biodiversity; and (2) that the presence of Lophocladia lallemandii can affect the molluscan assemblage inhabiting the canopy-forming Gongolaria montagnei. The fourth case study, performed along the northwestern coast of Sicily, shows how Halophila stipulacea can affect the growth of the co-occurring native seagrass Cymodocea nodosa. Overall, these case studies demonstrate various ways in which NIS can interact with native biodiversity and habitats. Furthermore, they emphasize that MPAs are ineffective at preventing the introduction and spread of NIS.

Effects of anthropogenic pressures on the seagrass Halophila stipulacea and its associated macrozoobenthic communities in the northern Gulf of Aqaba

Halophila stipulacea is a tropical seagrass species, native to the Red Sea, Persian Gulf, and Indian Ocean, while invasive to the Mediterranean and Caribbean Seas. The benthic fauna assemblages associated with H. stipulacea in its native habitats and the potential effects of anthropogenic stressors on these assemblages remain unknown. We compared meadow characteristics, associated fauna assemblages and trophic niche structures of H. stipulacea from an impacted and a pristine site in the northern Red Sea. Seagrass cover and biomass were higher in the impacted site, however, the associated fauna community was more abundant and diverse in the pristine site. Both meadows showed comparable trophic niches based on stable isotope analysis. This study provides first insights into the macrozoobenthos associated with H. stipulacea in its native habitat and highlights the importance of better understanding the relationship between seagrasses and their associated biota and the potential effects of urbanization on this relationship.

Searching for the competitive ability of the alien seagrass Halophila stipulacea with the autochthonous species Cymodocea nodosa

The tropical seagrass Halophila stipulacea (Forsskål) Ascherson, 1867 entered in the Mediterranean Sea through the Suez Canal more than 100 years ago. In coastal-marine ecosystems the spatial niche of H. stipulacea is often overlapped with that of native Mediterranean Sea seagrasses and therefore it might out-compete them. Aiming to better understand its invasiveness potential, we monitored a Southern Mediterranean shallow coastal-marine water habitat from August 2010 to August 2011, where H. stipulacea co-occurred with the native seagrass Cymodocea nodosa (Ucria) Ascherson, 1870. Besides, the year-round dynamics of H. stipulacea was also monitored in four periods. To test the hypothesis that the presence/absence of H. stipulacea may have an effect on C. nodosa density, we analyzed the shoot density of C. nodosa in 8 sites, 4 sites where H. stipulacea was present (impacted sites) and 4 where H. stipulacea was absent (control sites). The results showed significant differences in C. nodosa shoot density according to the presence/absence of H. stipulacea, with the lowest values observed in sites where it co-occurred with H. stipulacea. We hypothesize that the dense rhizome-sediment net created by H. stipulacea can interfere with C. nodosa density, pushing down its rhizomes in the anoxic layer. The leaf features of H. stipulacea were generally comparable to those of other Mediterranean populations. In January 2011 a significant decline of H. stipulacea was observed, maybe related to changes in the environmental conditions that have become unfavorable (e.g. hydrodynamics, turbidity) and, unexpectedly, the seagrass disappeared in April 2011. In January, we also observed the occurrence of the green alien alga Caulerpa cylindracea Sonder, 1945 which rapidly invaded the bare substrate left by H. stipulacea.

Superior growth traits of invaded (Caribbean) versus native (Red sea) populations of the seagrass Halophila stipulacea.

The online version contains supplementary material available at 10.1007/s10530-023-03045-z.