Intertidal Populations Research Articles

Biological traits of a commercial mussel species Mytilus platensis from the South Atlantic Ocean: Environmental drivers affecting on its intertidal population

Population density, morphometric relationships, maximum age and shell growth rate were studied in an intertidal commercial mytilid species, the blue mussel Mytilus platensis d’Orbigny, 1842, at Villa Gesell (37° 15′S; 56° 57′W), Argentina, Southwest Atlantic Ocean. The population density and mean shell length showed variation across the year, i.e., the month with the highest density (December) was mostly characterized by smaller sized individuals, representing the newly settled recruits during the austral summer months. Shell cross sections and acetate peels of M. platensis revealed an internal shell growth pattern alternating broad opaque and narrow translucent bands, corresponding to fast and slow-growing periods, respectively. Translucent bands representing external rings were formed mostly during July, coinciding with a decrease in sea water temperature (austral winter) and gonadal maturation processes. Data confirmed the annual formation of translucent bands in this species, showing a maximum age of 8 years. The generalized von Bertalanffy growth curve showed values of SL∞ = 38.46 mm, k = 0.28 yr−1 and t0 = −0.57 yr−1 (R2 = 0.80). Compared to studies of subtidal populations, lower shell growth rate in this intertidal population were observed, which may be associated with the stressful environmental conditions in this habitat, such as desiccation, exposure to waves and fluctuating sea water temperature. The characteristics described throughout the study highlight that the variability associated with intertidal environments could limit the sustainability of commercial exploitation of M. platensis in these ecosystems compared with conspecific subtidal populations.

Habitat characteristics shaping zoantharians’ distribution at intertidal habitats of the Canary Islands

Ocean warming is facilitating the proliferation of zoantharians worldwide due to their thermophilic affinities. However, other anthropogenic factors, such as declining water quality, have been appointed to favour the occurrence of zoantharians dominated areas and barrens. Subtropical and tropical intertidal habitats are especially vulnerable to zoantharians outbreaks due to the progressive increase in ocean temperature and because it is where most human pressure is concentrated. The aim of this study was to evaluate the abundance of intertidal zoantharians populations, related to habitat characteristics (rugosity and submerged substrate) and the presence of other benthic organisms, such as sessile invertebrates and macroalgae. Number and coverage of zoantharian colonies were recorded in transects (20 × 4 m) along 25 intertidal rocky platforms of the Canarian Archipelago, and photographic techniques in quadrats were used to examine the relationships between the percentage of substrate cover of zoantharians, macroalgae and sessile invertebrates by means of correlations analyses. Results indicated that high levels of rugosity favour the abundance and coverage of Palythoa aff. clavata and Zoanthus pulchellus, and the proportion of submerged substrate was also important in determining Zoanthus coverages. The presence of extensive macroalgae communities was seen to reduce zoantharian coverages, being the only sessile organisms that were able to outcompete them for space in the Canary Islands. Thus, this study provided valuable information about patterns of abundances and coverages of intertidal zoantharians useful for future monitoring of intertidal populations of zoantharians in Canary Islands and their interaction with surrounding marine organisms.

Environmental factors have stronger effects than biotic processes in patterns of intertidal populations along the southeast coast of Brazil

Rocky shore communities are shaped by complex interactions among environmental drivers and a range of biological processes. Here, we investigated the importance of abiotic and biotic drivers on the population structure of key rocky intertidal species at 62 sites, spanning ∼50% of the Brazilian rocky shoreline (i.e., ∼500 km). Large-scale population patterns were generally explained by differences in ocean temperature and wave exposure. For the gastropod species Lottia subrugosa, differences at smaller scales (i.e., 0.1–1 km) were better explained by other abiotic influences such as freshwater discharge and substrate roughness. Based on the general population patterns of intertidal species identified, three main oceanographic groups were observed: a cold-oligotrophic grouping at northern sites (Lakes sub-region), a eutrophic group associated with large estuaries and urban zones (Santos and Guanabara bays); and a transitional warm-water group found between the two more productive areas. Larger individuals of Stramonita brasiliensis, L. subrugosa and Echinolittorina lineolata were generally found in the cold-oligotrophic system (i.e., upwelling region), while small suspension feeders dominate the warm-eutrophic systems. Evidence of bottom-up regulation was not observed, and top-down regulation effects were only observed between the whelk S. brasiliensis and its mussel prey Pernaperna. Environmental drivers as compared to biotic interactions, therefore, play a key role determining the population structure of multiple intertidal species, across a range of spatial scales along the SW Atlantic shores.

Adaptation of functional traits in Gracilaria dura with the local environment: implications for resource management and exploitation

Seaweed functional traits provide insights on natural populations, their adaptations to the local environment, which can be utilized for commercial exploitation. Here, we analyzed the functional traits of Gracilaria dura from two intertidal populations in Veraval and Adri, from the coast of Gujarat, India, over a period of three months. Functional traits were measured by analyzing growth rates, respiration, primary production, antioxidant activity and the content of plant growth hormones. The weight-to-length allometric exponent ≈3 indicated that G. dura grew almost isometrically. Furthermore, frond shape was not significantly different. Fronds in Veraval, resting submerged, grew faster than the fronds in Adri, which, lay exposed in the flat bare rock during low tide. Accordingly, the simultaneous increase in antioxidant activity, O2 production and chlorophyll content in fronds from Adri suggests that stress from desiccation and UV led to the detrimental accumulation of Reactive Oxygen Species, leading to decreased growth and decreased production of growth hormones. The increased Chl-a may indicate enhanced non-photochemical quenching (NPQ) for the dissipation of excess absorbed light. These results aid in establishing the best practices for maximizing biomass yield or the yield of specific molecules. For maximized biomass yield, fronds should not be subject to emersion nor cultivated on the sea-surface. On the other hand, fronds grown subject to emersion or at the sea-surface yield less biomass but more content on molecules such as antioxidants (flavonoids, phenolics, enzymes), that fight stress from desiccation high temperatures and UV. In this case, stress should still be avoided during the initial growth, at the onset of the growth season, in order to not disrupt the production of growth hormones. The increased O2 production at Adri was initially mistakenly perceived as enhanced Net Primary Production. Only a posterior holistic perspective over the whole data allowed to conclude that it was likely the stress-induced detrimental accumulation of Reactive Oxygen Species. More robust experiments are required to establish if the differences observed between locations have led to the evolution of genetic strains specific to each habitat that may show different performances and yields when cultivated in similar environments.

Saccharina sporophytes settling on kelp cultivation rafts come from wild intertidal populations and not from the farmed cultivars

Abstract Wild-type sporophytes of Saccharina japonica have been recurrently observed on the infrastructures of kelp farming rafts in China’s northern production region (Liaoning and Shandong provinces). However, there are few detailed reports delineating their origin, genetic attributes and affiliations with farmed populations. Using microsatellite molecular analyses, we furnish a comparative analysis of the populations sampled from these rafts at one of the historical kelp farming regions Rongcheng city (Shandong province), both before and after the “2021–2022 kelp farming disaster”, in conjunction with two field populations from nearby intertidal zones where natural populations prevail. The results show that the wild kelp populations on the kelp farming rafts in 2023 had originated from the wild populations dwelling in intertidal zones several kilometers away, rather than from the artificially cultivated kelp cultivars.

Size at first maturity on the mussel Mytilus platensis d'Orbigny, 1842 at the Southwest Atlantic

The size at first maturity of the bivalve Mytilus platensis d'Orbigny, 1842 was estimated in the intertidal population of Villa Gesell (37°15′S; 56°57′W), Buenos Aires, Argentina. We hypothesize a variation in population maturity associated with shell length (SL) and differences between females and males. Individuals were analyzed histologically, sexed (females and males), and classified as mature and immature. We carried out a kernel density estimation to allow us to visualize the distribution of the classifications based on the SL and used a generalized linear model to study the size at first sexual maturity for the population and between sexes. Our results showed immature specimens with reduced development in acini and expanded connective tissue. Mature males and females showed the acini expanded and filled with vitellogenic oocytes and spermatozoa, respectively. The size at first sexual maturity at which 50% of the population was mature (SL50) was 11.80 mm SL for all population, while SL50 was 11.42 mm in females and 10.83 mm in males. We determined significant interaction between SL and sex ( P < 0.05). We conclude that the knowledge of the reproductive aspects in intertidal populations is essential for the development of a potential fishing resource.

Restoring seagrass meadows in Basque estuaries: nature-based solution for successful management

Seagrass meadows provide a wide variety of ecosystem goods and services (biodiversity maintenance, carbon sequestration, sediment oxygenation and enrichment, filtering, coastal protection, and nursery area for other species). For this reason, they are areas of high economic development while historically highly impacted by human activity. Therefore, there is a growing need for the restoration and conservation of these habitat engineers as a nature-based solution for climate change mitigation and biodiversity conservation. The Basque Country region (northern Spain) has experienced a decline in seagrass meadows driven by increases in nutrient concentrations and decreases in water clarity. In this region, only 3 out of its 12 estuaries currently host natural intertidal populations of the protected species Zostera noltei. However, there is written evidence of the previous presence of this species in other estuaries within the region. Despite significant improvement in water quality has been observed in most Basque estuaries, these plants show a low capacity for a successful natural recolonization, requiring human restoration planning and implementation. To ensure a successful seagrass restoration a plan is needed which encompasses four phases: feasibility and pre-project planning, project design, pre-restoration tasks, and restoration/monitoring. This work focuses on the first and second phases, which include the selection of the most suitable reception sites and restoration trials. Therefore, a restoration and monitoring plan is proposed for a Basque estuary which includes further involvement of local stakeholders (users, managers) to increase the probability of success.

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.

Status of intertidal populations of Pismo clams (Tivela stultorum) across their range in California

Long-term monitoring programs are critical to understanding a species’ population dynamics through time and to inform policy decisions. Pismo clams, Tivela stultorum (Mawe, 1823), are an iconic species in California and once supported commercial, recreational, and subsistence harvest for many decades. Despite their economic and cultural importance, little is known about their current population status and monitoring efforts in California are spatially and temporally limited. This project aimed to: (1) collect standardized baseline data on Pismo clam abundance across their range in California; (2) identify factors associated with Pismo clam presence to inform future monitoring efforts and to begin developing testable hypotheses about population drivers; and (3) synthesize historical surveys of Pismo clam populations in California. We conducted intertidal field surveys for Pismo clams at 38 sites in California during 2018 and 2019. We observed no legal sized clams (4.5 in/114 mm) and only 2% of individuals were half of legal size across all survey locations. Clam populations are in lower abundance and comprise smaller sizes than historical accounts. We detected small individuals at most sites, suggesting that successful recruitment has occurred on the majority of beaches where clams were present; however, population recovery may require many years. Finally, we found that median sediment grain size is an important predictor for clam presence on open coast beaches in California. Future research will need to examine a number of factors and their interactions across spatial scales to disentangle Pismo clam population drivers.

Intertidal populations of Ulva spp. and Undaria pinnatifida are good habitat providers for invertebrates but not for fish

Intertidal populations of Ulva spp. and Undaria pinnatifida are good habitat providers for invertebrates but not for fish

Population Dynamics of Intertidal Resident Species during a Reduction in Asian Shore Crab Population

Hemigrapsus sanguineus (Asian shore crab), which is native to coastal estuarine habitats along the east coast of Asia, have overtaken a wide range of the coast in the northeastern region of the United States. Intertidal population densities of Hemigrapsus sanguineus, Cancer irroratus (Atlantic rock crab), Littorina littorea (Common periwinkle), and Modiolus demissus (Atlantic Ribbed Mussel), among others, were measured at three sample sites from 8/2/22 to 11/14/22 using quadrat sampling techniques. A period of removal was enacted solely upon Sample site #1 from 9/28/22 to 11/2/22 to cause a reduction in the Asian shore crab population. During the period of removal, no statistically significant changes in Asian shore crab demographics (Carapace size/sex ratios) would be observed (p = 0.181 for carapace size changes). There would however be an 8% decrease in Asian shore crab population density within Sample site #1, comparative to a 1% increase at Sample site 2. Rock crab population density at Sample site #1 would increase by a relatively drastic 31%, whereas the Rock crab population would experience only a 7% increase at Sample site #2. The Shannon-Weiner diversity index was used to derive the effective number of species (ENS). There would be a drastic difference in change of ENS following the removal period at Sample site #1 (increase of 0.248 ENS), and the change of ENS at Sample site #2 (decrease of 0.024 ENS), suggesting the reduction in Asian Shore Crab population played a role in increased population diversity of intertidal resident species.

Cryptic species complex or an incomplete speciation? Phylogeographic analysis reveals an intricate Pleistocene history of Priapulus caudatus Lamarck, 1816

We detected five divergent COI lineages in Priapulus caudatus Lamarck, 1816: four from the Arctic and one from the Sea of Okhotsk. K2P distances ranged from 6.87% to 14.35% with a mean distance of 10.03%, which typically corresponds to inter-species distances. However, four of five lineages (excluding the Okhotsk one) were found in syntopy, and no morphological differences between potential species were found. Single-gene phylogenies for the mitochondrial 16S, as well as nuclear 28S, 18S, and FoxQ2 markers, had different topologies and were non-congruent with the COI tree. This may point to an incomplete speciation due to a series of long-time geographical isolations during Pleistocene glaciations in different parts of range of P. caudatus. The 28S and 18S sequences fell into two genotypes, one being found mainly in the intertidal, and the other exclusively in the subtidal, which indicates the prevalence of unidirectional migrations from the intertidal into the subtidal. Because Arctic intertidal populations disappear during glaciations and recover in interglacial periods, and due to their partial reproductive isolation from each other, they give rise to new genetic lineages. Subtidal populations are permanent and have less significant isolation boundaries, therefore they accumulate genetic diversity.

Once-iconic Pismo clams persist in southern California at low intertidal population densities and with variable recruitment

The Pismo clam (Tivela stultorum) has experienced substantial population decline in California over the past century, extinguishing most public participation in a once-iconic recreational fishery before the end of the 20th century. A subsequent decrease in data collection has led to uncertainty about the current population status of this species. We conducted 6 years of intertidal Pismo clam population assessment surveys in Orange, San Diego, and southern Los Angeles Counties to provide a current dataset that could help guide research and management efforts in southern California. Pismo clams were observed at 19 out of 27 study sites during 57 days of surveys. Average clam bed density was low (mean 2.0 ± 1.1 clams/m2, median 0.1 ± 0.7 clams/m2, n = 21 sites), especially when considering larger clams ≥ 35 mm (mean 0.3 ± 0.1 clams/m2, median 0.1 ± 0.4 clams/m2), and varied greatly between sites (0.0–98.5 clams/m2), with Orange County densities approximately one order of magnitude lower than those in San Diego County. Juvenile recruitment was generally low or undetectable, except for consistent recruitment within a < 10 km beach area in San Diego County and a much larger, widespread recruitment event in 2022. Multi-year observations at several sites failed to indicate any consistent seasonal or inter-annual population trends. Densities and abundances were similar to recent historic data (< 30 years old), but are substantially lower than populations prior to the 1980s. We conclude that the Pismo clam persists on many southern California beaches at generally low densities and that recruitment is occurring throughout the southern California region with high spatial and temporal variability. This study provides foundational data to help inform Pismo clam conservation management decisions and to which additional monitoring, ecological research, and fishery data collection should be added.

Hierarchical genetic structure in a direct-developing whelk (Buccinum undatum) throughout the western North Atlantic

Direct-developing marine species lack a pelagic larval phase and are therefore predicted to have limited dispersal capacity and exhibit strong genetic structure over small spatial scales. Nevertheless, empirical work suggests that developmental mode, alone, cannot always explain realized connectivity patterns. We characterized spatial genetic structure within the western North Atlantic lineage of the waved whelk ( Buccinum undatum) , a direct-developing gastropod that is the target of an emerging fishery. We genotyped individuals throughout Atlantic Canada using 1052 single nucleotide polymorphisms, documenting strong hierarchical genetic structuring throughout this region. Overall, pairwise genetic structure estimates were high, with two major genetic clusters separating southwestern and northeastern sites. But more complex patterns emerged at smaller spatial scales. Notably, pairs of sites on deeper offshore habitat exhibited panmixia, despite being separated by up to 100 km, whereas inshore and intertidal populations spaced at comparable distances were strongly genetically divergent. Collectively, our multi-scale analyses revealed that direct developers can exhibit genetic patterns that do not conform to simple geographic expectations. Future exploration of potential depth-variable connectivity is warranted, especially among harvested populations.

Larval dispersal from an energetic tidal channel and implications for blue mussel (Mytilus edulis) shellfisheries

Complex coastal currents control early-stage larval dispersal from intertidal populations, and late-stage settlement patterns, but are often poorly resolved in larval dispersal models. Generally, there is high uncertainty in the timing of larval spawning, which markedly affects larval dispersal. In this study, we describe the physical parameters that induce spawning events in the blue mussel, Mytilus edulis, using a variation of the Condition Index (which relates the mass of meat to the mass of the shell) as a proxy. We developed a high-resolution Eulerian coastal hydrodynamic model, coupled with a Lagrangian particle tracking model, to quantify the potential dispersal of early-stage mussel larvae based on differing spawning dates obtained from field data. Our results showed that (1) the timings of larval spawning cannot be explained solely by ‘thermal shocks’ in the sea or air temperatures (i.e. fluctuations in temperature causing stress); (2) larger spawning events generally occurred during neap tides; (3) the simulated larval dispersal was largely but not always predicted by averaged current pathways (calculated over two weeks period); and (4) simulated self-recruitment was low at sites associated with strong tidal currents. These results have important implications for shellfisheries stock management and sustainability. Specific to this study, simulated mussels from shellfishery beds off North Wales dispersed more than 25 km in one week and so could feasibly contribute to the wider population throughout the northern part of the Irish Sea.

Gametogenic cycle of the marine gastropod Trophon geversianus in a harsh Patagonian intertidal environment

ABSTRACT The edible marine gastropod Trophon geversianus shows a large geographic distribution along the southern tip of South America, inhabiting both rocky intertidal and shallow subtidal zones, with distinct behavioural responses to environmental stressful conditions (desiccation, temperature amplitude and high irradiation). Nevertheless, its reproductive biology has been poorly reported to date. We studied the gametogenic cycle of an intertidal rocky shore population from northern Patagonia (Argentina), in order to investigate the relation between gonadal changes and seasonal environmental variables. Mature individuals were taken monthly during an annual reproductive season and processed by standard histology procedures (haematoxylin and eosin). Adults of T. geversianus registered different gonadal maturation stages along the year, showing a marked seasonal variation without a resting period. The highest gamete evacuation season in both sexes was registered when the environmental stress was minimum (winter), while a greater number of individuals with gonads in proliferation and growth were predominant during maximum environmental stress (summer). The reproduction pattern presented an extended period of evacuation and a short phase of non-evacuation, associated to drastic seasonal environmental changes in the intertidal zone of Atlantic Patagonia. This information is useful to protect the resource and assure the sustainability of intertidal populations.

An Individual-Based Model of the Red Alga Agarophyton chilense Unravels the Complex Demography of Its Intertidal Stands

Algal demographic models have been developed mainly to study their life cycle evolution or optimize their commercial exploitation. Most commonly, structured-aggregated population models simulate the main life cycle stages considering their fertility, growth and survival. Their coarse resolution results in weak predictive abilities since neglected details may still impact the whole. In our case, we need a model of Agarophyton chilense natural intertidal populations that unravels the complex demography of isomorphic biphasic life cycles and be further used for: (i) introduction of genetics, aimed at studying the evolutionary stability of life cycles, (ii) optimizing commercial exploitation, and (iii) adaptation for other species. Long-term monitoring yield 6,066 individual observations and 40 population observations. For a holistic perspective, we developed an Individual-Based Model (IBM) considering ploidy stage, sex stage, holdfast age and survival, frond size, growth and breakage, fecundity, spore survival, stand biomass, location and season. The IBM was calibrated and validated comparing observed and estimated sizes and abundances of gametophyte males, gametophyte females and tetrasporophytes, stand biomass, haploid:dipoid ratio (known as H:D or G:T), fecundity and recruitment. The IBM replicated well the respective individual and population properties, and processes such as winter competition for light, self-thinning, summer stress from desiccation, frond breakage and re-growth, and different niche occupation by haploids and diploids. Its success depended on simulating with precision details such as the holdfasts’ dynamics. Because “details” often occur for a reduced number of individuals, inferring about them required going beyond statistically significant evidences and integrating these with parameter calibration aimed at maximized model fit. On average, the population was haploid-dominated (H:D > 1). In locations stressed by desiccation, the population was slightly biased toward the diploids and younger individuals due to the superior germination and survival of the diploid sporelings. In permanently submerged rock pools the population was biased toward the haploids and older individuals due to the superior growth and survival of the haploid adults. The IBM application demonstrated that conditional differentiation among ploidy stages was responsible for their differential niche occupation, which, in its turn, has been argued as the driver of the evolutionary stability of isomorphic biphasic life cycles.

Ultrastructure and functional morphology of the dorsal organs in Scoloplos armiger (Annelida, Sedentaria, Orbiniida)

Annelids and particularly polychaetes possess a great variety of sensory organs and respond to numerous sensory stimuli. Although eyes and nuchal organs are comparatively well studied, the so-called dorsal organs are among the lesser-known sense organs in aquatic annelids. Moreover, they are known to be restricted to only two out of approximately 80 families of polychaetes—Orbiniidae and Spionidae—which are not closely related. These organs have been regarded as segmentally repeated nuchal organs in the latter taxon, but in Orbiniidae, data are lacking, although it is known that the organs occur almost along the entire trunk except for the anterior-most segments. Furthermore, although the nuchal organ ultrastructure is known to be comparatively uniform for many polychaete species, a comparative investigation has not been conducted in Orbiniidae. To bridge this data gap, we examined an intertidal population of the widely distributed species Scoloplos armiger. Although not completely identical, nuchal and dorsal organs show a high degree of correspondence in the examined specimens. Moreover, both organs correspond to the general structure of nuchal organs. They comprise ciliated supportive cells and bipolar receptor cells and are innervated directly from the brain. The supportive cells form subcuticular spaces and olfactory chambers apically protected by specialized microvilli that house the sensory processes—cilia and microvilli—of the monociliated receptor cells. Therefore, it can be concluded that nuchal and dorsal organs are also identical in Orbiniidae. However, despite general correspondence with spionids, convergent evolution in the two taxa appears to be the most parsimonious interpretation.

Evidence for self-sustaining populations of Arcuatula senhousia in the UK and a review of this species\u2019 potential impacts within Europe

The invasive Asian date mussel (Arcuatula senhousia) inhabits diverse global coastal environments, in some circumstances posing significant ecological and economic risks. Recently recorded in the Greater North Sea ecoregion, an established population has not previously been confirmed. Combining historical and field data, we provided baseline information from the UK and recorded colonisation in a variety of habitats. Gonadal development was assessed using the gonadosomatic index (GSI) to determine if an intertidal soft-sediment population is self-sustaining. Arcuatula senhousia records from subtidal muddy/mixed-sediment within a major estuarine system from 2007 to 2016 were also analysed. First detected in 2011, spatial distribution was variable across the years within the subtidal, with individuals found at 4–9 out of 25 sites, and densities per site varying from 10 to 290 individuals per m2. The intertidal population was, in part, associated with seagrass (Zostera spp.) and attached to bivalves. In marinas, individuals were attached to concrete tiles, associated with live Mytilus edulis, and to dead Ostrea edulis. Mean GSI from the intertidal population differed across months, peaking in July before declining in September/October, but with high inter-individual variability. Arcuatula senhousia is reproducing and maintaining viable populations. Using a natural capital approach, we identify the potential impacts on Europe’s functionally important habitats, fisheries and aquaculture if its spread continues.

Lack of match between nutrient-enriched marine seafoam and intertidal abundance of long-lived invertebrate larvae

Since most marine benthic species experience a pelagic larval phase, scales of dispersal are key determinants of population dynamics. Biologically derived marine foam has been suggested to increase chances of fertilisation and reduce dispersal of larvae of short-live duration (hours), thus maintaining localised intertidal populations. The present study examined the role sea-foam plays as a mechanism of physical retention/accumulation for long lived (weeks) invertebrate larvae that are relatively long-lived (weeks). Larvae were collected using a submersible pump at two sites along the south-east coast of South Africa, where intertidal assemblages are dominated by beds of mussels and barnacles. Sampling took place on six occasions in 2015–2016, during events of high sea-foam production and periods of no foam accumulation. Foam/water was collected from the surface and bottom of tidal channels. There was no difference in abundance of larvae of any of the invertebrate taxa examined, whether foam was present or absent. Regardless of foam state, barnacle and polychaete larvae were mostly associated with the surface of the short water column at the tidal channels. This study highlights how the very nearshore environment may play a key role in limiting scales of larval dispersal, but especially how the effects of physical processes can be taxon-specific, depending on the larval duration and characteristics.