Impact of sewage pollution on two species of sea urchins in the Mediterranean Sea (Cortiou,France): Radial asymmetry as a bioindicator of stress

BackgroundThe surroundings of the Cortiou sewage are among the most polluted environments of the French Mediterranean Sea (Marseilles, France). So far, no studies have precisely quantified the impact of pollution on the development of organisms in this area.MethodsWe used a fluctuating asymmetry (FA) measure of developmental instability (DI) to assess environmental stress in two species of radially symmetric sea urchins (Arbacia lixula and Paracentrotus lividus). For six sampling sites (Cortiou, Riou, Maire, East Maire, Mejean, and Niolon), levels of FA were calculated from continuous and discrete skeletal measures of ambulacral length, number of pore pairs and primary tubercles.ResultsFor both species, the most polluted sampling site, Cortiou, displayed the highest level of FA, while the Maire and East Maire sampling sites displayed the lowest levels. A. lixula revealed systematic differences in FA among sampling sites for all characters and P. lividus showed differences in FA for the number of primary tubercles.ConclusionsStatistical analyses of FA show a concordance between the spatial patterns of FA among sampling sites and the spatial distribution of sewage discharge pollutants in the Cortiou area. High developmental stress in these sampling sites is associated with exposure to high concentrations of heavy metals and many harmful organic substances contained in wastewater. FA estimated from structures with complex symmetry appears to be a fast and reliable tool to detect subtle differences in FA. Its use in biomonitoring programs for inferring anthropogenic and natural environmental stress is suggested.     

Seagrass Proliferation Precedes Mortality during Hypo-Salinity Events: A Stress-Induced Morphometric Response

Halophytes, such as seagrasses, predominantly form habitats in coastal and estuarine areas. These habitats can be seasonally exposed to hypo-salinity events during watershed runoff exposing them to dramatic salinity shifts and osmotic shock. The manifestation of this osmotic shock on seagrass morphology and phenology was tested in three Indo-Pacific seagrass species, Halophila ovalis, Halodule uninervis and Zostera muelleri, to hypo-salinity ranging from 3 to 36 PSU at 3 PSU increments for 10 weeks. All three species had broad salinity tolerance but demonstrated a moderate hypo-salinity stress response – analogous to a stress induced morphometric response (SIMR). Shoot proliferation occurred at salinities <30 PSU, with the largest increases, up to 400% increase in shoot density, occurring at the sub-lethal salinities <15 PSU, with the specific salinity associated with peak shoot density being variable among species. Resources were not diverted away from leaf growth or shoot development to support the new shoot production. However, at sub-lethal salinities where shoots proliferated, flowering was severely reduced for H. ovalis, the only species to flower during this experiment, demonstrating a diversion of resources away from sexual reproduction to support the investment in new shoots. This SIMR response preceded mortality, which occurred at 3 PSU for H. ovalis and 6 PSU for H. uninervis, while complete mortality was not reached for Z. muelleri. This is the first study to identify a SIMR in seagrasses, being detectable due to the fine resolution of salinity treatments tested. The detection of SIMR demonstrates the need for caution in interpreting in-situ changes in shoot density as shoot proliferation could be interpreted as a healthy or positive plant response to environmental conditions, when in fact it could signal pre-mortality stress.     

Posidonia australis seed predation in seagrass habitats of Two Peoples Bay,Western Australia

Posidonia australis seed predation experiments conducted in three seagrass habitats (P. australis, Posidonia sinuosa, Halophila ovalis) and bare sand in Two Peoples Bay, Western Australia, showed higher predation rates in seagrass than bare sand, supporting general conclusions from two previous predation studies in Western Australia. However, much higher rates were noted in H. ovalis, compared to previous observations of very low rates in H. ovalis on Rottnest Island, Western Australia. We attribute these differences to gammaridean amphipods (family Lysianassidae) that were present in a detrital layer within the H. ovalis in Two Peoples Bay. Our data from Two Peoples Bay continues to add to the growing body of information showing high seed predation rates in most seagrass habitats by a diverse group of crustacean species.     

Development of a ‘sediment-stress’ functional-level indicator for the seagrass Halophila ovalis

Understanding mechanistic relationships between seagrass and their environmental stressors should be considered for effective management of estuaries and may inform on why change has occurred. We aimed to develop indicators for seagrass health in response to sediment conditions for the Swan-Canning Estuary, south-west Australia. This article describes the development of a new sediment-stress indicator, relating aspects of seagrass productivity with sediment sulfur dynamics. Sulfur stable isotope ratio and total sulfur were measured monthly within the roots, rhizomes and leaves of Halophila ovalis, and significantly varied across sites and months. The growth of seagrass over the summer months appeared restricted by sediment condition, with growth of seagrass lower when sediment derived sulfur and/or total sulfur within rhizome of leaf tissues was higher. H. ovalis appeared quite tolerant of sulfide intrusion within the root compartment, but growth was compromised when sulfide breached the root–rhizome barrier. The tightest correlation between potential sulfur metrics and seagrass growth was observed for the ratio (δ³⁴S_leaf + 30)/(TS_leaf), and it is this ratio that we propose may be a useful sediment-stress indicator for seagrass. The study also highlights that sediment condition needs to be considered at the meadow scale.     

Confirmation bias in studies of fluctuating asymmetry

Fluctuating asymmetry (FA) represents small, non-directional deviations from perfect symmetry in morphological characters. FA is generally accepted to increase in response to stress; therefore, FA is frequently used in ecological studies as an index of stress experienced by an organism, in particular due to environmental pollution. We experimentally tested the hypothesis that the outcomes of studies based on FA measurements may have been influenced by confirmation bias, i.e. the tendency of humans to seek out evidence in a manner that confirms their hypotheses and beliefs. We collected 100 leaves of downy birch (Betula pubescens) from a single tree, grouped them haphazardly into ten samples, scanned every sample, and then asked each of 31 scientists (experienced in studying FA) to measure FA from the scanned images of all 100 leaves. Three groups of participants were provided with false information about the origin of each sample and about the hypothesis to be tested, and one group (control) was provided with true information. The participants who believed that the leaves originated from a heavily polluted site reported significantly higher values of FA when compared to the participants who believed that the leaves were collected from an unpolluted site. When the participants were told that half the samples originated from a polluted site and half from a clean site and were asked to attribute each sample to either of these sites based on leaf FA, the differences in FA between samples classified as ‘polluted’ and ‘unpolluted’ were much higher than the differences obtained from measurements of the same sets of samples made by the control group. We conclude that when scientists expected to find high FA in some samples, the results of their measurements confirmed their expectations. This effect, classified as confirmation bias, may considerably influence the outcomes of the research on FA. This confirmation bias can be avoided by using a blind method, where the person conducting measurements is not aware of the origin of samples being measured. We argue that the use of blind methods is critically important for any study addressing environmental or genetic impacts on FA.     

Leaf fluctuating asymmetry of common plantain as an indicator of habitat quality

Abstract

Although developmental instability (DI), measured as fluctuating asymmetry (FA), is expected to be positively related to environmental stress and negatively to habitat quality, the pattern found here was the reverse. Developmental instability of leaf traits (leaf width and vein distances within a leaf) was estimated (using two indices of FA: FA₄ and σ_i ²) and compared between three populations of Plantago major L. (Plantaginaceae) from northern Serbia. Two of the populations are from chronically polluted areas (Karaburma & Zemun), while Crni Lug is from an unpolluted, natural area. Results obtained using both FA indices were the same; higher asymmetry levels in the unpolluted area than in the polluted sites, were found for both traits. Between the two polluted sites, FA values were significantly higher in Karaburma site for vein distances within a leaf. Concerning differences in FA₄ values between samples, in two cases, results are similar to those found for σ_i ² values, for vein distances within leaf. These are the first quantitative data on P. major indicating that (i) plants living in the stressful sites are more symmetrical and (ii) leaf FA for plant species with wide ecological distribution such as P. major should be considered as an ‘index of habitat quality.’     

Fluorescence imaging application: effect of leaf age on seagrass photokinetics

We used the Imaging-PAM fluorometer to map spatial variability of photosynthesis in three seagrass species, Halophila ovalis, Zostera capricorni and Posidonia australis. Photosynthesis was described by relative photosynthetic rate (PS/50), effective quantum yield (Φ_PSII), non-photochemical quenching (NPQ and qN), electron transport rate (ETR) and leaf absorptivity. Photosynthetic patterns were linked to leaf age and light climate but patterns were not consistent across species. Longitudinal heterogeneity in photosynthesis was apparent along the leaves of all three species while lateral spatial heterogeneity was found only across Z. capricorni and H. ovalis leaves. Age of leaf tissue, determined by longitudinal location on the leaf, strongly influenced photosynthetic activity of Z. capricorni and P. australis. A comparison of H. ovalis leaves of differing maturity demonstrated the influence of leaf age on photosynthetic activity, yet a comparison of Z. capricorni leaves of differing maturity showed no leaf-age effects.Variations in stress-induced changes across a seagrass leaf can be used to identify areas or particular regions of the leaf, which are more susceptible to photodamage. Clear evidence of substantial within-leaf heterogeneity in photosynthetic activity (i.e., a two-fold variation in half saturation constant along a leaf of P. australis) has serious implications for use of small sections of leaf for photosynthetic incubations (such as O₂ or single-point chlorophyll a fluorescence measurements).     

Halophila ovalis in the Tropical Atlantic Ocean

A species of seagrass in the genus Halophila was found growing in a shallow lagoon on the west shore of Antigua in the Caribbean West Indies. Genetic analysis showed the plants were Halophila ovalis. In addition, the samples had no genetic deviation (using nrDNA sequences) from Halophila johnsonii, considered to be an endemic and endangered species in Florida, USA. Morphological analysis demonstrated the Antiguan Halophila to be well within the range of plant characteristics previously described in the literature for H. ovalis, except for leaf width and number of seeds per fruit, and again, not different from H. johnsonii and very closely related to H. ovalis from the Indo-Pacific. Ours is the first report of H. ovalis in the Tropical Atlantic bioregion.     

Does fluctuating asymmetry constitute a sensitive biomarker of nutritional stress in house sparrows (Passer domesticus)?

Small random deviations from left–right symmetry in bilateral traits, termed fluctuating asymmetry (FA), are theoretically predicted to increase with environmental stress and believed to constitute a potential biomarker in conservation. However, reported relationships between FA and stress are generally weak and variable among organisms, traits and stresses. Here we test if, and to what extent, FA increases with nutritional stress, estimated from independent feather growth measurements, in free-ranging house sparrows (Passer domesticus). Ptilochronological feather marks showed significant heterogeneity among study plots, indicating that house sparrow populations were exposed to variable levels of nutritional stress during development. However, individuals from more stressed populations did not show increased levels of fluctuating asymmetry in tarsus or rectrix length, nor was there evidence for significant between-trait concordance in FA at the individual or the population level. Lack of support for FA in tarsus and rectrix length as estimator of nutritional stress in house sparrows may indicate that developmental instability is insensitive to nutritional stress in this species, poorly reflected in patterns of fluctuating asymmetry due to ecological or statistical reasons, or highly context-specific. Such uncertainty continues to hamper the use of FA as a biomarker tool in conservation planning.     

Cutting out the middle clam: lucinid endosymbiotic bacteria are also associated with seagrass roots worldwide

Seagrasses and lucinid bivalves inhabit highly reduced sediments with elevated sulphide concentrations. Lucinids house symbiotic bacteria (Ca. Thiodiazotropha) capable of oxidising sediment sulphide, and their presence in sediments has been proposed to promote seagrass growth by decreasing otherwise phytotoxic sulphide levels. However, vast and productive seagrass meadows are present in ecosystems where lucinids do not occur. Hence, we hypothesised that seagrasses themselves host these sulphur-oxidising Ca. Thiodiazotropha that could aid their survival when lucinids are absent. We analysed newly generated and publicly available 16S rRNA gene sequences from seagrass roots and sediments across 14 seagrass species and 10 countries and found that persistent and colonising seagrasses across the world harbour sulphur-oxidising Ca. Thiodiazotropha, regardless of the presence of lucinids. We used fluorescence in situ hybridisation to visually confirm the presence of Ca. Thiodiazotropha on roots of Halophila ovalis, a colonising seagrass species with wide geographical, water depth range, and sedimentary sulphide concentrations. We provide the first evidence that Ca. Thiodiazotropha are commonly present on seagrass roots, providing another mechanism for seagrasses to alleviate sulphide stress globally.Subject terms: Microbial ecology, Plant ecology, Soil microbiology     

Nutrient dynamics in 3 morphological different tropical seagrasses and their sediments

Seasonality of nutrient dynamics in three morphologically different seagrass species and their sediments was examined for 1 year between November 2006 and November 2007 at four sites in the Andaman Sea, Thailand. The smaller species, Cymodocea serrulata and Halophila ovalis, showed major seasonal variation in shoot density, above- and belowground biomass, much more than expected from seasonal changes in water temperature and light conditions. All parameters showed minimum values in the dry season due to desiccation during neap tides. In contrast Enhalus acoroides showed less seasonal variation. Only limited seasonality was found in tissue N content of all species, whereas tissue P content responded to the low P concentration in the water column during the wet season. There were no differences in sediment conditions among species, and nutrient pools were generally low. Furthermore there were no significant spatial differences in seagrass and sediment nutrient dynamics, despite varying anthropogenic activity at the study sites, reflecting the oligotrophic conditions in this region.     

Improving accuracy and efficiency in seagrass detection using state-of-the-art AI techniques

Seagrasses provide a wide range of ecosystem services in coastal marine environments. Despite their ecological and economic importance, these species are declining because of human impact. This decline has driven the need for monitoring and mapping to estimate the overall health and dynamics of seagrasses in coastal environments, often based on underwater images. However, seagrass detection from underwater digital images is not a trivial task; it requires taxonomic expertise and is time-consuming and expensive. Recently automatic approaches based on deep learning have revolutionised object detection performance in many computer vision applications, and there has been interest in applying this to automated seagrass detection from imagery. Deep learning–based techniques reduce the need for hardcore feature extraction by domain experts which is required in machine learning-based techniques. This study presents a YOLOv5-based one-stage detector and an EfficientDetD7–based two-stage detector for detecting seagrass, in this case, Halophila ovalis, one of the most widely distributed seagrass species. The EfficientDet-D7–based seagrass detector achieves the highest mAP of 0.484 on the ECUHO-2 dataset and mAP of 0.354 on the ECUHO-1 dataset, which are about 7% and 5% better than the state-of-the-art Halophila ovalis detection performance on those datasets, respectively. The proposed YOLOv5-based detector achieves an average inference time of 0.077 s and 0.043 s respectively which are much lower than the state-of-the-art approach on the same datasets.     

A Miniature Bioassay for Testing the Acute Phytotoxicity of Photosystem II Herbicides on Seagrass

Photosystem II (PSII) herbicides have been detected in nearshore tropical waters such as those of the Great Barrier Reef and may add to the pressure posed by runoff containing sediments and nutrients to threatened seagrass habitats. There is a growing number of studies into the potential effects of herbicides on seagrass, generally using large experimental setups with potted plants. Here we describe the successful development of an acute 12-well plate phytotoxicity assay for the PSII herbicide Diuron using isolated Halophila ovalis leaves. Fluorescence images demonstrated Diuron affected the entire leaf surface evenly and responses were not influenced by isolating leaves from the plant. The optimum exposure duration was 24 h, by which time the inhibition of effective quantum yield of PSII (∆F/F_m’) was highest and no deterioration of photosystems was evident in control leaves. The inhibition of ∆F/F_m’ by Diuron in isolated H. ovalis leaves was identical to both potted and hydroponically grown plants (with leaves remaining attached to rhizomes), indicating similar reductions in photosynthetic activity in these acute well-plate assays. The sensitivity of the assay was not influenced by irradiance (range tested 40 to 400 μmol photons m^-2 s^-1). High irradiance, however, caused photo-oxidative stress in H. ovalis and this generally impacted in an additive or sub-additive way with Diuron to damage PSII. The bioassay using isolated leaves is more rapid, uses far less biological material and does not rely on specialised aquarium facilities in comparison with assays using potted plants. The development and validation of this sensitive bioassay will be useful to reliably screen and monitor the phytotoxicity of existing and emerging PSII herbicides and contribute to risk assessments and water quality guideline development in the future.     

Fluctuating asymmetry of leaves is a poor indicator of environmental stress and genetic stress by inbreeding in Silene vulgaris

Fluctuating asymmetry (FA) is often used as a measure of developmental instability and has been proposed as an indicator of both environmental and genetic stress. However, the empirical evidence for the effects of stress on FA in plants is inconsistent, and there are few controlled experimental studies. We analyzed different distance- and shape-based measures of the fluctuating asymmetry of the leaves of clonally replicated self- and cross-pollinated lineages of Silene vulgaris (Moench) Garcke (Caryophyllaceae) grown under a control and seven different stress treatments (drought, copper, simulated herbivory, and two levels of nutrient deficiency and of shade). Overall, FA differed among stress treatments, but was not generally higher under stress and even reduced in some treatments. Different measures of FA were only weakly correlated. Inbreeding increased only one measure of leaf FA, and only under high stress intensities. Our findings suggest that in S. vulgaris leaf fluctuating asymmetry does not serve as an indicator of environmental stress or of genetic stress by inbreeding.     

Fluctuating asymmetry as an ecological indicator of heavy metal stress in Lythrum salicaria

Fluctuating asymmetry (FA), a measure of random deviation of organismal traits from perfect bilateral symmetry, can be induced by environmental and/or genetic stress. We have conducted a replicated experiment to test the effect of a locally abundant heavy metal pollutant, lead, on the FA of a common wetland invasive species, Lythrum salicaria (purple loosestrife). The exposure to lead significantly reduced the shoot length, number and length of leaves as well as the biomass, but increased FA of L. salicaria leaves. The investigation suggests that FA in L. salicaria may be used as an ecological indicator to identify environmental stress caused by certain heavy metal pollutants. However, more studies are needed for the use of FA in L. salicaria as a comprehensive tool for biomonitoring of a wider array of environmental pollutants.     

Impacts of dugong foraging on benthic animal communities in a Thailand seagrass bed

We studied the community structure and abundance of benthic animals in intertidal seagrass beds in Thailand, where dugongs (Dugong dugon) feed on the seagrass Halophila ovalis. Our objective was to examine whether the benthic community was affected by the foraging activity of the dugongs, which can cause disturbance to the sediment. We collected a total of 4156 benthic animals from three study sites and classified them into 77 different taxa. A lucinid bivalve, Pillucina sp., was the most abundant taxa (approximately 80% of individuals). Taxa richness of the benthic animals was higher in intact H.ovalis vegetation compared to dugong trails at all sites. In general, the densities of five functional groups (classified by position on/in the sediment and motility) and five dominant taxa were higher in intact seagrass vegetation compared to dugong trails, although the magnitude of this difference varied among functional groups and taxa. Densities were at least threefold higher in the seagrass vegetation than in the dugong trails for epifauna and shallow infauna, but were at most twofold higher for deep infauna. Taxa richness and abundance of the benthic animals in the dugong trails were similar between the day following, and 7–9days after, the creation of the trails. Thus, delayed and/or indirect effects of dugong herbivory on benthic communities were not detected in this study. These findings demonstrate that the foraging activity of dugongs in seagrass can have considerable impacts on the abundance and diversity of benthic organisms, although this effect varies among benthic animals with different lifestyles.     

Magnitude of within-patch variation in seagrass Halophila ovalis growth affected by adjacent Thalassia hemprichii vegetation

Seagrass beds in South-east Asia sometimes consist of a mosaic of different species in monospecific patches. We examined whether the magnitude of within-patch variation in the seagrass Halophila ovalis is affected by the presence or absence of surrounding vegetation consisting of another seagrass species Thalassia hemprichii in an intertidal flat in Thailand waters. We measured biomass and growth rates of H. ovalis at the edges and centers of two different types of patches: (i) H. ovalis patches adjoining T. hemprichii vegetation (HT patches), and (ii) H. ovalis patches adjoining unvegetated sand flats (HS patches). Furthermore, we examined the possible effects of interspecific interactions on the growth of H. ovalis by experimentally removing adjoining T. hemprichii at the edges of HT patches. The biomass of H. ovalis was greater at the patch centre than the patch edge in both types of patches. For the growth rate of H. ovalis, significant interactions were detected between patch types and positions in patches. The difference in growth was significant and more than 4-fold between edges and centers of the HS patches, whereas the growth was not significantly different between edges and centers of the HT patches. The removal of T. hemprichii did not significantly affect the growth rate of H. ovalis at the edge of the HT patches. These findings demonstrate that the magnitude of within-patch variation in H. ovalis growth is affected by the conditions of adjoining habitats. However, any effects of local competition with T. hemprichii on H. ovalis growth were not evident in this short-term manipulative experiment.     

Environmental stress as an indicator of anthropogenic impact across the African Albertine Rift: a case study using museum specimens

We examined fluctuating asymmetry (FA) and body condition (BC) as two measures of environmental stress using museum specimens of Lophuromys aquilus, a rodent species complex wide-spread across the African Albertine Rift. We related FA and BC to a spatially-derived index of anthropogenic impact using a principal components analysis (PCA). We found no relationship between the four PCA scores and mean FA or BC, but did find that FA variance was higher in areas with lower anthropogenic impact. There was also a negative, albeit non-significant, trend for PC3, suggesting that populations with higher than average BC were in areas with higher anthropogenic impact. Overall, our case study does not support FA and BC as effective predictors of environmental stress with low to moderate habitat disturbance. In fact, L. aquilus, as a habitat generalist, may be positively affected by some aspects of anthropogenic change. Studies relating environmental stress to anthropogenic impact should examine sites with a wide range of habitat qualities and human impact and utilize multiple measures of environmental stress to characterize the health of one or more populations.     

Fluctuating Asymmetry as an Indicator of Environmental Stress From Off-Highway Vehicles

Abstract: With human activities increasingly impacting natural resources in relatively remote locations, there is a need for simple and efficient methods to explore the ecological consequences of these activities. Little is understood about the influences of off-highway vehicle (OHV) use on wildlife populations. We examined the effect of OHV activity on developmental instability in a phrynosomatid lizard (i.e., western fence lizard [Sceloporus occidentalis]) in the western Great Basin, USA. We measured fluctuating asymmetry (FA) of bilateral head-scale patterns in populations of lizards at 3 OHV and 3 non-OHV sites. Fluctuating asymmetry was higher at OHV sites relative to non-OHV sites, supporting the idea that OHV activity can stress wildlife populations. We found FA to be a good tool for uncovering responses to stress in natural populations, and we recommend exploring FA as a means of uncovering developmental instability in other systems that merit conservation interest.     

New record for Halophila decipiens Ostenfeld in Kenya based on morphological and molecular evidence

The seagrass Halophila decipiens Ostenfeld was recorded for the first time in Kenya in 2003. It was growing in subtidal mixed meadows with H. ovalis in protected bays at a depth of 3 m. DNA sequence analysis and morphological characteristics confirmed the identification. It is possible that this species is more widely distributed in the east African region. There was very little sequence differentiation between the Kenyan H. decipiens and the other samples in the Indo-West Pacific, and between this region and the Atlantic Ocean, further validating from an evolutionary perspective recent dispersal of this species.