Benthic Research Articles

Tracking bottom-fishing activities in protected vulnerable marine ecosystem areas and below 800-m depth in European Union waters.

Numerous studies have highlighted bottom-contact fishing gears as the primary threat to vulnerable marine ecosystems (VMEs). In November 2022, the European Commission closed 87 VME protection polygons to bottom fishing in European waters. Using public automatic identification system (AIS) data, we found an 81% decrease in bottom-contact fishing effort within these areas in the year following the closures. However, approximately 3500 hours of bottom-contact fishing persisted within the closures. We also quantified up to 17,600 hours of bottom-contact fishing in unprotected areas where VMEs are known or likely to occur. Last, our analysis revealed ongoing bottom trawling below 800 meters in European waters totaling 19,200 hours over 2 years. These findings underscore the urgent need for states to enhance surveillance and monitoring of their fleets to ensure effective fisheries management.

Surveying Nearshore Bathymetry Using Multispectral and Hyperspectral Satellite Imagery and Machine Learning

Nearshore bathymetric data are essential for assessing coastal hazards, studying benthic habitats and for coastal engineering. Traditional bathymetry mapping techniques of ship-sounding and airborne LiDAR are laborious, expensive and not always efficient. Multispectral and hyperspectral remote sensing, in combination with machine learning techniques, are gaining interest. Here, the nearshore bathymetry of southwest Puerto Rico is estimated with multispectral Sentinel-2 and hyperspectral PRISMA imagery using conventional spectral band ratio models and more advanced XGBoost models and convolutional neural networks. The U-Net, trained on 49 Sentinel-2 images, and the 2D-3D CNN, trained on PRISMA imagery, had a Mean Absolute Error (MAE) of approximately 1 m for depths up to 20 m and were superior to band ratio models by ~40%. Problems with underprediction remain for turbid waters. Sentinel-2 showed higher performance than PRISMA up to 20 m (~18% lower MAE), attributed to training with a larger number of images and employing an ensemble prediction, while PRISMA outperformed Sentinel-2 for depths between 25 m and 30 m (~19% lower MAE). Sentinel-2 imagery is recommended over PRISMA imagery for estimating shallow bathymetry given its similar performance, much higher image availability and easier handling. Future studies are recommended to train neural networks with images from various regions to increase generalization and method portability. Models are preferably trained by area-segregated splits to ensure independence between the training and testing set. Using a random train test split for bathymetry is not recommended due to spatial autocorrelation of sea depth, resulting in data leakage. This study demonstrates the high potential of machine learning models for assessing the bathymetry of optically shallow waters using optical satellite imagery.

Sediment Characteristics Study at Sharm Obhur, the Red Sea using Multibeam Backscatter

Sharm Obhur, situated approximately 35 km north of Jeddah City along the Red Sea coast, is a prominent recreational area with diverse benthic habitats. Accurate mapping of seabed sediments is critical for characterizing marine ecosystems and elucidating benthic habitat distribution in this region. This study underscores the importance of assessing sediment distribution patterns in Sharm Obhur, particularly due to the prevalence of recreational activities and navigational traffic. This research investigates sediment dynamics in Sharm Obhur, a coastal creek near Jeddah, utilizing Multibeam Echo Sounder (MBES) backscatter data and grab sample analyses. The study focuses on understanding sediment distribution patterns and their implications for coastal management. MBES technology provides high-resolution data on seafloor composition and roughness, essential for assessing sediment types and bedforms. Through spatial alignment of MBES data with ground truth sediment samples, we analyze sediment characteristics and validate findings. Results indicate predominant sand presence with detectable mud in deeper channels and northern anthropogenically impacted areas. Gravel deposits correlate with coral formations near the creek edges. Tidal influence is evident, with increased sand presence at the creek entrance. The study demonstrates MBES backscatter's efficacy in mapping sediment distributions and seabed characteristics, benefiting marine science and coastal management. This research enhances knowledge of sediment transport processes, facilitating informed decision-making for sustainable coastal development and conservation efforts.

Physiological and behavioral responses of the Baltic clam Macoma balthica to a laboratory simulated CO2-leakage from a subseabed carbon storage site.

Carbon capture and storage in sub-seabed geological reservoirs is now officially included in the atmospheric CO2 emissions reduction policy and meets the agenda of Sustainable Development Goals (SDGs). Over the last few years biological risk assessment studies have delivered substantial empirical data on possible consequences of CO2 leakages from underwater storage sites on benthic systems. Current knowledge on Carbon Capture and Storage CCS associated risks is limited to marine systems. Yet there are multiple areas identified as suitable for carbon storage, but their hydrogeochemical features are so distinct that they should be studied as separate cases. Baltic Sea is one example of an area but is host to a unique - in a world scale - ecosystem with low salinity in combination with reduced oxygen availability in the benthic zone. Geological surveys have designated a potential storage site in the Southern Baltic Sea, namely the B3 oil field. Thus, this study focuses on biological effects of seawater acidification caused by a simulated CO2 leakage scenarios under laboratory conditions on a model macrobenthic in-faunal species. Baltic clams Macoma balthica were exposed to different environmental pH scenarios: pH7.7 (no leakage), pH7.0 (moderate hypercapnia) and pH6.3 (severe hypercapnia) in three independent experiments conducted with the use of a hyperbaric tank (Karl Eric Titank) mimicking hydrostatic pressure of 900kPa, relevant to conditions at the B3 field. Selected physiological aspects of the Baltic clam, such as survival, shell growth rate, morphometric condition and biochemical composition were investigated along with their behavioral responses, i.e. sediment burrowing activity. The results showed modest effects of hypercapnia on physiological performance of the clams that did not lead to greater mortality in neither of the tested leakage scenarios. Apart from high survival of the clams even in the lowest seawater pH (6.3) there were only little changes observed in the burrowing depth of the clams and biochemical composition of their soft tissues related to seawater acidification. The most evident physiological responses of the clams to prolonged hypercapnia (40days at pH6.3) were manifested in decreased shell growth.

The Lingering Menace: How Legacy Organochlorine Pesticides Still Threaten Our Rivers and Food Chains

The bioaccumulation of organochlorine pesticides (OCPs) in aquatic organisms has been extensively explored in recent years due to associated ecological risks and potential threats to human health through food. This study aims to assess the bioaccumulation of organochlorine pesticides (OCPs) in 18 freshwater fish species from the Pearl River Delta, China. The observed OCP concentrations ranged from 0.69 to 43.15ng/g wet weight, indicating low to moderate pollution levels. A linear mixed-effects model revealed the contribution of the feeding habits and geographic distributions at 58.67 and 0.67% of OCP accumulation, respectively. Bottom-dwelling carnivorous fish species exhibited greater OCP residue levels than those in the upper layers of the freshwater. Estuarine organisms showed higher Dichlorodiphenyltrichloroethane (DDT) residue levels than those inland species. On the other hand, although there were no direct carcinogenic or exposure risks associated with OCPs in the study area, potential related hazards were observed. Consumers are, therefore, advised to select fish species from the middle and upper delta rather than bottom fish, shellfish, and shrimp. The current research provides valuable guidance for aquatic food consumption.

A changing marine lightscape: Two decades of satellite data to map the global benthic mesophotic zone and explore trends of change

A changing marine lightscape: Two decades of satellite data to map the global benthic mesophotic zone and explore trends of change

Influence of habitat enhancement and habitat degradation on spiny lobster populations in the Caribbean: An overview

Spiny lobsters are one of the most valuable fishing resources worldwide. These lobsters have a complex life cycle with a lengthy pelagic phase followed by a lengthier benthic phase, with several habitat shifts in some species. Finding adequate habitats is complicated for these lobsters, as they interact with different prey, predators, and competitors in subsequent habitats. This paper provides an overview of the characteristics of spiny lobster benthic habitats, focusing on two species from the wider Caribbean region, Panulirus argus and P. guttatus. Tropical spiny lobsters are omnivorous; therefore, food is not as limiting a component of habitat as is shelter, on which their survival greatly depends. Indeed, lack of appropriate shelter may result in population bottlenecks. This observation underlies the use of artificial structures (such as “casitas”) to enhance lobster habitats for fisheries and conservation purposes. Pros and potential cons of habitat enhancement are also reviewed. The other side of the coin is habitat degradation. Throughout the wider Caribbean and elsewhere, climate change –mostly resulting from anthropogenic stressors– and coastal development are increasing coastal pollution and eutrophication, seagrass loss, algal blooms, habitat fragmentation, coral reef degradation, and marine diseases, changing the habitat landscapes with potentially dire effects for spiny lobster populations. Because of the different habitats used by these two species throughout their benthic lives, P. guttatus is likely to be more vulnerable to coral reef degradation and the settling postlarvae and early juveniles of P. argus to degradation of seagrass and shallow hard bottom communities. Conservation of appropriate habitats for target species should be considered an important aim of ecosystem-based fisheries management.

Some Likely behaviours for Axisymmetric Rising Plumes in a Quiescent uniform Ambient through Warm Discharge

An investigation into some possible behaviours of rising plumes with a quadratic dependence relation assumption was made with the consideration that the plumes arises from a virtual sources. Our results showed that both forced and pure plumes provides buoyancy flux at the source. Meanwhile, the lazy plume rises with a significant volume flux and finite temperature with zero volume flux for both the forced plume and pure at the source. The forced plume also possesses some significant upward momentum flux and zero momentum flux for the pure plume. Determination of zero-buoyancy height and the fountain-top height are key when it comes to rising plumes. If the ambient water depth into consideration is less than the zero-buoyancy height then the warm water will definitely spread outwards as surface gravity current; otherwise, it will form a fountain.Our results showed that the upward momentum flux attains its pick at; and changes sign in the buoyancy force at this point while the momentum flux tends to zero with a finite final volume flux Qf from its maximum rise height. Meanwhile, as M0 approaches the value , so the Qf and the Q0 tends to the value. It is worth stating also that the fountain-top height will be of great significant if the focus is on erosion of an ice cover. We therefore suggest maintaining a low source Froude number which will result to minimising impact on the lake floor or on the surface. Though, some level of carefulness is needed whenever we intend to make use of the present results to foretell the behaviour of real plumes. In particular, if laboratory experiments are required either to examine the theory or to model larger-scale flows in the environment. Lastly, with the entrainment model, it is required that the plume will be fully turbulent, a condition usually obtained with a Reynolds number Re > 2000. This can be likened to the power station warm discharge which will definitely be fully turbulent with it large volume flux. These are basic conditions on the validity of the assumptions. There are some limitations as stated in the conclusion. But then, the overall numerical results here are very good as they present us with more insight into rising plume with the quadratic dependence relation assumption.

A New Interpretation of the Dominance and Diversity of Horseshoe Island (Antarctic Peninsula) Epibenthic Communities

This study investigates the epibenthic communities of Lystad Bay on Horseshoe Island (Western Antarctica) using underwater video imaging techniques during the eighth Turkish Antarctic Expedition in 2024. The primary objective of the study was to assess species diversity across different depths, substrate structures and potential ice scouring. Six sampling stations were selected, varying in depth (3–34 m) and substrate type (rocky, sandy, and muddy). Results show that a general tendency for an increase in species diversity with depth, particularly below 19 m, and is higher on sandy-muddy substrates than on rocky ones. The analysis of video footage identified 30 species from six phyla, with the highest species richness recorded at deeper stations (St 6) and the lowest at shallower rocky areas (St 1). The analysis of diversity indices (Shannon-Weiner, Margalef, and Pielou's evenness) inticates that substrate type exerts a significant influence on species richness, with sandy-mud habitats exhibiting higher diversity. Station 4 exhibited a low species richness, possibly as a result of local ice scouring impacts despite its depth and the presence of suitable benthic habitat. This study highlights the importance of oceanographic and environmental factors, such as ice scouring and terrestrial inputs, are in determining the organization of Antarctic benthic communities. Further research is required to understand the long-term effects of comprehend these elements on the biodiversity of the Antarctic ecosystem. This study is the inaugural comprehensive study in this region and will serve as a pivotal reference point for future research endeavours.

The record of the Early Late Paleocene Event (ELPE, 59.5 Ma) in shallow-water Tethys Himalayan carbonates (Zongpu Formation, South Tibet)

ABSTRACT The Early Late Paleocene Event (ELPE, 59.5 Ma) was a short-lived climatic perturbation accompanied by prominent biotic changes. Although the ELPE has been widely recognized from pelagic and continental archives, shallow-marine records remain scarce and poorly documented. To constrain the pattern and magnitude of the ELPE and associated environmental changes, we here present a detailed sedimentologic, biostratigraphic, and stable-carbon-isotope study of upper Paleocene platform carbonates continuously exposed in the Jiajin section of southern Tibet. Two distinct negative carbon-isotope excursions (CIEs) are documented: the first one occurred during shallow benthic zone (SBZ) 3 with a magnitude of ∼ 1.0‰, the second one began near the SBZ3–SBZ4 boundary with a magnitude of ∼ 1.5‰. Carbonate microfacies dominated by packstones with rotaliids and/or dasycladacean algae testify to an open shallow-marine environment. Neither a significant change in paleo–water depth nor evidence of early diagenetic dissolution are documented across the ELPE. The microfacies change observed during the ELPE, characterized by a decrease in the abundance of larger benthic rotaliid foraminifera relative to calcareous green algae, is related to environmental perturbations and increased nutrient supply. The changed trophic level may have resulted from intensified continental weathering promoted in turn by global warming. Compared with biocalcification crises observed in deep-water sedimentary records, shallow-water carbonate production remained relatively unaffected by changes in ocean-water chemistry.

Editorial: Experiments in benthic ecology: using experimental manipulations to study the effects of pressures on benthic organisms

Editorial: Experiments in benthic ecology: using experimental manipulations to study the effects of pressures on benthic organisms

Recovery and expansion of rhodoliths beds and Laminaria rodriguezii forests after bottom trawl ban

AbstractRhodolith beds and Laminaria rodriguezii forests constitute marine habitats of high conservation value. In the Menorca Channel (Balearic Islands, Western Mediterranean) rhodolith beds predominate in the sedimentary bottoms of the continental shelf and in some areas, these bottoms present an erect stratum with L. rodriguezii. Bottom trawling has negative impacts on rhodolith beds and, specially, on L. rodriguezii forests due to the direct effects of extraction and mechanical destruction and indirect effects related to sediment resuspension and posterior settlement that promotes burial. In this work we compare the distribution of rhodolith beds and L. rodriguezii forests in the Site of Community Importance (SCI) of the Menorca Channel before and after 2016, when a Fishing Protection Zone (FPZ) banning trawling was established. Since the implementation of the FPZ the extension of both rhodolith beds and L. rodriguezii forests have shown an increase of 6% and 54% respectively, and biomass of rhodolith-forming species and L. rodriguezii showed higher values than before the FPZ. The improvement of these habitats in the SCI Menorca Channel evidences that the bottom trawl ban is an effective measure for the conservation and restoration of benthic communities and habitats. This is particularly relevant in the case of rhodolith beds and L. rodriguezii forests due to their ecological importance and their role as essential fish habitats that improve the sustainability of marine living resources.

Macrobenthic assemblages of anthropogenically active subtidal zones contiguous to a marine protected area: a 9-year revisit.

Traditional taxonomical methods, an appropriate Benthic Biotic Index incorporating the Taxonomical Sufficiency concept and Biological Traits Analyses, were applied concurrently to evaluate the long-term variations in the benthic ecology of subtidal zones of a Marine-Protected Area subjected to prevalent oil and gas industrial activities. Our evaluation spanned two comparable surveys conducted 9years apart (2010 vs. 2019). Results indicated temporally consistent environmental factors, with exceptions noted in the bottom water dissolved oxygen, temperature, pH, and salinity. Hydrocarbon levels in water and sediments remained low and comparable between the two periods, aligning with defined baselines. Benthic studies indicated consistent taxonomical indices, polychaete family compositional changes, consistency in the relative proportion of functional trait modalities, significantly improved functional parameters, and prevalence of dominant k-selected sensitive traits. Moreover, the Benthic Opportunistic Polychaetes Amphipods index conferred a good to high ecological status to all stations in both years, reinforcing the continued favorable ecological quality of the area. Hence, it is surmised that the protectionary benefits of marine protected areas have extended to the subtidal zones as well.

Recreational Anglers’ Specialization and Perception of Management Measures in Yucatan, Mexico

ABSTRACT Understanding anglers’ diversity can help to improve the effectiveness of governing recreational fisheries. Online and in situ surveys were applied to evaluate perceptions and preferences among anglers’ fishing frequency (occasional, regular, and frequent) as an indicator of their specialization in Yucatan, Mexico. Occasional anglers’ most reported fish species are associated with bottom fishing (non-selective), while regular and frequent with trolling and casting (selective). The primary fate of the catches was to preserve the sizes they considered large enough and release the small ones. Unlike occasional, the frequent does not consider overexploited species to be affected by this fishery. The fish size regulation was chosen as the most effective management measure among anglers of all specialization levels. The social and ecological context of the Yucatecan coast must be considered before introducing management measures, and resource managers must establish collaboration strategies with the most specialized anglers to promote best fishing practices.

Analyzing Archive Transit Multibeam Data for Nodule Occurrences

We show that analyzing archived and future multibeam backscatter and bathymetry data, in tandem with regional environmental parameters, can help to identify polymetallic nodule fields in the world’s oceans. Extensive archived multibeam transit data through remote areas of the world’s oceans are available for data mining. New multibeam data will be made available through the Seabed 2030 Project. Uniformity of along- and across-track backscatter, backscatter intensity, angular response, water depth, nearby ground-truth data, local slope, sedimentation rate, and seafloor age provide thresholds for discriminating areas that are permissive to nodule presence. A case study of this methodology is presented, using archived multibeam data from a remote section of the South Pacific along the Foundation Seamounts between the Selkirk paleomicroplate and East Pacific Rise, that were collected during the 1997 Foundation–Hotline expedition on R/V Atalante. The 12 kHz Simrad EM12D multibeam data and the other forementioned data strongly suggest that a previously unknown nodule occurrence exists along the expedition transit. We also compare the utility of three different backscatter products to demonstrate that scans of printed backscatter maps can be a useful substitute for digital backscatter mosaics calculated using primary multibeam data files. We show that this expeditious analysis of legacy multibeam data could characterize benthic habitat types efficiently in remote deep-ocean areas, prior to more time-consuming and expensive video and sample acquisition surveys. Additionally, utilizing software other than specialty sonar processing programs during this research allows an exploration of how multibeam data products could be interrogated by a broader range of scientists and data users. Future mapping, video, and sampling cruises in this area would test our prediction and investigate how far it might extend to the north and south.

Discovering the Bathylithology and Bioengineering Organisms of the Punta Coles Marine Natural Reserve, Moquegua, Peru

The Punta Coles Natural Reserve (PCNR), located on the southern coast of the province of Ilo, Moquegua Region, Peru, houses an area rich in marine biodiversity which is composed of natural banks of benthic invertebrates, which constitute the potential and dynamic fishery of commercial interest for artisanal fishermen that impact the area. To mitigate this impact, all stakeholders need to know, understand, value, and balance the dependence of benthic biodiversity with the ecosystem for its preservation and the promotion of sustainable fishing. Along these lines, the components studied are bathymetry, lithology, and the tapestry of bioengineers that make up the reserve’s seabed. Mapping of the complexity of the benthic habitat of the seabed was developed on the basis of an integrated analysis of multibeam bathymetric data, geoscientific information associated with the antecedents of benthic biodiversity of the PCNR, sediment maps of the seabed, subtidal sedimentological texture, and photographic records. The characterization and granulometry of the sediments that make up the PCNR were determined according to the Udden–Wentworth classification criterion (Φ). The utility of the GPSMAP 178C multibeam echosounder for the interpretation of seabed sediments stands out. The kriging interpolation method was used to extrapolate the characteristics of the benthic habitat, which is suitable for large areas of the sea. As a result, three habitats were derived and mapped, and three structures (bioengineers), namely, “subtidal kelps” (Lessonia trabeculata), “mussels” (Aulacomya atra), and “red sea squirt” (Pyura chilensis), were associated mainly with sedimentary rock habitats and mixed substrates. In the sand substrate, these algae were found to a lesser extent. The results show that this study is a conceptual model that summarizes the understanding of ecology and prospects for the sustainability of profitable, ecoefficient fishing activities in the Moquegua Region, Peru.

Automatic Detection of Seafloor Bedforms for 3D Bathymetric Data

Seafloor bedforms are sedimentary structures that can reveal the local hydrodynamics conditions as they are the result of the bottom sediments' response to the dominant flow. The studying of these dynamic bottom shapes is important given that they can provide auxiliary information for the mapping of benthic habitats and can present a risk to navigation and marine structures. Mapping of the sea bottom is often done with dense and spatially extensive 3D bathymetry data (point clouds) resulting in a more precise representation of the targeted area. However, the most common praxis in this field is to rasterize the original data for the easiness in computation and lack of yet well-established methodology for 3D bathymetric data processing. As a consequence, the rasterization process causes information loss and increase of data processing time. The advantage of points clouds is that they comprise a larger volume of information in the same file, e.g. depth, intensity, RGB, and point classes, enabling a closer representation of reality and simultaneous generation of multiple products as potential habitat maps, Landscape Information Model (LIM), and denser Digital Bathymetry Model (DBM). Therefore, the purpose of this work is to apply a modified U-Net convolutional neural network for detecting and classifying bedform types in the bathymetry point cloud. The methodology will be applied to two datasets collected on the Espirito Santo Continental Shelf: Recifes Esquecidos (RE) and Doce River (DR).The following methodological steps will be carried out: (1) data collection, (2) generation of bathymetry derivatives as slope, curvature, geomorphons, aspect, and data tiling up for data augmentation, (3) image labeling, (4) model implementation including the training, validation, and testing steps, (5) calculation of model’s performance metrics: Intersection over Union (IoU), mean Average Precision (mAP), recall, and precision. This study will also present a performance comparison between the modified U-Net and a Random Forest (RF) forest. The selected areas are very rich in sedimentary features, so it is expected as the final product a classified points cloud in transversal, parallel, transitional, and artifacts (errors related to the surveying method) classes. It is also expected to have better performance from the non-convolutional model, RF.

Seafloor sediment microtopography as a surrogate for biodiversity and ecosystem functioning.

Marine soft sediments play crucial roles in global biogeochemical cycles and biodiversity. Yet, with organisms often hidden in the sediment, they pose challenges for effective monitoring and management. This study introduces a novel approach utilizing sediment microtopography as a proxy for ecosystem functioning and biodiversity. Combining field sampling, benthic chamber incubations, and advanced Structure-from-Motion photogrammetry techniques, we investigated the relationships between microtopographic features and various ecological parameters across diverse subtidal habitats. Our findings reveal strong associations between sediment microtopography and environmental variables, benthic fluxes, biodiversity metrics, and community functional traits, with microtopography consistently explaining more than 50% of the variance in the data. This research demonstrates the potential of sediment microtopography as a cost-effective and scalable tool for assessing soft-sediment ecosystem dynamics and informing conservation strategies. By providing insights into the links between habitat structure and ecological processes, this study advances our understanding of marine benthic ecology and opens new possibilities for habitat assessment applications worldwide.

Heterogeneous long-term and seasonal brine evolution, and Artemia fecal pellet controls on Urmia Lake (NW Iran) salt-crust formation and mineralogy

ABSTRACT Urmia Lake resides as a substantial hypersaline lake characterized by notable fluctuations in water salinity, brine composition, and water level over long-term, annual, and seasonal intervals. Extremely rapid water elevation fall (> 7 m) in the last three decades has caused the formation of a salt crust on the lake floor. A manmade stone causeway divided the lake into two relatively deeper northern parts with minimal water inputs and a shallower southern part with maximal river inflows. Restricted water flow through the narrow water passage of the causeway leads to complex salinity processes, brine evolution, and salt-crust formation in Urmia Lake. This research analyzes the ionic composition of lake-sediment and salt-crust pore water, the mineralogy of salt crusts, and the ionic composition of both surface and deep lake waters during both the wet and dry seasons of 2019. The findings indicate that the northern and southern parts of the lake undergo stratification during wet seasons due to significant freshwater input, whereas they become homogenized during dry seasons through progressive evaporative concentration and water mixing. The spatial and temporal variations in the lake brine type (primary Na-Mg-Cl) and ionic composition contribute to the formation of a halite salt crust (NaCl > 97%) with heterogeneous mineralogy and thickness. In Urmia Lake, the variable thickness and mineralogy of the exposed marginal salt crust suggest rapid salt-crust reorganization by annual and seasonal deposition and dissolution processes. Conversely, the submerged central salt crust, with continuous thickening and constant mineralogy, remains unaffected by seasonal variations in brine type and dissolution processes. It is noteworthy to mention that Artemia (a brine shrimp) controls the mineralogy of the lake salt crust through the deposition of calcium and carbonate ions in the form of biochemical fecal pellets.

Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability

This study describes two new species of the subgenus Desmoscolex (Nematoda: Desmoscolecidae) from subtidal benthic habitats in Korea, located in the Northwestern Pacific Ocean. Both species are distinguished by the absence of cephalic setae—a rare feature previously documented in only three species within this subgenus: Desmoscolex (Desmoscolex) rostratus Timm, 1970; D. (D.) asetosus Decraemer, 1975; and D. (D.) obscurus Bussau, 1993. The new species exhibit distinct morphological features. Desmoscolex (D.) lanceosetatus sp. nov. is characterized by an asymmetrical, dorsally curved head; large amphidial fovea that extend to the posterior margin of the first main ring; and lance-shaped subdorsal setae. In females, the absence of subventral setae on the 14th ring deviates from the typical arrangement of 17 main rings. Desmoscolex (D.) rotundicephalus sp. nov. is identified by its globular head, oval amphidial fovea, and tapering subdorsal setae with an open tip. In females, the absence of subventral setae on the 14th ring further differentiates it from closely related species. This paper provides detailed morphological descriptions supported by scanning electron microscopy (SEM) and differential interference contrast (DIC) images. Additionally, it includes a comprehensive morphological comparison with previously described species, species diagnoses, and a pictorial key for identification.