Marine Caves Research Articles

Lost in the Dark: Current Evidence and Knowledge Gaps About Microplastic Pollution in Natural Caves

In this study, a systematic review of the scientific literature was carried out to summarize the emerging evidence on microplastic pollution in natural caves. After the screening of 655 papers on the topic from a combined search on the Web of Knowledge and the Scopus databases, we found only 14 studies reporting quantitative data on microplastics from a total of 27 natural caves. Most of the assessments focused on water and sediment, with very limited investigations concerning the cave biota. Overall, the most common types of particles found in caves were small (<1 mm) fibers (~70–90% of items), transparent or light-colored, mostly made of polyethylene and polyethylene terephthalate. Anthropogenic cellulosic materials, however, represented a non-negligible portion of particles (i.e., ~20–30%). Microplastic concentrations in caves varied between 0.017 and 911 items/L for water and 7.9 and 4777 items/kg for sediment, thus falling within the levels of microplastic pollution found in other terrestrial, freshwater, and marine environments. Levels of microplastic pollution appear largely variable among caves, stressing the need to extend the geographic and environmental ranges of the assessments, which are currently concentrated on Italian caves on land, with very few case studies from other regions of the world and from marine caves. Despite their putative isolation, natural caves have a high vulnerability to microplastic contamination, requiring much more research effort to understand the potential risk that plastics pose to these fragile ecosystems.

An update on the genus Ischiomysis with a description of I. proincisa sp. nov. (Crustacea, Mysida) from a sublittoral marine cave in the Gulf of Guinea (tropical E-Atlantic)

An update on the genus Ischiomysis with a description of I. proincisa sp. nov. (Crustacea, Mysida) from a sublittoral marine cave in the Gulf of Guinea (tropical E-Atlantic)

Baseline for marine cave monitoring strategies in the Alboran Sea using modified Cave Ecosystem-Based Quality Index (CavEBQI)

Marine caves constitute vulnerable habitats with unique and diverse biocoenoses. Monitoring these habitats is still challenging, which hinders the ability to evaluate global and local pressures that threatens their ecological value. In this study, ecological quality is estimated in twenty-one marine caves distributed along the northern and southern coasts of the Alboran Sea, a highly understudied area regarding marine caves. For that purpose, adjustments on the original Cave Ecosystem Based Quality Index (CavEBQI) are suggested in order to efficiently estimate the ecological quality of marine caves. Several methodological aspects regarding the assessment of biotic coverage, the visual census of motile fauna and the plasticity of the index usability were evaluated. Ecological quality of marine caves was in average “good” although it ranged between “poor”, “moderate” and “good” depending on the features of each cave. This study emphasizes the importance of adapting biotic indicators to biogeographical differences and technological advancements.

Habitat partitioning and first microplastic detection in the Argentarola marine cave (Tyrrhenian Sea, Italy)

Due to their location, at the boundary between the marine and terrestrial domain, marine caves are very sensitive to environmental changes, but, at the same time, they are extremely vulnerable due to the enclosed setting. Despite their hostile conditions, they are reservoirs of biodiversity, and there is a need to improve their knowledge for conservation and protection purposes. This research studied sediment characteristics (grain size and organic geochemistry) and benthic foraminiferal assemblages of the Argentarola cave (Tuscany, Italy) to define cave ecozones and characterize their environmental variability. Because microplastic pollution is considered a potential threat for the cave, its presence was researched in sediment samples; moreover, the tests of agglutinated foraminifera were analyzed through Fourier Transform Infrared Spectroscopy (FTIR) to verify the possible inclusion of these contaminants.Result indicated the presence in the whole cave of fine-grained sediment with a low organic matter content of clear marine origin, pointing to oligotrophic conditions. Microplastic items at low concentrations were found in sediments, mainly constituted by very small (0.1–0.5 mm) polyethylene and polypropylene fragments and fibers. Foraminifera were abundant and diversified with dominant Spirillina vivipara and Patellina corrugata, like in other small Mediterranean marine caves. Despite the scarce variability of environmental parameters, foraminifera developed two distinct assemblages in relation to sediment texture and organic matter content. In the relatively more organically enriched one, two agglutinated deep-water species, Lagenammina difflugiformis and Glomospira charoides, never recorded before in marine caves, were common. The first one included polyethylene items among the particles of the agglutinated test. Then, this species may be considered an early indicator of microplastic pollution. However, this pollution did not influence the Ecological Quality Status, which was found to be high to good when applying the Foram-Ambi index. This study constitutes a new approach to studying marine caves, providing new tools for the environmental assessment and microplastic monitoring of this protected habitat.

First Survey of the Sponge Community of a Semi-Submerged Marine Cave along the Adriatic Apulian Coast

Mediterranean marine caves have been categorized as both biodiversity reservoirs and vulnerable habitats. However, only a few studies have focused on Porifera assemblages within marine caves along the Adriatic Apulian coast (southern Italy). In this study, the sponge fauna of the Rondinella cave, a semi-submerged marine cave along the coast of Bari (Southern Adriatic Sea), was investigated for the first time. The use of advanced image analysis in combination with targeted sampling has made it possible to determine the spatial distribution and diversity of Porifera along a transect from the entrance to the end of the cave. Data analysis clustered the stations into two groups, separated according to the distance from the entrance and corresponding to the cave entrance and the semi-dark zone. Sponges were found at all stations covering a considerable part of the substrate, with the highest cover values occurring in the semi-dark zone. A total of 54 sponge taxa were identified: 49 Demospongiae, 3 Homoscleromorpha, and 2 Calcarea. Six species are new records for the Apulian marine caves, one species represents a new record for marine caves, and two species are new findings for the southern Adriatic Sea.

Sponge Communities of Submarine Caves and Tunnels on the Fernando de Noronha Archipelago, Northeast Brazil

Submarine caves are important biodiversity reservoirs, but there is little information about the biota of marine caves in the Southwestern Atlantic. Here, we describe three submarine cavities and their sponge communities on the Fernando de Noronha Archipelago, Northeast Brazil. The underwater cavities were explored and collections were made through scuba diving from 5 to 18 m depths. Sapata Cave has a wide semi-dark zone near the entrance, a narrow transition zone, and a dark chimney, which is closed at the top. Ilha do Meio Cave is narrower and shallower than Sapata Cave, but has a long passage that leads to two completely dark rooms. Pedras Secas Tunnel has only a semi-dark zone with high water movement. The sponge communities in the semi-dark zones of the three cavities are rich and dominated by the classes Demospongiae and Homoscleromorpha, but Calcarea are also common. The transition zones of both caves are dominated by a desma-bearing sponge, thinly encrusting spirastrellids, and small Homoscleromopha and Calcarea. The dark zone in Ilha do Meio Cave is almost azoic, with only three species. This study has increased the number of sponge species known in submarine cavities on Fernando de Noronha from 29 to 69, highlighting the great richness of the sponge communities in these cryptic environments.

From cultural and natural heritage to reservoir of biomedicine: Prospection of bioactive compounds produced by bacterial isolates from caves

Caves, with distinctive features and specialized ecosystems, harbour microorganisms that have adapted metabolically to survive in seemingly harsh conditions with limited essential elements. Despite the potential threat of biodeterioration to subterranean heritage due to microbial proliferation, caves serve as vital reservoirs of bioactive compounds produced by subsurface-adapted microorganisms. These microorganisms form biofilms, producing diverse bioactive compounds with properties such as antimicrobial, antioxidant, and anticancer capabilities. We conducted a comprehensive study with different bacterial strains isolated from terrestrial and marine caves (Escoural cave and in different caves on the Lanzarote island and on Selvagem Grande island). Extracts from these strains demonstrated potential in inhibiting the growth of pathogenic bacteria, showed robust antioxidant effects, and exhibited antiproliferative activity against breast cancer cells, moreover some of these extracts (Knoellia sp. CI04-s2.2, Brevibacillus sp. MR02B-s3, Bacillus sp. CK03-s2 and Bacillus sp. SD03-s6) present more than one bioactivity. This study unveiled the potential of cave bacterial strains as valuable sources of bioactive compounds, with implications for the development of novel biocides and pharmaceuticals. Bioprospecting within these unique environments offers a distinct opportunity not only to explore and safeguard these ecosystems but also to promote the development of eco-friendly, safe, and sustainable solutions.

A new species of the genus Trichopagurus de Saint Laurent, 1968 (Crustacea: Decapoda: Anomura: Paguridae) from a semi-submerged marine cave in Okinawa Island, southwestern Japan.

A new hermit crab species of Paguridae, Trichopagurus spinibrachium n. sp., is described on the basis of a single male specimen collected from the anchialine environment of a semi-submerged marine cave in Okinawa Island, Ryukyu Islands, southwestern Japan. It is morphologically similar to T. macrochela Komai & Osawa, 2005 and T. tenuidactylus Komai, 2013, but differs in the shape and armature of the ocular, antennular and antennal peduncles, chelipeds, and ambulatory legs. The new species may be distinguished from all known congeners by the stronger armature of the carpi and meri of both chelipeds. Trichopagurus spinibrachium n. sp. is the third species of the genus recorded from submarine caves, although it may also occur on nearby coral reefs as does T. macrochela and T. trichophthalmus (Forest, 1954).

A new genus and species of a submarine cave crab of the family Plagusiidae Dana, 1851 (Crustacea: Brachyura: Grapsoidea) from Okinawa Island, Ryukyu Islands, southwestern Japan.

Caligoplagusia okinawa n. gen. & n. sp., assigned to the brachyuran family Plagusiidae Dana, 1851 is described on the basis of a male and a female specimens collected from a semi-submerged marine cave on the limestone shore of Okinawa Island, Ryukyu Islands, southwestern Japan. The new monotypic genus is most similar to Euchirograpsus H. Milne Edwards, 1853 and Miersiograpsus Trkay, 1978 in that the frontal margin lacks a deep sublateral cleft adjacent to the orbit, but differs significantly by its subhexagonal carapace shape, the anterolateral margin of the carapace with three teeth including the external orbital tooth, the anterior margin of ambulatory meri each armed with a row of large teeth, and the greatly reduced eyes. The pale body colour, reduced eyes, the very elongate antennal flagellum, and long ambulatory legs are all typical brachyuran adaptations to living in cave environments. This is the first cave-dwelling species of the family Plagusiidae.

Amphipods in Mediterranean Marine and Anchialine Caves: New Data and Overview of Existing Knowledge

Marine and anchialine caves host specialized faunal communities with a variable degree of endemism and functional specialization. However, biodiversity assessments on this habitat are scarce, particularly in relation to small-sized cryptic fauna (such as amphipods), which often play a key role in benthic ecosystems. The present article compiles all records of marine and brackish-water amphipods inhabiting marine and anchialine caves along the Mediterranean basin, combining information extracted from a literature review with newly acquired records. A total of 106 amphipod species has been reported (representing approximately 20% of the Mediterranean amphipod species), mostly from the North-Western Mediterranean. Examination of new material from marine caves in Greece has yielded 14 new records from the East Ionian and Aegean Sea. Most of the reported species display wide ecological amplitude in terms of habitat and substrate preferences, feeding habits as well as bathymetric and geographical distribution. In contrast, only 17 amphipod species have been reported from marine-brackish waters in anchialine caves, predominantly represented by cave specialists with a narrow spatial distribution and distinct morphological traits. Our overall knowledge on amphipods inhabiting Mediterranean caves is far from complete so that new and valuable findings are expected to occur as new caves are explored.

Paleoecological reconstruction during the Holocene in the Middle Branch of Bue Marino Cave (Sardinia, Italy)

This study was conducted in the Middle Branch of Bue Marino Cave (Sardinia, Italy) to reconstruct paleoecological conditions during the Holocene through microfaunal proxies in seven surface sediment samples and a short sediment core (BMD-2018, 18 cm), all collected in 2018 and another core sampled in 2021 (BMD-2021, 28 cm). The first attempt at dating cave sediment through luminescence was conducted on BMD-2021; the derived age was 6.04 ± 0.47 ka at 18 cm depth. Although continuous sedimentation and constant rates are not probable in the cave, this indicates that the sedimentary record dates to the early times after the Holocene flooding of the cave by the sea. Benthic foraminifera and grain size were analyzed in all surface samples and core BMD-2018, while only benthic foraminifera were studied in BMD-2021. The recent foraminiferal assemblages, studied from an ecological point of view, were applied as modern analog to reconstruct the paleoecological conditions in sediment cores. Significant changes in the sedimentary environment were excluded; the two cores showed a similar foraminiferal turnover at a similar depth, from an older assemblage with prevailing indifferent Ammonia inflata to a younger one with prevailing opportunist Eggerelloides advena. This turnover was attributed to changes in the amount and/or quality of available nutrients. Based on these results, this event is likely related to possibly attributable to the effects of climate changes that occurred in the Holocene. However, more extensive studies are necessary to better understand the effects of the climatic/environmental events in the Late-Holocene in the caves. Although it represents a first attempt at paleoenvironmental reconstruction based on sediments from a Mediterranean marine cave, this study demonstrated that benthic foraminifera are refined paleoenvironmental proxies and that the integrated approach with the luminescence dating produces reliable results for studying the effects of global changes in these environments.

Ecological gradients in a marine cave revisited 26 years after

Marine caves are unique and vulnerable habitats threatened by multiple global and local pressures. Changes in the sessile epibenthic community of the Grotta Azzurra of Capo Palinuro (Salerno, Italy) during the last 26 years have been evaluated using growth forms and trophic guilds as non-taxonomic descriptors, informative of the structure and the functioning of the cave ecosystem, respectively. Photographic samples were collected in 1992 and 2018 to investigate the stability of the ecosystem response to the three main environmental gradients occurring inside the cave: i) confinement, ii) depth, and iii) light. Benthic communities were well spatially distributed along the three gradients inside the cave according to both the morphology and the feeding strategy of organisms, although the zonation was less marked considering trophic guilds. Gradients persisted and zones remained distinct through time, notwithstanding significant changes in community composition. The trophic organization was preserved throughout the cave, but significant changes in the spatial organization of benthic assemblages were widely observed, mainly due to 3-dimensional growth forms replaced by 2-dimensional (i.e., encrusting) growth forms and by increased sedimentation, thus causing the structural homogenization of the cave community. The outermost and the deepest sectors of the cave experienced higher changes compared to the most confined, blind-ended, sector and to the shallowest depths, which resulted comparatively more stable. From a functional point of view, filter feeders always dominated the communities (even considering their shift from erect to encrusting morphologies) and the few observed changes were due to the reduction of photophilic autotrophs and active ciliate without lophophore and the increase of mixotrophic sponges. As local human pressures remained low through the period considered, the changes observed are likely to be mainly due to global warming, which caused increased sea surface water temperatures and repeated summer heat waves (reported in 1994, 2003, 2008, 2012, and 2015). Regular monitoring programs on marine caves should be promoted to create historical series of data precious to better understand magnitude and pattern of change in the long-term evolution, and to estimate the recovery time of such priority habitats.

Revealing the Mediterranean Monk Seal (Monachus monachus)’s Cave Preference in Gökova Bay on the Southwest Coast of Türkiye

The first cave-monitoring studies to be carried out on the southwest (SW) coast of Türkiye on endangered Mediterranean monk seals using camera traps occurred between 2017 and 2021 in five marine caves within Gökova Bay. The visual data obtained from the monitoring studies were evaluated to reveal the Mediterranean monk seals’ seasonal and diel cave use and identify the individual seals who were using the caves. Moreover, the necessary features and measurements of the identified caves were recorded to determine whether there were any correlations between the determined variables and monthly cave use by the monk seals. The results showed that cave use occurred mainly nocturnally, with the seals showing a diurnal activity pattern in the area. We evaluated 108,280 images/videos in total and identified 18 individuals using five caves in Gökova Bay. Three of these caves provided suitable characteristics for pupping, and two of them were used for pupping. A beta regression model revealed that the monthly cave use ratios varied seasonally, with more use in the fall season. Furthermore, the trends in annual cave use ratios, seasonality, wind speed, size of the wet area, luminance, number of cave-entry paths, and human activity were the best variables with which we could forecast the cave preferences of the seals.

Monk seal faeces as a non-invasive technique to monitor the incidence of ingested microplastics and potential presence of plastic additives

Anthropogenic debris, including plastics, has recently been identified as a major threat for marine mammals and the Marine Strategy Framework Directive aims to achieve the good environmental status of European waters by addressing among other criteria, the effects of marine litter on biota.This study implemented for the first time a non-invasive technique for collecting monk seal samples to assess microdebris ingestion in combination with identifying plastic additives and porphyrins biomarkers. A total of 12 samples of monk seal faeces were collected from marine caves in Zakynthos Island, Greece.A total of 166 microplastic particles were identified; 75 % of the particles were smaller than 3 mm. Nine phthalates and three porphyrins were detected. A strong correlation was found between the number of microplastics and the concentration of phthalates. The values of both phthalates and porphyrins were found lower than in other marine mammal tissues, suggesting that seals might not be impacted by them yet.

Density estimation of the main structuring sessile species in underwater marine caves with a deep learning approach

Monitoring marine biodiversity is a challenge in some vulnerable and difficult-to-access habitats, such as underwater caves. Underwater caves are a great focus of biodiversity, concentrating a large number of species in their environment. However, most of the sessile species that live on the rocky walls are very vulnerable, and they are often threatened by different pressures. The use of these spaces as a destination for recreational divers can cause different impacts on the benthic habitat. In this work, we propose a methodology based on video recordings of cave walls and image analysis with deep learning algorithms to estimate the spatial density of structuring species in a study area. We propose a combination of automatic frame overlap detection, estimation of the actual extent of surface cover, and semantic segmentation of the main 10 species of corals and sponges to obtain species density maps. These maps can be the data source for monitoring biodiversity over time. In this paper, we analyzed the performance of three different semantic segmentation algorithms and backbones for this task and found that the Mask R-CNN model with the Xception101 backbone achieves the best accuracy, with an average segmentation accuracy of 82%.

Photogrammetry as a promising tool to unveil marine caves’ benthic assemblages

Traditionally, monitoring approaches to survey marine caves have been constrained by equipment limitations and strict safety protocols. Nowadays, the rise of new approaches opens new possibilities to describe these peculiar ecosystems. The current study aimed to explore the potential of Structure from Motion (SfM) photogrammetry to assess the abundance and spatial distribution of the sessile benthic assemblages inside a semi-submerged marine cave. Additionally, since impacts of recent date mussel Lithophaga lithophaga illegal fishing were recorded, a special emphasis was paid to its distribution and densities. The results of SfM were compared with a more “traditional approach”, by simulating photo-quadrats deployments over the produced orthomosaics. A total of 22 sessile taxa were identified, with Porifera representing the dominant taxa within the cave, and L. lithophaga presenting a density of 88.3 holes/m2. SfM and photo-quadrats obtained comparable results regarding species richness, percentage cover of identified taxa and most of the seascape metrics, while, in terms of taxa density estimations, photo-quadrats highly overestimated their values. SfM resulted in a suitable non-invasive technique to record marine cave assemblages. Seascape indexes proved to be a comprehensive way to describe the spatial pattern of distribution of benthic organisms, establishing a useful baseline to assess future community shifts.

Sponge Fauna of the Apulian Marine Caves (Southern Italy): Current State of Knowledge

Submerged and semi-submerged marine caves are considered a European habitat of Community Interest as they preserve one of the most important biodiversity heritages in the Mediterranean and serve as refugia for endemic and/or “relict” species. Among sessile benthic taxa, caves represent significant reservoirs of sponge species richness and are well representative of the entire poriferan Mediterranean fauna. In order to assess the current knowledge of sponge species in marine caves along the Apulian coast, this study gathered data from the available literature (national and international scientific publications and grey literature) with original data, surveying 26 marine caves in the area. A total of 145 Porifera species were reported in marine caves on the Apulian coast, including 117 in the Tremiti archipelago, 33 along the Adriatic coast of Bari and 73 along the Salento Peninsula. Original data includes new records for nine species in the Corvine cave, two in the Murene cave and one in the Zinzulusa cave. Our results suggest that marine cave communities along the Apulian coast are not uniformly surveyed, being the caves of the Tremiti Islands and those of the Salento Peninsula among the best studied, while large stretches of the regional coast, although particularly rich in marine caves, are poorly investigated for their sponge fauna.

Introduced species in Mediterranean marine caves: an increasing but neglected threat

Marine caves are protected habitats with high biodiversity and low ecological resilience, vulnerable to multiple pressures including biological invasions. Therefore, comprehensive lists of alien species in sea caves and assessments of their impacts are urgently needed. This study aimed to provide an updated and validated list of introduced species in marine caves of the Mediterranean Sea based on the latest available checklist (2014). The number of introduced species in the updated list (December 2022 status) has doubled in the last eight years. The updated checklist includes 126 species (107 alien, 15 cryptogenic, two crypto-expanding, and two questionable) belonging to 12 phyla. The highest number of introduced species in caves was evidenced in the eastern Mediterranean (105), specifically in the Levantine (99) and the Aegean (34) ecoregions. Most introduced species originated from the Indo-Pacific. Most species entered the Mediterranean Sea through the Suez Canal, followed by species transferred via vessels. Most introduced species were found at the entrance and in the semi-dark zone of caves (60 and 52 species, respectively), with only 19 species being reported from the dark zone. Approximately 28% of the introduced species have a high impact on ecosystems, including nine of the ten worst invasive species in the Mediterranean Sea. Despite the lack of data on the impact of introduced species on marine cave biodiversity, given their high species richness and the recently reported population explosions of some alien fish in the eastern Mediterranean, our results highlight the need for continuous inventorying and monitoring.

First record of microplastic in the environmental matrices of a Mediterranean marine cave (Bue Marino, Sardinia, Italy)

This study investigates for the first time the presence of microplastics in sediment, water, and benthic organisms (foraminifera) of a marine cave in the Gulf of Orosei (Sardinia, Italy). Microplastics were found in all water, and sediment samples with similar shapes, sizes, and compositions; identified items were mainly fragments and fibers constituted by PVC and polyethylene. Their provenance was supposed to be predominantly from the sea than from the seasonal freshwater supplies from the karst system. Foraminiferal assemblages were mainly constituted by calcareous hyaline taxa in the outer station, while in the inner ones, the agglutinated Eggerelloides advenus was dominant. FTIR analyses on agglutinated shells identified polyethylene. Microplastic items are collected by the foraminifers and sediment grains building the shell chambers. This is the first study providing evidence that marine caves may be collectors of microplastics and that, in these habitats, microplastics enter the biotic matrix at the protist's level.

Geomorphological, sedimentological, and ecological characterization of marine caves from Capraia Island (Tremiti Archipelago, southern Adriatic Sea, Italy): An integrated approach

Marine caves are sites of great interest from geomorphological and ecological viewpoints due to their significant environmental variability, being located at the boundary between marine and terrestrial domains. Most of the Mediterranean marine caves resulted from karst phenomena which widely affected the carbonate landscapes. At Tremiti Archipelago (southern Adriatic Sea, Italy), the karst activity is widespread, determining, in combination with existing tectonic features and long-term geomorphological evolution, the presence of several marine caves, partly or completely submerged. Two of these marine caves, located at Cala Caffè and Cala Sorrentino (Capraia Island), were considered for a multidisciplinary study focused on their geomorphological, sedimentological, and ecological characterization. These submerged blind-ended caves, located between 19 and 25 m water depth, show morphologies clearly linked to the tectonic-karst geomorphological setting. Superficial sediment samples were analyzed to derive grain size distribution, total carbon and nitrogen, isotopic ratios, living and dead benthic foraminifers, and dead brachiopods; water parameters were also considered. Sediment distribution showed a similar trend in the two caves, with finer textures, higher organic content in the inner stations, and increased biogenic components. Foraminifers showed a faunal change from the outer to the inner sectors associated with the decreasing grain size and the increased organic matter. Differently, the distribution of brachiopods did not display evident patterns; the differences in absolute abundance between the two sites were mainly attributed to the entrance dimensions and the morphology of the two caves, which deeply affected light penetration. This integrated approach demonstrated that geoforms, due to karst and tectonic processes, create small-scale coastal geomorphological habitats, greatly increasing biodiversity.