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

Citation: Navarro-Barranco, C.; Martínez, A.; Sempere-Valverde, J.; Chebaane, S.; Digenis, M.; Plaitis, W.; Voultsiadou, E.; Gerovasileiou, V. Amphipods in Mediterranean Marine and Anchialine Caves: New Data and Overview of Existing Knowledge. Diversity 2023,15, 1180. https:// doi.org/10.3390/d15121180 Academic Editor: Viatcheslav Ivanenko Received: 3 November 2023 Revised: 22 November 2023 Accepted: 23 November 2023 Published: 29 November 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article Amphipods in Mediterranean Marine and Anchialine Caves: New Data and Overview of Existing Knowledge Carlos Navarro-Barranco 1,* , Alejandro Martínez 2, Juan Sempere-Valverde 1, Sahar Chebaane 3,4 , Markos Digenis 5, Wanda Plaitis 6, Eleni Voultsiadou 7and Vasilis Gerovasileiou 5,6 1Laboratorio de Biología Marina, Facultad de Biología, Universidad de Sevilla, Avenida Reina Mercedes, 41012 Sevilla, Spain; [email protected] 2 Molecular Ecology Group (MEG), Water Research Institute (IRSA), National Research Council of Italy (CNR), 28922 Verbania Pallanza, Italy; [email protected] 3 Marine and Environmental Sciences Centre (MARE), ARNET—Aquatic Research Network, Regional Agency for the Development of Research, Technology and Innovation (ARDITI), Edifício Madeira Tecnopolo, Caminho da Penteada, 9020-105 Funchal, Portugal; [email protected] 4Departamento de Biologia Animal, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal 5Department of Environment, Faculty of Environment, Ionian University, 29100 Zakynthos, Greece; [email protected] (M.D.); [email protected] (V.G.) 6Hellenic Centre of Marine Research (HCMR), Institute of Marine Biology, Biotechnology and Aquaculture (IMBBC), 71500 Heraklion, Greece; [email protected] 7Department of Zoology, School of Biology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; [email protected] *Correspondence: carlosnavarr[email protected] Abstract: 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 brackishwater 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. Keywords: Crustacea; biodiversity; benthic ecology; stygobionts; biogeography; Mediterranean Sea 1. Introduction Motile macrofauna constitute a ubiquitous and key component of all coastal marine ecosystems. Many motile invertebrate species live in association with living primary substrates (e.g., macroalgae, seagrasses, and sessile invertebrates), whereas others are specialised to burrow or crawl amongst seafloor sediments or the small crevicular spaces within rocky or biogenic reefs [ 1 ]. Those epifaunal and infaunal communities frequently account for the greatest proportion of invertebrate species and abundance in benthic communities and, given their rapid temporal turnover rates, they are major contributors to secondary production in a wide range of habitats and geographical regions [1–6]. Diversity 2023,15, 1180. https://doi.org/10.3390/d15121180 https://www.mdpi.com/journal/diversity Diversity 2023,15, 1180 2 of 23 Amphipod crustaceans dominate benthic macroinvertebrate communities both in terms of abundance and numbers of species. The Order Amphipoda comprises approximately ten thousand species described within more than 200 families [ 7 ]. They inhabit both marine, brackish and freshwater habitats at all latitudes and bathymetric ranges, although the highest species richness occurs in benthic marine shallow water environments [ 7 , 8 ]. The numerical dominance of amphipods in comparison with other major macrofaunal groups (e.g., molluscs, annelids and other crustaceans) has been reported in a wide variety of habitats such as macroalgal and seagrass beds, gorgonian forests, fouling communities, as well as intertidal and shallow unvegetated sediments [ 5 , 9 – 14 ]. Many amphipod mesograzers affect macrophyte biomass and, as a prey source in benthic ecosystems, they constitute a relevant link between primary producers and higher-order consumers [ 15 – 17 ]. On the other hand, suspension-feeding amphipods also contribute to benthic–pelagic coupling and carbon dynamics by consuming a significant quantity of particles from the water column [ 18 , 19 ]. Amphipod species also exhibit a great diversity of life history strategies and variability in their tolerance to environmental conditions [ 20 , 21 ]. For all these reasons, they are reliable bioindicators used in laboratory ecotoxicology tests, as well as in field biomonitoring studies [ 21 ]. Unfortunately, amphipod communities are often overlooked in biodiversity inventories and ecological studies in most marine habitats [1]. Marine caves are common features of rocky coastlines. Despite the lack of cave inventories for a large stretch of the Mediterranean coastline, more than 3000 marine caves have been reported in the region so far [ 22 ]. Because they host a biota rich in cave-exclusive, threatened and relict species, caves have been considered as “biodiversity reservoirs” for many animal groups [ 23 , 24 ]. Darkness and isolation mostly drive the composition and distribution of cave-dwelling communities [ 22 , 25 , 26 ]. These environmental parameters vary horizontally along the cave, establishing gradients that also depend on the specific topography of each cave [ 27 , 28 ]. Accordingly, marine caves significantly contribute to the overall heterogeneity of coastal areas, thereby favouring an increased species richness at regional scales. Moreover, caves provide numerous ecosystem services to humans, insofar as they act as habitat and nursery grounds for many commercially important species and serve as popular locations for marine recreational activities. However, the biodiversity of motile invertebrates within such environments and their role in the functioning of cave ecosystems remain largely unexplored. Indeed, most studies investigating motile macroinvertebrates in benthic habitats neglect marine caves. Simultaneously most studies focused on caves predominantly concentrate on conspicuous animals [ 1 , 4 , 29 , 30 ]. In the Mediterranean Sea, where marine caves have been historically better investigated than elsewhere in the world [ 22 ], recent reviews have summarized the diversity of cave sponges [ 23 ], fish [ 31 ] and decapods [ 32 ]. The synthesis conducted by Romano et al. [ 33 ] on benthic foraminifera within Mediterranean marine caves probably constitutes the only review specifically addressing small-sized inconspicuous biota, although it is primarily concerned with highlighting the reliability of this group as a paleoecological indicator rather than characterizing their biodiversity or ecological patterns on a regional scale. Mediterranean cave-dwelling amphipods were first studied by Sandro Ruffo of the Verona National History Museum during the 1940s and 1950s. Ruffo worked on species from both marine and freshwater cave environments [ 34 , 35 ], see also references in [ 36 ]. Many of the sites sampled during that period were described as “marginal caves” or “mixohaline hypogean waters” [ 37 , 38 ], corresponding to the term “anchialine caves”, which was introduced in the 1980s to characterize subterranean water bodies with an underground connection to the ocean [ 38 , 39 ]. Ruffo’s investigations on anchialine habitats resulted in the description of novel taxa belonging to genera Metahadzia and Salentinella from Puglia (Southern Italy). Later on, exhaustive field and taxonomical studies were conducted by Karaman and Sket along the Dalmatian karstic coast (see [ 40 ] and references therein). Marine cave species, instead, were studied in the Tyrrhenian Sea and the French coast [ 35 , 37 , 41 – 43 ]. Studies on Mediterranean amphipod cave fauna continue to this date, but information on this topic is scattered among different sources such as taxonomic Diversity 2023,15, 1180 3 of 23 descriptions of new species (e.g., [ 44 ]), species inventories (e.g., [ 45 , 46 ]) or ecological studies published in grey literature (e.g., [ 47 ]). In addition, most published research focused on the Western Mediterranean basin and the Adriatic Sea, while amphipod fauna in marine caves of the Eastern basin and the African coast remain understudied [48]. In the context of baseline assessments and ecological studies, it is imperative to pay more attention to the motile benthic macrofauna, since this group holds significant importance in comprehending the biodiversity and functioning of marine habitats. In an extensive review about Mediterranean marine caves, Gerovasileiou and Bianchi [ 22 ] reported on the occurrence of 83 amphipod taxa from 19 studies, but no detailed checklist was provided. Here, we summarize the existing knowledge about marine and brackish-water amphipods in Mediterranean marine and anchialine caves. First, we collected new information on the amphipod diversity of marine caves of the understudied Eastern Mediterranean basin through targeted samplings. New and previous data for other Mediterranean regions were combined in order to provide an updated inventory of the species reported from cave environments. Finally, we present the geographical and ecological patterns of amphipod fauna in Mediterranean marine and anchialine caves. 2. Materials and Methods 2.1. Amphipod Sampling New records of amphipods were gathered between 2010 and 2022 from 13 marine caves, spanning from the East Ionian Sea (eight marine caves in Zakynthos Island) to the Aegean Sea (two caves in Lesvos Island, two in Crete and one in Rhodes) (Figure 1). In each cave, samples of dominant primary substrates (e.g., sponges, cnidarians, and bryozoans) were collected with SCUBA diving from different cave zones (a brief description of the depth, cave topography, etc., is provided in Supplementary Table S1). The primary substrates were first enclosed in plastic containers to prevent loss of epifauna, and then were carefully detached. In the caves of Lesvos Island, samples were collected using a quadrat sampler (20 × 20 cm) [ 49 ] at 10 stations (six in Fara cave and four in Agios Vasilios cave; with three replicate samples per station), located on the cave walls and ceilings along the horizontal cave axis (Figure 2—for detailed information see [ 50 – 52 ]). In the laboratory, all samples (already preserved in 70% ethanol) were washed through a 0.5 mm mesh sieve. Amphipods were identified to species level and counted. Diversity 2023, 15, x FOR PEER REVIEW 4 of 25 Figure 1. Location of the marine caves providing new material included in the present study. Figure 2. Lateral and plan views of Fara (left) and Agios Vasilios caves (right) of Lesvos Island. Location of collected samples is indicated in relation to distance from the entrances. Open blue circles: ceiling samples; full violet circles: wall samples. Modified from [51]. Figure 1. Location of the marine caves providing new material included in the present study. Diversity 2023,15, 1180 4 of 23 Diversity 2023, 15, x FOR PEER REVIEW 4 of 25 Figure 1. Location of the marine caves providing new material included in the present study. Figure 2. Lateral and plan views of Fara (left) and Agios Vasilios caves (right) of Lesvos Island. Location of collected samples is indicated in relation to distance from the entrances. Open blue circles: ceiling samples; full violet circles: wall samples. Modified from [51]. Figure 2. Lateral and plan views of Fara ( left ) and Agios Vasilios caves ( right ) of Lesvos Island. Location of collected samples is indicated in relation to distance from the entrances. Open blue circles: ceiling samples; full violet circles: wall samples. Modified from [51]. 2.2. Literature Review We reviewed the existing knowledge about marine and brackish-water amphipods in Mediterranean marine and anchialine caves. The initial dataset was sourced from the Mediterranean marine cave biodiversity database by [ 24 ] and Stygofauna Mundi [ 53 ], followed by a thorough literature review from a wide variety of sources, including peerreviewed articles and grey literature sources (e.g., PhD theses and technical reports) (a full reference list is available in Supplementary Tables S1 and S2). Original genus and species names were updated to the World Register of Marine Species taxonomic backbone (WoRMS, last accessed on 28 July 2023; [ 54 ]). The final amphipod checklist will be uploaded to the World Register of marine Cave Species (WoRCS, http://www.marinespecies.org/worcs/, last accessed on 28 July 2023; [ 55 ]), a thematic species database of WoRMS which aims at creating a comprehensive taxonomic and ecological database of species from marine caves worldwide. The resulting dataset consisted of occurrence data by cave. Separate datasets were compiled for marine and anchialine caves since they are characterized by different environmental constraints and harbour a distinctive fauna. Marine caves, which open directly on the coastline or on the seafloor, present a greater water exchange with the open sea, whereas anchialine caves often lack such entrances, resulting in a greater isolation. Consequently, anchialine caves typically exhibit distinct freshwater, brackish and marine water bodies, with varying residence times [ 56 ]. In this review, records from interstitial habitats, coastal wells or other freshwater hypogean habitats were not considered. Consequently, only those amphipods collected from marine or brackish waters within anchialine cave systems are considered. Nearby anchialine caves located in limestone terrains are often part of the same karstic systems. Therefore, multiple reports of the same species from different but presumably connected caves were considered as single-study sites. Marine caves show high heterogeneity with regard to their submersion level (i.e., fully or semi-submerged), morphology (i.e., blind-ended or tunnel-shaped), bathymetry, substrate type (hard or soft bottom) and ecological zonation (i.e., entrance, semi-dark and dark zones, and associated biocoenoses) [ 22 ]. When available, this type of information was also catalogued to explore potential ecological patterns (detailed information about cave features and references is provided in Supplementary Table S1. Diversity 2023,15, 1180 5 of 23 2.3. Ecological Characterization of the Species and Data Analysis Once the species occurrence dataset was compiled, the data were grouped in eight Mediterranean regions according to [ 22 ] to investigate regional diversity patterns, namely Alboran Sea, Algero-Provençal Basin, Tyrrhenian Sea, Tunisian Plateau, Adriatic Sea, Ionian Sea, Aegean Sea and Levantine Sea. Spearman’s rank correlation coefficient was calculated using the Social Science Statistics website (https://www.socscistatistics.com/, accessed on 1 August 2023) to investigate the relationship of amphipod species number with the number of publications and number of caves studied in each region, as a non-parametric measure of statistical dependence. In addition, all identified amphipod species were assigned to different ecological categories with regard to their zoogeographic and bathymetric distribution, substrate category and feeding habits in order to explore possible patterns. Precisely, the species were classified into four zoogeographic categories based on the classifications suggested by [ 57 – 59 ] and the World Amphipoda Database of WoRMS [ 54 ]: (i) Mediterranean endemics; (ii) Mediterranean–Atlantic; (iii) Mediterranean–Atlantic–Arctic; and (iv) Cosmopolitan species. The classification of bathymetric distribution included four zones according to [ 57 ], namely intertidal, infralittoral, circalittoral and bathyal. However, many species presented a wide bathymetric range comprising several of the aforementioned zones. Concerning substrate preference, soft bottom amphipods were categorized into those reported from coarse sands, fine/medium sands, muddy bottoms and indifferent (i.e., those present in a wide spectrum of sediment types). In the case of amphipods inhabiting hard bottoms, information was compiled about the primary substrate where they were reported (macroalgae, Porifera, Cnidaria, Bryozoa, Annelida, Mollusca, etc.), based on data obtained from different sources [ 29 , 57 , 60 – 73 ]. It should be noted that the above ecological characterization applies to the overall distribution of amphipod fauna across the Mediterranean Sea in all types of marine habitats, given the limited ecological knowledge about cave-dwelling amphipods. Amphipods were assigned according to their feeding habits based on previous data from gut content analyses [ 20 ]. Four categories were considered, specifically, detritivores (species with more than 95% of detritus in their gut content), herbivores (>50% of algae), carnivores (>50% of prey) and omnivores (>5% of different trophic sources). In a few cases where data were not available for a given species, the trophic category was assigned based on data about other species of the same genus. New data from Eastern Mediterranean marine caves was mainly qualitative (occurrence data by cave or substrate type), except for the two marine caves of Lesvos Island, where amphipod abundance (N) and species richness (S) were also obtained. One-way permutational ANOVA (PERMANOVA) was used to examine the variability of the above measures across the stations of each cave (factor, station; fixed with six levels for Fara cave and four levels for Agios Vasilios cave). Statistical analysis was undertaken using the PRIMER-E v.6 software package [74]. 3. Results 3.1. Research Effort Overview Information on marine and brackish-water amphipods reported from Mediterranean marine and anchialine caves was sourced from 98 studies (33 about marine and 66 about anchialine caves; Supplementary Tables S1 and S2). These studies included information about 42 marine caves and 50 anchialine systems (including complex karstic systems with multiple nearby entrances). Marine caves have been studied in six countries, namely Spain (6 studies in 10 caves), France (5 studies in 6 caves), Italy (14 studies in 10 caves), Croatia (1 study in 1 cave), Malta (5 studies in 3 caves) and Greece (3 studies, including this work, in 13 caves), and six regions: the Adriatic Sea (1 cave), the Tunisian Plateau (2 caves), the Aegean Sea (5 caves), the Alboran Sea, the Algero-Provençal Basin and the Ionian Sea (9 caves in each). In contrast, anchialine caves have been investigated in six countries, namely Spain (31 studies in 20 sites, all located in the Balearic Islands), France (2 studies in 2 sites), Italy (21 studies in 6 sites), Croatia (20 studies in 18 sites), Montenegro (1 study in Diversity 2023,15, 1180 6 of 23 1 site) and Greece (4 studies in 3 sites), comprising the Algero-Provençal Basin (41 studies in 25 sites), the Adriatic Sea (20 studies in 19 sites) and the Ionian Sea (18 studies in 6 sites). 3.2. New Data from Marine Caves of the Eastern Mediterranean Sea Our survey provided 14 new records of amphipods from marine caves of Greece (only eight species were previously known from marine caves of Greece). Considering the new records obtained in the present study, a total of 16 species has been reported from the Aegean Sea and 13 from the East Ionian Sea. Agios Vasilios cave (11 species) and Shrimp cave (8 species) had, respectively, the highest number of species in these two regions. Our quantitative survey in the caves of Lesvos Island yielded a total of 247 individuals (64 from Agios Vasilios cave and 183 from Fara cave) that belonged to 13 species (including two taxa identified at the genus level and an unidentified taxon) (Table 1). Abundance per scraped quadrat sampler (400 cm 2 ) ranged from zero to 16 individuals in Agios Vasilios cave and up to 45 individuals in Fara cave. Mean amphipod density in Agios Vasilios cave was 5.333 individuals ± 1.597 (SE, standard error), while that of Fara cave was double, with 10.167 individuals ± 2.949 (SE) in the scraped quadrates. The most abundant species was Leptocheirus bispinosus (48.2% of all individuals), followed by Perrierella audouiniana (20.2%), and Liljeborgia dellavallei (11.3%). The highest abundance and species richness were found on cave walls and ceilings of the entrance and semi-dark cave zones, dominated by sponges and scleractinians, whereas extremely low abundances were retrieved from the dark cave zone (Table 1). Abundance and species richness differed significantly among sampling stations in both the Fara and Agios Vasilios caves (Table 2). Table 1. Amphipods recorded in the Fara and Agios Vasilios caves. The mean number of individuals is presented for each species along with the total number of individuals per sampling station. For each sampling station, the location inside the cave (L w, Left wall; R w, Right wall; C, Ceiling; and Entr, Entrance), distance from the entrance (m), biocoenosis (Cor, Coralligenous Biocoenosis; SD, Semidark Cave Biocoenosis; Trans, Transitional Zone; and Dark, Dark Cave Biocoenosis) and main sessile biota (Rh, Rhodophytes; Sc, Scleractinian corals; Sp, Sponges; Sr, Serpulid polychaetes; and Br, Bryozoans) are indicated. Fara Cave Agios Vasilios Cave Sample location L w C R w L w C Walls C Walls C Walls Distance from entrance (m) Entr 5 5-10 5-10 15-20 20-30 5-10 5-10 15-20 15-20 Biocoenosis Cor SD SD SD Trans Dark SD SD Dark Dark Dominant encrusters Rh Sc-Sp Sc-Sp Sp Sr-Br Sr-Sp-Br Sc-Sp Sp Sc-Sp-Sr Sp-Sr Species Sampling stations F1 FC1 F2 F3 FC2 F4 VC1 V1 VC2 V2 Apherusa sp. 0.3 Colomastix pusilla 0.3 0.3 0.7 0.3 3.0 Gammarus subtypicus 0.7 Iphimedia carinata 0.3 0.3 Iphimedia sp. 0.3 Leptocheirus bispinosus 10.0 21.7 5.3 2.3 0.3 Leptocheirus pectinatus 3.0 2.3 0.3 0.3 Leucothoe spinicarpa 1.3 0.3 Liljeborgia dellavallei 0.3 3.7 2.3 2.7 0.3 Lysianassina longicornis 0.3 Perrierella audouiniana 2.0 10.0 0.7 3.0 1.0 Stenothoe antennulariae 0.7 Amphipoda unid. 1.0 0.3 Total number of individuals 8 73 74 12 16 0 37 22 3 2 Mean abundance 1 8 8.3 1.3 1.7 0 2.3 4.0 0.3 0.3 Species richness 3 5 3 3 1 0 6 9 2 3 In the studied Ionian caves, since the primary substrate type of most samples consisted of sponges (Agelas oroides and Chondrosia reniformis), sponge-associated species such as Colomastix pusilla were those with the highest frequency of occurrence (it was found in 55% of the sponge samples collected) and the highest abundance (26 individuals) (Table 3). Other common species were Gitana sarsi (15 individuals found in four caves) and Leptocheirus pectinatus (8 individuals collected in two caves). Other erect invertebrates such as bryozoans Diversity 2023,15, 1180 7 of 23 and hydroids proved to be a suitable habitat for amphipods (16 amphipod individuals in a single bryozoan colony in the Korakonisi cave), but the scarcity of these substrates in most of the caves explored prevented a detailed exploration of their associated fauna. Table 2. One-way PERMANOVA results for amphipod species richness and abundance across the sampling stations of Agios Vasilios and Fara caves. Species Richness Abundance Cave Source df MS Pseudo-F P (Perm) MS Pseudo-F P (Perm) Agios Vasilios Station 3 12.56 10.76 0.005 93.56 13.37 0.006 Res 8 1.17 7.00 Fara Station 5 2.89 5.78 0.015 379.17 5.95 0.010 Res 12 0.50 63.72 Table 3. Abundance of each amphipod species collected along the new targeted biodiversity surveys in the Ionian Sea. CS = Cape Skinari; FC = Four caves; SC = Sulphur cave; BC = Butterfly cave; PL = Porto Limnionas; AO = Agelas oroides; Br = Bryozoans; CO = Chondrosia reniformis; Hy = Hydroids. CS Blue Cave FC Shrimp Cave SC BC Korakonisi PL Substrate AO AO CO AO AO Hy AO AO AO Br AO Species Apocorophium acutum 1 1 Apolochus picadurus 1 Colomastix pusilla 1 17 1 6 1 Coxischyrocerus inexpectatus 1 Gitana sarsi 5 9 1 Leptocheirus bispinosus 3 1 Leptocheirus guttatus 1 Leptocheirus pectinatus 1 2 5 Leucothoe spinicarpa 1 Phtisica marina 1 1 Pseudoprotella phasma 1 1 Stenothoe monoculoides 1 Stenothoe sp. 1 3.3. Amphipod Diversity in Mediterranean Marine Caves A total of 106 amphipod species belonging to 38 different families was reported from 42 Mediterranean marine caves in six countries (Table 4). Families with higher numbers of species were Ischyroceridae (7 species), as well as Maeridae, Phoxocephalidae and Stenothoidae (6 species each). Approximately half (49%) of the species are known from a single marine cave, whereas 80% were found in less than five locations (Supplementary Table S3). Highest species richness was recorded in Cueva de Cerro-Gordo in Spain (32 species), followed by Grotte de L’île Plane in France and Grotta del Mago in Ischia, Italy (26 species each) and Grotta di Bergeggi in Liguria, Italy (20 species). The most widespread species were Colomastix pusilla (known from 16 caves), Phtisica marina (13), Pseudoprotella phasma (12) and Leucothoe spinicarpa (10), often found in high densities. Other species with a documented high abundance at cave entrances or semidark cave sectors were Autonoe rubromaculatus,Coxischyrocerus inexpectatus,Harpinia crenulata, Harpinia pectinata,Lembos websteri, and Stenopleustes nodifera [ 42 , 75 , 76 ]. A few species, such as Aristias neglectus and Harpinia pectinata, were reported as common or highly abundant in the inner dark zones of marine caves [ 76 , 77 ]. However, most studies lack information about abundance estimations and/or the ecological zone where the amphipod species were found. Overall, 41 amphipod species were reported from cave entrances, 80 from semi-dark cave sectors, and 27 inhabited the dark cave biocoenosis. Diversity 2023,15, 1180 8 of 23 Table 4. Number of marine caves where amphipod species have been reported in each Mediterranean biogeographical region. * New records from marine caves of Greece. †This record most likely corresponds to Elasmopus vachoni Mateus & Mateus, 1966. Family Species Biogeographical Region Alboran Sea Algero-Provençal Basin Tyrrhenian Sea Tunisian Plateau Adriatic Sea Ionian Sea Aegean Sea Ampeliscidae Ampelisca rubella A. Costa, 1864 - - 1 - - - - Ampelisca serraticaudata Chevreux, 1888 - - - - - - - Ampelisca truncata Bellan-Santini & Kaim-Malja, 1977 - - - 1 - - - Ampelisca typica (Spence-Bate, 1856) 1 - - - - - - Amphilochidae Amphilochus manudens Spence Bate, 1862 - 1 - - - - - Apolochus neapolitanus (Della Valle, 1893) - - 1 - - - - Apolochus picadurus (J.L. Barnard, 1962) * - - - - - 1 - Gitana cf. abyssicola G.O. Sars, 1892 1 - - - - - - Gitana sarsi Boeck, 1871 * - 1 - - - 4 - Ampithoidae Ampithoe ramondi Audouin, 1826 - 2 1 - - - - Pleonexes helleri (Karaman, 1975) - - 1 - - - - Aoridae Aora spinicornis Afonso, 1976 - 1 1 - - - - Autonoe rubromaculatus (Ledoyer, 1973) - - 1 - - - - Lembos websteri Spence Bate, 1857 4 3 - - - - - Microdeutopus algicola Della Valle, 1893 1 - - - - - - Aristiidae Aristias neglectus Hansen, 1988 - 5 - - - - - Perrierella audouiniana (Spence Bate, 1857) * - 1 1 - - - 2 Atylidae Nototropis swammerdamei (H. Milne-Edwards, 1830) - - 1 - - - - Nototropis vedlomensis (Spence Bate & Westwood, 1862) - 1 - - - - - Bogidiellidae Marinobogidiella tyrrhenica (Schiecke, 1979) - - 1 - - - - Calliopiidae Apherusa bispinosa (Spence Bate, 1857) - 2 1 - - - - Caprellidae Caprella hirsuta Mayer, 1890 - - 1 - - - - Caprella liparotensis Haller, 1879 - 1 1 - - - - Liropus minimus Mayer, 1890 - - 1 - - - - Phtisica marina Slabber, 1796 3 5 - - - 3 2 Pseudolirius kroyeri (Haller, 1879) 4 - - - - - - Pseudoprotella phasma (Montagu, 1804) * 4 2 2 - - 3 1 Colomastigidae Colomastix pusilla Grube, 1861 - 5 3 - - 6 2 Corophiidae Apocorophium acutum (Chevreux, 1908) * 1 1 1 - - 1 - Leptocheirus bispinosus Norman, 1908 1 1 - - - 2 2 Leptocheirus guttatus (Grube, 1864) * - - - - - 1 - Leptocheirus longimanus Ledoyer, 1973 1 - - - - - - Leptocheirus pectinatus (Norman, 1869) * 2 3 - - - 2 1 Monocorophium sextonae (Crawford, 1937) - 1 - - - - - Cressidae Cressa cristata Myers, 1969 - - - 2 - - - Cressa mediterranea Ruffo, 1979 - 2 - 1 - - - Cyproideidae Peltocoxa marioni Catta, 1875 - - 1 - - - - Dexaminidae Dexamine spiniventris (Costa, 1853) 1 5 1 - - - - Diversity 2023,15, 1180 9 of 23 Table 4. Cont. Family Species Biogeographical Region Alboran Sea Algero-Provençal Basin Tyrrhenian Sea Tunisian Plateau Adriatic Sea Ionian Sea Aegean Sea Dexamine spinosa (Montagu, 1813) 1 3 1 - - - - Tritaeta gibbosa (Spence Bate, 1862) 1 - 3 - - - - Gammaridae Gammarus subtypicus Stock, 1966 - - - - - - 1 Hyalidae Apohyale crassipes (Heller, 1866) - 1 - - - - - Protohyale (Protohyale) schmidtii (Heller, 1866) - 1 1 - - - - Iphimediidae Iphimedia carinata Heller, 1866 * - - - - - - 2 Iphimedia eblanae Spence Bate, 1857 - 2 - - - - - Iphimedia minuta G.O. Sars, 1883 - 4 - - - - - Ischyroceridae Centraloecetes dellavallei (Stebbing, 1899) - 1 - - - - - Coxischyrocerus inexpectatus (Ruffo, 1959) * 3 - 1 - - 1 - Ericthonius brasiliensis (Dana, 1853) - 1 1 - - - - Ericthonius punctatus (Spence Bate, 1857) 1 - - - - - - Jassa marmorata Holmes, 1905 - 1 1 - - - - Jassa slatteryi Conlan, 1990 1 - - - - - - Microjassa cumbrensis (Stebbing & Robertson, 1891) 2 - - - - - - Plumulojassa ocia (Spence Bate, 1862) * - - - - - - 1 Kamakidae Cerapopsis longipes Della-Valle, 1893 4 - - - - - - Leucothoidae Leucothoe oboa Karaman, 1971 1 - - - - - - Leucothoe spinicarpa (Abildgaard, 1789) 1 5 1 - - 1 2 Liljeborgiidae Idunella nana (Schiecke, 1973) - - 1 - - - - Liljeborgia dellavallei Stebbing, 1906 1 3 2 - - - 1 Lysianassidae Lysianassa caesarea Ruffo, 1987 - - - - - - 1 Lysianassa costae H. Milne Edwards, 1830 - 5 - - - - - Lysianassa pilicornis Heller, 1866 - 2 - - - - - Lysianassina longicornis (Lucas, 1846) * - 3 - - 1 - 1 Maeridae Elasmopus rapax Costa, 1853 - 1 - - - - - Elasmopus pectenicrus (Spence Bate, 1862) †1 - - - - - - Elasmopus pocillimanus (Spence Bate, 1862) - 1 - - - - - Maera grossimana (Montagu, 1808) - - 1 - - - - Maeropsis revelata (Krapp-Shcickel, Martì& Ruffo, 1996) - - - 1 - - - Othomaera othonis (H. Milne Edwards, 1830) - 1 - - - - - Quadrimaera inaequipes (A. Costa in Hope, 1851) 1 4 1 1 - - - Melitidae Melita palmata (Montagu, 1804) - 1 - - - - - Nuuanuidae Gammarella fucicola (Leach, 1814) 3 2 - - - - - Oedicerotidae Deflexilodes acutipes (Ledoyer, 1893) - - - - - - - Deflexilodes griseus (Della Valle, 1893) 3 1 - - - - - Monoculodes packardi Boeck, 1871 1 - - - - - - Perioculodes longimanus (Spence-Bate & Westwood, 1868) 5 - - - - - - Pontocrates arenarius (Spence Bate, 1858) 1 - - - - - - Synchelidium cf. longidigitatum Ruffo, 1947 1 - - - - - - Diversity 2023,15, 1180 16 of 23 descriptive faunistic studies conducted in any marine cave, and especially those addressing cryptic fauna, will probably provide novel and valuable information. 4.2. Anchialine Cave Fauna Amphipod diversity patterns among anchialine caves in different Mediterranean regions are largely affected by the distribution of these fragmented habitats as well as sampling effort. To date, three areas within these regions stand out in terms of amphipod records: the Dinaric karst (Croatia and Montenegro), Puglia (Italy) and the Balearic Islands (Spain). In contrast to the wide geographical and ecological distribution exhibited by marine cave amphipods, many of the species reported from marine and brackish waters in anchialine caves had limited distribution ranges. Psammogammarus burri was described from brackish waters of Cova des Burri (Cabrera, Balearic Islands) and has never been found elsewhere [ 118 ]. The species Bogidiella cerberus is known only from its type locality in Alepotrypa cave, in Peloponnese, Greece [ 119 ]. Racovella birramea is found exclusively in Cova des Coll (Mallorca Island). Bogidiella balearica, another Balearic endemic species, inhabits several anchialine caves in a small region of Mallorca and Cabrera Islands [ 120 ]. Pseudoniphargus leucatensis is only known from an anchialine cave on the French coast, which is also the only locality in our study where Niphargus angelieri is present. However, the latter has also been reported from other freshwater localities [ 121 ]. Despite being reported in a higher number of locations (>10 anchialine caves), Metacrangonyx longipes and Niphargus hebereri are also confined to a specific region. The former is exclusively distributed in Balearic groundwaters but, due to its tolerance to a wide range of salinity, it extends from freshwaters up to 200 m above the sea level to marine littoral caves [ 122 ]. Niphargus hebereri is endemic to the Dinaric karst, where it tolerates wide variations in salinity, oxygen concentration, and pollutants [ 123 , 124 ]. Moreover, N. hebereri and its congener N. pectencoronatae (also reported from brackish waters in anchialine caves) are listed as endangered in the Red List of Croatian Fauna due to their narrow distribution range and habitat specialization [ 125 ]. In any case, both Niphargus and Pseudoniphargus are highly diversified along the Mediterranean basin, but their distribution in anchialine caves is mainly restricted to freshwater areas [ 126 – 128 ]. Similarly, the family Hadziidae includes many subterranean species but only Hadzia fragilis and Metahadzia minuta are recorded in brackish waters of anchialine caves near the coast (e.g., [123,129,130]). The only species with a wide distribution in anchialine caves is Salentinella angelieri, recorded in our census at the Algero-Provençal Basin, the Adriatic and the Ionian Sea. Some species of the genus Rhipidogammarus (e.g., R. rhipidiophorus) also display a wide circum-Mediterranean distribution associated with coastal groundwaters, including records from brackish waters in littoral caves with marine influence [ 131 , 132 ]. However, these taxa were not considered in the present study, since the aforementioned locations do not fall within the concept of anchialine caves. The family Salentinellidae currently includes 14 species of strictly subterranean aquatic amphipods but only Salentinella angelieri and S. gracillima are common inhabitants of brackish-water coastal aquifers and anchialine caves [ 133 ]. Salentinella gracillima is endemic to the coast of Puglia (Southern Italy), although its synonymy with S. angelieri has been recently suggested [ 133 ]. Overall, the distribution of S. angelieri encompasses the coasts of Spain, France, Italy, Croatia, Greece, Algeria, and Morocco [ 120 , 133 – 135 ]. Findings of this species in different subterranean biotopes and regions point out its high adaptability, so that it can be considered as a generalist species in coastal phreatic systems rather than as an anchialine specialist. In any case, S. angelieri may represent a species complex (Stoch, personal communication in [ 124 ]), which may be resolved by applying molecular and morphological approaches. Phylogenetic analyses proved to be valuable tools for revealing cryptic diversity and clarifying the phylogeography and evolutionary relationships of anchialine species [ 136 ]. Such tools have been already applied to some of the species herein reported, disclosing, for example, the Diversity 2023,15, 1180 17 of 23 existence of fragmented population structures and independent lineages of Pseudoniphargus mercadali and Metacrangonyx longipes in different Balearic Islands [137,138]. Among the most common adaptations of anchialine amphipods to cave environments is the reduction of eyes and pigmentation, which could be observed in genera such as Bogidiella,Racovella,Niphargus and Pseudoniphargus. Other troglomorphic features associated with darkness include the elongation of body appendages and an increasing number of mechano- and chemoreceptors. The typically elongated third uropod of Niphargus and Pseudoniphargus, the long antennae and uropods of Psammogammarus burri, as well as the presence of peduncular protuberances in pleopods and uropods of Bogidiella and Metahadzia have also been interpreted in this context [ 120 , 139 ]. As they live in a buffered environment, cave species might exhibit lower tolerance to environmental changes than their surface counterparts. Some Niphargus species might be obligate stenothermic, particularly vulnerable to pollutants [ 123 , 140 ]. On the other hand, some environmental parameters may also fluctuate in anchialine caves affecting their amphipod fauna. As expected, anchialine amphipods such as Niphargus hebereri,Salentinella angelieri and Metacrangonyx longipes proved to be more tolerant to salinity changes than other species thriving on cave streams or at the boundary between the surface and subterranean ecosystems [ 141 ]. Oxygen availability may be also limited spatially or at periods (due to the lack of primary production) and some species (e.g., N. hebereri) develop behavioural and metabolic responses to such fluctuations [ 142 ]. Other life history and physiological adaptations to these extreme environments include reduced metabolic rate, increased longevity, delayed maturity, fewer eggs per clutch and infrequent reproduction [ 139 , 143 , 144 ]. However, these adaptations have not been thoroughly examined in anchialine amphipods, which constitute an interesting model group in order to study species diversification and test evolutionary and ecological hypotheses. 5. Conclusions The present work highlights the high diversity of amphipods inhabiting Mediterranean marine caves. Our review showed that despite the relatively low number of studies dealing with amphipod fauna in marine caves, this habitat encompasses approximately 20% of the known Mediterranean amphipod species. The highest number of cave amphipod species has been found in the northwestern Mediterranean, which is the most studied area. Concerning the understudied Eastern Mediterranean basin, our 14 new records of cave amphipod species indicate the need for further exploration of invertebrate diversity inhabiting these unique ecosystems. On the other hand, the ecological characterization of the cave amphipod fauna compiled herein unveiled the wide ecological amplitude of these assemblages. None of the 106 amphipod species reported in marine caves are exclusive of cave habitats, being also present in other marine habitats such as macroalgal forests, seagrass meadows or coralligenous outcrops. Most of the species may inhabit different substratum types and exhibit a generalist diet and wide bathymetric and geographical range. In contrast to marine cave amphipods, species reported from Mediterranean anchialine caves often had narrow distribution range and showed habitat specialization. Many of the 17 amphipod species reported from marine-brackish waters in anchialine caves have been found exclusively in one or few cave localities and display morphological and physiological adaptations to the cave environment. These biological and ecological patterns provide valuable insights that may contribute to the conservation of this highly diverse and yet overlooked biotic component of marine caves. Furthermore, the species inventory presented can serve as a baseline study to further increase our knowledge of Mediterranean cave amphipods. Supplementary Materials: The following supporting information can be downloaded at https://www. mdpi.com/article/10.3390/d15121180/s1. Table S1: Marine cave description and references; Table S2: Amphipods in anchialine caves and references; Table S3: Amphipods in marine caves; Table S4: Ecological characterization of marine amphipods. Diversity 2023,15, 1180 18 of 23 Author Contributions: Conceptualization, C.N.-B. and V.G.; methodology, C.N.-B., A.M., J.S.-V., S.C., M.D. and V.G.; sampling: C.N.-B., J.S.-V., S.C., M.D. and V.G.; identified samples, C.N.-B., M.D., W.P., E.V. and V.G.; formal analysis, C.N.-B., A.M., J.S.-V., S.C., M.D., W.P., E.V. and V.G.; writing—original draft preparation, C.N.-B.; writing—review and editing, A.M., J.S.-V., S.C., M.D., V.G., W.P. and E.V.; funding acquisition, C.N.-B. and V.G. All authors have read and agreed to the published version of the manuscript. Funding: The present article benefited from two mobility grants awarded by the University of Seville and Spanish Ministry of Science and Innovation to C.N.-B. and J.S.-V., respectively. Additional funding was also provided by the project US-1381059 (Junta de Andalucía Proyectos I + D + I en el marco del Programa Operativo FEDER Andalucía) and TD2021-129725A (Spanish Ministry of Science and Innovation—Proyectos de Transición Ecológica y Transición Digital). S.C. was financially supported by doctoral fellowships by Agência Regional para o Desenvolvimento da Investigação, Tecnologia e Inovação (ARDITI-M1420-09-5369-FSE-000002). The material collection in Aegean marine caves was supported by (a) the Research Funding Programme “Heracleitus II: Investing in knowledge society” (EU Social Fund and Greek national funds); and (b) the project “Centre for the study and sustainable exploitation of Marine Biological Resources (CMBR)” (MIS 5002670), implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020) and co-financed by Greece and the EU (European Regional Development Fund). Data Availability Statement: Data supporting all the reported results are publicly available at the Supplementary Materials here provided. Acknowledgments: We are grateful to the Management Unit of Zakynthos and Ainos National Parks and Protected Areas of the Ionian islands of the Natural Environment and Climate Change Agency (NECCA) for providing sampling permission in marine caves of the Natura sites GR 2210001 and GR 2210002. We would also like to thank Dennis Mohr and the staff of Nero Sport Diving Center (Limni Keriou, Zakynthos), Charalampos Dimitriadis and Maria Sini for their valuable knowledge and help during sampling in Zakynthos and Lesvos. We would also like to thank Margaret Eleftheriou for reviewing the English language of the text and three anonymous referees for their helpful comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest. Funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References 1. Chen, Y.Y.; Edgar, G.J.; Fox, R.J. 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