Abstract
Marine caves are widely acknowledged for their unique biodiversity and constitute a typical feature of the Mediterranean coastline. Herein an attempt was made to evaluate the ecological significance of this particular ecosystem in the Mediterranean Sea, which is considered a biodiversity hotspot. This was accomplished by using Porifera, which dominate the rocky sublittoral substrata, as a reference group in a meta-analytical approach, combining primary research data from the Aegean Sea (eastern Mediterranean) with data derived from the literature. In total 311 species from all poriferan classes were recorded, representing 45.7% of the Mediterranean Porifera. Demospongiae and Homoscleromorpha are highly represented in marine caves at the family (88%), generic (70%), and species level (47.5%), the latter being the most favored group along with Dictyoceratida and Lithistida. Several rare and cave-exclusive species were reported from only one or few caves, indicating the fragmentation and peculiarity of this unique ecosystem. Species richness and phylogenetic diversity varied among Mediterranean areas; the former was positively correlated with research effort, being higher in the northern Mediterranean, while the latter was generally higher in caves than in the overall sponge assemblages of each area. Resemblance analysis among areas revealed that cavernicolous sponge assemblages followed a pattern quite similar to that of the overall Mediterranean assemblages. The same pattern was exhibited by the zoogeographic affinities of cave sponges: species with Atlanto-Mediterranean distribution and Mediterranean endemics prevailed (more than 40% each), 70% of them having warm-water affinities, since most caves were studied in shallow waters. According to our findings, Mediterranean marine caves appear to be important sponge biodiversity reservoirs of high representativeness and great scientific interest, deserving further detailed study and protection.
Highlights
Coastal marine hard substrata have a great ecological, scientific, and economic value due to their high structural complexity, which supports rich marine communities
The current research in caves of the North and South Aegean revealed the presence of 80 sponge taxa, 61 of which were identified to the species level
Zoogeographical analysis preformed in the present study showed that cave sponge assemblages of the Mediterranean seem to follow the pattern of the overall Mediterranean sponge fauna concerning the temperature range of species distribution
Summary
Coastal marine hard substrata have a great ecological, scientific, and economic value due to their high structural complexity, which supports rich marine communities. Marine caves are acknowledged for their rich biodiversity and in several instances are an integral part of hotspot areas [4] They constitute a relatively discrete, difficult to approach environment, evidence on impacts of human activities [5] and warming temperatures attributed to climate change [6,7] emphasizes the need to evaluate and protect their biodiversity. This becomes more urgent in the Mediterranean Sea, which is a recognized hotspot from both the biodiversity and the socioeconomic point of view, since it i) hosts 7% of the world’s marine biodiversity (with only 0.82% of the oceans’ surface), ii) supports a large number of endemic species, and iii) suffers intense and variable anthropogenic pressure predicted to increase in the future [8]. The high endemism of the Mediterranean Sea provided strong motivation for the development of a coastal network of Marine Protected Areas (MPAs), but the current system of protection is not yet representative, since several areas and habitats are underrepresented [9], including the cave habitat
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