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INTRODUCTION
One of the most important roles of sponges in the marine ecosystem is that they function as ecosystem engineers or “living hotels”, providing micro-habitat to diverse organisms (e.g. Koukouras et al. 1985, Ribeiro et al. 2003, Padua et al. 2013). This ecological role of sponges has gained increasing research interest over the last few decades for both scientific and conservation purposes. Sponge-associated fauna has been studied in a variety of habitats, ranging from shallow rocky beds in temperate seas (Koukouras et al. 1985, 1992, 1996) to tropical coral reefs (Villamizar and Lauchlin 1991), seagrass meadows (Çinar et al. 2002), polar regions (Amsler et al. 2009) and deep-sea ecosystems (Klitgaard 1991).
Although sponges constitute the dominant sessile organisms in marine cave environments (Gerovasileiou and Voultsiadou 2012, 2016), their functional role as ecosystem engineers in this habitat type has received little attention. Recently, Navarro-Barranco et al. (2016) studied the crustaceans associated with invertebrate hosts, among which the sponge Ircinia variabilis, in a marine cave in the Alboran Sea and found that all the examined species were equally used as refuges by crustaceans.
Sessile assemblages of marine caves are characterized by a decrease in diversity, biomass and three-dimensional complexity towards inner dark cave sectors, driven by the extreme oligotrophy due to the reduction of light and the increasing water confinement (Harmelin et al. 1985, Bianchi and Morri 1994). On the other hand, sessile invertebrates have been shown to increase the structural complexity of hard substrates (Voultsiadou et al. 2010, Navarro-Barranco et al. 2014, 2016). This complexity in the space-limited oligotrophic environment of the dark caves could be particularly significant for the motile cave dwellers. However, the motile invertebrate diversity of Mediterranean marine caves has received limited interest compared with the sessile biota; the few relevant studies have mainly taken place in the northwestern Mediterranean Sea (Ledoyer 1965, Navarro-Barranco et al. 2014, 2016), while such data are lacking from the eastern basin (but see Gerovasileiou et al. 2015b).
From the trophic point of view, marine caves are considered simple systems in which organic supply depends on the circulation of water entering the cave, given the absence of primary production due to lack of light (Bianchi et al. 2003). The trophic structure of Mediterranean caves is insufficiently studied,...