ORIGINAL PAPER Shallow-water sponge grounds along the Apulian coast (central Mediterranean Sea) Francesca Strano 1,2 & Valerio Micaroni 1,2 & Gabriele Costa 3 & Iacopo Bertocci 4,5 & Marco Bertolino 3 Received: 25 March 2019 /Revised: 8 October 2019 /Accepted: 19 December 2019 # Senckenberg Gesellschaft für Naturforschung 2020 Abstract Sponge grounds are complex three-dimensional benthic habitats dominated by sponges. These sponge-dominated assemblages have been reported worldwide, from the intertidal zone to the deep sea. In shallow euphotic waters, dense sponge aggregations have been mainly found in tropical areas, and their presence is in some cases related to environmental degradation and coral decline. The Mediterranean Sea is globally recognised as a biodiversity hotspot, where light-exposed rocky reefs are typically dominated by photophilous algae. However, high local anthropogenic pressures, coupled with climate change, are leading to the reorganisation of benthic communities and the occurrence of regime shifts in several areas. Here we report the first description of unusual, shallow-water sponge grounds in Mediterranean light-exposed rocky reefs, in an area previously impacted by the destructive date-mussel fishery. These assemblages, found along the Apulian coast (central Mediterranean Sea), are now (2017) characterised by a mean coverage of sponges ranging between 3% and 33%, with maximum values up to 85%. Variation in the structure of assemblages and in the abundance of individual taxa between depths has been tested by multivariate and univariate techniques. The spatial characterisation has been complemented with the taxonomic analysis of the sponge assemblages, which resulted in the identification of 14 sponge taxa. These findings are compared with results of previous research in the same area and discussed with particular reference to the potential variables involved in sponge dominance and spatial distribution in the present system and elsewhere. Keywords Porifera . Spatial distribution . Rocky subtidal . Barren habitat . Sponge-dominated habitat Introduction Sponges diverged from other metazoans over 600 million years ago and are now common sessile organisms in tropical, temperate, and polar benthic ecosystems (Brien et al. 1973; Srivastava et al. 2010; Bell et al. 2015). Sponges are consid- ered important members of benthic marine ecosystems, as a result of their extensive distribution, large biomass in many habitats, and the number of ecological roles that they play (Bell 2008; Heip et al. 2009). Sponge aggregations may create complex three- dimensional habitats, which, when characterised by large sponge biomass and space occupancy, are described as ‘sponge grounds’ (Heyward et al. 2010; Hogg et al. 2010; Bo et al. 2012; Murillo et al. 2012; Beazley et al., 2013; Maldonado et al. 2017). Sponge grounds are found in a variety of habitats, from the intertidal to the deep-sea environment (Maldonado et al. 2017). In shallow waters, dense sponge aggregations have been reported in polar, temperate, and tropical ecosystems, and Francesca Strano and Valerio Micaroni are first co-authors. Communicated by D. Janussen Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12526-019-01026-x) contains supplementary material, which is available to authorized users. * Francesca Strano francesca.strano.mare@gmail.com 1 Avamposto MARE, University of Salento, Lungomare Cristoforo Colombo s.n.c., I-73039 Tricase, Lecce, Italy 2 School of Biological Sciences, Victoria University of Wellington, Wellington 6140, New Zealand 3 Department for the Earth, Environment and Life Sciences (DISTAV), University of Genoa, Corso Europa 26, 16132 Genoa, Italy 4 Department of Biology, University of Pisa, CoNISMa, via Derna 1, 56126 Pisa, Italy 5 Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy Marine Biodiversity (2020) 50:7 https://doi.org/10.1007/s12526-019-01026-x