Tolerance of Sponge Assemblages to Temperature Anomalies: Resilience and Proliferation of Sponges following the 1997–8 El-Nin ˜ o Southern Oscillation Francisco Kelmo 1 *, James J. Bell 2 , Martin J. Attrill 3 1 Instituto de Biologia, Universidade Federal da Bahia, Campus Universita ´rio de Ondina, Salvador, Bahia, Brazil, 2 School of Biological Sciences, Victoria University of Wellington, Wellington, New Zealand, 3 Marine Institute, Plymouth University, Drake Circus, Plymouth, United Kingdom Abstract Coral reefs across the world are under threat from a range of stressors, and while there has been considerable focus on the impacts of these stressors on corals, far less is known about their effect on other reef organisms. The 1997–8 El-Nin ˜o Southern Oscillation (ENSO) had notable and severe impacts on coral reefs worldwide, but not all reef organisms were negatively impacted by this large-scale event. Here we describe how the sponge fauna at Bahia, Brazil was influenced by the 1997–8 ENSO event. Sponge assemblages from three contrasting reef habitats (reef tops, walls and shallow banks) at four sites were assessed annually from 1995 to 2011. The within-habitat sponge diversity did not vary significantly across the study period; however, there was a significant increase in density in all habitats. Multivariate analyses revealed no significant difference in sponge assemblage composition (ANOSIM) between pre- and post-ENSO years for any of the habitats, suggesting that neither the 1997–8 nor any subsequent smaller ENSO events have had any measurable impact on the reef sponge assemblage. Importantly, this is in marked contrast to the results previously reported for a suite of other taxa (including corals, echinoderms, bryozoans, and ascidians), which all suffered mass mortalities as a result of the ENSO event. Our results suggest that of all reef taxa, sponges have the potential to be resilient to large-scale thermal stress events and we hypothesize that sponges might be less affected by projected increases in sea surface temperature compared to other major groups of reef organisms. Citation: Kelmo F, Bell JJ, Attrill MJ (2013) Tolerance of Sponge Assemblages to Temperature Anomalies: Resilience and Proliferation of Sponges following the 1997–8 El-Nin ˜ o Southern Oscillation. PLoS ONE 8(10): e76441. doi:10.1371/journal.pone.0076441 Editor: Roberto Pronzato, University of Genova, Italy Received June 7, 2013; Accepted August 23, 2013; Published October 7, 2013 Copyright: ß 2013 Kelmo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work is funded by the corresponding author, Francisco Kelmo, with additional support from the Plymouth University Marine Institute to FK. The funder coordinated the overall research objectives and organised the sampling, identification of organisms and data collation throughout the survey, developed sampling methodology, input information on species biology and wrote the paper. Competing Interests: Co-author James Bell is a PLOS ONE Editorial Board member, however, this does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials. * E-mail: kelmo@ufba.br Introduction Coral reefs around the world are under threat from a range of local- and global-scale threats [1,2,3]. At local scales, these include habitat destruction, overfishing, pollution, sedimentation and invasive species. While these threats can have devastating impacts on reefs, in most cases management intervention and mitigation is possible at the scale of the impact. However, for global scale threats, particularly those related to climate change, ocean acidification and climatic variation (e.g. El-Nin ˜ o events), mitigat- ing and managing these impacts is much more challenging. While the consequences of these larger scale impacts on corals have received considerable attention, far less is known about their likely impacts on other non-calcifying reefs organisms (see [4]). For example, while the effects of large-scale El-Nin ˜o Southern Oscillation (hereafter ENSO) events on coral communities have been well described, particularly as a result of the 1997–8 event that had devastating impacts of many coral reefs ([5,6] many others), the influences of ENSO events on other dominant reef organisms are much less well known. Sponges are a major component of coral reef communities across the world (e.g. [7,8,9]), with a range of important functional roles, from efficiently processing vast quantities of water and stripping food particles, to acting as a major eroders of the carbonate reef structure [10,11]). Sponges also form important relationships with a range of microorganisms [12], which can facilitate high levels of benthic primary production and nutrient cycling. Despite the fact that sponges are such important components of reefs, we still understand comparatively little about their ecology and stress responses compared to corals (highlighted by [4,13]). Declines in coral abundance have been well-documented world- wide (e.g. [14,15]), and while there are many reports of increased algal abundance as coral cover and herbivorous fish abundance decline, through so-called ‘phase-shifts’, there is increasing recognition that other non-coral states are possible (see [13,16,]). Interestingly, in contrast to the reports of long-term declines in coral abundance, increases in sponge abundance have also been documented (e.g. [17]) and there have been further reports of small-scale (km 2 ) changes from coral-dominated to sponge dominated states within the Caribbean (e.g. [18,19,20]) and Pacific Oceans [21,22]. Some declines in tropical sponge abundance have also been reported (e.g. [23]). In most of the cases where sponges have increased in abundance, they appear to PLOS ONE | www.plosone.org 1 October 2013 | Volume 8 | Issue 10 | e76441