Commentary, J Mar Biol Oceanogr Vol: 7 Issue: 1
Are Some Photosymbiotic Bioeroding Sponges More Bleaching-Tolerant than Hard Corals?
*Corresponding Author : James K.H. Fang
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Tel: +852 3400 8703
Fax: +852 2364 9932
Received: December 18, 2017 Accepted: January 11, 2018 Published: January 16, 2018
Citation: Fang JKH, Schönberg CHL, Ormond RFG (2018) Are Some Photosymbiotic Bioeroding Sponges More Bleaching-Tolerant than Hard Corals? J Mar Biol Oceanogr 7:1. doi: 10.4172/2324-8661.1000187
Are Some Photosymbiotic Bioeroding Sponges More Bleaching-Tolerant than Hard Corals?
Some bioeroding sponges of the Cliona viridis species complex are aggressive space competitors on coral reefs, which can often kill hard corals. Like hard corals, C. viridis complex spp. are symbiotic with the dinoflagellates Symbiodinium spp., which provide vital energy for the hosts’ survival. In this study, C. viridis complex spp. and hard corals were observed on a Caribbean reef over a year. Many hard corals were found bleached (tissue whitening due to the loss of Symbiodinium spp.) at elevated temperatures. However, C. viridis complex spp. appeared to be unaffected and remained dark brown at all times, indicating abundant Symbiodinium within the sponges. Our observations suggest a higher tolerance of some C. viridis complex spp. to bleaching compared to hard corals. This finding also implies a competitive advantage of these bioeroding sponges over hard corals during increasingly frequent and severe mass bleaching events under climate change, potentially leading to net reef erosion.
Keywords: Cliona; Bioerosion; Caribbean; Reefs; Symbiodinium; Zooxanthellae; Temperature; Acidification
Hard corals live in symbiosis with dinoflagellates of the genus Symbiodinium, microbial partners which provide vital energy for the hosts' survival and calcification performance . Failure of this symbiosis can be lethal to the hosts and is commonly referred to as bleaching, a phenomenon of tissue whitening due to the damage or loss of Symbiodinium spp. [2,3]. Mass bleaching events and subsequent large-scale mortality of hard corals have been observed in various habitats under climate change, and rising temperature has been identified as one of the main threats [4,5]. Another significant but often ignored putative threat to future coral reefs is accelerating bioerosion, i.e. enhanced erosion of reef carbonate frameworks by biological activities . Among many bioeroding organisms, sponges of the Cliona viridis species complex represent aggressive excavators and space competitors on coral reefs that can often kill hard corals .Like hard corals, C. viridis complex spp. are also symbiotic with Symbiodinium spp., but some of these sponges may be more tolerant to bleaching. This may partly be due to the sponges' ability to relocate symbionts within their three-dimensional tissue structures, and partly to the traits of their symbionts [8-11]. This tolerance to bleaching can be a critical factor to determine the competitive strength of C. viridis complex spp. against hard corals under climate change.
Here we report observations on C. viridis complex spp. and hard corals around Little Cayman in the central Caribbean during the third global coral bleaching event [5,12]. Judging from their surface colour, up to 30% of hard coral colonies were partially or completely bleached in November 2015 (30−31°C, average daily seawater temperature; e.g. Figure 1). The extent of coral bleaching was much reduced by May/June 2016 (27−28°C), but partial bleaching was again evident on up to 15% of hard coral colonies, principally below 15 m depth, in November/December 2016 (28-29°C) following warmer seawater temperatures in July/August 2016 (30−32°C). In contrast, observed C. viridis complex spp. remained dark brown at all times, which was indicative of abundant Symbiodinium within the sponges (e.g. Figure 1). These field observations are in line with the hypothesis that some C. viridis complex spp. are more bleaching-tolerant than hard corals during thermal events [8-9,13]. In this regard, natural bleaching of C. viridis complex spp. appears to be rare and to date has only been observed on Cliona varians in the Florida Keys . However, personal communication of authors confirmed that bleached C. varians had ‘mostly recovered’ three months later. In an earlier field experiment, C. varians had also bleached when transplanted from 20 m to 1 m depth (possibly due to light stress), but the sponge regained high densities of Symbiodinium within three months [15, C. varians as Anthosigmella varians]. The ability of C. viridis complex spp. to effectively prevent or recover from bleaching suggests a great resilience of their association with Symbiodinium. However, this relationship can fail as well under extreme conditions [16-18, for Cliona orientalis].
Figure 1: A coral reef community containing partially bleached corals, (A) Agaricia agaricites and (B) Orbicella faveolata, and (C) an adjacent unbleached bioeroding sponge of the Cliona viridis species complex at 15 m depth on the Little Cayman Wall in Crawl Bay, northwest of Little Cayman on the 8th of November 2015. The scale bar indicates approximately 10 cm.
Overall, our field observations suggest a more robust symbiosis with Symbiodinium in some C. viridis complex spp. compared to that in hard corals, a conclusion which is however dependent on the combination of species/clades between Symbiodinium and the hosts. This also implies at least a temporary competitive advantage of some C. viridis complex spp. over hard corals during mass bleaching events, which will likely become more frequent and severe under future climate change [3-5,19]. Further development may lead to selective survival of the more resilient C. viridis complex spp., as previously observed at various sites in the Caribbean and on the Great Barrier Reef after major coral bleaching events [20-21]. The success of C. viridis complex spp. can be due to enhanced sponge physiology under moderately changed future conditions, along with improved settlement in environments with bleached/dead corals compared to those with unbleached/living corals [6-7,22-25]. Both of these pathways will accelerate reef bioerosion, which may eventually outweigh calcification and shift some coral reefs into net erosional states.
Fieldwork in Cayman was funded by a Darwin Plus grant from the UK Department of Environment, Food and Rural Affairs (DPLUS036). Thanks are due for field assistance to M. Gore, C. Millar and staff of the Cayman Islands Department of the Environment.
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