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Molluscs on Acid: Gastropod Shell Repair and Strength in Acidifying Oceans

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Molluscs on Acid: Gastropod Shell Repair and Strength in Acidifying Oceans Vol. 509: 203–211, 2014 MARINE ECOLOGY PROGRESS SERIES Published August 27 doi: 10.3354/meps10887 Mar Ecol Prog Ser Molluscs on acid: gastropod shell repair and strength in acidifying oceans Daniel W. Coleman1,*, Maria Byrne2, Andrew R. Davis1 1Institute for Conservation Biology & Environmental Management, School of Biological Sciences, University of Wollongong, New South Wales 2522, Australia 2Schools of Medical and Biological Sciences, University of Sydney, New South Wales 2052, Australia ABSTRACT: The importance of ‘top-down’ regulation of assemblages by predators is well doc- umented at a variety of spatial and temporal scales on rocky-shores. Predators have consump- tive and non-consumptive impacts on their prey; however, much remains to be discovered about how climate change may affect predator-prey interactions and processes related to these interactions. We investigated the effect of predicted near-future ocean acidification on a molluscan defence mechanism: shell repair. We simulated non-consumptive damage by a durophagous (shell crushing) predator to 2 common intertidal gastropod species: Austrocochlea porcata and Subninella undulata. Our data show a stark contrast in the response of these 2 gastropods to simulated ocean acidification; A. porcata exhibited a depressed shell repair rate, compromised shell integrity and reduced condition. These 3 critical attributes for survival and protection against predators were all severely affected by ocean acidification. In contrast S. undulata was unaffected by ocean acidification. These results suggest that if atmospheric CO2 levels continue to rise, and ocean pH subsequently drops, then less resistant species such as A. porcata may face increased predation pressure and competition from more successful taxa within the same community. This could affect predator-prey relationships, with the potential to cascade through intertidal communities. KEY WORDS: Climate change · Predation · Shell strength · Shell growth · pH · Ocean acidification Resale or republication not permitted without written consent of the publisher INTRODUCTION tionally, cooler mid-high latitude environments will experience relatively rapid undersaturation of arag- The impact of predicted climate change on onite, placing these marine zones under greater organisms has been investigated for decades; how- threat than low latitude systems (Caldeira & ever, marine scientists are only beginning to under- Wickett 2005, Kleypas et al. 2005). This altered stand how the acidification of the world’s oceans marine chemistry has the potential to reduce or dis- might affect marine life (Dupont & Pörtner 2013). rupt calcification in marine organisms, threatening By the end of this century, ocean acidification, due shell-forming taxa such as molluscs (Byrne et al. to elevated partial pressure of carbon dioxide 2009, Gaylord et al. 2011, Lischka et al. 2011). (pCO2) levels from anthropogenic emissions, is However, the effect of elevated pCO2 on key com- expected to reduce the global mean surface pH ponents of consumer-prey interactions has been (currently around 8.2) by up to 0.5 pH units tested for only a few species (Landes & Zimmer (Caldeira & Wickett 2005), as well as diminish the 2012, Allan et al. 2013, Poore et al. 2013). As pre- saturation of 2 calcium carbonate (CaCO3) species: dation is one of the key processes controlling popu- aragonite and calcite (Kleypas et al. 2005). Addi- lation dynamics and structuring communities in *Corresponding author: [email protected] © Inter-Research 2014 · www.int-res.com 204 Mar Ecol Prog Ser 509: 203–211, 2014 marine ecosystems (Paine 1966, Sih et al. 1985, MATERIALS AND METHODS Harley 2011, Knights et al. 2012), understanding the effect of future acidification on predator-prey Study species and artificial damage associations is important. In the intertidal environment, durophagous (shell Austrocochlea porcata and Subninella undulata crushing) crabs are dominant predators, preying on a were collected in the Illawarra region on the New range of gastropod species (Chilton & Bull 1984, South Wales south coast (Towradgi reef: 34° 23’ 11” S, Moody & Aronson 2007). However, a large propor- 150° 54’ 54” E). Both species are common Australasian tion of crab attacks fail, inflicting damage to the endemic molluscs in the intertidal environment with shells of intertidal molluscs (Vermeij 1982a), expos- broad distributions in temperate and sub-tropical ing vulnerable soft tissue to the threat of further pre- Australia (http://seashellsofnsw. org. au, accessed on dation and abiotic stress (Blundon & Vermeij 1983). 31 August 2013). Field populations of both species The shell repair process re-establishes the function show a high incidence of shell repair following un- and integrity of shells, increasing the survival of prey successful attacks by predators: more than 20% of (Blundon & Vermeij 1983). This repair strategy A. porcata and 25% of S. undulata show evidence of appears to have been highly successful throughout aperture repair (D. W. Coleman unpubl. data). To geological time, including in current climatic condi- minimise the confounding influence of size variation, tions, with more than 20% of individuals showing only A. porcata with a maximum length (ML), i.e. evidence of shell repair in some modern species pop- from tip of apex to the shell lip/aperture of 17 ± 2 mm ulations (Vermeij et al. 1981, D. W. Coleman unpubl. and S. undulata measuring 26 ± 2 mm ML were col- data). In addition, the integrity (strength) of repaired lected. Hence our focus was on adults of these shells is equivalent to undamaged shells (Salgëback species. Ap proxi mately 7 ± 1 mm2 of shell was re- 2005). Given the potential for near-future elevated moved from the aperture of A. porcata while 102 ± 1 2 CO2 environments to compromise shell calcification mm was re moved from the larger species S. undu- (Kleypas et al. 2005, Kroeker et al. 2013), the effec- lata. Damage was inflicted to simulate typical ‘peel’ tiveness of this repair process may be under threat. damage inflicted to the shell aperture by the Mid-latitude temperate marine waters surround durophagous crab Ozius truncatus as observed in the southeast coast of Australia, supporting a diver- laboratory and field specimens. The snails were held sity of gastropod grazers, often in high abundance. in a vice and needle-nose pliers used to break the Duraphagous crabs including the black-fingered shell aperture. All individuals were quarantined for crab Ozius truncatus (Edwards 1834) have been 48 h to ensure they were alive and behaving normally identified as dominant predators on small gastropods before being randomly allocated to treatments. and inflict characteristic damage patterns on their shells (Chilton & Bull 1984, authors’ pers. obs.). In this study, we focus on how ocean acidification Experimental conditions affects shell repair in gastropods after a simulated unsuccessful predatory attack. We assessed this in 2 Experiments were carried out in 3 large recirculat- common gastropods: a trochid, Austrocochlea por- ing seawater tables (3 × 1.5 × 0.25 m) with sump tanks cata (Adams 1853) and a turbinid, Subninella undu- of equal size totalling 520 l per system. Two acidifica- lata (Lightfoot, 1786). Both species are extremely tion treatments were applied based on Intergovern- abundant, and show high levels of shell damage and mental Panel on Climate Change pH (pCO2) pre - repair on intertidal rocky reefs (D. W. Coleman dictions for 2100 and 2050 (IPCC 2007): pH 7.7 unpubl. data). Additionally, they may also be under (~840 ppm) and pH 7.9 (~560 ppm), respectively. A threat from ocean acidification given the high arago- control treatment, with pH 8.2 (~380 ppm), simulated nite:calcite ratios in their shells (98.65:1.35% and present-day conditions. Seawater collected from the 98.2:1.8% in S. undulata and A. porcata, respec- ocean (pH 8.2) was acidified, using a RedSea pro CO2 tively) and the sensitivity of aragonite to dissolution system and a TUNZE™ Model 7074/2 pH controller, at high pCO2 concentrations (Orr et al. 2005). We to the desired level within the rearing system. pH was tested the hypothesis that reduced pH would affect recorded weekly and remained relatively constant the rate of shell repair, changes in shell thickness, in all treatments for the duration of the experiment. shell integrity (strength), gastropod condition and Water temperature was maintained at 20°C for all CaCO3 polymorph composition relative to control treatments with 10% water changes performed (current) conditions. weekly in addition to constant filtration and oxy- Coleman et al.: Ocean acidification and molluscs 205 genation to maintain >90% oxygen. Salinity was accreted shell was very clear and could be meas- maintained at 35 to 37‰ using distilled water. These ured using Vernier calipers with high precision temperature and salinity values represent ambient (0.05 mm). Shell growth was recorded every 5 d for conditions for this region during the experimental each species over a period of 55 d for A. porcata period (May 2009). and 30 d for S. undulata. At the end of both these Water samples were taken at the beginning, mid- respective periods, the shell aperture was fully dle and end of the experiment to determine total repaired; further growth consisted of thickening of alkalinity (using the Gran plot calculation method). the repaired shell. We present data for the most Aragonite (Ωar) and calcite (Ωca) saturation levels and extreme (pH 7.7) and control (pH 8.2) conditions
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