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Galaxea, Journal of Coral Reef Studies 20: 19-28（2018） Original paper Impact of increased seawater pCO2 on the host and symbiotic algae of juvenile giant clam Tridacna crocea Haruko KURIHARA* and Tomoaki SHIKOTA Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan * Corresponding author: H. Kurihara E­mail: [email protected]­ryukyu.ac.jp, harukoku@e­mail.jp Communicated by Saki Harii (Editor­in­Chief) Abstract Increases in atmospheric CO2 cause decreases in calcium carbonate saturation, which is predicted to Introduction affect the calcification process of most marine calcifiers. At the same time, the increase of seawater pCO2 is also There is now a critical need to understand the effects of known to increase the productivity of primary producers. environmental change on marine organisms caused by Giant clams host symbiotic dinoflagellates (‘zoo xan­ increasing anthropogenic atmospheric CO2. When oceans thellae’: Symbiodinium spp.) that provide nutrition and uptake atmospheric CO2, seawater pH declines, a pheno­ use CO2 as their primary source for photosynthesis. This menon which is called ocean acidification (OA, Caldeira leads to the hypothesis that increased seawater pCO2 rise and Wickett 2003). Because decreased seawater pH also could positively affect the production of giant clam reduces calcium carbonate saturation state (Ω, Orr et al. zooxanthellae, and dampen effects of CO2 on host giant 2005), OA is projected to adversely impact the physiology clams. To test this hypothesis, we measured the shell and biology of principally marine calcifiers such as growth rate, photosynthesis rate, respiration rate and mollusks (excluding cephalopods), echinoderms, and zooxanthellae density of the juvenile Tridacna crocea corals (Doney et al. 2009). reared under three different pCO2 conditions. Results The effect of OA on mollusks has garnered attention, revealed that negative shell growth of juvenile Tridacna not only for scientific interest, but also due to the economic crocea was observed once seawater Ωarag reached less than value of mollusks (Narita et al. 2012; FAO 2016). A num­ 2.33. Additionally, although zooxanthellae density in T. ber of studies have evaluated the effect of OA on several crocea increased with seawater pCO2 rise, zooxanthellae commercially important mollusk and have illustrated productivity did not change, suggesting that the produc­ negative impacts on their survival (Berge et al. 2006), tivity per zooxanthella decreased in high pCO2 seawater. growth (Michaelidis et al. 2005; Berge et al. 2006), cal­ Our findings suggest future seawater pCO2 rise will not cification (Gazeau et al. 2007), shell hardness (Beniash et increase productivity of zooxanthellae, thus giant clam al. 2010), and metabolism (Michaelidis et al. 2005). will be negatively impacted in the coming centuries. Mean while, despite the socio­economic value of giant clams, in the Indo­Pacific region, only a few studies have Keywords ocean acidification, giant clam, symbiotic algae, evaluated their susceptibility to OA. CO2 Giant clams are classified in the order Cardiida and the 20 Kurihara and Shikota: Ocean acidification impacts on giant clam subfamily Tridacninae and are the largest living bivalves. hosts was dampened by the increase photosynthetic rate Within the group, there are 12 recognized living species, of zooxanthellae. However, until now, there has been no comprised of ten species of the genus Tridacna and two of study examining the direct effect on the net productivity the genus Hippopus, which all commonly inhabit in the of giant clams under high pCO2 conditions. For corals, coral­reef ecosystems of the Indo­Pacific Ocean (Mol­ several studies have reported that elevated pCO2 does not lusca Base 2018). Due to their economic value as a food enhance the photosynthesis and productivity of zooxan­ source and for ornaments, they have been over­harvested thellae (Goiran et al. 1996; Schneider and Erez 2006; for centuries, and, together with other human impacts on Marubini et al. 2008; Takahashi and Kurihara 2012). coral reef environments, this has led to giant clams being How ever, one study reported that photosynthetic rates listed in the Red List of Threatened Species (Wells 1996; increased in under high pCO2 (Suggett et al. 2013), while Neo et al. 2015). Giant clams are known to show the another study reported a reduction in photosynthetic rate highest growth rate among mollusks, up to 16 times higher under high pCO2 (Anthony et al. 2011). One reason for than oysters (Bonham 1965) and the shells of giant clams this lack of response, or conflicting responses, in coral are mainly composed of aragonite calcium carbonate photosynthetic rates may be because zooxanthellae of － (Taylor et al. 1973). Watson et al. (2012) first reported corals utilize mainly HCO3 rather than CO2 for photo­ effects of OA and indicated that both high seawater pCO2 synthesis (Al­Moghrabi et al. 1996; Allemand et al. 1998). and temperature reduced the survival of the juvenile In contrast to corals, the zooxanthellae of giant clams Tridacna squamosa. Another study indicated that the have been hypothesized to predominantly use CO2 for growth rate of the juvenile giant clams T. squamosa, T. photosynthesis (Yellowlees et al. 1993; Goiran et al. 1996; crocea and T. maxima cultured for one year at Waikiki Leggat et al. 2000). Additionally, unlike corals where the Aquarium, under conditions with seawater with high zooxanthellae are intracellular and located at the oral pCO2 (700 to 1,400 μatm) and high nutrient concen tra­ endoderm (Yellowlees et al. 2008), the endosymbiotic tions, was slower compared to the previously reported zooxanthellae of giant clams are reported to be extra­ values of the same species cultured under normal seawater cellular, living within the Z­tubes connected to the conditions (Toonen et al. 2012). stomach (Norton et al. 1992). Therefore, it is possible that Meanwhile, giant clams have the distinct characteristic OA may have different effects on coral and giant clam of a symbiotic relationship with dinoflagellates (‘zooxan­ physiologies. thellae’: Symbiodinium spp., Yonge 1953). Establishment Here, we examine the effect of OA on both shell growth of the symbioses is reported to occur at the juvenile stage and metabolism of giant clams, including respiration and just after metamorphosis from the veliger larval stage photosynthetic rates to test the possibility that high pCO2 (Hirose et al. 2006). Similar to hermatypic corals, giant will stimulate the net production of zooxanthellae. Juve­ clams receive a significant amount of energy from photo­ niles of the giant clam Tridacna crocea were cultured at 3 synthetic products produced by zooxanthellae (Trench et different pH / pCO2 conditions predicted to occur in future al. 1981). The influence of OA on these symbiotic marine coral reef ecosystems. calcifiers is of great interest because while high seawater pCO2 can decrease calcification rates, it may enhance the photosynthetic rates of symbiotic algae. Indeed, Watson Material and methods (2015) demonstrated that although the growth rate of juvenile Tridacna squamosa was reduced when reared at Organisms low light intensity (35 μmol photons m－2 s－1) in high Juveniles of the giant clam Tridacna crocea were pro­ pCO2 seawater, the impact was less pronounced when the vided by the Okinawa Prefectural Fisheries Research and clams were reared at high light intensity (304 μmol Extension Center, Okinawa Island, Japan. Individuals －2 －1 photons m s ) and high pCO2. This result has been were artificially fertilized, infected by monoculture Sym­ interpreted that the negative effect of OA on giant clam biodinium taken by the giant clam T. crocea and cultured Kurihara and Shikota: Ocean acidification impacts on giant clam 21 for 1 y in a flow­through outdoor tank. Sixty one­year­old verify that there was no variation among containers. juveniles of similar sizes were selected from the cultured Seawater volume and seawater exchange rate of the con­ stock at the Center, and transported to Sesoko Station, tainer was large enough so that the metabolism of the University of the Ryukyus. All 60 juveniles were imme­ giant clam did not influence seawater pH within the con­ diately moved to an outdoor tank (187 L) continuously tainer. The pH electrode was calibrated every day before supplied with seawater pumped (2.5 L min－1) from 4-5 m measurement using NBS scale buffer solutions. Salinity depth in front of Sesoko Station and acclimatized for 3 were measured every week for all containers using a weeks before starting the experiment. refractometer (Atago, 100­S, Japan). Total alkalinity (AT) were measured every week for 6 randomly selected con­ Experimental set-up tainers using an autoburette titrator (Kimoto, ATT­05, － 2－ Just before starting the experiment, 30 juvenile T. Japan). Sea water pCO2, HCO3 , CO3 and Ωarag were cal­ crocea of approximately the same size (7.2±0.65 mm) culated based on pH temperature, alkalinity and salinity were selected and placed individually in 30 culture con­ using the CO2SYS program of Lewis and Wallace (1998), tainers (130 ml in volume). Each of 10 culture containers with dissociation constants K1 and K2 from Mehrbach et were supplied with control (pCO2/pH: 400 μatm/pH 8.2), al. (1973) and the aragonite solubility (Kspa) of Mucci Mid CO2 (1,000 μatm/pH 7.8), or High CO2 (2,000 μatm/ (1983) (Table 1). pH 7.6) seawater at the rate of 250 ml min－1. T. crocea juveniles were cultured for 4 weeks under these conditions. Measuring shell growth Seawater pCO2 was adjusted by continuously bubbling The growth rates of the clams were evaluated as the flowing pumped seawater (2.5 L min－1) in the bubbling change in shell size during the 4 weeks culture period. －1 tank with air or air­CO2 mixed gas at a rate of 3.0 L min Pictures of the clams were taken just before introducing made up by mixing air and pure CO2 using a mass flow the clams to the different pCO2 conditions (time 0: all controller (HORIBASTEC, Japan).

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