Abstract
Elevated pCO2 threatens coral reefs through impaired calcification. However, the extent to which elevated pCO2 affects the distribution of the pelagic larvae of scleractinian corals, and how this may be interpreted in the context of ocean acidification (OA), remains unknown. We tested the hypothesis that elevated pCO2 affects one aspect of the behavior (i.e., motility) of brooded larvae from Pocillopora damicornis in Okinawa (Japan), and used UV-transparent tubes that were 68-cm long (45 mm ID) to incubate larvae on a shallow fringing reef. Replicate tubes were filled with seawater at ~ 400 µatm or ~ 1000 µatm pCO2, stocked with 50 larvae each, and incubated vertically for 12 h with their midpoints at 0.3-m (shallow) or 3.3-m (deep) depth over a reef at 4-m depth. Larval behavior was assayed through their position in the tubes, which was scored in situ every 4 h beginning at 08:00 hrs. Lipid content was measured at the end of the experiment as a potential driver of behavior through its effects on larval buoyancy. Larval position in the tubes varied between depths and time of day at ~ 400 µatm pCO2 and ~ 1000 µatm pCO2. At ~ 400 µatm, larvae moved towards the top (0.1-m) of shallow tubes throughout the day, but in the deep tubes they aggregated at the bottom of the tubes from 08:00 hrs to 20:00 hrs. In contrast, larvae incubated at ~ 1000 µatm pCO2 aggregated at the bottom of shallow tubes at 08:00 hrs and 20:00 hrs, however in the deep tubes they were found in the bottom throughout the day. As lipid content of the larvae declined 23–25% at ~ 1000 versus ~ 400 µatm pCO2, loss of lipid may be a cause of modified larval behavior at high pCO2. If the pCO2-mediated changes in behavior and lipid content during this short experiment occur during longer exposures to high pCO2, our results suggest OA could alter the dispersal capacity of brooded coral larvae.
Highlights
Understanding the processes contributing to the distribution of organisms is a cornerstone of ecology (Hastings and Harrison, 1994), and in the marine environment, behaviorally-modulated dispersal of pelagic larvae plays a critical role in determining the spatial distribution of benthic populations (Levin, 2006)
To quantify the movement of pelagic larvae in the marine environment, their vertical position in the water column must be measured with a high degree of temporal resolution, because their vertical position can change within hours (Harii et al, 2002), and vertical changes as small as Larval Behavior in High pressure of CO2 (pCO2)
While the present study shows an effect of high pCO2 on vertical position of the pelagic larvae of a brooding coral, this outcome should be interpreted within the constraints of the experimental design employed
Summary
Understanding the processes contributing to the distribution of organisms is a cornerstone of ecology (Hastings and Harrison, 1994), and in the marine environment, behaviorally-modulated dispersal of pelagic larvae plays a critical role in determining the spatial distribution of benthic populations (Levin, 2006). Behaviorally-mediated dispersal of pelagic larvae plays a strong role in determining benthic and pelagic community structure (reviewed in Jones et al, 2009), with these effects evident over areas as small as ≤300 m2 (Jones et al, 2005; Almany et al, 2007) and as big as tens-to-hundreds of kilometers through connectivity mechanisms (Cowen et al, 2006). One main finding in studies on connectivity among benthic communities on coral reefs is strong spatiotemporal heterogeneity (Cowen and Sponaugle, 2009), which is an enduring feature of community ecology (Cornell and Lawton, 1992) that emerges in long-term analyses of community structure (Edmunds, 2000; Bruno and Selig, 2007; Pratchett et al, 2011) and studies of the settlement choices of sessile invertebrate larvae (Whalan et al, 2015)
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