No abstract

Thecosomatous pteropods, a group of aragonite shell-bearing zooplankton, are becoming an important sentinel organism for understanding the influence of ocean acidification on pelagic organisms. These animals show vulnerability to changing carbonate chemistry conditions, are geographically widespread, and are both biogeochemically and trophically important. The objective of this study was to determine how increasing duration and severity of CO treatment influence the physiology of the thecosome , integrating both gene expression and organism-level (respiration and calcification) metrics. We exposed pteropods to over-saturated, near-saturated or under-saturated conditions and sampled individuals at 1, 3, 7, 14 and 21 days of exposure to test for the effect of duration. We found that calcification was affected by borderline and under-saturated conditions by week two, while respiration appeared to be more strongly influenced by an interaction between severity and duration of exposure, showing complex changes by one week of exposure. The organismal metrics were corroborated by specific gene expression responses, with increased expression of biomineralization-associated genes in the medium and high treatments throughout and complex changes in metabolic genes corresponding to both captivity and CO treatment. Genes associated with other physiological processes such as lipid metabolism, neural function and ion pumping had complex responses, influenced by both duration and severity. Beyond these responses, our findings detail the captivity effects for these pelagic organisms, providing information to contextualize the conclusions of previous studies, and emphasizing a need for better culturing protocols.

16Shelled pteropods are planktonic molluscs that may be affected by ocean 17 acidification. Limacina retroversa from the Gulf of Maine were used to investigate the 18 impact of elevated carbon dioxide (CO2) on shell condition as well as swimming and 19 sinking behaviours. Limacina retroversa were maintained at either ambient (ca. 400 µatm) 20 or two levels of elevated CO2 (800 and 1200 µatm) for up to four weeks, and then 21 examined for changes in shell transparency, sinking speed, and swimming behaviour 22 assessed through a variety of metrics (e.g., speed, path tortuosity, wing beat frequency). 23After exposures to elevated CO2 for as little as four days, the pteropod shells were 24 significantly darker and more opaque in the elevated CO2 treatments. Sinking speeds were 25 significantly slower for pteropods exposed to medium and high CO2 in comparison to the 26 ambient treatment. Swimming behaviour showed less clear patterns of response to 27 treatment and duration of exposure, but overall, swimming did not appear to be hindered 28 under elevated CO2. Sinking is used by L. retroversa for predator evasion, and altered 29 speeds and increased visibility could increase the susceptibility of pteropods to predation. 30 31 Introduction 32The chemistry of the oceans is rapidly changing due to the infiltration of 33 anthropogenic carbon dioxide (CO2) into the surface ocean, a process known as ocean 34 acidification. One of the effects of ocean acidification is a decrease in the availability of 35 carbonate ion (CO3 2-) which affects calcifying organisms that use calcium carbonate 36 (CaCO3) to build shells and other structures (e.g. Orr et al. 2005, Royal Society 2005. A 37 shifting balance of dissolution and calcification as saturation state decreases due to ocean 38 3 acidification jeopardises the shell structure that, in many cases, provides protection from 39 predators (e.g. Fabry et al. 2008). Ocean acidification could also change the way that some 40 organisms move in their environment since calcified structures govern the movements of 41 certain planktonic organisms, including echinoderms and molluscs (e.g. Chan et al. 2011, 42 Wheeler et al. 2013. 43Thecosomes, or shelled pteropods (Order Euthecosomata; henceforth referred to 44 simply as pteropods), are planktonic molluscs that build calcium carbonate shells in the 45 crystal form of aragonite, which is less stable than the other common form, calcite. saturation state have also shown signs of dissolution under scanning electron microscopy 56 (Bednaršek, et al. 2012, Bednaršek, et al. 2014, Bednaršek, and Ohman. 2015. 57Shelled pteropods are a food source for many marine organisms, including 58 seabirds, whales, salmon, trout, mackerel, cod, myctophids, and other zooplankton 59 (LeBrasseur 1966, Ackman et al. 1972, Conover and Lalli 1974, Levasseur et al. 1996, 60 Pakhomov et al. 1996, Armstrong et al. 2005, Hunt et al. 2008, Karnovsky et al. 2008, 61 4 Pomerleau et al. 2012, Sturdevant et al. 2013, and hence any effects of ocean acidifica...

Natural cycles in the seawater partial pressure of carbon dioxide (CO2) in the Gulf of Maine, which vary from ~250-550 µatm seasonally, provide an opportunity to observe how the life cycle and phenology of the shelled pteropod Limacina retroversa responds to changing food, temperature and carbonate chemistry conditions. Distributional, hydrographic, and physiological sampling suggest that pteropod populations are located in the upper portion of the water column (0-150 m) with a maximum abundance above 50 m, allowing them to generally avoid aragonite undersaturation. Gene expression and shell condition measurements show, however, that the population already experiences biomineralization stress in the winter months even when aragonite is slightly oversaturated, reinforcing the usefulness of this organism as a bio-indicator for pelagic ecosystem response to ocean acidification. There appear to be two reproductive events per year with one pulse timed to coincide with the spring bloom, the period with highest respiration rate, fluorescence, and pH, and a second more extended pulse in the late summer and fall. During the fall there is evidence of lipid storage for overwintering, allowing the second generation to survive the period of low food and aragonite saturation state. Based on these observations we predict that in the future pteropods will likely be most vulnerable to changing CO2 regionally during the fall reproductive event when CO2 concentration already naturally rises and there is the added stress of generating lipid stores.

17Natural cycles in the seawater partial pressure of carbon dioxide (CO2) in the Gulf of Maine, 18 which vary from ~250-550 µatm seasonally, provide an opportunity to observe how the life cycle 19 and phenology of the shelled pteropod Limacina retroversa responds to changing food, 20 temperature and carbonate chemistry conditions. Distributional, hydrographic, and physiological 21 sampling suggest that pteropod populations are located in the upper portion of the water column 22 (0-150 m) with a maximum abundance above 50 m, allowing them to generally avoid aragonite 23 undersaturation. Gene expression and shell condition measurements show, however, that the 24 population already experiences biomineralization stress in the winter months even when 25 aragonite is slightly oversaturated, reinforcing the usefulness of this organism as a bio-indicator 26 for pelagic ecosystem response to ocean acidification. There appear to be two reproductive 27 events per year with one pulse timed to coincide with the spring bloom, the period with highest 28 respiration rate, fluorescence, and pH, and a second more extended pulse in the late summer and 29 fall. During the fall there is evidence of lipid storage for overwintering, allowing the second 30 generation to survive the period of low food and aragonite saturation state. Based on these 31 observations we predict that in the future pteropods will likely be most vulnerable to changing 32 CO2 regionally during the fall reproductive event when CO2 concentration already naturally rises 33 and there is the added stress of generating lipid stores. 34 73 patchy high abundances of small individuals appearing in the fall that persisted through the 74 spring. Redfield noted a correlation between the seasonal progression of the location of L. 75 retroversa patches and major surface currents, and based on the bimodal and non-linear growth 76 of their size frequency distributions he suggested that the population is maintained by pulses of 77 lateral advection from an open ocean source. The species has been reported to feed on detritus, 78 diatoms and dinoflagellates (Lalli & Gilmer, 1989; Morton, 1954) with the capacity to ingest 79 large quantities of phytoplankton (~4000 ng of chl a pigment individual -1 day -1 , Bernard & 80 Froneman, 2009). Limacina retroversa is eaten by a variety of fish (Bigelow, 1924; Lebour, 81 1932; Suca et al., 2018) and can become such a prevalent component of the diet of mackerel and 82herring that they can act as a vector for toxins and non-palatable compounds, negatively 83 affecting fisheries (Ackman et al., 1972;Foster et al., 2012;White, 1977). They also appear to 84 play a role in modifying the alkalinity budget and carbonate chemistry in deep basins in the Gulf 85 of Maine (Wang et al., 2017b). 86 The local ecological importance of L. retroversa, its natural seasonal exposure to strong variation 87 in CO2 levels, and the fact that it is widely distributed in temperate portions of the Atlantic open 88 ocean position the species as a valuable mod...