Amy E. Maas

Energetic plasticity underlies a variable response to ocean acidification in the pteropod, Limacina helicina antarctica.

Ocean acidification, caused by elevated seawater carbon dioxide levels, may have a deleterious impact on energetic processes in animals. Here we show that high PCO2 can suppress metabolism, measured as oxygen consumption, in the pteropod, L. helicina forma antarctica, by ∼20%. The rates measured at 180–380 µatm (MO2  = 1.25 M−0.25, p = 0.007) were significantly higher (ANCOVA, p  =  0.004) than those measured at elevated target CO2 levels in 2007 (789–1000 µatm,  =  0.78 M−0.32, p  =  0.0008; Fig. 1). However, we further demonstrate metabolic plasticity in response to regional phytoplankton concentration and that the response to CO2 is dependent on the baseline level of metabolism. We hypothesize that reduced regional Chl a levels in 2008 suppressed metabolism and masked the effect of ocean acidification. This effect of food limitation was not, we postulate, merely a result of gut clearance and specific dynamic action, but rather represents a sustained metabolic response to regional conditions. Thus, pteropod populations may be compromised by climate change, both directly via CO2-induced metabolic suppression, and indirectly via quantitative and qualitative changes to the phytoplankton community. Without the context provided by long-term observations (four seasons) and a multi-faceted laboratory analysis of the parameters affecting energetics, the complex response of polar pteropods to ocean acidification may be masked or misinterpreted.

Reexamination of the species assignment of Diacavolinia pteropods using DNA barcoding.

Thecosome pteropods (Mollusca, Gastropoda) are an ecologically important, diverse, and ubiquitous group of holoplanktonic animals that are the focus of intense research interest due to their external aragonite shell and vulnerability to ocean acidification. Characterizing the response of these animals to low pH and other environmental stressors has been hampered by continued uncertainty in their taxonomic identification. An example of this confusion in species assignment is found in the genus Diacavolinia. All members of this genus were originally indentified as a single species, Cavolinia longirostris, but over the past fifty years the taxonomy has been revisited multiple times; currently the genus comprises 22 different species. This study examines five species of Diacavolinia, including four sampled in the Northeast Atlantic (78 individuals) and one from the Eastern tropical North Pacific (15 individuals). Diacavolina were identified to species based on morphological characteristics according to the current taxonomy, photographed, and then used to determine the sequence of the “DNA barcoding” region of the cytochrome c oxidase subunit I (COI). Specimens from the Atlantic, despite distinct differences in shell morphology, showed polyphyly and a genetic divergence of <3% (K2P distance) whereas the Pacific and Atlantic samples were more distant (∼19%). Comparisons of Diacavolinia spp. with other Cavolinia spp. reveal larger distances (∼24%). These results indicate that specimens from the Atlantic comprise a single monophyletic species and suggest possible species-level divergence between Atlantic and Pacific populations. The findings support the maintenance of Diacavolinia as a separate genus, yet emphasize the inadequacy of our current taxonomic understanding of pteropods. They highlight the need for accurate species identifications to support estimates of biodiversity, range extent and natural exposure of these planktonic calcifiers to environmental variability; furthermore, the apparent variation of the pteropods shell may have implications for our understanding of the species’ sensitivity to ocean acidification.

Transcriptome-wide analysis of the response of the thecosome pteropod Clio pyramidata to short-term CO2 exposure

Thecosome pteropods, a group of calcifying holoplanktonic mollusks, have recently become a research focus due to their potential sensitivity to increased levels of anthropogenic dissolved CO2 in seawater and the accompanying ocean acidification. Some populations, however, already experience high CO2 in their natural distribution during diel vertical migrations. To achieve a better understanding of the mechanisms of pteropod calcification and physiological response to this sort of short duration CO2 exposure, we characterized the gene complement of Clio pyramidata, a cosmopolitan diel migratory thecosome, and investigated its transcriptomic response to experimentally manipulated CO2 conditions. Individuals were sampled from the Northwest Atlantic in the fall of 2011 and exposed to ambient conditions (~380ppm) and elevated CO2 (~800ppm, similar to levels experienced during a diel vertical migration) for ~10h. Following this exposure the respiration rate of the individuals was measured. We then performed RNA-seq analysis, assembled the C. pyramidata transcriptome de novo, annotated the genes, and assessed the differential gene expression patterns in response to exposure to elevated CO2. Within the transcriptome, we identified homologs of genes with known roles in biomineralization in other mollusks, including perlucin, calmodulin, dermatopontin, calponin, and chitin synthases. Respiration rate was not affected by short-term exposure to CO2. Gene expression varied greatly among individuals, and comparison between treatments indicated that C. pyramidata down-regulated a small number of genes associated with aerobic metabolism and up-regulated genes that may be associated with biomineralization, particularly collagens and C-type lectins. These results provide initial insight into the effects of short term CO2 exposure on these important planktonic open-ocean calcifiers, pairing respiration rate and the gene expression level of response, and reveal candidate genes for future ecophysiological, biomaterial and phylogenetic studies.

The effect of elevated carbon dioxide on the sinking and swimming of the shelled pteropod Limacina retroversa

The effect of elevated carbon dioxide on the sinking and swimming of the shelled pteropod Limacina retroversa Alexander J. Bergan, Gareth L. Lawson*, Amy E. Maas, and Zhaohui Aleck Wang Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA Bermuda Institute of Ocean Sciences, St. George’s GE01, Bermuda Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA *Corresponding author: tel: þ508 289 3713; fax: 508 457 2134; e-mail: glawson@whoi.edu.

A transcriptomic resource for the northern krill Meganyctiphanes norvegica based on a short-term temperature exposure experiment

The northern krill, Meganyctiphanes norvegica, is an important component of the pelagic food web across the North Atlantic. Widespread from the Mediterranean to the Subarctic Atlantic, populations appear to be strongly adapted to local temperatures, and seem to have very little plasticity. The goal of this study was to create and annotate a de novo transcriptome assembly to allow for comparative and physiological studies and to explore the gene expression response of M. norvegica from the Gulf of Maine to two different temperature conditions. Our Trinity assembly produced 405,497 transcripts with ~16% annotation success versus nr with a stringent cutoff (>1e-10), and substantial cross-annotation versus FlyBase and other published pelagic crustacean transcriptomes. There were 122 transcripts that were differentially expressed based on our 2-day 9 versus 12°C temperature exposure, and their annotation suggested changes in energetic metabolism and molting. These results generate a useful molecular resource for further more directed studies as well as provide initial insight into the physiological processes that may shape the temperature response of the northern krill.

Assembly of a reference transcriptome for the gymnosome pteropod Clione limacina and profiling responses to short-term CO2 exposure

The gymnosome (unshelled) pteropod Clione limacina is a pelagic predatory mollusc found in polar and sub-polar regions. It has been studied for its distinctive swimming behavior and as an obligate predator on the closely related thecosome (shelled) pteropods. As concern about ocean acidification increases, it becomes useful to compare the physiological responses of closely-related calcifying and non-calcifying species to acidification. The goals of this study were thus to generate a reference transcriptome for Clione limacina, to expose individuals to CO2 for a period of 3days, and to explore differential patterns of gene expression. Our Trinity assembly contained 300,994 transcripts of which ~26% could be annotated. In total, only 41 transcripts were differentially expressed following the CO2 treatment, consistent with a limited physiological response of this species to short-term CO2 exposure. The differentially expressed genes identified in our study were largely distinct from those identified in previous studies of thecosome pteropods, although some similar transcripts were identified, suggesting that comparison of these transcriptomes and responses may provide insight into differences in responses to ocean acidification among phylogenetically and functionally distinct molluscan lineages.

Seasonal controls of aragonite saturation states in the Gulf of Maine

The Gulf of Maine (GoME) is a shelf region especially vulnerable to ocean acidification (OA) due to natural conditions of low pH and aragonite saturation states (Ω-Ar). This study is the first to assess the major oceanic processes controlling seasonal variability of the carbonate system and its linkages with pteropod abundance in Wilkinson Basin in the GoME. Two years of seasonal sampling cruises suggest that water-column carbonate chemistry in the region undergoes a seasonal cycle, wherein the annual cycle of stratification-overturn, primary production, respiration-remineralization and mixing all play important roles, at distinct spatiotemporal scales. Surface production was tightly coupled with remineralization in the benthic nepheloid layer during high production seasons, which results in occasional aragonite undersaturation. From spring to summer, carbonate chemistry in the surface across Wilkinson Basin reflects a transition from a production-respiration balanced system to a net autotropic system. Mean water-column Ω-Ar and abundance of large thecosomatous pteropods show some correlation, although patchiness and discrete cohort reproductive success likely also influence their abundance. Overall, photosynthesis-respiration is the primary driving force controlling Ω-Ar variability during the spring-to-summer transition as well as over the seasonal cycle. However, calcium carbonate (CaCO3) dissolution appears to occur near bottom in fall and winter when bottom water Ω-Ar is generally low but slightly above 1. This is accompanied by a decrease in pteropod abundance that is consistent with previous CaCO3 flux trap measurements. The region might experience persistent subsurface aragonite undersaturation in 30–40 years under continued ocean acidification.

Daily cycle in oxygen consumption by the sea anemone Nematostella vectensis Stephenson

ABSTRACT In bilaterian animals, the circadian clock is intimately involved in regulating energetic metabolism. Although cnidarians exhibit diel behavioral rhythms including cycles in locomotor activity, tentacle extension and spawning, daily cycles in cnidarian metabolism have not been described. To explore a possible circadian metabolic cycle, we maintained the anemone Nematostella vectensis in a 12 h light/dark cycle, a reversed light cycle, or in constant darkness. Oxygen consumption rates were measured at intervals using an optical oxygen meter. Respiration rates responded to entrainment with higher rates during light periods. During a second experiment with higher temporal resolution, respiration rates peaked late in the light period. The diel pattern could be detected after six days in constant darkness. Together, our results suggest that respiration rates in Nematostella exhibit a daily cycle that may be under circadian control and that the cycle in respiration rate is not driven by the previously described nocturnal increase in locomotor activity in this species. Summary: This study demonstrates that the sea anemone Nematostella exhibits a daily cycle in respiration rate that is not driven by the previously described nocturnal increase in movement in this species.

Exposure to CO2 influences metabolism, calcification and gene expression of the thecosome pteropod Limacina retroversa

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 CO2 treatment influence the physiology of the thecosome Limacina retroversa, 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 CO2 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. Summary: Severity and duration of CO2 exposure influence calcification and respiration differentially in Limacina retroversa, as shown by organismal and gene expression responses, while captivity interactively influences respiration and gene expression.

Using a shell as a wing: pairing of dissimilar appendages in Atlantiid heteropod swimming

ABSTRACT Atlantiid heteropods are zooplanktonic marine snails which have a calcium carbonate shell and single swimming fin. They actively swim to hunt prey and vertically migrate. Previous accounts of atlantiid heteropod swimming described these animals sculling with the swimming fin while the shell passively hung beneath the body. Here, we show, via high-speed stereophotogrammetric measurements of body, fin and shell kinematics, that the atlantiid heteropod Atlanta selvagensis actively flaps both the swimming fin and shell in a highly coordinated wing-like manner in order to swim in the intermediate Reynolds number regime (Re=10–100). The fin and shell kinematics indicate that atlantiid heteropods use unsteady hydrodynamic mechanisms such as clap-and-fling and delayed stall. Unique features of atlantiid heteropod swimming include the coordinated pairing of dissimilar appendages, use of the clap and fling mechanism twice during each stroke cycle, and the fins extremely large stroke amplitude, which exceeds 180 deg. Highlighted Article: Atlantiid heteropod swimming is unique in that these planktonic snails use a swimming fin and shell as paired wings to execute complex appendage kinematics, including a double clap-and-fling.