18. Vulnerability of Pteropod Shell In The Arctic Ocean: A Result of Culture Experiment Under Natural Seawater

Katsunori Kimoto (1)*, Jonaotaro Onodera (1), Naomi Harada (1), Kohei Matsuno (2), Takahito Ikenoue (3), Osamu Sasaki (4)

1 Institute of Arctic Climate and Environment Research (IACE), JAMSTEC, 2-15, Natsushima-cho, Yokosuka, 237-0061, Japan.
2 National Institute of Polar Research (NIPR), 10-3, Midori-cho, Tachikawa-shi, Tokyo 190-8518, Japan.
3 Marine Ecology Research Insitiute, 300 Iwawada, Onjuku-machi, Isumi-gun, Chiba 299-5105 Japan.
4 The Tohoku University Museum, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan.

The pteropods (thecosome), planktic molluscs having aragonite shell is major component of marine zooplankton and it plays important roles for oceanic carbon cycles and marine ecosystems. In particular, the aragonite shell is more soluble in seawater, and could be affected by ocean acidification (OA). In this study, we performed simple culture experiment of pteropod Limacina helicina living in the Arctic Ocean and described shell degradation processes under different water mass in the natural seawater.

Culture experiment had carried out on the R/V Mirai during the Leg MR13-06 Arctic cruise in 2013. In this cruise, we stayed two weeks at fixed station (Sta. 41: 72° 45’N, 168°16’W, water depth: 52 m) from 11 Sep. to 25 Sep., 2013 and collected L. helicina by vertical plankton tow for culture experiments. During observations, we observed drastic changes of carbonate chemistry in the water column within several days. In particular, degrees of aragonite saturation (Ωara) was shown from 0.6 (around seafloor) to 2.1 (surface). We used these natural seawater with different three conditions of Ωara (0.7, 1.4, and 2.1), and cultured L. helicina in nine culture vessels (batches) during two weeks.

The texture of shell surface of L. helicina showed remarkable changes under stereomicroscope within 48 hours from starting experiment: All shells of L. helicina kept hyaline under 2.1 and 1.4 of Ωara conditions. On the other hand, cloudy and whitish (damaged) shells were appeared under 0.7 Ωara condition. Seven days after, shell surface became whitish moderately in 1.4 Ωara conditions. It had never appeared whitish colored shells under 2.1 Ωara condition through the period of culture experiment. Onshore laboratory, all cultured shell density of L. helicina had analyzed to detect aragonite shell density by the Microfocus X-ray CT (MXCT). We will discuss about shell density changes during the culture experiment.