The effects of climate change on the heart rates & growth of sea slugs in the Gulf of Maine

In the next 80 years, sea surface temperatures are expected to increase by 1.5o to 2oC and ocean pH is expected to drop by 0.06 to 0.32 units, with exacerbated effects seen in coastal waters. Temperature increase has already forced organisms to shift their range polewards and ocean acidification has negatively affected calcifying organisms. Interactive effects, only more recently studied, vary depending on phylum and life cycle stage. This study examined both the upper thermal tolerance and interactive effect of temperature and acidification on the heart rate of five cold-water species of nudibranchs (Aeolidia papillosa, Cuthona gymnota, Dendronotus frondosus, Flabellina verrucosa, and Onchidoris bilamellata) and one species of sacoglossan (Placida dendritica) from the Gulf of Maine. Thermal tolerance was determined by recording heart rate for each organism starting at 4oC and increasing the temperature by increments of 4oC until the organism’s heartbeat slowed or ceased. For interactive effects, pH levels used were pH 8 (control) and pH 7 at temperatures: 4o, 8o (control), 12o, and 16oC. Upper thermal tolerance limits ranged from 16o to 20oC for the nudibranchs and 24oC for the sacoglossan. The combined effects of increasing temperature and lower pH were neutral, negatively additive, and antagonistic. Only F. verrucosa exhibited an interactive effect, with higher temperature and lower pH leading to decreased heart rate. Although no interactive effect was demonstrated in C. xgymnota, D. frondosus, and O. bilamellata, lower pH slowed heart rates across all temperatures. Subsequently, the relationship between temperature and growth rates was examined in D. frondosus and F. verrucosa. The nudibranchs were reared for eight weeks at 4o, 10o, or 16oC and growth was measured weekly. The ideal temperature for growth appeared to be 10oC, whereas 16oC was lethal. Additionally, an unsuccessful attempt was made to culture A. papillosa, but the number of embryos per egg capsule and larval growth rates were examined. Size of adult sea slug positively impacted the number of embryos per egg capsule, with embryos increasing in length by 50% over the first week and 10% over subsequent weeks. With an interactive effect only seen in one species and upper temperatures being lethal if held constant for a month, temperature appears to be the greatest threat to survival. What is happening to these sea slugs in the GOM is likely happening to other snails and marine invertebrates throughout the ocean. Knowing how organisms will react to the projected changes can help inform future policies and practices.

Leigh Gibson J., 2019. The effects of climate change on the heart rates & growth of sea slugs in the Gulf of Maine. MSc thesis, University of New Hampshire, 79 p. Thesis.

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Ocean acidification in the IPCC AR5 WG II

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