Abstract
Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope‐based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope‐based Bayesian niche metrics to investigate the trophic relationships of three co‐occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean—a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell‐less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ13C and δ15N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.
Highlights
Human‐driven climate change is causing significant chemical and physical changes to global oceans, leading to demonstrable direct and indirect ecological impacts to Southern Ocean assemblages across all trophic levels (Constable et al, 2014)
While the Southern Ocean supports a range of diverse ecosystems, it is among the most rapidly changing regions on a global scale, and it is an ongoing challenge to predict how these ecosystems and species will respond (Murphy et al, 2012)
Given the complex spatial and temporal dynamics of the Southern Ocean, future research focused on understanding species responses due to human‐induced physical and chemical changes need to be small scale and regionally comparative (Allan et al, 2013)
Summary
Human‐driven climate change is causing significant chemical and physical changes to global oceans, leading to demonstrable direct and indirect ecological impacts to Southern Ocean assemblages across all trophic levels (Constable et al, 2014) Among these changes is ocean acidification, which causes dissolution of shell‐secreting organisms (Fabry, Seibel, Feely, & Orr, 2008; Orr et al, 2005). Gut content and fatty acid analyses of gymno‐ some (shell‐less) pteropods point to a highly specialized carnivorous diet, preying exclusively on thecosomes (Conover & Lalli, 1972) Both theco‐ somes and gymnosomes are important food for higher trophic level organisms (Lalli & Gilmer, 1989; Pakhomov, Perissinotto, & McQuaid, 1996), so changes to pteropod assemblages in the Southern Ocean will have flow on effects up the food web (Suprenand & Ainsworth, 2017). Co‐occurring Southern Ocean pteropods could provide a unique model community to understand how different functional traits (e.g., feeding structures, body size) correlate with the trophic structure of communities consisting of both monophagous specialists and generalists
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