Anthropogenic release of CO2 into the atmosphere is causing ocean acidification (OA) and ocean warming (OW) at a rate unprecedented during the last 66 million years. Planktonic organisms are amongst the most vulnerable to these changes, living close to the atmosphere/ocean boundary where CO2 dissolves into the ocean and direct warming occurs. As a vital component of all open ocean food chains and an essential food source for commercially fished species, understanding the effects of ocean changes upon plankton is crucial if we are to plan for the future.
The central aim of this study was to assess the atlantid heteropods, a group of planktonic snails, for their use as bio-indicators of ocean changes in a past, present and future framework. These snails build a shell of aragonite, a form of calcium carbonate that is extremely sensitive to OA, being more difficult to grow and maintain as pH decreases. To meet this aim, the project had three main objectives. 1) Assess species boundaries. Responses to ocean changes are likely to be species-specific, so we need to understand diversity within the atlantids. 2) Determine environmental tolerances in the present by modeling geographical distributions (e.g. temperature tolerance) and carrying out experiments (e.g. responses to changing pH). 3) Evaluate the effects of ocean changes in the past and the future. Using the fossil record we can assess how past ocean changes affected the diversity and distribution of atlantid species. Using Species Distribution Models (SDMs) we can also forecast changes in distribution for predicted future environmental scenarios.
Through integrated analysis of shell morphology, molecular phylogeny and geographic distribution patterns of atlantids, we found >35% more diversity than previously thought. Our results identified 37 hypothetical species (clades), while there are only 24 atlantid species currently described. One of these species, Atlanta vanderspoeli, was described as part of this project. Geographical distributions were updated, and used to determine species-specific environmental tolerances to physical ocean parameters. For example, distributions of Atlanta ariejansseni indicate a temperature tolerance of 7.9–20.1°C, whereas the closely related Atlanta inflata has a broader temperature tolerance of 7.1–31.1°C. A fossil calibration of our atlantid phylogeny suggests that great diversification has occurred over the last ~25 Ma, potentially in response to environmental changes. These results demonstrate that confident species identification is crucial for OA experiments. If species containing multiple cryptic clades (where they cannot be identified from each other) are used, species/clade specific responses to experimental conditions could produce misleading experimental results.
During the Atlantic Meridional Transect 2017 (AMT27) expedition, eight atlantid species were kept under laboratory conditions. Analysis of one growth rate experiment and one OA experiment, both on the most southerly distributed atlantid species, A. ariejansseni indicate a negative response to decreasing ocean pH, but a positive response to increased food availability. Together, our results show that atlantids would be excellent indicators of many different ocean parameters.
Fossil analysis of two sediment cores collected offshore of the Balearic Islands (Mediterranean Sea) demonstrates that variation in abundance and diversity of atlantids over the last 15-20 ka (since the LGM) reflects known changes in ocean temperature as indicated by commonly used proxies (planktonic foraminifera abundance, diversity and oxygen isotope ratios). This suggests that atlantids could be used as palaeo-temperature indicators. Initial modelling of future atlantid biogeography shows that distributions change considerably with predicted future conditions, often showing a reduction in potential habitat.
The results of this study demonstrate that the atlantid heteropods would be excellent bio-indicators of past and future ocean changes. The study also highlights that juvenile atlantids are good experimental organisms, being easily maintained under laboratory conditions and having a growth rate that is measurable on a scale of days.
