Pteropods, abundant aragonitic calcifying plankton, are the equivalent of "canaries in the coal mine" for our acidifying oceans. Their sensitivity to high CO2 levels and limited fossil record has led to the widely held view that pteropods only became abundant after the Paleocene-Eocene Thermal Maximum (PETM, ~56 million years ago), the closest analogue to modern-day ocean acidification and warming. An international team of researchers have shown, based on an extensive molecular dataset (2654 genes) and new fossil evidence, that the two major groups of pteropods (sea butterflies and sea angels) have Cretaceous origins and hence, must have survived previous global change events in Earth's history. These findings indicate that pelagic aragonitic calcifiers have shown remarkable resilience to perturbations in the Earth's carbon cycle over evolutionary timescales. This research is published in the Proceedings of the National Academy of Sciences.
Tiny sea snails swimming against the climate crisis current
Many people don’t give slugs and snails a second thought, or the admiration they deserve. However, slugs and snails (more properly known as gastropods) are truly remarkable and incredibly diverse, having colonised almost every nook and cranny of our planet, except the sky.
One group of snails that really deserve some appreciation are the holoplanktonic gastropods, the pteropods and atlantid heteropods. These small snails spend their entire lives swimming in the ocean, never settling on a solid surface. These planktonic gastropods are known to be extremely sensitive to ocean changes, with some pteropods being described as sentinels to indicate changes in ocean acidity. Yet despite this sensitivity, two new studies show that both groups survived large-scale global change crises in the past, which led to widespread extinctions of many other marine taxa. Could this resilience to past crises give a glimmer of hope in our current climate crisis?
Swimming snailsdiverged in the Early Cretaceous
The holoplanktonic gastropods are an amazing example of adaptation to a particular environment. In order to live a planktonic lifestyle, swimming fins or wing-like appendages have developed from their snaily foot, and their shell has become thin and small (sometimes absent in the adult stage). The pteropods, commonly known as sea butterflies and sea angels, and the heteropods, known as sea elephants, are mesmerising creatures with unique swimming styles. Surprisingly, although these two groups look superficially similar, they are not closely related, having independently colonised the upper ocean by finding similar solutions to a planktonic problem.
The small, thin shells of holoplanktonic gastropods leave a patchy fossil record, and so for a long time, the timing of their origins has been disputed. Two new articles published recently by an international group of researchers, explored the evolution of the two groups using a combination of genetic information and the ages of key fossils. The results were surprisingly similar for the two groups of planktonic gastropods. Both the pteropods and the atlantid heteropods were found to be much older than previously thought, with both groups originating in the Early Cretaceous (~100.5–145.0 Million years ago). This early origin reveals that swimming snails survived through two large-scale global change crises, the Cretaceous-Paleogene extinction event (KT) and the Paleocene-Eocene Thermal Maximum (PETM).
Resilience in a changing ocean
These fascinating and beautiful snails may seem small and insignificant, but they provide vital links in the ocean food web and in the cycling of carbon in the ocean. Unfortunately, experimental studies indicate that they are also likely to be catastrophically affected by imminent ocean changes, including ocean warming and ocean acidification.
Nevertheless, could the fact that they have lived through past climate crises give some hope for the swimming snails of the future? The PETM is considered the most analogous geological event to the current Anthropogene climate crisis, and their survival through this event suggests a higher resilience than previously thought. However, current rates of climate change are now unprecedented, even in comparison to the PETM. Pteropod expert, and lead author of one study Dr Katja Peijnenburg stresses that "past performance is no guarantee of future results" particularly because many marine organisms are unlikely to have sufficient time to adapt at the current rate of change. So, while the new research gives new hope for swimming snails in a future ocean, their fate is still very much uncertain.
Passionate guest researcherArie Janssen
This publication would not have been possible without the knowledge and expertise of Arie Janssen, retired curator (1997) and guest researcher of Naturalis. He has mapped a complete overview of all recent and fossil species.
- Image credits: Katja Peijnenburg, Erica Goetze, Deborah Wall-Palmer, Lisette Mekkes. More images and video footage of these organisms are available from: Katja Peijnenburg.
- Article details: The origin and diversification of pteropods precede past perturbations in the Earth's carbon cycle. Authors: Katja T.C.A. Peijnenburg, Arie W. Janssen, Deborah Wall-Palmer, Erica Goetze, Amy Maas, Jonathan A. Todd, Ferdinand Marlétaz.
- And: Fossil-calibrated molecular phylogeny of atlantid heteropods (Gastropoda, Pterotracheoidea). Authors: Deborah Wall-Palmer, Arie W. Janssen, Erica Goetze, Le Qin Choo, Lisette Mekkes & Katja Peijnenburg.