- A recent study has revealed four species of Diacavolinia sea butterflies or Diacavolinia pteropods, new to India.
- Pteropods are a type of zooplankton that live in the water column of the ocean. They play an important role in processes in the marine ecosystem, from the food chain to carbon cycling.
- Scientists warn that pteropods are threatened due to global warming and note that their populations need to be monitored for marine conservation.
In the fall of 2019, Kiran Shah was sorting through small bottles containing sand and rocks, which had been gathering dust for years in a lab located on a hilltop in Port Blair. These bottles with samples were collected by marine scientists in 2011. During this sorting exercise, Shah stumbled upon tiny brown shells, smooth as glass, their size as small as the seeds of a guava, hidden in the sediment. Little did she know that she was about to find pteropods new to India.
Pteropods are molluscs – invertebrates that do not have a backbone. There are two types of pteropods: sea angels (without shells) and sea butterflies (with shells). Shah’s description of four species of sea butterflies – Diacavolinia pteropods (D. deshayesi, D. grayi, D. mcgowani, and D. strangulata) discovered in Indian waters for the first time – was recently published in the journal Zoologischer Anzeiger. These species have been previously reported only from the Atlantic, Pacific, Caribbean, and Banda Seas.
Pteropods play a crucial role in the marine food web and carbon cycling, apart from being indicators of ocean acidification. This discovery increases India’s pteropod species count from 22 to 26 and comes at a critical time when pteropod populations are showing signs of decline globally because of warming oceans.
“The availability of pteropods signifies that the region of Andaman and Nicobar Islands has a healthy ecosystem,” said Shah, a marine biologist. The research was the subject of her PhD dissertation at Pondicherry University, supervised by P.M. Mohan, the co-author of the study.
In December 2011, the samples containing these species were collected by the FORV Sagar Sampada, a government-owned research vessel, from the depth of 200 metres around the Nicobar Islands. These were preserved in formaldehyde and stored for years, largely unexamined. When Shah began to check the samples, she came across the pteropods. “They appeared exclusively in the offshore waters and I couldn’t find them in the nearshore samples,” said Shah, who studied 100 cruise samples.
The Diacavolinia pteropods are visible to the naked eye but require a microscope for identification. Typically, live specimens of these pteropods have a transparent shell but the specimens Shah studied had fossilised with time and turned brown in colour, with their fragile shells requiring careful handling with a paint brush. “Some of the structures were broken and their colours were missing. I had a very small number of shells to represent the species I was identifying,” she told Mongabay-India.
Since there is no consolidated checklist or key of pteropods available and even the World Register of Marine Species (WoRMS), a database of marine organisms, had not been updated with the names of pteropods in the Indian Ocean, Shah had to refer to their descriptions from the Pacific and Atlantic Oceans. She noted that while there is research available from Thailand, Singapore, and the Arabian Sea on these species, very little research exists from India. “The problem with these types of studies is that there are very few experts [on pteropods] for identification,” said Sijin Kumar A.V., a geology professor at Central University of Kerala, while noting that the study was done “as per standard protocol”. A marine geologist by specialisation, he has been researching pteropod fossils from the Indian Ocean for a long time and is not associated with the study.
From food chains to carbon cycling, pteropods do it all
Shah and Kumar explained why pteropods are key to maintaining a balance in the ocean. First, they are the basis of the food chain and act as food for bigger organisms such as baleen whales, other fish and seabirds. They are high in lipids and calcium, making them a nutritious prey. Sea angels also feed on sea butterflies.
Second, pteropods also contribute to the carbon cycle in the ocean through the way they travel in the water vertically. At night, they travel up from the deep waters to feed. As the sun rises and temperature increases, they travel down, thus contributing to the ocean carbon cycling in the water column.
Their shells are made of aragonite, a mineral containing a form of calcium carbonate that is highly soluble. At the end of their lives, these aragonite shells (a form of carbon) settle on the ocean floor, trapping and storing the carbon in the form of sediment eventually. This is how pteropods remove carbon from the atmosphere indirectly.
Third, pteropods serve as paleoenvironmental records. Since they are preserved in their original state as fossils when they die, their chemical composition tells a story about the environment and period of time they lived in, Shah explained.
Finally, the delicate aragonitic shells of pteropods are sensitive to changes in the ocean’s chemistry, making them indicators of ocean acidification events in the past and present. Kumar noted that there is a decline in pteropod populations through warm periods during the geological ages. His studies on pteropod fossils found that glacial sediments contain abundant shells as compared to sediments from the Holocene period (the last 11,000 years), which contain far fewer shells.
Sea butterflies are on the climate frontlines
Climate change however is accelerating the decline of these sea butterflies. As carbon emissions increase due to human activities, the ocean is absorbing more carbon and this lowers the pH level of ocean water, making it more acidic. This directly impacts pteropod shells. “They start getting ruptured in certain places or become thin with time,” explained Shah. Or the organism may not be able to grow its shell in the first place. Also, pteropods cannot regenerate broken shells and their shells begin to dissolve in acidic ocean water, a phenomenon that scientists have already seen in the Southern Ocean around Antarctica.
“Once it goes to an extreme level, acidified water and pollutants will directly go into their systems through their soft body. That is when these shells will die,” said Shah. Kumar warned that pteropods could be “the first plankton species to become extinct if we do not take any measures to reduce carbon emissions”. The disappearance of pteropods could have cascading impacts on the ecosystem: the marine food chain will be impacted, the carbon cycle will get disrupted, and the overgrowth of phytoplankton, which the pteropods feed on, could lead to harmful algal blooms.
The study also has its limitations, the foremost being the sampling constraints. Shah’s work is based on the morphological analysis of samples obtained from a single cruise and these are unable to capture the live population of pteropods in the Nicobar Sea, she said. Kumar explained that this is a common problem: “In marine studies, it’s very difficult to get the samples. On land, we can collect samples easily, but in the ocean, we need specific platforms or research vessels.”
A genetic analysis of the species would have made Shah’s research more robust but because the specimens were preserved in formaldehyde, doing a DNA analysis to confirm the identity of the species was not possible. “However, future studies using fresh plankton samples could employ integrative taxonomy to further validate these records,” the study noted.
A foundation for future ocean acidification studies
The discovery of these pteropod species is significant because not only does it fill a research gap in marine taxonomy but it also lays the foundation for future research on paleoclimatology (the study of ancient climates), along with providing a baseline for future ocean acidification and climate studies in the region.
According to Shah, researchers can aim to collect live samples and study their food preferences, reproduction and life cycle. They could also conduct ocean acidification experiments by testing temperature variations on the behaviour and swimming patterns of pteropods in the lab, she told Mongabay-India. Such experiments, in turn, will create knowledge that can be applied to study fossil records to aid geological studies, said Kumar. Researchers can also analyse pteropod fossil records to study both past and modern ocean acidification, he added.
With the push to develop the Andaman and Nicobar Islands for trade and tourism, its huge variety of endemic species face enormous risks. This makes it even more important to ensure biodiversity monitoring in the region to ensure existing species including pteropods do not suffer, said Shah. In a world where mammals such as dolphins and whales have become symbols of marine conservation, pteropods, no matter how small, deserve our attention.
Banner image: Lateral views of D. grayi, D. deshayesi and D. strangulata. Images by Kiran Shah.
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