When iceberg A68 broke away from Antarctica’s Larsen C Ice Shelf in 2017, it exposed seafloor that had been hidden beneath the ice. In January 2019, a remotely operated vehicle surveyed part of that newly accessible habitat and recorded 1,036 active fish nests. The count comes from five dives—not a census of the entire region—and the survey documented the nests without witnessing fish build them.
How did the iceberg reveal the nests?
A68 calved from the Larsen C Ice Shelf in 2017, opening access to seabed that had been covered by the iceberg. During the Weddell Sea Expedition 2019, researchers surveyed waters around the ice shelf while also seeking the wreck of Shackleton’s Endurance. On five days in January 2019, they used the remotely operated vehicle (ROV) Lassie from the research vessel SA Agulhas II to examine the seabed. The study reports 27 hours of ROV video. Frontiers’ account describes A68 as covering 5,800 square kilometres; that figure is not a measurement of the surveyed nest habitat. Frontiers’ account of the discovery
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The expedition dataset dates the five ROV dives to 14–23 January 2019. The authors of the 2025 study describe the Weddell Sea as “one of Earth’s most remote and least studied regions.” PANGAEA dive dataset · Connelly et al., 2025 study
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Across those five dives, the researchers recorded 1,036 active nests in 277 groups, plus 93 nests they classified as inactive. They observed larvae in or around 72 active nests. Active and inactive nests were found at depths of 290–411 metres. These are counts of what the ROV recorded in its surveyed areas, not an estimate of all nests in the western Weddell Sea or the full size of a breeding colony.
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For the analysis, researchers reviewed the video, photographed nests with timestamps, and measured them from images. Two lasers mounted 10 centimetres apart provided a scale reference. Depressions smaller than 6.5 centimetres were excluded from the nest count. A nest was classified as active when detritus was absent from its depression, and inactive when detritus had accumulated there. This classification relied on video; the team did not collect sediment samples and judged sediment size visually. Study methods and findings
What kinds of fish nests were filmed?
The researchers grouped nest arrangements into six observed patterns. Clusters accounted for more than 42% of nests and were the most common arrangement. Singular nests were the second most abundant and had the largest mean diameter.
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| Arrangement | What the name describes | Reported frequency or size comparison |
|---|---|---|
| Cluster | Nests grouped together | More than 42% of nests; the most common arrangement |
| Crescent | Nests arranged in a crescent shape | Frequency not stated in the study summary |
| Line | Nests arranged in a line | Frequency not stated in the study summary |
| Oval | Nests arranged in an oval | Frequency not stated in the study summary |
| Sharp U | Nests arranged in a sharp U shape | Frequency not stated in the study summary |
| Singular | An individual nest rather than a group arrangement | Second most abundant; largest mean nest diameter |
The names describe spatial patterns in the video. They do not, by themselves, establish why fish made each arrangement. Connelly et al., 2025
Which fish made the nests?
The researchers attributed the nests primarily to yellowfin noties (Lindbergichthys nudifrons). It was the only species seen occupying or maintaining nests during the 27 hours of video. That observation supports the attribution, but does not prove that every nest was made by this species: the authors could not exclude other species as creators of a minority of nests where neither a fish nor eggs were visible.
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The ROV footage did not show fish actively constructing nests. The study also leaves questions about social structure and nest ownership unresolved, so parental roles or ownership should not be treated as observations from these dives. Study discussion
Why are the nests grouped together?
Predator protection is a proposed explanation, not a cause demonstrated by the survey. The authors suggest that grouping might lower an individual fish’s danger, consistent with the “selfish-herd” idea that individuals can reduce their own risk by being among others. They also hypothesize that dense groups of nests might make it harder for the predatory ribbon worm Parborlasia corrugatus to identify a particular nest using chemical cues.
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The ROV survey did not capture nest construction or predation in action, so it cannot establish either mechanism. It documents where nests occurred and how they were arranged; testing why fish group them remains a separate question.
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Did temperature or the time since the ice moved explain nest locations?
Not consistently. At four sites, the study found no significant local temperature difference between nesting areas and nearby areas without nests; Site A was the exception. The researchers also found no relationship between the duration of ice-free conditions and nest characteristics. Those results do not identify a single environmental explanation for where the nests formed. Study results
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What does the discovery mean for Antarctic conservation?
The study presents the findings as evidence of structured breeding habitat that meets key criteria for a Vulnerable Marine Ecosystem. The authors say the evidence strengthens the case for the proposed Weddell Sea Marine Protected Area. It is not itself a new protection designation, and the nest counts do not establish the size of the wider population or the full extent of the habitat. Connelly et al., 2025
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