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All four krill an ROV collected at an Antarctic vent field were mated, egg-carrying females
Antarctic krill had never been recorded at deep-sea hydrothermal vents until 2024. Every one the team collected at Hook Ridge was a mated, egg-carrying female, and the study asks whether vents are spawning habitat.
The Scientist · Science desk
What happened
- Oceanographers recorded Antarctic krill at deep-sea hydrothermal vents for the first time in 2024, at the Hook Ridge and Quest Caldera vent fields, and have now published the account in Communications Biology.
- All four krill collected at Hook Ridge with a remotely operated vehicle were mature, egg-carrying females that had recently mated, and no males appeared on video there during nearly nine hours.
- At Quest Caldera, about 21 hours of video showed many pregnant krill near the vents, along with dense swarms at the active venting site.
- The comparison sample, taken at a non-vent seafloor site in the Antarctic Sound, held the usual mix of juveniles, males and females.
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Why it matters
- constraint This is a presence result built on four collected animals and about 30 hours of video. It does not measure what share of the spawning population is involved, and it leaves the biomass numbers the fishery is argued over where they were.
- decision Anyone drawing critical-habitat boundaries for reproductive krill now has to weigh seafloor vent fields, which sit outside the surface layer the species was assumed to occupy.
- capability Vent-associated gut microbes and tissue chemistry give a marker for identifying krill that have visited vents, so how often those trips happen can be asked of animals caught elsewhere.
- exposure The largest concentration of the species sits in the Peninsula and Scotia Sea region, where warming is fastest and fisheries already press on the population. Any reproductive habitat there is contested ground.
Why a gravid female would pay for the trip down is the part the team reasons through without testing it. Temperatures near the vents fall within the thermal tolerance of Antarctic krill and below the upper threshold for their embryos, and at those temperatures embryos should develop faster [12][13]. Warm-water incubation is something other marine animals do, octopuses among them [13]. Vents also carry zinc and manganese, metals krill need and that are otherwise hard to find [14]. The team says the benefit to spawning females was not directly tested [12].
What the eggs actually do is open. The study authors wrote: "Whether these females would have released their eggs at depth or ascended to spawn nearer the surface remains unknown." [1] Either way, they wrote, "reproductively active females are undertaking deep vertical migrations during the spawning season when their metabolic resources are generally focused on reproductive output." [2]
Gut contents from the Hook Ridge females held microbe types associated with vent ecosystems. The team infers recent feeding on chemosynthetic microbes. Stable chemical signatures and trace metals in their tissues indicated repeated exposure to vent-derived food and mineral-rich water [9].
Scale is why the fishery question follows at all. The researchers put the species' cumulative biomass at around half a billion tons, likely the highest of any wild animal on Earth [15]. About a third of it sits in the water between the northern Antarctic Peninsula and the Scotia Sea, one of the fastest-warming areas on the planet [16]. One third of half a billion tons is about 167 million tons in that one region [1]. Whales, seals, penguins and seabirds eat krill, and most predators that do not eat krill eat animals that do [17].
The authors frame their finding as a knowledge gap. They wrote that "we still lack fundamental knowledge on what constitutes critical habitat for key parts of the Antarctic krill life cycle" [3], and they describe mounting international recognition of the need to conserve critical krill habitat [10]. The finding says where to look. The candidate location for part of the reproductive cycle moves out of the upper layer where the species was thought mostly to live [1]. It moves down to two active vent fields sampled by remotely operated vehicle [4].
What to watch
- A depth-stratified count of gravid females through a spawning season would show whether vent visits are routine or rare.
- A direct test of whether krill embryos incubated at vent temperatures develop faster than at surface temperatures.
- Whether krill habitat-conservation proposals start naming seafloor vent fields alongside surface waters.