Two New Deep-Sea Drifters Split the Ocean by Depth

Two Deep-Sea Drifters have just joined the ocean's strangest family, according to researchers at the Monterey Bay Aquarium Research Institute. The newly named siphonophores, one with stretchy orange stomachs and one with a fiery red body, show how life in the open ocean can split apart not by distance, but by depth. Their discovery, published in the Zoological Journal of the Linnean Society, adds two species to a group that already ranks among the most dominant predators of the midwater.

Siphonophores are not single animals in the usual sense. Each is a colony of specialised units called zooids, working together to swim, catch food, and reproduce. Relatives of jellyfish and corals, they drift through the water between the surface and the seafloor, a realm with no fences, no mountains, and no obvious borders. That lack of barriers makes their diversity a puzzle: without land in the way, what stops one widespread population from staying one species forever?

Depth, not distance, draws the lines

The new study offers a clear answer for this group. The team combined video from underwater robots, detailed study of museum specimens, and genomic sequencing to sort out species boundaries. Seven species turned up in the institute's video archive, with an eighth recognised from museum collections. Although their geographic ranges overlap, the species separate along depth gradients, with different members occupying different layers of the water column. In other words, vertical space does the work that horizontal distance does on land.

That pattern, known as vertical niche partitioning, matters because the midwater is the largest living space on Earth by volume, yet it remains far less explored than the surface or the seabed. If depth alone can drive speciation there, the true diversity of open-ocean life may be much greater than current counts suggest. The researchers also found two major evolutionary lineages within the group, one of which shows a striking acceleration in mitochondrial genome evolution alongside a shift in its reproductive system. That link between genome change and reproductive change gives evolutionary biologists a rare, concrete case to study.

Shape told part of the story too. Measurements of the swimming bells, the jetting structures that propel these colonies in unison, showed variation strongly tied to lineage, with each branch following its own growth trajectory. Differences in stinging-cell size and tentacle architecture pointed to functional divergence as well, suggesting that related species are not just living at different depths but hunting in subtly different ways. Form, genes, and habitat line up, which is why the team could confidently draw new species boundaries rather than merely splitting hairs.

Meet the two newcomers

The first newcomer, Bargmannia marlena, is recognised by its stretchy orange stomachs, a feature that stands out even in the dim footage captured by remotely operated vehicles. The second, Bargmannia pyrosa, carries a fiery red body and was recorded in the Gulf of California at a depth of 1,696 metres. Both were previously unknown to science, hidden in plain sight among footage and specimens that had been collected but not yet fully sorted. Their naming brings the group's known diversity into sharper focus and gives future surveys a reference point for recognising them again.

There is a practical lesson here for ocean exploration. The institute's robots have now contributed to the discovery of well over three hundred new species in the depths of Monterey Bay and beyond, many of them recognised only after years of video sat in archives waiting for careful review. New species are not only found on daring expeditions to unvisited places; they are also found by revisiting existing evidence with better tools, from genome sequencing to precise shape measurement. Every archived dive is a potential discovery waiting for the right question.

According to the institute, understanding this hidden diversity is also a conservation question. The midwater supports fisheries, regulates carbon, and hosts animals that migrate vertically each night in one of the largest daily movements of biomass on the planet. Protecting a habitat that people rarely see starts with knowing what lives there and how its communities are structured. A depth-layered community cannot be protected by a single blanket rule, because different layers face different pressures from warming, fishing, and future deep-sea activity.

What the discovery changes

For evolutionary science, the work shows how biodiversity can emerge from the interplay of genomic innovation, body-shape divergence, and depth structuring, without any geographic barrier at all. That is a useful correction to the textbook picture in which new species mainly arise when populations are split by a mountain range or an ocean basin. In open water, the gradient itself is the barrier, and it is invisible to anyone who only maps life in two dimensions.

For readers, the takeaway from these Deep-Sea Drifters is simpler and more wonderful: the deep sea is still naming new neighbours. Colonies of tiny specialists, jetting in unison through cold darkness, have been quietly dividing the water column into neighbourhoods for millions of years, and science is only now learning the addresses. The next archived video, the next museum drawer, or the next robot dive could hold another one. Readers curious about ocean and space frontiers can also explore a mission nearing its moment of truth around an asteroid, an hourly watch from space that follows bad air days, and how ice grains from a distant moon could hold clues to alien life. The siphonophore findings were reported by Phys.org and by the Monterey Bay Aquarium Research Institute.