It’s not uncommon for scientists to find the occasional oddity in the middle of the North Pacific. But just North of Hawaii, they spotted something they weren’t expecting: a giant virus that carries the longest tail ever documented in a virus.
The study was published in bioRxiv and reports that the tail can extend up to 2.3 microns — longer than many bacteria and more than twice the length of the tail on Tupanvirus, the previous record-holder.
PelV-1’s story began at Station ALOHA, a long-term monitoring site in the North Pacific Subtropical Gyre. Scientists collected seawater 25 meters below the surface, isolating a dinoflagellate called Pelagodinium and, to their surprise, its viral hitchhiker.
The virus infects dinoflagellates, a group of plankton, which is pretty rare in itself.
Dinoflagellate-infecting viruses are notoriously elusive. Only two other large DNA viruses are known to infect members of this group, and neither has a sequenced genome. That gap matters because dinoflagellates are central players in marine ecosystems — they form symbiotic partnerships, fuel food webs, and sometimes trigger harmful algal blooms. Understanding their viral foes could reveal how energy and nutrients move through the ocean.
But what’s really curious is the virus’ tail.
The Longest Tail in the Virosphere
Electron microscopy revealed PelV-1’s most striking feature: a thin, 30-nanometer-wide tail that can stretch longer than 10 times the virus’s 200-nanometer capsid. Some virions also carry a shorter, stubby protrusion on the opposite end, emerging from a star-shaped “stargate” opening.
Time-lapse imaging suggests it uses its tail to latch onto host cells early in infection. But intriguingly, newly formed viruses inside host cells lack tails entirely, implying the appendage is assembled later, after the cell bursts open.
This unusual life cycle raises questions about what the tail does in the open ocean. The researchers suspect it boosts the virus’s chances of bumping into a host in the nutrient-poor gyre. “It increases the effective diameter of the virus,” the team writes, “increasing the probability of contacting a potential host.”
Sequencing the viral DNA brought another surprise: the cultures harbored a second, rarer virus. Dubbed co-PelV, it belongs to the same family, Mesomimiviridae, but lacks tail genes. It carries its own arsenal of metabolic tools — including enzymes for breaking down chlorophyll and cellulose — and may influence not just its host’s metabolism but also its behavior.
PelV-1’s genome is huge for a virus — 459,000 base pairs containing 467 genes. Some of these genes carry out functions usually seen in living cells, including parts of the cell’s energy cycle, proteins that capture light, and even rhodopsin — a light-sensitive molecule that could help the virus harness energy from sunlight.
Viruses Like We’ve Never Seen Before
The discovery of PelV-1 underscores just how little we know about giant ocean viruses. Giant viruses have already upended textbook definitions of what a virus can do, with genomes that encode metabolic pathways, stress-response proteins, and photosynthetic machinery. PelV-1 adds a record-breaking morphological twist to the story.
PelV-1’s unusual combination of features — from its trademark tail to its extensive set of metabolic genes — broadens the range of forms and functions known among marine viruses. By adding both morphological and genomic extremes to the catalog, it puts into perspective how little we know of the oceans’ viral world.
Future work will probe exactly how PelV-1’s tail assembles, how it influences infection success, and whether similar long-tailed viruses lurk elsewhere in the sea. For now, the discovery is a reminder that even in some of the most studied waters on Earth, the microbial world could still holds surprises.
