‘Living fossil’ reveals ancient link between squids and octopuses

The deep sea often challenges our ideas about how life evolves. Some creatures carry stories that stretch across unimaginable time. The vampire squid sits among these quiet survivors. Its soft body and drifting lifestyle hide a complex past written in its genome.

A new study now brings that history into view, revealing how this species links the worlds of squids and octopuses.

Vampire squid reveals family story

Researchers sequenced the vampire squid genome and uncovered traits shared with both squids and octopuses. Its appearance earned it a dramatic name, though it feeds gently on drifting debris.

“Interestingly, in Japanese, the vampire squid is called kōmori dako, which means bat octopus,” said Masa aki Yoshida from Shimane University.

Despite its octopus classification, it carries traits seen in squids and cuttlefish. The new analysis shows that this species preserved pieces of an ancient chromosomal pattern once common in early coleoids.

Such patterns support the idea that modern octopuses arose from squid-like ancestors.

The experts noted that early cephalopods passed through an intense chromosomal reshaping event known as ACCRE.

This reshaping created new regulatory landscapes that helped coleoids develop advanced traits. The vampire squid now helps clarify what happened after that event. 

The split of cephalopods

Cephalopods split into two major branches more than 300 million years ago. Squids and cuttlefish formed one path. Octopuses and the vampire squid formed the other. The genome shows the vampire squid sits between these groups.

Though placed with octopuses, it retains structures typical of the squid and cuttlefish branch.

The study shows that octopus genomes underwent major fusions and rearrangements over time. Many chromosomes fused and formed new mixed units.

These irreversible changes shaped the lineage after it split from the vampire squid.

The study also notes that ancient vampyropod species once filled many marine niches before most vanished. The vampire squid now stands as the last of that line. 

Giant genome of the vampire squid

The vampire squid’s genome spans more than 11 billion base pairs. This makes it one of the largest animal genomes ever sequenced.

Despite this size, its structure remains stable. This stability makes the vampire squid a genomic living fossil. Its genome keeps parts of an old squid like layout.

Octopuses, in contrast, show signs of intense fusion events that reorganized chromosomes into new shapes. “The vampire squid sits right at the interface between octopuses and squids”, says Oleg Simakov.

The study shows that gene mixing within fused chromosomes cannot reverse. This makes such mixing a strong marker of evolutionary history.

The team also found that transposable elements expanded throughout the genome without disrupting its basic structure.

This pattern contrasts with many systems where such elements promote rearrangements. The vampire squid therefore separates genome size growth from structural chaos. 

Vampire squid’s ancient heritage

The study compared the vampire squid with the pelagic octopus Argonauta hians. The genome of Argonauta hians was presented in detail for the first time. Early coleoids likely carried a squid like chromosomal pattern.

Later fusions shaped the genomes of modern octopuses. These fusions formed mixed chromosomes with combined gene sets from several ancestral units.

This process also shaped regulatory regions that influence arm specialization and shell loss.

“Although it is classified as an octopus, the vampire squid retains a genetic heritage that predates both lineages,” said Emese Tóth from the University of Vienna.

According to the researchers, octopus genomes show an extra period of intense rearrangements after their split from the vampire squid.

These patterns explain why octopuses show greater chromosomal mixing than their deep sea cousin. 

New insights into cephalopod evolution

The findings highlight that the shared ancestor of squids and octopuses looked more squid like than expected. Changes in chromosomal layout, rather than new genes, shaped the evolution of modern cephalopods.

The study proposes that demographic changes or shifts in genome stability may have driven these large transitions.

The research also suggests that regulatory changes, not gene loss, explain major traits like shell loss in octopuses.

The vampire squid now helps track how these transformations unfolded. Its preserved chromosomal layout shows where the modern cephalopod story began and how later rearrangements shaped the diversity we see today.

These results give scientists a rare view into very old evolutionary steps. They show how tiny shifts in chromosomes can shape entire body plans. The study also reveals how ancient lineages adapted to changing oceans.

The vampire squid acts like a living archive of those changes. Its genome captures clues that other species lost. This lets researchers compare past and present patterns with far more clarity.

The study is published in the journal iScience.

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