New genomes have finally answered the puzzle of squid evolution.

Cuttlefish and squid have always been somewhat enigmatic. They solve problems in ways that appear almost too brilliant for animals without backbones, flash colours, and move with abrupt bursts of speed.

For years, scientists have been attempting to put together the origins of these species, but the narrative has remained frustratingly unfinished. Lack of curiosity wasn't the issue. There was insufficient proof. Fossils are uncommon and frequently ambiguous. There has been inconsistent genetic data.

For a very long time, scientists could only speculate about the relationships between various species and the times at which they split off. Now that image has begun to sharpen.

The difficulty of researching squid evolution

In order to create a more accurate evolutionary history, a recent study compiles vast amounts of genomic data, including three recently sequenced squid genomes. The Okinawa Institute of Science and Technology is the source of the work, where scientists have finally been able to put many of the puzzle pieces together.

The first author of the study, Dr. Gustavo Sanchez, stated, "Squid and cuttlefish are amazing creatures, yet their evolution has been notoriously difficult to study."

"Many research groups have proposed different evolutionary hypotheses based on different morphological characteristics and molecular datasets, and the question of their ancestry has been under investigation for decades." "Some of the mysteries surrounding their origins have been solved with our new genomic information."

A family in several forms

Decapodiformes, or ten-limbed cephalopods, is the group that includes squid and cuttlefish. From deep, dark seas to shallow reefs, they can be found practically anywhere in the ocean.

But they don't all have the same appearance. An interior shell is one characteristic that unites many of them. Even that varies greatly. Some have rounded, smooth cuttlebones. Others have a gladius, which is a narrow, blade-like structure. Some have completely lost their shell, while others have spiral shells.

Previous attempts to map their family tree failed due to insufficient data.

According to Dr. Sanchez, "earlier reconstructions of decapodiform evolution were built from datasets with limited resolution and were prone to biased signals, obscuring the true relationships between different species." "A clearer, more consistent picture of how these animals evolved is now provided by whole genome data."

The genomes of squid are enormous.

It wasn't simple to get that crisper image. The genomes of cuttlefish and squid are enormous. Some are twice as big as the human genome. Because of this, sequencing them is a laborious and slow procedure.

The issue of gathering samples is another. Many species are found deep underwater or far from the coast. Sequencing requires fresh DNA, which isn't always readily available.

According to Dr. Sanchez, "some lineages are mysterious and only known in the deep sea, while others are abundant and highly diverse in tropical reef systems like the Ryukyu Archipelago."

"We worked with colleagues who had access to more difficult samples, and we were lucky to find some important species on our doorstep in Okinawa."

The first evolutionary tree constructed from genomes representing almost all major decapodiform groups is the outcome of that endeavour. It took five years and international cooperation to come together.

A peculiar squid fills the void.

The ram's horn squid, or Spirula spirula, is one of the study's most perplexing creatures. It has a tightly wound interior shell that has long baffled scientists. It is rarely seen.

"In the past, some scientists mistakenly believed that the ram's horn squid was closely related to cuttlefish due to its shell structure," stated Dr. Fernando Á. Fernández- Állvarez of the Spanish Institute of Oceanography, who co-authored the paper.

"I thought this genome could help close a significant gap and shed light on the more general evolutionary questions of cephalopods."

For cephalopods, the sea surface would have been an extremely hostile habitat. There would have been relatively few suitable oxygen-rich environments close to the coast at that time, according to Dr. Sanchez.

"The fact that this feature has persisted throughout their evolutionary history is evidence of their deeper oceanic origins because intense ocean acidification in shallower waters would also likely have degraded their shells."

An increase in diversity

Conditions close to the surface improved following the extinction catastrophe. As coral reefs recovered, new habitats became available. Cuttlefish and squid invaded these regions and quickly started to diversify.

We don't see much branching for many tens of millions of years after the first lineage separates in the Cretaceous. However, as species adapt and develop to new and changing habitats, we suddenly witness rapid diversification during the K-Pg recovery period, according to Dr. Sanchez.

"This is an illustration of a "long fuse" model, which is characterised by a brief period of change followed by a burst of diversity."

What this implies for upcoming studies

This new evolutionary map does more than just provide answers to long-standing queries. It provides a framework for researchers to examine how the peculiar characteristics of squid and cuttlefish evolved.

These creatures are renowned for their sophisticated neurological systems, intricate behaviour, and characteristics like dynamic camouflage. It may be possible to explain how certain skills developed by knowing their genetic background.

According to Daniel Rokhsar, co-author of the study, "squids and cuttlefish are an endless source of inspiration for scientists because they have so many unique features compared to other animal groups."

"We can make meaningful comparisons to uncover the molecular changes associated with major cephalopod innovations, from the emergence of novel organs and dynamic camouflage to the neural complexity that supports their remarkable behaviour, thanks to these genomes and a clear picture of their evolutionary relationships."