Squids are mollusks, full members of the phylum Mollusca alongside snails, clams, and slugs. They belong to the class Cephalopoda, a group that also includes octopuses, cuttlefish, and nautiluses. The classification surprises many people because squids look nothing like the shelled, slow-moving creatures most of us picture when we hear “mollusk,” but the evolutionary evidence connecting them runs deep, from their embryonic development to the remnants of a shell hidden inside their bodies.
Why Squids Count as Mollusks
The word “mollusk” comes from the Latin mollis, meaning soft, and the defining features of the phylum are a soft body, a muscular foot, and a structure called the mantle that in many species secretes a shell. Squids check every one of these boxes, though each feature has been modified dramatically by hundreds of millions of years of evolution toward an active, predatory lifestyle.
The muscular foot is a good example. In a snail, the foot is the flat, slimy sole the animal creeps on. In a clam, it is the fleshy wedge that digs into sand. In a squid, that same embryonic tissue has been reshaped into the ring of arms and tentacles surrounding the mouth. Research on cephalopod limb development shows that arms and tentacles have no direct structural equivalent in other mollusks but likely formed from the ventral embryonic foot, a core feature of the molluscan body plan, making them evolutionary novelties derived from an ancient conserved structure.1eLife. Evolution of limb development in cephalopod mollusks Embryological work on the nautilus, the most primitive living cephalopod, confirms this: the distinct cephalic compartment, foot, brain cords, mantle, and shell in a developing nautilus embryo resemble the body plans of monoplacophorans and basal gastropods, and the tentacles develop not from the head but from the foot.2PubMed. Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development
Squids also retain a radula, the ribbon-like feeding structure lined with tiny teeth that is one of the hallmark features of mollusks. In snails it works like a cheese grater, scraping algae off rocks. In squids it sits inside the beak apparatus and helps pull food into the esophagus. The radular apparatus in cephalopods has been studied alongside the chitinous beaks to understand how these structures function during feeding within the broader mollusk phylum.3Royal Society Publishing. The radular apparatus of cephalopods And the mantle, the muscular tissue cloak that in a clam lines the inside of the shell, has become in squids a powerful jet-propulsion system. By contracting the mantle and forcing water through a narrow funnel, a squid can accelerate rapidly or hover in place, a system that researchers have likened in some respects to jellyfish propulsion in its low-pressure mode and true high-pressure jetting when the animal seals the mantle tightly around its head.4Journal of Experimental Biology. Aperture effects in squid jet propulsion
Where Did the Shell Go
If squids are mollusks, the obvious question is: where is the shell? The answer is that it is still there, just buried inside the body. The thin, transparent, feather-shaped structure running along a squid’s back is called the pen or gladius, and it is the evolutionary remnant of an external shell. The pen serves as a site of attachment for major muscle groups and as a protective barrier for the internal organs. Its durability and flexibility come from a unique combination of chitin and protein.5PubMed Central. Construction and Composition of the Squid Pen from Doryteuthis pealeii
The story of shell loss connects squids to other cephalopods along a spectrum. Nautiluses still carry a full, coiled external shell with gas-filled chambers that control buoyancy. Cuttlefish have the cuttlebone, a rigid internal structure that also manages buoyancy. Squids reduced the shell further to the flexible pen. Octopuses have gone the furthest, losing the structure almost entirely in most species. This progression illustrates a broader evolutionary trend in the cephalopod lineage: shedding shell weight to gain speed and agility. A study of the shell matrix proteins in nautilus shells found that while nautilids retain the biomineralization toolkit shared across shelled mollusks, most extant cephalopods have lost their external mineralized shells entirely.6PubMed Central. Hydrophilic Shell Matrix Proteins of Nautilus pompilius and the Identification of a Core Set of Conchiferan Domains
The Evolutionary Path from Shelled Ancestor to Squid
Cephalopods first appeared during the Cambrian period, roughly 530 million years ago, evolving from a small, monoplacophoran-like mollusk whose conical external shell was modified into a chambered buoyancy apparatus.7PubMed. Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules That chambered shell was the key innovation: by pumping gas in and out of sealed chambers, early cephalopods could control their depth, freeing them from the seafloor and opening an entirely new lifestyle as active swimmers and predators. The nautilus still uses exactly this system today.
From that common shelled ancestor, the cephalopod lineage split into two main groups. One branch kept the external shell and eventually produced nautiluses as the sole living survivors. The other branch, the coleoids, progressively internalized or lost the shell and developed the fast, flexible body plans we see in modern squids, cuttlefish, and octopuses. Molecular and fossil evidence together indicates that the major coleoid lineages, including the orders containing octopuses and the various squid groups, diverged during the Paleozoic era.8PubMed. Divergence time estimates for major cephalopod groups: evidence from multiple genes That is a very long time for independent evolution to sculpt these animals into forms that look nothing like their clam and snail relatives, even though the foundational blueprint remains the same.
A Brain Unlike Any Other Invertebrate
One of the most striking things about squids, and cephalopods generally, is their nervous system. Among invertebrates, nothing else comes close. The coleoid cephalopod nervous system rivals those of vertebrates in size but has an entirely different organization. The brain sits between the eyes, with two large optic lobes flanking a central brain mass that is divided into upper and lower regions around the esophagus. The brain regions of cephalopods appear to correspond to the simpler paired ganglia found in other mollusks like snails, clams, and tusk shells, but they have been enormously expanded and concentrated.9Current Biology. Evolution of cephalopod nervous systems
This concentrated nervous system gives squids remarkable abilities. They can change color and skin texture in milliseconds using chromatophores controlled directly by the brain. They can learn, remember, and solve problems. And their arms contain substantial local neural circuitry, allowing semi-independent processing of touch and taste information at the limb level. All of this evolved from the same simple paired-nerve-cord arrangement shared by slugs and snails.
Camera Eyes That Evolved Independently
If you look at a squid’s eye, it looks eerily like your own: a single lens focusing light onto a retina. This similarity is one of the most famous examples of convergent evolution in biology. Squids and humans arrived at nearly the same optical design through completely independent evolutionary paths. Coleoid cephalopods evolved a camera eye whose structure is very similar to that found in vertebrates, yet other mollusks possess much simpler mirror, pinhole, or compound eyes that differ in their degree of complexity.10PubMed Central. Genetic mechanisms involved in the evolution of the cephalopod camera eye revealed by transcriptomic and developmental studies
What makes the convergence even more interesting is the degree to which the same genes are involved. A comparative study of gene expression in octopus and human eyes found that roughly 69% of the genes active in the octopus eye were also expressed in the human eye, and the vast majority of those genes were already present in the common ancestor of all bilaterally symmetric animals.11PubMed Central. Comparative analysis of gene expression for convergent evolution of camera eye between octopus and human In other words, evolution did not invent two completely separate toolkits for building camera eyes. It reached into the same ancient genetic toolbox and assembled similar solutions in two lineages separated by over half a billion years. The squid’s eye is a mollusk eye built from mollusk development, but one that converged on a design shared with animals on a very distant branch of the tree of life.
A Closed Circulatory System in an Invertebrate
Most mollusks have open circulatory systems, where blood is pumped into body cavities and sloshes loosely around the organs before draining back to the heart. This works fine for a sedentary clam but would never support the oxygen demands of a squid chasing prey at speed. Squids and other coleoid cephalopods evolved a closed circulatory system, with blood contained within vessels, which delivers oxygen far more efficiently. They even have three hearts: one systemic heart that pumps blood around the body and two branchial hearts that push blood through the gills.
Research on the molecular basis of this system found that squids express a growth factor receptor in their developing blood vessels that plays a similar role to the one used by vertebrates to build their own closed circulatory systems. The highly restricted expression pattern of this receptor in vascular cells of both cephalopods and vertebrates suggests a shared molecular signature of closed circulatory systems that was independently elaborated during evolution.12PubMed Central. Squid vascular endothelial growth factor receptor: a shared molecular signature in the convergent evolution of closed circulatory systems Like the camera eye, the closed circulatory system is a case where squids independently arrived at an engineering solution also found in vertebrates, drawing on some of the same molecular building blocks despite evolving separately for hundreds of millions of years.
RNA Editing and the Cephalopod Genome
One of the more extraordinary discoveries about squids and their coleoid relatives in recent years involves how they use their genetic information. Most animals rely on DNA mutations as the main engine of protein diversity: a change in the DNA sequence leads to a change in the protein. Coleoid cephalopods have supplemented that strategy with an unusually extensive use of RNA editing, in which the cell chemically modifies RNA messages after they are copied from DNA, producing altered proteins without changing the underlying genetic code.
The scale of this editing in squids is striking. Squids have over 57,000 RNA editing sites, and the process affects most transcripts in the nervous system.13PLOS Genetics. The evolution and adaptation of A-to-I RNA editing The editing is concentrated in genes related to nerve cell excitability and structure. Researchers found that the DNA sequences surrounding these editing sites are unusually conserved across cephalopod species, suggesting the editing is actively maintained by natural selection. But this conservation comes at a cost: because the DNA near editing sites cannot change freely, the overall rate of genetic mutation in protein-coding regions is reduced. The result is a trade-off between genome evolution and transcriptome plasticity, one that highlights RNA recoding as a strategy for diversifying proteins, particularly those associated with neural function.14PubMed Central. Trade-off between transcriptome plasticity and genome evolution in cephalopods
This strategy appears to be largely unique to the coleoid cephalopods among mollusks. Nautiluses, which split from the coleoid lineage before these innovations arose, do not show the same degree of RNA editing. Neither do other mollusk groups. It may be one of the molecular innovations that helped underwrite the complex behaviors and sophisticated nervous systems that set squids and their relatives apart from their shelled cousins.
Squids Among the Cephalopods
Within the class Cephalopoda, “squid” is not a single, tidy taxonomic group. The animals we casually call squids are spread across two major orders. True squids belong to the order Myopsida (like the common market squid) and Oegopsida (like the giant squid and Humboldt squid), both within the larger superorder Decapodiformes, which also includes cuttlefish and bobtail squids. All decapodiforms share the trait of having ten limbs: eight shorter arms and two longer tentacles used for prey capture. This distinguishes them from octopuses, which have eight arms and belong to the separate superorder Octopodiformes.
Molecular dating studies support a Paleozoic origin for both the octopus and squid lineages, meaning these two branches of cephalopod evolution diverged hundreds of millions of years ago.8PubMed. Divergence time estimates for major cephalopod groups: evidence from multiple genes The reproductive strategies also differ between the groups: phylogenetic analyses suggest that the ancestors of the squid and cuttlefish lineage used asynchronous ovulation, producing eggs in batches over time, while the ancestors of octopuses used synchronous ovulation, producing all their eggs in a single bout.15Molecular Phylogenetics and Evolution. A phylogenetic approach to understand the evolution of reproduction in coleoid cephalopods These differences in life history have profound consequences. Many octopus species die after a single reproductive event, while various squid species can reproduce multiple times, contributing to the explosive population dynamics for which squids are known.
Ecological Heavyweights Despite Short Lives
Squids occupy a peculiar ecological niche for a mollusk. Most mollusks are grazers, filter feeders, or slow-moving scavengers. Squids are active predators that grow at extraordinary rates. Modeling work comparing squid and fish population dynamics found that squids have an average somatic growth rate roughly five times higher than that of bony fish, and their faster growth more than compensates for a lower maximum reproductive rate, giving squid populations the ability to expand rapidly when conditions allow.16bioRxiv. The role of squid for food web structure and community-level metabolism This growth comes at a price: squids require higher food density to sustain their populations compared to fish.
Most squids live only one to two years. In that brief window they hatch, grow to full size, reproduce, and die, cycling biomass through ocean food webs at a pace that dwarfs longer-lived predators. They are major prey for marine mammals, seabirds, and large fish, and they are themselves voracious consumers of smaller fish and invertebrates. Some fisheries biologists have noted that as fish stocks have been depleted in parts of the ocean, squid populations have expanded to fill the gap, a dynamic consistent with their fast growth and high food requirements. Their ability to boom and bust in response to environmental conditions makes them one of the more volatile but ecologically important groups of marine animals.
Why the Classification Sometimes Confuses People
The confusion around whether squids are “really” mollusks often comes down to a mismatch between everyday categories and evolutionary ones. In common experience, mollusks are shellfish: mussels, oysters, snails. A streamlined, jet-propelled predator with camera eyes, a brain that rivals a fish’s, and the ability to change color in a fraction of a second simply does not fit that mental image. But biological classification is not about what things look like on the surface. It is about shared ancestry, and the evidence for cephalopods being mollusks is overwhelming, from the conserved embryonic body plan to the radula to the homologous mantle tissue to the molecular phylogenetics confirming the group’s placement within Mollusca.
If anything, the squid is a testament to how much evolutionary change a single body plan can accommodate. The same basic blueprint that produces a sedentary oyster also produced an animal with a closed circulatory system, a complex brain, and tens of thousands of RNA editing sites fine-tuning its nervous system on the fly. The distance between a garden snail and a Humboldt squid is not a gap in classification. It is a measure of what roughly 530 million years of evolution can do with a muscular foot, a mantle, and a radula.