These worm-like deep-sea creatures are actually mollusks hiding in plain sight

Nature · Published 3 October 2026 Swipe more like this →

In short: Aplacophorans look nothing like the clams or snails we associate with mollusks. These cylindrical, shell-less marine animals inhabit the deep ocean floor, often disguised as simple worms. Despite their strange appearance, they possess key molluscan features and may hold the secret to the evolutionary origins of the entire phylum.

Aplacophora is a superclass of marine mollusks that thrive in deep-water environments across all global oceans. While they lack the characteristic shells of their relatives, they are defined by their worm-like bodies, which are typically small—often under 5 centimeters—though some species can reach up to 30 centimeters in length. Instead of a shell, their skin is embedded with calcified spicules, which are often composed of aragonite and can change in shape as the animal matures.

The group is divided into two distinct classes: the Solenogastres and the Caudofoveata. Their lifestyles differ significantly; caudofoveates generally burrow into the seafloor to feed on detritus or microorganisms, while solenogasters are typically epibenthic carnivores that prey on cnidarians. These animals were once misidentified as echinoderms, specifically sea cucumbers, before being officially recognized as mollusks in 1987. Their classification has been a subject of intense debate, with some theories suggesting they represent a primitive ancestral state, while others argue they evolved from more complex, shelled ancestors.

The evolutionary history of aplacophorans has been clarified by recent evidence. While their relationship to other mollusks was long considered unclear, molecular studies now support the group as a monophyletic clade. A pivotal discovery in 2012 of a fossil named Kulindroplax provided crucial evidence, revealing that these creatures evolved from ancestors that possessed valves. This finding links them to the clade Aculifera, positioning them as a sister group to the polyplacophorans and confirming that their current shell-less form is a secondary development rather than a primitive trait.

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