These marine invertebrates are our closest invertebrate relatives and build houses from cellulose.

Nature · Published 29 September 2026 Swipe more like this →

In short: Tunicates are strange, sac-like marine animals that start life as tadpole-like larvae with notochords, linking them to vertebrates. While many spend their adult lives anchored to rocks, they possess a unique, cellulose-based exoskeleton and complex circulatory systems that periodically reverse direction, challenging our understanding of simple invertebrate life.

Tunicates, or Tunicata, are marine invertebrates that share a common ancestor with vertebrates, forming a clade known as Olfactores. Despite their simple, often sessile adult forms, they are classified as chordates because their mobile larvae possess key features like a notochord and a dorsal nerve cord. This relationship is so profound that they are considered our closest invertebrate relatives, more closely related to us than to other groups like lancelets or echinoderms.

Their biology is marked by remarkable oddities. Most tunicates are filter feeders that process water through a mucus net secreted by an endostyle. Uniquely among animals, they produce cellulose for their protective outer tunic, a trait once thought exclusive to plants. Their circulatory systems are equally unusual; their hearts periodically stop and reverse the direction of blood flow. Some species even concentrate rare metals like vanadium in their blood cells to levels ten million times higher than the surrounding seawater, potentially as a chemical defense.

The group is diverse, with roughly 3,051 described species. While many are solitary, others form complex colonies of integrated units called zooids. Their life cycles can be incredibly intricate, involving asexual budding and sexual reproduction, with some species exhibiting complex alternations of generations. Fossils are rare due to their soft bodies, but evidence like the Middle Cambrian Megasiphon thylakos confirms their basic design has existed for 500 million years. Whether they are the sessile sea squirts that 'eat their own brains' during metamorphosis or the free-swimming salps, tunicates remain a vital, if enigmatic, branch of the chordate tree.

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