A survivor from the Cambrian
The animal known as Lingula anatina is a brachiopod, a phylum of marine animals that may superficially resemble clams but are biologically distinct. Its lineage dates back to the early Cambrian Period, over 500 million years ago. Specimens found in the waters around Japan are almost identical in shape to their ancient Cambrian ancestors. This lack of morphological change led Charles Darwin to called them "living fossils." While its external appearance has remained static, genetic studies show its genome is actively evolving.
Lingula lives in vertical burrows dug into the muddy and sandy bottoms of shallow coastal seas and intertidal flats. Its body consists of two shells, or valves, that are typically green or brown and grow to about 3-3.5 cm long. From the posterior end of the shells emerges a long, muscular, fleshy stalk called a pedicle. The animal uses this pedicle to anchor itself deep in the sediment and to retract its body for protection. Unlike most other shelled sea creatures that have calcium carbonate shells, Lingula's valves are composed of a mix of calcium phosphate, protein, and chitin. This unique composition is thought to have helped it survive periods of high ocean acidity, such as during the end-Permian mass extinction.
The biology of endurance
Lingula's primary survival tool is its burrowing lifestyle. By retreating deep into the sediment, it could shelter from the catastrophic surface events that characterized Earth's five major mass extinctions. Its physiology is well-suited for this existence. It can tolerate periods of low oxygen (anoxia), a condition likely common during extinction events when marine ecosystems collapsed. Following the Permian-Triassic extinction, the most severe in Earth's history—Lingula survived but thrived, expanding its range globally into the vacated ecological space.
The internal anatomy of Lingula is also distinctive. It feeds using a specialized structure called a lophophore, a crown of ciliated tentacles that filters food particles from the water current it generates. It possesses an open circulatory system and its blood contains a copper-based respiratory pigment called hemerythrin. When oxygenated, hemerythrin gives the blood a violet-pink color; it is colorless when deoxygenated. This is different from the iron-based hemoglobin in vertebrates. The animal has a simple nervous system, with two main ganglia and nerve cords extending to the mantle and lophophore. Some species possess statocysts, which are balance sensory receptors that help the animal orient itself within its burrow.