Why nature keeps reinventing the crab: the strange evolutionary phenomenon of carcinisation
In short: If you see a crab with only eight visible legs, you are likely looking at an Anomuran. While true crabs belong to the Brachyura, Anomurans are a diverse group that repeatedly evolved crab-like bodies. This evolutionary process, known as carcinisation, has occurred independently across multiple distinct lineages within the infraorder.
The infraorder Anomura, often called Anomala, represents a fascinating case of evolutionary convergence. Unlike true crabs, which share a consistent body plan, Anomurans exhibit a wide variety of forms. The name itself reflects this, stemming from an old classification system that grouped decapods by their tail shapes: Macrura for long-tailed, Brachyura for short-tailed, and Anomura for those with differently-shaped tails. Despite their morphological diversity, they are united by their decapod heritage and a specific anatomical quirk: their tenth pair of legs is typically reduced and tucked away inside the gill chamber to assist with cleaning.
The most striking feature of this group is carcinisation, the independent development of a crab-like body with a concealed tail. This transformation has occurred multiple times, creating lineages that appear to be crabs but are not closely related to the Brachyura. Specifically, king crabs (Lithodidae), porcelain crabs (Porcellanidae), and the hairy stone crab (Lomisidae) each represent separate instances where evolution converged on the armoured crab form. This process highlights how nature can arrive at the same functional solution through entirely different genetic and developmental pathways.
Molecular and morphological data confirm that Anomura and Brachyura are sister taxa, forming the clade Meiura. This evolutionary split likely occurred during the Late Triassic period. The earliest evidence of this divergence is found in the fossil record, specifically the Platykotta akaina, which dates back to the Norian–Rhaetian stages of the Ghalilah Formation in the United Arab Emirates. Today, the infraorder is categorized into seven extant superfamilies, showcasing the enduring success of their varied, and often deceptively crab-like, body plans.
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- Summary by Intellect · source: Anomura
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