The CSIC documents for the first time a wild octopus with a functional bifurcated arm in its natural habitat

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Santiago de Compostela, 29 April 2025. The Spanish Research Council (CSIC) has documented the most complete case to date of an octopus with a fully functional bifurcated arm in its natural habitat. The finding supports previous research by showing that octopuses, even those with atypical arm morphologies, simplify arm use by linking it to behavioural categories, while also displaying specialised usage.

The discovery, reported in the journal Animals, results from a five-month monitoring of the specimen by researchers from the ECOBIOMAR group at the Institute of Marine Research (IIM-CSIC, Vigo) and the Balearic Oceanographic Centre (COB-IEO-CSIC) in the framework of the ECOSUMA project, funded by the Ministry of Science, Innovation and Universities.

It reveals the astonishing adaptability of wild octopuses after severe injury, with potential implications for neuroscience, marine conservation and bioengineering.

The study examined the individual in situ, which is noteworthy since current methods generally rely on ex situ or laboratory experiments to analyse the behaviour of the species.

"While it provides an almost complete overview of this exceptional individual, it is important to acknowledge that it was conducted on a single subject, with no possibility of replication in multiple individuals. However, the extreme rarity of live specimens with bifurcated appendages creates a situation where this is currently not possible," the researchers explain.

"Octopuses have flexible arms with numerous nerve cells, enabling them to explore and interact with their surroundings in unique ways. Occasionally, these animals develop unusual traits, such as additional or split arms, but little is known about how this affects their movement and behaviour. This study observed a wild octopus with a naturally bifurcated arm, using underwater video recordings to analyse how it used its arms over time," the authors add.

The study applied behavioural quantification methods to describe and examine the behavioural repertoire of this unique individual through underwater video footage. Special attention was paid to behaviours and actions involving arm use, with a particular focus on the two bifurcated arms. The main research questions were: Does the presence of a fully functional bifurcated arm alter the behaviour of Octopus vulgaris? To what extent is the bifurcated arm used in different behaviours? Does arm use change over time?

The results showed that the split arms were initially used more often for actions beneath the body, but became less specialised as the octopus grew. The availability of video recordings of a living cephalopod with a fully functional bifurcated arm in the wild allows for the description and analysis of this anomaly.

“This milestone not only reveals the unusual regeneration of a split arm into two, but also the differentiated and adaptive use of both regenerated arms—a feat never before documented in cephalopods. The specimen, a male Octopus vulgaris, showed signs of having survived a previous attack, having lost several limbs. During regeneration, one of its arms (R1) bifurcated, generating two separate appendages: R1a and R1b. Both grew over time and were used in specialised ways—one more frequently for feeding and the other for exploratory behaviours,” explain the study authors.

Arm bifurcation had been previously described from an anatomical point of view, but this is the first study to document how those regenerated and bifurcated arms are used in a wild setting.

“This documentation suggests two key findings. First, the bifurcated arms were initially used for tasks close to the body, but over time their involvement in more complex actions such as foraging or exploration increased, suggesting progressive and specific adaptation. Secondly, damaged arms were used less frequently in risky behaviours, which could indicate a form of pain memory or experiential learning,” they note.

Additionally, the observation of lateralised and highly individualised arm use for specific tasks highlights the surprising motor plasticity of octopuses.

“The fact that this octopus adapted its behaviour and reorganised the use of its arms functionally suggests complex neural mechanisms that could inspire new applications in robotics, neuroscience and regenerative medicine,” say the researchers.

These findings suggest that octopuses may adapt their arm use in response to injury and recovery, possibly displaying changes in how their nervous system controls movement.

According to the researchers, this opens new questions about the species, such as how their nervous system integrates new limbs, and new avenues of research into neurogenesis in regenerated appendages or how the central and peripheral nervous systems reorganise in response to such anomalies. Understanding these adaptations may shed light on how animals respond to physical challenges and could even inspire new designs in robotics and prosthetics by mimicking the octopus's arm regeneration capabilities.

“One research avenue could explore what happens to the neurons in a lost octopus arm—are they redistributed throughout the body or simply lost? If neurons are lost, do they regenerate slowly? Does the presence of an additional arm result in twice the usual number of regenerated neurons? Alternatively, if neurons are redistributed throughout the body and then allocated to arms as they grow, when an arm bifurcates, is it possible that neurons are distributed to only one branch and not the other? If so, this redistribution could explain why bifurcation normally results in a non-functional limb. Other questions that arise include: Could an octopus with newly bifurcated arms be operating with reduced innervation and effectively relearning how to use these limbs? Or are these limbs fully innervated but simply inhibited by their length?” the researchers conclude.