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Characterization of Arm Autotomy in the Octopus, Abdopus aculeatus (d'Orbigny, 1834)


Autotomy is the shedding of a body part as a means of secondary defense against a predator that has already made contact with the organism. This defense mechanism has been widely studied in a few model taxa, specifically lizards, a few groups of arthropods, and some echinoderms. All of these model organisms have a hard endo- or exo-skeleton surrounding the autotomized body part. There are several animals that are capable of autotomizing a limb but do not exhibit the same biological trends that these model organisms have in common. As a result, the mechanisms that underlie autotomy in the hard-bodied animals may not apply for soft bodied organisms. A behavioral ecology approach was used to study arm autotomy in the octopus, Abdopus aculeatus. Investigations concentrated on understanding the mechanistic underpinnings and adaptive value of autotomy in this soft-bodied animal.

A. aculeatus was observed in the field on Mactan Island, Philippines in the dry and wet seasons, and compared with populations previously studied in Indonesia. Frequency of encountered arm loss and habitat characteristics were recorded and compared between sites, seasons, sex, and arm position. The incidence of autotomy was highest in Indonesia; nearly half of the sampled octopuses were found missing or regenerating arms from the base of the body. Most of the octopuses were male. There was no significant difference in the incidence of autotomy between Indonesia and the Philippines or between the wet and dry seasons in the Philippines. Individuals lost between one and five arms, one arm being the most common. The eight arms from each individual were not equally lost. The anterior pairs of arms were more susceptible to loss due to their function as exploratory arms. The male third right arm used for reproduction was lost less frequently.

Attempts to fully describe the behaviors associated with autotomy besides limb detachment have not been made for any organism. Multiple arm autotomy events were observed in A. aculeatus to generate an inventory of behaviors that are characteristic of autotomy. Sequential analysis of these behaviors was conducted and found to follow a particular order during autotomy. Some of the classified behaviors include other secondary defense mechanisms, such as jetting and inking.

To understand the mechanism for arm loss at the tissue level, histological sections and force measurements were collected from autotomized arms to characterize the autotomy zone. Individuals were also decerebrated to establish the relative importance of the central nervous system in the occurrence of autotomy. Histology revealed a zone of weakness with no specialized adaptations for loss between proximal suckers four and eight. An average tensile force of 0.645 ± 0.145 MPa was required for autotomy to occur. Eight out of ten decerebrated individuals did not autotomize an arm post-decerebration suggesting that the central nervous system plays a role in arm autotomy.

Lastly, the costs and benefits of arm autotomy to A. aculeatus were assessed by studying the effects of arm loss in whole body and autotomized arm locomotion. Individuals with fewer arms were not found to exhibit any negative impact to their crawling or jetting behaviors. The autotomized arms post detachment performed complex behaviors and suctioned to various surfaces for prolonged periods of time suggesting potential adaptations in the arm to distract a predator while the octopus escapes.

Overall, the octopus provided a good model system for studying the mechanisms and adaptive value of autotomy in a soft-bodied animal. There multitude of arms were easy to manipulate and induce autotomy in the lab. The relatively high frequency of individuals missing arms in the wild suggest that autotomy plays an important defense role for A. aculeatus and needs further characterization to identify the underlying cause. Efforts to study autotomy in other non-model organisms will broaden and enhance our knowledge on what seems like an extreme, but common defense mechanism.

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