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BIOL630 TERM PROJECT
MUSCULAR SYSTEM
Image courtesy of Tumblr.
Octopuses do not have an internal skeleton and a completely soft body. Many have studied how they are able to still maneuver with their large body size while lacking this skeletal system. Fortunately, the octopus’s muscular system is complex and able to allow for these adaptations. A major portion of the body mass of an octopus are the four pairs of arms. They operate via “muscular hydrostats”, the internal pressure within the arm structures that create support and fluid movements. In fact, researchers believe that the octopus’s arm has the most flexibility of any limb found in nature (Kennedy et al, 2020). The arrangement of the muscle tissues, as seen in figure 1, is unique in that there is control in all directions. This, combined with the lack of skeleton rigidity leads to it having great potential for a wide range of motion. These researchers pinpointed different movements and which muscles are responsible for those possible movements:
Elongation/Shortening: Transverse and longitudinal muscles, antagonistic activation
Stiffening: Transverse and longitudinal muscles, co-activation
Bending: Contraction of longitudinal muscle, resistance of transverse muscle
Torsion/Twisting: Contraction of oblique muscles
The most common motion that was observed in studied octopuses was bending at 65% of all movements recorded at 11,074 out of 16,563 movements (Kennedy et al, 2020).
Figure 1. Diagram showing a cross-section of an octopus arm. Legend: TM, transverse muscle. LM, longitudinal muscle. OM, oblique muscle. CM, circumferential muscle. SU, sucker. AN, axial nerve. (Courtesy of Kennedy et al, 2020)
On each of the 8 arms are about 280 suckers lining up and down. The suckers consist of two chambers and three different layers of muscles: radial muscles lining the outside of the upper and lower chamber, circular muscles along the lower chamber, and meridional muscles arranged within the radial muscle. To form a tight seal, the radial muscles in the upper chamber contract and the sucker flattens around the surface it is suctioning to. As the muscle contracts, the water inside of the chambers drops. Because the water pressure outside of the arm is higher now than the water inside of the sucker, the adhesion to the object is forced tighter (Hall, Smithsonian). A study conducted in 2002 that focused on the suckers found that this worked best on wettable surfaces, and then the octopus was exposed to a surface that was less wettable did not allow for adhesion to be maintained (Kier and Smith, 2002).
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