Diverse musculature layers in three species of octopus support precise motor control yet lack smooth muscle.

Octopus intrinsic arm musculature is often referred to as entirely
obliquely striated muscle. However, only three muscle layers have been
systematically shown as striated muscle. Because molluscan muscle control
can vary greatly (i.e. smooth vs striated types), a systematic examination
of each arm muscle layer is necessary to understand its neuromuscular
control. Here, we use 2-photon microscopy to determine if striations are
present in the eight layers and trabeculae of intrinsic arm muscle in
Octopus bimaculoides (California two-spot octopus) and Abdopus aculeatus
(“prickly” octopus). We also use scanning electron microscopy (SEM) to
examine the arm of Octopus bocki (Bock’s pygmy octopus). We confirm for
the first time that each intrinsic arm muscle layer from multiple octopus
species is obliquely striated. Furthermore, we find that the two layers of
longitudinal muscle-- divided by the median oblique layer--exhibit
different morphology. This occurs in all three species examined, with
significantly lower muscle fiber density in the internal longitudinal
layer of O. bimaculoides and A. aculeatus, and smaller internal
mitochondria cores (with larger muscle area) compared to the external
longitudinal layer in O. bocki. This suggests additional functional muscle
layers that would give octopuses even greater precision in motor control
of their arms. The data provided as part of the manuscript will be
arranged as a series of images that support each of the subpanels of the
figures.