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 versus striated types), a systematic examination of each arm muscle layer is necessary to understand its neuromuscular control. Here, we use two-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 to examine the arm of Octopus bocki (Bock's pygmy octopus). We confirm for the first time, to our knowledge, 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 fibre density in the internal longitudinal layer of O. bimaculoides and A. aculeatus, and smaller internal mitochondria cores (with larger muscle area) compared with 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.