Cross references:     Globin   Heme      Muscle Innervation       
Motor Neuron Evolution    Motor Nerve Organization      
Medial Motor Column
Fast vs. Slow Twitch Muscles    

Shark Muscles

Myoglobin (Wiki)   
    "Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Myoglobin is only found in the bloodstream after muscle injury. It is an abnormal finding, and can be diagnostically relevant when found in blood. [2]

Myoglobin is the primary oxygen-carrying pigment of muscle tissues.[3] High concentrations of myoglobin in muscle cells allow organisms to hold their breaths longer. Diving mammals such as whales and seals have muscles with particularly high myoglobin abundance.[2

Evolutionary implications: myoglobin-like proteins found in ancient microorganisms - 2000 (Goog)