Myosin was purified to a homogeneity from sea sponge, Halichondria okadai. The myosin consisted of 220 kDa heavy chain, 18 kDa calcium binding light chain and 21 kDa phosphorylatable light chain. Rotary shadowed images showed the two headed myosin (myosin II) with a 160 nm tail. The myosin was less soluble in a KCl solution as compared to rabbit skeletal myosin.

The K -stimulated and Ca² -stimulated ATPase activities of sea sponge myosin were 0.46 and 0.07 μmol Pi min⁻¹ mg⁻¹, respectively. The Mg² -activated myosin ATPase activity showed no significant enhancement by the addition of rabbit skeletal muscle actin despite that the light chain was phosphorylated by myosin light chain kinase from chicken gizzard. Sea sponge myosin 18 kDa light chain bound to Ca² ion but was not phosphorylated like Physarum plasmodia myosin light chains.