Dye coupling in the muscles controlling squid chromatophore expansion

Dye coupling between the cone-shaped radial muscle fibres, which control the expansion and closing of a squid chromatophore organ, was investigated in the squid Loligo vulgaris. Particular attention was paid to the role of the myomuscular junctions located between the muscle fibres. Lucifer Yellow was injected ionophoretically into single muscle fibres under normal artificial sea water (ASW) and under various concentrations of calcium in ASW. Under ASW, 44% of muscle fibres examined were dye-coupled, 82% were coupled under calcium-free sea water and 67% were coupled under sea water containing high concentrations of calcium. Dye transfer was blocked by octanol. Muscle fibres were never seen to link adjacent chromatophore organs. Results are discussed in terms of the role of the myomuscular junctions in the regulation of chromatophore expansion in the living animal.