Short Communication Neural Correlates of Colour Change in Cuttlefish

This paper presents the first direct demonstration in a living cephalopod of neural activity associated with chromatic behaviour. It has long been known that colour change in these molluscs is unique in that it is brought about by chromatophores controlled neurally rather than hormonally (e.g. Hofmann, 1907). The chromatophore muscles are innervated directly by nerves whose cell bodies lie in the brain itself (Sereni & Young, 1932; Boycott, 1953); the muscles respond tetanically when these motoneurones are stimulated above 10-15 Hz (Florey, 1966); electrical stimulation in the chromatophore lobes of the brain causes darkening of the skin (Boycott, 1961); stimulation in the optic lobes can elicit patterning (the expansion of some sets of chromatophores and the simultaneous retraction of others) (Boycott, 1961; Chichery & Chanelet, 1976). Darkening, paling and patterning can also be elicited by introducing specific neurotransmitters into the blood supplying the brain (Andrews, Messenger & Tansey, 1983). But until now no-one has successfully recorded from the nerves supplying a specific set of chromatophores in the living animal and shown changes in activity as the display is switched on and off. The common cuttlefish, Sepia officinalis L., has an especially large repertoire of discrete body patterns (Holmes, 1940; R. T. Hanlon & J. B. Messenger, in preparation). When threatened, the adult displays a bold pattern that in its fullest expression comprises dark fins, expanded pupils, dark rings round the eyes and a pair of large black spots on the otherwise pale mantle (Fig. 1). We term this response the deimatic pattern and the spots, which really characterize it, the deimatic spots (DS). [We prefer deimatic to dymantic (both from the same root meaning 'I terrify') because this term seems to have gained general acceptance for arthropods and vertebrates (Maldonado, 1970; Edmunds, 1974; McFarland, 1981).] Cuttlefish in captivity readily display the DS (bilaterally or unilaterally) to an approaching large object and, because the nerves to the DS lie very superficially under the skin (Kier, Messenger & Miyan, 1985), we have begun to use this system to examine the nature of chromatophore nerve activity before and during a display. Adult Sepia of between 130 and 200 mm were anaesthetized in isotonic MgC^ in sea water (Messenger, Nixon & Ryan, 1985). The skin over the left fin nerve foramen