Camp. Biochem. Phssiol. Vol.94C.No. 2, pp. 351-355, 1989 Printed in Great Britain 0306-4492/89 $3.00 + 0.00 0 1990 Pergamon Press plc CATECHOLAMINE-INDUCED COLOUR CHANGES IN THE CORNEAL IRIDOPHORES OF THE SAND GOBY, zyxwvutsrqponmlkji PCMATOSCHISTUS MINUTUS JULIASHAND*, JULIAN C. PARTRIDGE and JOHN N. LYTHGOE Department of Zoology, University of Bristol, Woodland Road, Bristol BS8 IUG, U.K. zyxwvutsrqponmlk (Received 19 December 1988) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Abstract-l. The iridescent cornea of the sand goby, Pomatoschistus minutus, responds to light by an increase in the amplitude and alteration of the maximum wavelength of spectral reflexions from the iridophores. 2. Two distinct physiological mechanisms are responsible for the two types of changes, one involving redistribution of material within the iridophore, the other involving a flow of material into the iridophore as a whole. 3. The types of colour change induced by catecholamines were investigated. 4. Alpha-receptor agonists increased the amplitude of reflexions whereas beta-receptor agonists decreased the amplitude of the reflexions. 5. The beta-antagonist propranolol interrupted the normal light response. 6. It is concluded that, in vitro, both light and catecholamines bring about a redistribution of material within the cornea1 iridophore. INTRODUCTION Many diurnal, shallow water, marine fishes possess corneas that are iridescent, the function of which is uncertain but may be to reduce intra-ocular flare caused by bright downwelling light (Lythgoe, 1971) or to camouflage the otherwise black pupil (Lythgoe, 1975). The iridescence is produced by the constructive interference of light reflected from a regular stack of transparent plates of alternating high and low refrac- tive index. The wavelength and intensity of the reflected light depends upon the thickness of the plates, their refractive indices and the angle of inci- dence of the light (Huxley, 1968; Land, 1972). The structures responsible for iridescence in fish corneas have been investigated in many species and seven distinct anatomical types have now been described (Lythgoe, 1976; Shand, 1988). Of these, four are known to be able to change colour in response to light @hand, 1988). The colour change often involves a shift in wavelength of the reflected light towards short wavelengths in darkness, and towards longer wavelengths in the light. However, in the sand goby, Pomatoschistus minutus, the light-response also in- volves an increase in the amplitude of the reflected light (Lythgoe and Shand, 1989). Using Huxley’s (1968) mathematical model for the iridescent reflec- tions from biological multilayers, Lythgoe and Shand (1989) have shown how two distinct physiological mechanisms might account for the wavelength and amplitude changes. The sand goby cornea1 iridophore is constructed from layers of thin, whole cells, separated by an *Address for correspondence: Department of Marine Biology, James Cook University, Townsville, Q. 48 11, Australia. extracellular matrix (Fig. 1.) and is situated between Descemet’s membrane and the stroma (Lythgoe, 1975). To produce iridescent reflexions the cytoplasm and extracellular matrix must differ in refractive index and a transfer of material between the high and low refractive index layers will alter the amplitude of reflexion, without altering the wavelengths. However, a shift in wavelength of the iridescent colour can be explained by an inflow of material into the iridophore as a whole (Lythgoe and Shand, 1989). Physiologically active iridophores which respond to light have been found in the skin of the killiefish, Fundulus heteroclitus (Foster, 1933, 1937), the cardi- nal tetra, Cheirodon innesi (Rohrlich, 1974) and the neon tetra, Paracheiroadon innesi (Lythgoe and Shand, 1982). In the neon tetra the colour change occurs in isolated strips of skin and an opsin-based pigment is thought to be located within the iri- dophores themselves (Lythgoe et al., 1984). Light, however, is not the only factor influencing the colour changes of iridophores. Foster (1937) showed that adrenaline (epinephrine) can cause a shift to longer wavelengths in the reflexions from the dermal iridocytes of the killiefish, and Lythgoe and Shand (1982) noted a similar response in the iri- descent lateral strip of the neon tetra. The dermal iridophores of the damselfish (Chry siptera cy anea) do not appear to respond to light but under certain behavioural conditions, in- cluding stress, there is a rapid change of hue (Oshima et al., 1985). The normal cobalt-blue colouration becomes a dark violet during excitement and greenish after handling. After detailed investigation of the mechanism regulating colour changes Ksukawa et al. (1986) attributed the rapid changes in hue to the sympathetic adrenergic system. A stress-induced red- dening of the goby cornea has also been observed cwc, 94,2--A 351