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Reflective properties of iridophores and fluorescent ‘eyespots’ in the loliginid squid Alloteuthis subulata and Loligo vulgaris

L. M. Mäthger^1,2,* and E. J. Denton¹

¹ The Marine Biological Association of the UK, Citadel Hill, Plymouth PL1 2PB, UK and
² Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

View larger version (18K): [in a new window]   Fig. 1. (A) Spectral irradiance (E) of downwelling daylight for the Gulf of California at 19m depth (solid line) (after Tyler and Smith, 1970) and for the English Channel at 20m depth (filled circles) (after Atkins, 1945). Spectral absorption (S) of the squid rhodopsin (broken line) for a maximum optical density of 0.5 (calculated after Knowles and Dartnall, 1977) with the wavelength for maximum absorption (max) at 499nm (Morris et al., 1993). Maximum values have been made equal to 100. (B) Angular distribution of daylight in the sea: grey solid line, after an equation and constants suggested to one of us by Dr J. E. Tyler for oceanic waters off the Azores (see Denton et al., 1972); short dashed line, for Lake Pend Oreille (Tyler, 1960); long dashed line, for turbid water at a depth of 40m off Japan (Sasaki et al., 1962, cited in Jerlov, 1976); black solid line, for Mediterranean waters (Lundgren, 1976, cited in Jerlov, 1976). Radiances for 0° (vertically downwards) have been made equal to 100.

View larger version (16K): [in a new window]   Fig. 2. (A) Apparatus used to measure spectral reflectivity and polarisation. F, interference filters; L1, L2, L3, light sources; M, dissecting microscope; P, preparation; PMT, photomultiplier tube; Pol., polarising filters; T, tilting table; W, Perspex wedge. (B) Apparatus used to measure reflectivity (after Denton and Nicol, 1962). Symbols as in A. O, opal glass; M1, M2, dissecting microscopes.

View larger version (93K): [in a new window]   Fig. 3. Calculated spectral reflectivities for an ideal quarter-wavelength (/4) stack with 10 platelets of chitin and spaces of cytoplasm at angles of incidence from 0 to 80° (max=660nm at normal incidence) (calculated by D. M. Rowe). The degree of shading at each point in the two diagrams represents reflectivity: the lighter the shading, the higher the reflectivity (ranges given below figure). (A) For light polarised in the plane perpendicular to the plane of incidence. (B) For light polarised in the plane parallel to the plane of incidence. Changing the angle of the incident light from 0 to 25° only shifts the wavelengths best reflected from 660nm to approximately 610nm, while a change in the incident light from 45 to 55° shifts the wavebands best reflected from 500 to 440nm.

View larger version (15K): [in a new window]   Fig. 4. Transparency in the squid Alloteuthis subulata; (x) measured through the entire mantle in a position anterior to the inksac; () measured through the entire mantle in a position posterior to the inksac; () measured through the mantle wall with intact skin; and () measured through the mantle wall from which the skin had been removed.

View larger version (35K): [in a new window]   Fig. 5. (A) Reflecting stripes of the squid Alloteuthis subulata and Loligo vulgaris. The names are given according to the colour reflected when the stripes are viewed in white light at near-normal incidence (see text for details). (B) Cross section perpendicular to the long axis of a squid’s mantle, in the region of the fins (dotted oval in A). d and v represent dorsal and ventral regions, respectively. For each stripe, solid lines indicate the iridophores. Broken lines, normal incidence to the iridophores; dotted lines, horizontal.

View larger version (38K): [in a new window]   Fig. 6. Diagram showing the orientation of iridophores of the ‘red’ stripe on the left side of the mantle. The group is made up of two pairs, each tilted by 10–15° along the antero-posterior plane of the mantle, one pair tilted towards the head, the other towards the ‘tail’.

View larger version (108K): [in a new window]   Fig. 7. Photographs of the reflecting stripes of squid. The ‘red’ stripe at the anterior end of the mantle in white light at (A) 15° incidence, (B) 45° incidence and (C) 55° incidence. Scale bars, 300µm. The mantle ‘green’ stripe in white light at (D) 15° incidence and (E) at 45° incidence. Scale bars, 5mm. (F) The ventral iridophores in white light at 15° incidence. Scale bar, 200µm. (G) The mantle ‘blue’ stripe in white light at 15° incidence. Scale bar, 200µm. (H) Photograph in white light showing the position of the fluorescent layer above the eyes of a squid (Alloteuthis subulata). The inset photograph shows the fluorescent layer excited with blue light. Scale bar, 1cm.

View larger version (22K): [in a new window]   Fig. 8. Spectral reflectivities for the perpendicular and parallel planes of polarisation of the mantle ‘red’ stripe at a position halfway along the length of the mantle at angles of incidence of (A) 15°; (B) 45° and (C) 55° for Alloteuthis subulata (thick lines) and Loligo vulgaris (thin lines). The diagrams at the side of each figure show a squid mantle in dorso-ventral section. They indicate the direction in which the light is reflected, i.e. the angle of incidence at which the measurements were made.

View larger version (20K): [in a new window]   Fig. 9. Same as Fig.8 but for the mantle ‘green’ stripe of Loligo vulgaris at (A) 15°, (B) 45° and (C) 55° incidence.

View larger version (18K): [in a new window]   Fig. 10. Squid mantle in dorso-ventral section (d and v represent dorsal and ventral regions) showing the directions in which light is reflected from the iridophores of the ventral side. Dashed double-dotted line, normal incidence to iridophore; solid line, incident and reflected light; dashed dotted line, 15° incidence; dotted line, 45° incidence; dashed line, 55° incidence.

View larger version (14K): [in a new window]   Fig. 11. Same as Fig.8 and Fig.9 but for the mantle ‘blue’ stripe of Loligo vulgaris at (thick line) 15° and (thin line) 30° incidence.

View larger version (12K): [in a new window]   Fig. 12. The excitation and emission spectra of an ethanol pigment extract of the fluorescent layers of Loligo vulgaris. Intensity has been normalized to the maximum value for each spectrum.

View larger version (24K): [in a new window]   Fig. 13. (A) Reflectivity (R) of the mantle ‘red’ stripe in the perpendicular plane of polarisation (see Fig.8). (B) Ordinate is ERS, where E is the spectral irradiance of submarine daylight, R is the reflectivity shown in A and S is the spectral absorption of the squid’s rhodopsin (see Fig.1A for E and S).

View larger version (12K): [in a new window]   Fig. 14. Diagram to illustrate the meaning of the symbols used in Ratio 1, showing a squid mantle in dorso-ventral section (d and v represent dorsal and ventral regions) and an observer looking at a reflective area on the surface of the squid. Dotted line, vertical line; dashed line, light (L) incident to the iridophore at an angle 1 with the vertical; dashed double-dotted line, normal incidence to the iridophore; solid line, reflected (R) and transmitted (T) light at an angle of incidence to the normal of the iridophore; the group of four arrows indicates the background light field at an angle 2 with the vertical.

View larger version (37K): [in a new window]   Fig. 15. (A) Iridophore of the mantle ‘red’ stripe (see Fig.5B) showing three observers (X, Y and Z), each viewing reflections of light from the iridophore. The line styles of this figure and the symbols given in the text are the same as shown in Fig.14. The thickness of each line roughly indicates intensity, i.e. the thicker the line, the greater the intensity. While the reflections seen by observer X are unpolarised, those seen by observers Y and Z are maximally polarised. m, mantle. (B) The reflective patterns seen looking along the length of the mantle ‘red’ stripe. The shading of the ‘red’ stripe indicates intensity of reflection, i.e. the lighter the shading, the higher the intensity. Dotted lines indicate the position of the eye of the observer opposite the midpoint of the ‘red’ stripe. Observer X: within an angle of view of less than ±30°, the intensities of the reflections will equal the background light. For a larger angle of view, the reflections (shown in white) become brighter than the background and polarised. Observer Y: the reflections seen within an angle of view of approximately ±30° will be polarised and approximately four times brighter than the background. Observer Z: the brightness of the reflections will equal the background light over the entire visual field.

View larger version (19K): [in a new window]   Fig. 16. Cross section through the mantle of a squid (from Fig.5B) showing how squid may maximise camouflage to observers from below. A large fraction of the incident light available at point O will pass through the spaces between the iridophores, through the mantle muscle (m) and through the internal organs (IO) to point P. Only a small fraction of the light is reflected by the iridophores (e.g. the ‘red’ stripe, at point R), allowing a large fraction to be transmitted into the mantle cavity. The ventral iridophores (V1) reflect weakly at angles around normal incidence, allowing a large part of the light to be transmitted. At oblique angles of incidence (V2), the ventral iridophores have high reflectivity and low transmission, channelling the incident light downwards.

View larger version (19K): [in a new window]   Fig. 17. The effects of position on the reflective patterns seen by observers X, Y and Z (see Fig.15). (A) At a distance of half a squid’s mantle length. F, fluorescent layers; R, ‘red’ stripe; B, ‘blue’ stripe; G, ‘green’ stripes; GP, ‘green’ patches (G and GP only for L. vulgaris). (B) For observers at a distance of 1.5mantlelengths.

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