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First published online October 5, 2007
Journal of Experimental Biology 210, 3624-3635 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006932

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Spectral and spatial properties of polarized light reflections from the arms of squid (Loligo pealeii) and cuttlefish (Sepia officinalis L.)

Tsyr-Huei Chiou^1,*, Lydia M. Mäthger², Roger T. Hanlon² and Thomas W. Cronin¹

¹ Department of Biological Sciences, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
² Marine Resources Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA

View larger version (11K): [in this window] [in a new window]   Fig. 1. (A) A diagrammatic view of the setup used to measure the spectral properties of the polarization reflection and to obtain images for imaging polarimetry. OF, optic fiber; CB, the attached film camera body; PL, linear polarizer; P, preparation; TT, tilting table as described by Denton and Nicol (Denton and Nicol, 1965); WT, water tank; L1, first illumination setting (Position 1); L2, second illumination position (Position 2); SP, spectrometer. (B) Geometry of the setup. The arrows marked L1 and L2 indicate the two illumination directions (at 90° and 45° to the observation axis, respectively). The dotted lines illustrate how the e-vector angles (which fall in the xy plane) were defined in terms of the x- and y-axes. All rotations of the tilting table were made about the y-axis. Thus, when the arm was perpendicular to the y-axis (i.e. it extended along the x-axis), changes in angle caused the arm to tilt upwards towards the light. When the arm axis was parallel to the y-axis, changes in angle produced rotation of the arm.

View larger version (241K): [in this window] [in a new window]   Fig. 2. Images of a squid (A) and a cuttlefish (B) showing the position of the arm stripes (as indicated by arrows). The differences in colors of the arm stripes are caused by differences in illumination of the animals in the photographs. (C,D) Close-up images of the squid (C) and the cuttlefish (D) arms tilted at various angles under the second illumination setup (Position 2, incident light from 45° above the horizontal). The left panels in C and D show the arm stripes in true-color images, while the right ones illustrate partial polarization values (%Pol) coded as in the key on the figure's right edge. The brightness of each color is proportional to the relative reflectance of the pixel in the original image (see Materials and methods for details of how relative reflectance values were obtained). The number at the left of each row of images indicates the tilt angle of the arm.

View larger version (39K): [in this window] [in a new window]   Fig. 3. Averaged reflectance spectra of the arm stripes of the squid (A) and the cuttlefish (B) and corresponding partial polarization spectra from the squid (C) and the cuttlefish (D). Data collected in Position 2 (see Fig. 1). Different colored curves in each figure represent spectra obtained from a sample tilted from 0° to 60° in increments of 10° (the key in A also applies to B–D). Note that identical reflectance values between spectra occurred in some cases (which are mathematically not possible to analyze); hence, some of the values in partial polarization curves are not plotted.

View larger version (24K): [in this window] [in a new window]   Fig. 4. Spectra of e-vector angles of the arm stripes of the squid and the cuttlefish oriented parallel (A,B) or perpendicular (C,D) to the y-axis. Data collected in Position 2 (see Fig. 1). As in Fig. 3, different colored spectra represent the calculated results from samples at various tilt or rotation angles (key in A also applies to B–D).

View larger version (186K): [in this window] [in a new window]   Fig. 5. Color-coded images of the arm stripes of the squid (A) and the cuttlefish (B) showing the e-vector angles of the arm stripes. The key for color coding of the e-vector angles is shown at the bottom right of the figure. As in Fig. 2C,D, the brightness of each coded color is proportional to the relative reflectance value at that point in the full-color image, and the arm stripes were tilted or rotated at different angles as indicated at the left side of each row. The original pictures were taken from the z-axis with illumination from right side of the image (Position 1; see Fig. 1).

View larger version (35K): [in this window] [in a new window]   Fig. 6. Spectral reflectance curves of the arm stripe of the squid with the longitudinal axis of the arm perpendicular to the y-axis (A) or parallel to the y-axis (B) and their respective partial polarization spectra (C) and (D). Data collected in Position 1 (see Fig. 1). As in Fig. 3, seven sets of spectra were obtained from the sample in both orientations (key in A also applies to B–D).

View larger version (36K): [in this window] [in a new window]   Fig. 7. Similar to Fig. 6, but obtained from the arm stripe of a cuttlefish (Sepia officinalis) oriented perpendicular to the y-axis (A,C) or parallel to it (B,D).

View larger version (210K): [in this window] [in a new window]   Fig. 8. Similar to Fig. 5A, but showing the real color images (A) and false color images (B) of the partial polarization (% Pol) of the arm stripes of the squid (Loligo pealeii). Data collected in Position 1 (see Fig. 1). The tilt or rotation angle is shown at the left side of each row. The color coding of the partial polarization values is shown at the right of the images. The color coded images were also weighted with relative reflectance values of the original images, as in the right panels of Fig. 2C,D. Note the apparent color change in the photos of squid arm stripes at greater angles of tilt. These changes are not seen in the reflectance spectra (Fig. 6), and the arms did not obviously change color as seen by eye; they apparently arise because of the extreme brightness of the reflections at these angles, which affected the digital camera's automatic white-balance setting.

View larger version (205K): [in this window] [in a new window]   Fig. 9. Similar to Fig. 8, but from the arm stripes of the cuttlefish (S. officinalis) at various orientations and tilt or rotation angles.

View larger version (138K): [in this window] [in a new window]   Fig. 10. Transmission electron micrographs of the iridophores in the arm stripes of the squid (L. pealeii) (A) and the cuttlefish (S. officinalis) (B). Sections were cut perpendicular to the longitudinal axis of the arm. The arrow in each graph indicates the direction toward the nearest skin surface. C, pigment granules of a chromatophore. Scale bars, 7.5 µm.