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Hebrew University, Interuniversity Institute for Marine Sciences, P.O. Box 469, Eilat 88103, Israel
Distinct polarization body patterns have been recorded in cephalopods. In cuttlefish (Sepia officinalis) and squid (Loligo pealeii) these patterns are postulated to constitute a discrete communication channel that may be "hidden" from some of their predators (1,2). In squid, the patterns of polarization are most prominent as long, narrow stripes along the arms (3). Examination of the skin of L. pealeii has now revealed very localized rows of iridophore cells that are reflecting and polarizing incident light and thus producing these patterns. Topical application of acetylcholine (ACh) to the arms of L. pealeii induced a change in the polarization reflection, as in other squid species (4). Moreover, silver staining and acetylcholinesterase histochemistry suggest that these iridophores are under direct neural control, unlike any known cephalopod iridophore.
Reflection and polarization of incident light by squid iridophores is accomplished by layers of intracellular platelets that are positioned parallel to each other (5). The spectrum (color) of the reflection can change from red/pink to blue and depends upon the distance between platelets, the orientation of the platelets, and the direction of viewing (6,7). In squid dermis, iridophores have been found heretofore only beneath the layer of chromatophores (4). Iridophores are found in many parts of squid skin, but in most species they are especially abundant on the mantle. Because the polarization patterns in Loligo pealeii are created within very localized areas on the arms (Fig. 1B), we examined the skin in those areas and looked for structures that could potentially reflect light to produce polarization patterns.
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For morphology, arms were preserved in 10% formalin in buffered seawater for 3 d and, after washing, they were cut, mounted, and transferred to 70% ETOH. Samples were then sectioned at 40–200 µm intervals and stained with Mayer’s hematoxylin and eosin, Masson’s trichrome, and silver (Holmes’ silver nitrate method). Sectioning and processing arm tissue for acetylcholinesterase histochemistry was done according to the method of Mesulam and Van Hoesen (8), using the acetylthiocholine medium specified by Geneser-Jensen and Blackstad (9). Images were then observed with a Zeiss Axioplan microscope equipped with an internal scaling and calibration system.
Strong partially linearly polarized reflection could be identified in specific lines along the animals’ arms (Figs. 1A–D) and was often associated with physical colors such as blue or pink. Microscopic examination of skin tissue at these locations revealed the existence of a new type of iridophore. These reflecting cells were located in very narrow areas of the skin, 60 ± 26 µm (n = 24) underneath the skin surface, and organized as long stripes. Cell length was 267 ± 131 µm (n = 22), and cell width was 14.8 ± 7.2 µm (n = 26). These long stripes of iridophores are consistent with the red and highly polarizing iridophores reported by Mäthger and Denton (7), but the squid arm iridophores are much narrower. Unlike other squid iridophores, which are found beneath the chromatophore layer, these cells were situated above the chromatophores (Fig. 1E). Platelets [1.8 ± 1.2 µm wide and 14.1 ± 6.4 µm long (n = 138)] were set in an angle inside the cell and were organized parallel to each other with a variability of 7.9° ± 4.0° (n = 25). Inter-platelet space was 1.2 ± 0.7 µm (n = 150), providing for an average density of 30.8 ± 9.0 platelets per 100 µm.
Previous studies have never furnished evidence of innervated squid iridophores (4). This is surprising considering the speed with which changes in color—even iridescent color—occur in cephalopods. Hanlon et al. (4) found that iridophores in the squids Lolliguncula brevis and Loligo plei became iridescent when treated with ACh, but no nerve fibers were found adjacent to ornear the iridophore cells. They therefore surmised that ACh would diffuse to the iridophore cell surfaces, would bind to ACh receptors there and thus would induce an ultrastructural change in the platelets to produce iridescence.
However, polarization patterns on cephalopods change in just a second or two, suggesting neural rather than hormonal control. Topical application of 10-3 M ACh to isolated skin patches induced polarization reflections. Silver staining revealed nerve fibers in very close proximity to the iridophores (Fig. 1F), suggesting that these cells may be innervated directly. Finally, staining for acetylcholinesterase revealed specific active areas at the attachment of potential nerve fibers to the iridophores (Fig. 1G).
Our results present quite a different cellular structure—and potential control mechanism—in which a polarization pattern is produced in the arms of squid. The control of these structures, and the significance of polarization patterns to squids, remain to be investigated.
We thank Michael Mitchell for sectioning some samples, John Messenger and William Kier for evaluating some microscopic tissue samples, and Louis Kerr and Rudi Rottenfusser for microscopy assistance. This study was sponsored by NSF grant IBN 9722805, BSF grant 1999040, and an MBL fellowship to NS.
Footnotes
1 Marine Resources Center, Marine Biological Laboratory, Woods Hole, MA 02543. 
2 Dept. of Biology, Rutgers, the State University of New Jersey, Camden, NJ 08102.
Literature Cited
This article has been cited by other articles:
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  S. D. Ramsden, L. Anderson, M. Mussi, M. Kamermans, and C. W. Hawryshyn Retinal processing and opponent mechanisms mediating ultraviolet polarization sensitivity in rainbow trout (Oncorhynchus mykiss) J. Exp. Biol., May 1, 2008; 211(9): 1376 - 1385. [Abstract] [Full Text] [PDF]
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T.-H. Chiou, L. M. Mathger, R. T. Hanlon, and T. W. Cronin Spectral and spatial properties of polarized light reflections from the arms of squid (Loligo pealeii) and cuttlefish (Sepia officinalis L.) J. Exp. Biol., October 15, 2007; 210(20): 3624 - 3635. [Abstract] [Full Text] [PDF]
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M. Mussi, T. J. Haimberger, and C. W. Hawryshyn Behavioural discrimination of polarized light in the damselfish Chromis viridis (family Pomacentridae) J. Exp. Biol., August 15, 2005; 208(16): 3037 - 3046. [Abstract] [Full Text] [PDF]
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