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Propagation of Bioluminescent Signals Underwater
Visibility of Aquatic Animals

The Relationship Between Tissue Ultrastructure and Transparency

The Visual Ecology of Polarization Vision

The Effect of Ultraviolet Vision on Predation

Optical Sampling Techniques for Zooplankton


Research Projects

The Relationship Between Tissue Ultrastructure and Transparency

Transparency is a remarkable characteristic of many oceanic zooplankton. Some degree of transparency is found in almost all pelagic animals that are not camouflaged by small size or mirrored surfaces, or protected by fast swimming speeds or chemical defenses, and it is generally accepted that transparency is an important method of camouflage from visual predators and/or prey in the optically featureless pelagic environment. Despite this, almost nothing is known about the physical basis of transparency in these animals. Using spectroscopy, electron microscopy, and electromagnetic field modeling, we investigate the relationships between tissue ultrastructure and transparency work to develop an understanding of the design principles of transparency in different tissues and species.

Figure 1: Electron micrographs of the cornea (left) and sclera (right) of the eye of a shark.

Transparency is found in two major types of tissues: 1) ocular tissues, and 2) the entire bodies of many pelagic animals. Due to its medical relevance, most research has focused on the former. Theoretical and experimental research on the physical basis of transparency in the vertebrate cornea and lens has shown that tissue ultrastructure and transparency are intimately related. For example, the cornea and the sclera (white) of the eye are both constructed primarily of collagen fibers. However, the collagen fibers in the cornea have a smaller diameter and are more regularly packed, leading to 95-99% transparency rather than opacity (Figure 1). The enucleation, elongation, and high protein concentrations of lens cells are additional structural modifications that are directly related to increased transparency. These modifications result in tissues that must be metabolically supported by surrounding tissue and are predisposed to an eventual decrease or complete loss of transparency over time (e.g. cataracts). Given the extremity of these modifications, it is likely that equally novel modifications have evolved in transparent zooplankton.

For a tissue to be transparent, light must pass through it without being scattered or absorbed. Except in specialized tissues that contain pigments, scattering is the more significant barrier to transparency because very few organic molecules absorb light. Scattering is caused by variations in refractive index. As light passes from one material to another, a change in refractive index alters the light's speed and direction. Highly scattering but non-absorbing substances can be completely opaque (e.g. snow, milk, clouds). Animal tissue normally has many variations in refractive index, due to various components required for life (cells, fibers, nuclei, nerves, etc.). Even gelatinous zooplankton, which contain a relatively large amount of water, still may have many refractive index variations (which generally lead to opacity after the animal dies).

Unfortunately, the relationship between refractive index variation and light scattering is extraordinarily complicated, and details about the refractive index distribution inside living tissue are not often known. Our works attempts to overcome these difficulties in a variety of ways, including electron microscopy, quantitative phase microscopy, and modeling light scattering using Mie and Fourier theory.

Figure 2. The salp Salpa maxima. Salps are pelagic tunicates often found in huge abundance.

Figure 3. The heteropod Pterotrachea sp.. These pelagic gastropods are highly visual predators with very interesting eyes.

Figure 4: Illustration by Hardy showing transparent gelatinous zooplankton.


Marsili, S., Salganik, R. I., Albright, C. D., Freel, C. D., Johnsen, S., Peiffer, R. L., and M. J. Costello (2004). Cataract formation in a strain of rats selected for high oxidative stress. Experimental Eye Research 79: 595-612.

Gilliland, K. O., Freel, C. D., Johnsen, S., Fowler, C., and M. J. Costello (2004). Random distribution of multilamellar bodies in human age-related nuclear cataracts. Experimental Eye Research. 79: 563-576.

Johnsen, S. (2001). Hidden in plain sight: the ecology and physiology of organismal transparency. Biological Bulletin 201: 301-138.

Johnsen, S., and E. A. Widder (2001). Ultraviolet absorption in transparent zooplankton and its implications for depth distribution and visual predation. Marine Biology 138: 717-730.

Johnsen, S. (2000). Transparent animals. Scientific American 282(2): 62-71.

Johnsen, S., and E. A. Widder (1999). The physical basis of transparency in biological tissue: ultrastructure and the minimization of light scattering. Journal of Theoretical Biology 199: 181-198.

Johnsen, S., and E. A. Widder (1998). The transparency and visibility of gelatinous zooplankton from the north west Atlantic and Gulf of Mexico. Biological Bulletin (Woods Hole) 195: 337-348.

Johnsen, S., and E. A. Widder (1998). The transparency and visibility of gelatinous zooplankton. Proceedings of the Fourteenth Conference of the Ocean Optics Society, Kailua-Kona, HI, USA.


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