Thursday, May 2, 2013

Birefringent structures in marine invertebrates

If one treats light as a transverse wave, as per Maxwell’s equations, a beam of unpolarized light can be thought of as a collection of these waves oscillating in all planes (Bennett, 1950). A polarizer placed over this ray would then block the transmission of all but one plane of oscillation. Normally a second polarizer, called an analyzer, placed perpendicular to the first would then remove that last plane of light, allowing no transmission. However, if that light is first passed through an anisotropic medium, that is one that is highly ordered, it causes light to refract in a secondary, angled path in addition to the one that will be cancelled out by the analyzer. This double refraction is called birefringence (Inuoe, 1970). This effect can be seen in the luminescence of various calcareous structures of marine invertebrates.

The image at left is a four-week old pluteus larva of the sand dollar, Dendraster excentricus, that I cultured in the lab. The larval spicules of the pluteus, which are made primarily of calcite, an anisotropic material, are illuminated. The larva’s left postoral arm can be seen in the plane of focus, one of its six arms at this stage of development.
Here, the two rings of calcite larval ossicles of a doliolaria, the non-feeding planktonic larva of a sea cucumber, can be seen to be birefringent. These ossicles will eventually be embedded in the epidermis of the adult holothuroid, toughening the exterior of the animal.

This picture shows the tri-radiate calcite spicules of the sea sponge, Leucilla nuttingi. These three-pointed rods are imbedded in the organism’s mesohyl, and are characteristic of sponges in the class Calcarea. A section of the sponge’s exterior was excised and then allowed to dissolve in a solution of 8% bleach, leaving only its calcareous skeleton.

Bennett, HS. Methods Applicable to the Study of Both Fresh and Fixed Material, in McClung’s Handbook of Microscopial Technique, 3rd ed., Paul Hoeber, New York, 1950. pp. 591-677

Inoué, Shinya. An Introduction to Biological Polarization Microscopy. Program in Biophysical Cytology. Woods Hole, MA 1970.

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