Extraocular sensitivity to polarized light in an echinoderm

This study tests the hypotheses that the birefringent calcite and stereom structure of the brittlestar (Ophiuroidea, Echinodermata) endoskeleton polarizes light and that certain brittlestars respond to polarized light. The first hypothesis was tested in Ophioderma brevispinum by examining ossicles from freshly killed specimens under polarized light. This analysis revealed that the lateral arm shields, oral arm shields, arm spines and aboral disk ossicles are dichroic and thus polarize light. The second hypothesis was tested in two orientation experiments under polarized light. The results from the first orientation experiment showed (1) that, under polarized light, animals oriented significantly and unimodally, (2) that, under polarized light with the e-vector perpendicular to that in 1, animals oriented significantly and unimodally to within 18° of the bearing of the animals in 1, and (3) that, under unpolarized light, animals did not orient significantly. The results from the second orientation experiment showed that, under polarized light, animals oriented significantly and unimodally to within 17° of an individual preference previously established under polarized light; but under unpolarized light, animals did not orient significantly to an individual preference established under polarized light. Thus, O. brevispinum orients under polarized, but not unpolarized, light. The unimodal orientation and lack of consistent alignment with the e-vector suggest that polarized light is not used as a directional cue but instead as a signal to sustain oriented behavior. The dichroism of the ossicles of O. brevispinum and the animal’s capacity (though eyeless) to discriminate between polarized and unpolarized light suggest that the mechanism of polarization sensitivity may rely on polarizing filters built from the animal’s skeleton.