Extraocular sensitivity to polarized light in an echinoderm

Area of Science:

• Marine Biology
• Echinoderm Research
• Sensory Biology

Background:

• Brittlestars (Ophiuroidea, Echinodermata) possess a stereom endoskeleton composed of birefringent calcite.
• The visual capabilities of brittlestars are poorly understood, particularly their potential to perceive polarized light.

Purpose of the Study:

• To investigate if the brittlestar endoskeleton polarizes light.
• To determine if brittlestars exhibit behavioral responses to polarized light.

Main Methods:

• Examined ossicles of Ophioderma brevispinum under polarized light to assess dichroism.
• Conducted orientation experiments with O. brevispinum in polarized and unpolarized light conditions.

Main Results:

• Brittlestar ossicles (lateral arm shields, oral arm shields, arm spines, aboral disk) were found to be dichroic, polarizing light.
• O. brevispinum demonstrated significant orientation in polarized light but not in unpolarized light.
• Orientation behavior in polarized light suggests a role in sustaining activity rather than directional navigation.

Conclusions:

• The calcite endoskeleton of O. brevispinum polarizes light, supporting the first hypothesis.
• O. brevispinum can discriminate between polarized and unpolarized light, indicating a novel sensory mechanism.
• The skeletal structure may function as integrated polarizing filters, enabling polarization sensitivity in eyeless echinoderms.