Morphology and spectral absorption characteristics of retinal photoreceptors in the southern hemisphere lamprey (Geotria australis)

The southern hemisphere lamprey Geotria australis possesses two types of cone photoreceptors and one type of rod photoreceptor.

• Both downstream and upstream migrants of Geotria contain distinct visual pigments with maximum absorbance wavelengths at 610 nm (long-wavelength-sensitive cone), 515 nm (medium-wavelength-sensitive cone), and 506 nm (medium-wavelength-sensitive rod).
• A 'yellow' photostable pigment was identified in the myoid region of all three photoreceptor types in the downstream migrant.
• In the upstream migrant, a short-wavelength-absorbing pigment was found in the rods and long-wavelength-sensitive cones, while medium-wavelength-sensitive cones contained a large transparent ellipsosome.
• This study provides the first evidence of a jawless vertebrate with two morphologically and physiologically distinct types of cone photoreceptors.
• The presence of a second type of cone in adult Geotria may reflect an adaptation to brightly lit surface waters during its marine life phase.

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