The morphology and spectral absorption characteristics of the retinal
photoreceptors in the southern hemisphere lamprey Geotria
australis (Agnatha) were studied using light and electron
microscopy and microspectrophotometry. The retinae of both downstream
and upstream migrants of Geotria contained two types of cone
photoreceptor and one type of rod photoreceptor. Visual pigments
contained in the outer segments of these three photoreceptor types had
absorbance spectra typical of porphyropsins and with wavelengths of
maximum absorbance (downstream/ upstream) at 610/616 nm
(long-wavelength-sensitive cone, LWS), 515/515 nm
(medium-wavelength-sensitive cone, MWS), and 506/500 nm
(medium-wavelength-sensitive rod). A “yellow” photostable
pigment was present in the myoid region of all three types of
photoreceptor in the downstream migrant. The same
short-wavelength-absorbing pigment, which prevents photostimulation of
the beta band of the visual pigment in the outer segment, was present
in the rods and LWS cones of the upstream migrant, but was replaced by
a large transparent ellipsosome in the MWS cones. Using
microspectrophotometric and anatomical data, the quantal spectral
sensitivity of each photoreceptor type was calculated. Our results
provide the first evidence of a jawless vertebrate, represented today
solely by the lampreys and hagfishes, with two morphologically and
physiologically distinct types of cone photoreceptors, in addition to a
rod-like photoreceptor containing a colored filter (a cone-like
characteristic). In contrast, all other lampreys studied thus far have
either (1) one type of cone and one type of rod, or (2) a single type
of rod-like photoreceptor.