Lampreys and hagfishes are the sole extant representatives of the early agnathan (jawless) vertebrates. We compared retinal function of fully metamorphosed, immature Mordacia mordax (which are about to commence parasitic feeding) with those of sexually mature individuals of its non-parasitic derivative M. praecox. We focused on elucidating the retinal adaptations to dim-light environments in these nocturnally active lampreys, using electroretinography to determine the temporal resolution (flicker fusion frequency, FFF) and temporal contrast sensitivity of enucleated eyecups at different temperatures and light intensities. FFF was significantly affected by temperature and light intensity. Critical flicker fusion frequency (cFFF, the highest FFF recorded) of M. praecox and M. mordax increased from 15.1 and 21.8 Hz at 9°C to 31.1 and 36.9 Hz at 24°C, respectively. Contrast sensitivity of both species increased by an order of magnitude between 9 and 24°C, but remained comparatively constant across all light intensities. Although FFF values for Mordacia spp. are relatively low, retinal responses showed a particularly high contrast sensitivity of 625 in M. praecox and 710 in M. mordax at 24°C. This suggests selective pressures favour low temporal resolution and high contrast sensitivity in both species, thereby enhancing the capture of photons and increasing sensitivity in their light-limited environments. FFF indicated all retinal photoreceptors exhibit the same temporal response. Although the slow response kinetics (i.e. low FFF) and saturation of the response at bright light intensities characterise the photoreceptors of both species as rod-like, it is unusual for such a photoreceptor to be functional under scotopic and photopic conditions.
The authors declare no competing or financial interests.
Conception: R.E.W., J.M.H., S.P.C., N.S.H. and I.C.P. Designed experiments: R.E.W. and J.M.H. Software development: J.M.H. and R.E.W. Performed experiments: R.E.W. Analysed data: R.E.W. and J.M.H. Provided resources: R.E.W., J.M.H., S.P.C. and I.C.P. Original draft preparation: R.E.W. Reviewed and edited manuscript: R.E.W., J.M.H., I.C.P., N.S.H. and S.P.C.
This work was supported by an Australian Research Council (ARC) Discovery Grant (DP110103294) to S.P.C., I.C.P. and N.S.H., an Australian Research Council Future Fellowship (FT110100528) to J.M.H. and the West Australian Premiers Fellowship (from the WA State Government and The University of Western Australia) to S.P.C. R.E.W. was supported by a Scholarship for International Research Fees (SIRF), a University International Stipend (UIS) and a UIS-Top Up from The University of Western Australia.
- Received September 26, 2016.
- Accepted January 11, 2017.
- © 2017. Published by The Company of Biologists Ltd