Changes in the spectral absorption of cone visual pigments during the settlement of the goatfish Upeneus tragula: the loss of red sensitivity as a benthic existence begins

Abstract

The goatfish Upeneus tragula undergoes an abrupt metamorphosis at settlement when the pelagic larvae begin a reef-associated benthic mode of life. A microspectrophotometric investigation of the retinal visual pigments was carried out on fish prior to, during, and following settlement. It was found that the visual pigment in the long wavelength-absorbing member of the double cones in the dorsal retina changed rapidly from a rhodopsin with a wavelength of maximum absorption (λmax) of 580 nm to that of 530 nm. The second member of the double cones always had a rhodopsin with the λmax absorbing at shorter wavelengths. Prior to settlement the average for this class of cones was 487 nm whereas during and immediately following the settlement period the λmax recorded from individual outer segments was found to vary between 480 nm and 520 nm, with two possible classes of cone absorbance emerging within this range. These two classes of absorbance had average λmax values of 487 and 515 nm. The average λmax of the paired cone classes in one larger wild-settled fish were found to be at 506 nm and 530 nm. No change was detected in the λmax of the single cones or the rods which were always found to have a λmax of about 400 nm and 498 nm respectively. The loss of the redabsorbing pigment occurred over the same time scale as the metamorphosis of morphological features associated with the settlement process. It is thought that the loss of this visual pigment is associated with the change in light environment of the fishes as they leave the surface waters to begin a benthic mode of life in deeper water.