Yellow filters and the absorption of light by the visual pigments of some amazonian fishes

YELLOW FILTERS AND THE ABSORPTION OF LIGHT BY THE VISUAL PIGMENTS OF SOME AMAZONIAN FISHES W. R. A. MUNTZ Laboratoryof Experimentalpsythology,SussexUniversity,Brighton (Receid

9 Muy 1973)

MANY different visual pigments have been extracted from the retinas of fishes. Sometimes, as in the case of deep sea fishes, there is a clear correlation between the spectral absorbance of the extracted pigment and the spectral characteristics of the environment in which the fish lives (DENTONand WARREN,1956, 1957; Mum, 1957; WALD, BROWN and BROWN, 1957), but in other cases no such correlation is apparent. The distribution of visual pigments in freshwater fishes, for example, has not yet been adequately explained. An added complication in this case is that such fishes often have two extractable visual pigments, one based on retinol (Al-based) and the other on 3dehydroretinoI (AZ-based), the relative proportion of which may vary with many factors, such as the salinity of the environment, light conditions, hormones, age and retinal focation sampled (see BRIDGES, 1972 and CRESCITELLS, 1972 for recent reviews). Despite considerable s~culation no completely satisfactory theory has yet been advanced as to the functional significance of these variations. In the present experiments the retinal pigments of a variety of Amazonian fishes were studied, in order to see whether there was any correlation between the pigments of the fishes from this region and the spectral qualities of the habitats in which they live. The Amazon river system is particularly interesting from this point of view because of the large number of species that it contains, and because, although daylength remains nearly constant throughout the year, there are wide variations between the colours of adjacent rivers (see MYERS,1947 and SIOLI, 1967 for general descriptions of the rivers in this area). Since the relative proportion of Al- and AZ-based pigment in the retinas of fishes may change very rapidly, the pigments must be studied in fishes that have been recently caught at known locations if the results are to be rnea~n~~ly related to the spectral qualities of the habitat. This may be diBcult, for interesting species are often only found in remote places, where suitable laboratory facilities for conventional extraction procedures are unavaifable. One common method of avoiding this difficulty is to freeze the retinas for later pigment analysis, but this is not always completely satisfactory because adequate methods for keeping the material frozen during transport are needed, and defective specimens cannot be replaced. In the present study the problem was avoided by measuring the spectral absorption of small areas of intact retinas with a technique similar to DENTON’S (1959), using an apparatus that was portable and needed a minimum of supporting facilities. Such measurements do not allow the accurate identification of visual pigments by partia1 bleaching techniques, but they do provide other kinds of information that are often not obtained from pigment extracts. Thus they can show the in situ density of the pigments, and indicate whether the pigments differ in different parts of the retina; and where more than one 2235

2236

W. R. A. Musrz

pigment is present, they can show the effective absorption of the mixture, which is the datum that should be compared to the spectral characteristics of the environment. Finally, the spectral absorptions of the lens and cornea, which will also affect spectra1 sensitivity, can be determined with the same apparatus. A detailed study of the visual pigments of 55 species of tropical freshwater fishes has been made by SCHWANZAIW (1967), using conventional extraction procedures. The specimens used in that study were mostly obtained from dealers, so their precise origins are unknown, and their visual pigments may also have been affected by the conditions under which they were kept. On the other hand, the techniques used by Schwanzara allowed the accurate identification of the visual pigments, so that her results are compIementa~ to the present data. Many of the cichlid fishes studied were found to have yellow corneas and lenses, and sometimes to have a photostable yellow pigment in the retina as well. Since such yellow filters will strongly affect spectral sensitivity, their spectral absorption was measured and the effect on the absorption of the visual pigment calculated. Yellow corneas have been reported previously for several teleost fishes (WALLS and JUDD, 1933; MORELA~?)and LYTHGOE, 1968; BRIDGES,1969). Visibly yellow lenses have only been reported for a single species (DENTON, 1956), but they are in fact common, at least among marine coral fishes (see Discussion). It has been demonstrated that the visual pigments of rudd (Scar&Gus erythrophthalmus) and trout (Sulmo fario and S. gairdneri) vary depending on the part of the retina sampled. These species have two extractable visual pigments, and in ali three cases the relative ~ropo~on of the Al-based pigment is greater in the dorsal part of the retina (Mui-rTZ and NORT~ORE, 1971; D~oN, MUPCZ and NORTHMORE,1971; REUTER,WI-KTEand WALD, 1971). A second purpose of the present experiments was to find out whether this variation within the retina is a widespread phenomenon among fishes.

hfETHODS Apparatus

The spectral absorbances of small areas of intact retina were measured using the double beam spectrophotometer shown diagrammatically in Fig. I. The filament of a 36 W 12 V car headlamp bulb (II) was focused on the entrance slit of a Hilger grating monochromator (MC). The emerging beam was divided into measuring (M) and compar~on (C) beams by suitable apertures in a baffle @I, and the light alternated between these at 150 c/s by a mechanically driven sectored wheel (SW). The intensity of the comparison beam could be varied by altering the size of its aperture, while that of the measuring beam was controlled by a neutral density wedge (W). The two beams were reflected upwards by means of right angle prisms (PI and PZ), and two images of the exit slit of the monochromator formed at the level of a microscope stage (ST) by the lenses Lr and L2. These images constituted the measuring and comparison spots; their size was @8 X 0.1 mm, and the distance between them could be varied by rotating the prisms Pr and P2. The relative intensity of the comparison and measuring spots was measured using an EM1 photomultiplier (PM), which had a corrugated S-20 photocathode. An opal Perspex window (Op) was included to further reduce any effects of light scattering by the specimen (DARTNALL, 1961). The photomultiplierwas mounted so that it could be swung out of the way while the specimen was positioned, and subsequently easily moved into position against a stop to make the measurements. The alternating light signal reaching the photomultiplier was converted by it into an alternating voltage, which was fed into an FET source follower, and thence into a band pass amplifier. The output of this amplifier was fed into a phase sensitive rectifier together with a signal from a small bulb and photodiode (D) which monitored the sectored wheel. The resulting d.c. signal was finally fed into a microammeter. Thus when the light reaching the photomultipIier from the comparison and measuring beams was of equal intensity no output was obtained, while imbalances caused deflections of the microa~eter needle, whose direction depended on which beam was more intense.

Yellow Filters and the Absorption

of Light

2237

FIG. 1. Diagram of the spectrophotometer. The bottom half of the figure shows a schematic plan view of the apparatus, and the top half a side view of the arrangement of the tinal lenses, prisms, and the photomultiplier. HG, heat glass; F, neutral density tilter. For the meaning of the other letters, see text.

The general procedure was as follows. The specimen (retina, cornea or lens) was dissected out and mounted on a microscope slide (MC), in glycerol, as described below. The slide was positioned by the mechanical movement of the microscope stage so that the measuring spot fell on the area of the specimen whose spectral absorbance was to be measured, while the comparison spot fell on a clear area of glycerol. In order to control against any instability of the preparation, measurements were made at 40 nm intervals through the spectrum, from 380 to 640 nm, and then at the same intervals in the reverse direction, from 620 to 400 run. At each wavelength the wedge (W) was adjusted until the measuring and comparison beams were balanced. The specimen was then removed, and all the measurements repeated. For any wavelength the difference between the two sets of wedge settings therefore shows the amount of neutral density that has to be added to the measuring beam in order to compensate for the removal of the specimen, which is of course the same as the density of the specimen at that wavelength. Since at each wavelength all the settings on the power supply and amplitier were kept the same during the two sets of measurements, and the same amount of light was reaching the photomultiplier from each beam, the accuracy of the measurements cannot be affected by any nonlinearities in the system, but depends only on the calibration of the neutral density wedge and the accuracy with which the null settings can be made. The wedge was calibrated at each wavelength in sifu by stopping the sectored wheel, blocking off the comparison beam, and measuring the attenuation produced at different settings with the photomultiplier d.c.-coupled toadigital voltmeter. Repeated settings of the wedge showed that an accuracy of @005 density units or better could be obtained. The spectrophotometer was designed to be easily transportable, and to need the minimum of supporting facilities. Both the power supply and measuring circuits were solid-state and ran off 24 V d.c. (two car batteries), and the whole apparatus when dismantled and packed weighed 13 kg. Material Fishes were collected from four localities near Manaus, using either hook and line or cast net. Most of the specimens came from the tirst two localities described. In some cases it was possible to measure the colour of the water in platinum units (e.g. WELCH, 1948) using a Hack Chemical Co. DR-EL series Portable Water Engineer’s Laboratory. (1) &rgo de Peixe Boi. This is a widening in the Rio Culheiras, which joins the Rio Negro about 30 km above Manaus. The water is clear and fairly coloured: a sample collected on 24/8/72 gave a measurement of 125 platinum units. Being part of the Rio Negro system, the water is acid and low in salts. (2) Logo C’usru/~nu.The Lago Castahna is a shallow lake, to the south of the Solimo& river, and upstream of the junction of the Solimo& and the Rio Negro. The chemical composition of the water is presumably much the same as that of the Solimo&, a “white water” river rich in salts and relatively neutral, but there is little suspended soil in the lake relative to the river and the water is comparatively clear. (3) Riserva Ducke. Some specimens were collected from a small clear acid stream in the Riserva Ducke, 10 km north of Manaus. The water from this stream eventually joins the Rio Negro. An analysis of water from the Riserva Ducke is given in KN~PPEL (1970). A sample collected on 3/8/72 gave a colourreading of 20 platinum units.

2238

W. R. A. MUXTZ

(4) A&ZLXLX One fish was collected from a stream entering the Rio Negro at Manaus. This water was clear and highly coioured, and a sample collected on t/9/72 gave a colour reading of IS5 ptatinum units. An analysis of water from the Tarum& a stream just above Manaus that is probably very similar, is given in KN6PPEL (1970).

The fishes were in al1 cases brought back to the iaboratory alive, where the measurements were made as soon as possible. Before an experiment a lish was dark adapted for at least 3 hr. It was killed either by decapitation, if small, or by immersion in a solution of Tricane Methanesulfonate (MS-222). The eyes were then removed, the lens and cornea cut away, and the retina gently freed from the underlying pigment epithelium. The retinas were mounted, receptor side up, in glycerol, in a shallow well on a microscope slide, and covered with a cover slip. On some occasions small cuts were made round the edge of the retina to help it to lie fiat, and with some larger eyes only part of the retina was mounted at a time. Corneas and lenses were similarly mounted in glycerol ; for the lenses a rather deeper well was used. During the measurements on lenses the spot was focused as nearly as possible at the centre of the fens. In fife the effective absorption woufd probably be rather less, because peripheral parts of the lens would also be involved, through which the Ii& path would be shorter. Since, however, the tenses are cut-off t%ters,the effect should be sm& provided that the pigment is uniformly distributed through the lens. This appeared to be the case by visual inspection, and DEXTON(1955) was unable to deteot any difference between the centre and periphery of the lens of Angrrirramdgaris. An uneven distribution of yehow pigmentation has, however, been reported for lenses from old specimens of Scurdinius erythro&halmu.s (VILLERMET and WEALE, X972), and for a frog (DEMON, 1955). The retinas of all the fishes studied absorbed maximally at wavelengths shorter than 535 nm. The visual pigments responsible will also have their Am,Xin this spectral region, and will absorb very little light at long wavelengths. The density at long wavelengths must therefore be due to substances other than visual pigments, or to scattering. Following the procedure of DEISTON et al. (1971), it has been assumed that the visual pigments absorb negligibly at 640 nm, and the density at this wavelength has accordingly been subtracted from the densities at other wavelengths in order to yield an estimate of the density of the visual pigment alone. This procedure assumes that the absorbance of the retina withaut visuai pigment is neutral; experiments on retinas from which the outer segments of the rods have been brushed away suggest that this is approximatefy true @ENTOS, i959).

RESULTS

(i) Distribution of pigment within the retina In 15 species measurements were made at more than one retinal position (Tables I, 3 and 4). In all cases most of the measurements m along a vertical line passing through the centre of the retina, with one measurement as near the dorsal edge as possible, and another as near the ventral edge as possible. When four or more positions were studied lateral regions were used as well. In no species was there any measurable difference in the spectral absorbances at different positions. This is illustrated in Fig. 2, which shows results for ~e~~~~~~~~~~~a~~. While there is some variability at short wavelengths, the agreement is ahnost perfect above 480 nm. Although the spectral absorbance of the retina does not vary with position in these species, the effective absorbance must vary in those cichlids that have yellow corneas, for the colour is deeper in the dorsal part of the cornea (see below). (ii) Pigments of CichlidJishes A. Visualpigments. Eleven fishes of the family Cichlidae were studied, and the results are summarized in Table I. The pigments are characterized in this table by the point on the long wavelength side of the curve at which the optical density has fallen to 50 per cent of its maximum (which will be referred to as the A,,), since this is easier to estimate accurately than the wavelength of maximum absorption f&.&l. SCHWANZAU (1957) analysed pigments from eight species of cichlid fishes. With one exception ~~~rnph~~odo~acq&&&atu, w& VP497, and Vl?516,), these ail had VP5001, and

Yellow Filters and the Absorption

of Light

2239

(nm)

WAVELENGTH

FIG. 2. Spectral absorbance of four points on the retina of a specimen of Ceophugus jurupuri. Some of the points have been displaced slightly from their true spectral position for clarity. The inset shows the approximate retinal positions of the different sets of measurements. and the figure beside each symbol on this inset shows the maximum optical density at each point. TABLE 1. VISUAL

Aequidens tetramerus (Heckel)

Acarichthys heckelii (Mueller & Trotschei) Geophagus jurupari Heckel Geophagus surinamensis (Bloch) Geophugus sp.* Cichla oceIIaris Schneider Uaru amphiacanthoides Heckel Cichlasoma festivum (Heckel) Astronotus ocellatus (Cuvier) Petenia spectabilis (Steindachner) Acuronia nussu (Heckel)

PIGMENTS

OF CICHLID

Length (cm)

Max. o.d.

No. of retinal loci Aso sampled

12.5 10.0

060 04s

585 560

17.8 15.0

0.75 0.76

573 565

* 22

0.59 O-38 0.69 O-40 O-30 0.39 040 0.68 0.52 060

565 570 550 570 567 563 576 582 550 557

15.0 12.5 30-j 18.0 7-6 28.0 10-O 17.5

:

FISHES

Calculated

h,.,

Per cent Al pigment

522.3 505-O

o-0 83.7

:

516.4 509.3

23.1 60.2

1 4 2 1 3 1 1 1 1 1

510.9 513.7 502.1 514-l 512.2 511.5 522.2 522.3 502.7 507.9

50.2 35.4 100.0 33.5 43.2 46.9 0.5 9z.i 68.2

r.m.s. error

Locality at which caught

l-575 Manaus O-052 Riserva Ducke O-015 Peixe Boi 0.085 Castahna O-017 0.016 O-922 0.029 0.013 0.020 O-016 l-513 O-013 0.025

Castahna Castahna Castahna Peixe Boi Castahna Castahna Castahna Castahna Peixe Boi Peixe Boi

’ Probably Geophagus wavriniGoss~ (1963). but the identification is not certain.

six species had VP5222 as well.’ An extract from Astronotus ocellatus, a species used in the present study, had both VP5001 and VP5222, and the same two pigments occurred in Aequidens portalegrensis and Cichlasoma meeki, two genera that are represented in the ’ In this paper the visual pigment nomenclature suggested by DARTNALL (1952) is used, in which each pigment is specified by its wavelength of maximum absorption together with subscript showing whether it is based on vitamin A1 (retinal) or vitamin AI (3-dehydroretinol).

present study by different species. It is therefore reasonable to assume that the retinal absorptions of the cichlids used in these experiments are due to mixtures of VP500, and VP5222 in different proportions. The table shows the X,,, and proportion of Al-based pigment in each case assuming that this is so, and finding the best fit using the computer program described in Mv;urz and NORTHMORE (1971). It can be seen from the table that there is a wide variation between the different members of this group, X,, ranging from 550 to 585 nm. Not only was there variation between species, there was also variation between different individuals of the same species in the two cases where this was studied (Geopkugus jurupari and Aequidens tetrametus). The results for Aequidens tetramerus are shown in Fig. 3, and are of especial interest because the specimens 100

p

zLv 2 ::

is

50 -

g.._.,.ils

iz

-0

2

I) t

0

0. 400 WAVELENGTH

500

600 (nm)

FIG. 3. Spectral absorbance of the retina for three specimens of Aequidens tetmmertcs. Filled circles, specimen caught at Manaus (water coiour 165 platinum units); half filIed circles, specimen caught in Lago do Peixe Boi (water colour 125 platinum units); empty circles, specimen caught at Riserva Ducke (water colour 20 platinum units).

were obtained from three localities which, although all part of the Rio Negro system, are very different in colour (see Methods). The fish from more coloured water had visual pigments absorbing at longer wavelengths. Since there would also be less light in the coloured water, this is similar to the effects of light on visual pigments originally described by DARTNALL,LANDERand MUNZ (1961) for a cyprinid (Scardinius eryt~zruphthaZmus),and subsequently confirmed for several other species (BRIDGES, 1972). To obtain further info~ation on this point some specimens of Aequidens tetramerus, caught at the Riserva Ducke, were kept either in total darkness or under normal laboratory illumination. Not many of these fish survived, but a specimen kept in darkness for 2 days had a X,, of 565 nm, compared with a AS0of 560 nm for a fish measured directly after being caught, and of 555 nm for one kept for 2 days in the iIluminated aquarium, Another specimen kept for 7 days in darkness had a h,, of 580 nm. It thus seems likely that this species can alter its visual pigments rapidly in response to alterations in the light environment. 3. Photostable yellow pigments. Many of the cichlids studied had yellow corneas and lenses. The most heavily pigmented cornea found occurred in a O-28 m long specimen of Astronotus ocellatus from Lago Castahna; in this case the vivid yellow colour extended over the whole cornea. Figure 4 shows the spectral absorbance for a point at the centre of the cornea of this specimen, for the lens, and for the retina. The retina contained visual pigment in high optical density, absorbing maximally at about 520 nm. The effective

Yellow Filters and the Absorption

of Light

2241

J

a

u 0.5 t 0

400

500

600

WAVELENGTH

FIG. 4. Spectral absorbance of cornea, lens, and retina of Astronotusocetlatus. In the main part of the figure the crosses show the absorption of the cornea, the empty circles that of the lens, and the filled circles that of the retina. The inset shows the percentage of the incident light that would beabsorbed by the visual pigment alone (tilled circles), and by the visual pigment in life when the lens and cornea have the characteristics shown in the main part of the figure (crosses).

absorption of light by this pigment will, however, be at much longer wavelengths and the inset to Fig. 4 shows the percentage of the incident light that would be absorbed by the retina alone, and that would be absorbed by the retina in life when the cornea and lens have the characteristics shown in the main part of the figure. It can be seen that the effective absorption is much reduced, and the position of maximum sensitivity shifted to about 560 nm. Other specimens of similar size caught at the same time were not subjected to detailed 0.0 0.7 t

0.6

400

500

600

WAVELENGTH

FIG. 5. Spectral absorbance

of cornea, lens and retina of Acarichthys heckeIii. Symbols as in the main part of figure (4).

W. R. A. MUNTZ

2242

analysis, but the depth of colour in the corneas and lenses appeared the same by visuai inspection. Figure 5 shows in contrast the resuIts for a 0-i 5 m long specimen of Acutichthys heckelii, also caught in the Lago Castabna. The lens and cornea of this species had no visible colour. Other species had degrees of pigmentation intermediate between those of Astronotus acellatus and Acurichthys heckelii (see Summary in Table 2). TABLE2. YELLOW~TRA-OCULARPIGMENTSOF CICHLIDS.THE LENSCVT-OFF WAVELENGTH ISTHATATWHICH THEOPTICAL DENSITYOF THE LENS IS0.3 GREATER TmN IT IS AT %oNM. CORNEA-t.OPTICAL DENSITIESWERE MEASURED AT 460?.% AT THE DORSAL MARGIN OF THf PUPIL, EXCEPT IN /iSttona&$OC&liUS WHERE IT WAS MEASURED AT TWE CfliTREOF THE PUPIL, AND IN &%b3mO ,.hiVUtTZ, WERE IT WAS MEASURED NEAR THE DORSAL EDGEOFTHEGORNE.4 Lens cut-&

Species

(W

Astronorusoceilatus (Cuvier) Aequidens terramews (Heckel)

Optical density cornea

425 435 -

0.74 0.96

Uaru amphiacanthoidesHeckel Pete& sperrabiiis (Steindachner)

440 430

0.87 0.53

Cich/asomafestiwm (Heckef)

455

0.63

C&&Q oce&is

Schneider

Crenicirhla letztiwiataHeckel Acuronia nussa (Hecket) Acarichrhys heckelii (Mueller & Trotschel) Geophagusjurupari Heckel Geophogussurinamensis(Bloch) Geophagussp?

I.4

No visible c&our

0%

405 (not visibly cofoured)

@40

No visible colour

No visible colour

Comments Calour extends over whole cornea.

Coiour in dorsal part of cornea, r?xtending over part of pupil. Colour extends over top third pupil. Colour only in donat part cornea, not reaching the pupil. Another smalter specimen had visible c&our in the cornea. Cornea1 colour restricted to patch over dorsal edge of pupif.

of of no a

i

f See footnote to Table (1).

The depth of the cornea1 pigmentation was greater at the top of the cornea, and in some cases restricted to this part. This is illustrated in Fig. 6, which shows how the depth of colouration increased towards the dorsal part of the cornea ie specimens of Aequidens tetramems, CicMu sdiaris and ~s~~~~~~~~oc&tm. In the first species the bottom haIf of the cornea was coiourIess, and there was an abrupt increase in pigmentation just above the mid-line. In contrast, the cornea of Astronotus ocel!atus showed a much more gradual increase in colour, with some pigment even near the bottom edge of the cornea. The specimen of Astronotus used in this experiment came from a population of fish that had been kept in captivity in the state of Sgo Paulo for 32 yr; they were much paler in body colour than the fish caught in Amazonia, and the corneas were also much less heavily pigmented. It can be seen from Fig. 4 that the cornea of Astronotus ocellatus absorbs maximally at about 460 nm, and this was afso so for all the other yellow corneas tested. A more detailed analysis, in which measurements were taken at 5 nm intervals, showed that three clear submaxima were present {Fig. 7), and the position of these submaxima agrees with the results presented by MURJXANRand LYTEGOE(1958) and by BR~GES fl969) for other fishes.

Yeilow Filters and the Absorption

OPTICAL

of Light

DENSITY

440-

840nm

6. Variations along the vertical axis in the depth of colouration of the corneas of specimens of Aeqtddens fetramerus (crosses), Cichla oce&rik (f&d circks) and Astronotus ocellatus (empty circles). The depth of colouration is shown as the difference in optical density at 440 and 640nm. FIG.

::

0.6

z

0.5

5 a

0.4

d

0.3

0

0.2

IS

0.

1

LII.*stIs,.tl*l 400

450

WAVELENGTH FIG. 7. Specfral

absorbance

500 (nml

of the cornea of a specimen of Cichlasoma festivum.

0.9 r 0.8 t

r

t

I

0.5 0.4

0.3

0.2

0.1

400 WAVELENGTH

500

600 fnm)

FOG.8. Spectral absorbance of lenses of various species. From right to left the symbols on the curves are as follows: V, Cichlasoma festivum; 0, Uaru amphiacanthoides; 0, Aequidens tetramerus; 0, Petenia spectabilis; A, Astronotus ocellatus; m, Crenicichla letttictdata; x, Acarichthys he&e&i.

2243

W. R. A. MUNTZ

2244

The spectral characteristics of the lenses were quite different, and cannot be due to the same pigment: in all cases they were cut-off filters (Fig. 8), which agrees with the data on yellow lenses of other animals (see MUX-Z, 1972 for summary). Many of the species that had yellow lenses and corneas also showed a marked hump on the spectral absorbance curve of the retina. This hump occurred between 440 and 480 nm, which are the wavelengths at which the cornea1 absorption is maximal, and was photostable, for it did not appear in difference spectra (Fig. 9). No hump at these wavelengths was ever seen in fishes that had colourless lenses and corneas. All these factors suggest that a photostable yellow filtering pigment also occurs in the retina, at about the level of the rod outer segments, and that this pigment is the same as that found in the cornea. One species, Crenicichla lenticulata, caught at the Lago do Peixe Boi, contained much larger quantities of yellow retina1 pigment than any of the other species studied; the retina when dissected out was a vivid yellow all over, and this colour was photostable. The lens was not visibly coloured (Fig. 8), and the colour of the cornea was restricted to a small patch over the dorsal edge of the pupil. The depth of the yellow colouration of the retina made it impossible to detect any visual pigment. It was, however, possible to measure the

JW3

a&I:

2,“;; t&a

50-

00x

0-O I

I

I

400

500

632

WAVELENGTH

(nm)

FIG. 9. Spectral absorbance and difference spectrum for the retina of a specimen of Uarzc amphiucunthoides. The crosses show the initial absorption spectrum of the retina, the circles the difference spectrum for a tungsten light bleach.

O-

450

500

WAVELENGTH

FIG. 10. Spectral absorbance

of the retina of Crenicichla lenticulata.

Yellow Filters and the Absorption

2’45

of Light

spectra1 absorption of the retinal yellow pigment in detail, and the result of doing this is shown in Fig. 10. Although the retina absorbs over the same spectral region as the corneas of other cichlids, the three sub-maxima are not apparent, suggesting that in fact a different pigment is involved. This was subsequently confirmed by making an ethanol extract from corneas of Astronofus ocellutus, and comparing this to an ethanol extract from retinas of Crenicichlu (Fig. 11). The pigments are clearly different in the two cases.

500

450

400

WAVELENGTH FIG. 11. Spectral absorbance curves for ethanol extracts of corneas of Astronotus ocellutus (filled circles) and retinas of Crenicichla Ienticulatu(crosses).

(iii) Pigments of charucins Seven fishes of the family Characidae were studied, and the results are summarized in Table 3. Here again there was a wide variation between species, X,, varying between 565 and 585 nm. SCHWANZARA (1967) analysed the visual pigments from 15 species of Characidae in all. With one exception (Metynnis schreitmuelleri, with VP530,) these a11 had VP5O31 or VP527,, or both. Included in SCHWANZARA’S sample are Prochilodus insignis (VFYO31 and VP527,), which occurs in the present sample, and Moenkhausia oligolepis (VP5O31 and VP527J and Rooseveltiella (= Serrasalino) nattereri (VP5272 only), two genera represented in the present sample by a different species. It therefore seems likely that the fishes in the present sample also had VP5O31 and VP527,, and the h,,, and proportions of Al-based pigment given in Table 3 have been calculated assuming that this is so. TABLE3.

Hoplias malabaricus@loch) Acestrorhynchusfaicatus (Bloch) Prochilodusinsignis Schomburgk Serrasalinus rhombeus &inn.) Moenkhauria iepiduru (Kner) Leporinusfasciatus (Bloch) Bryconops @Enis (Guenther)

VWJAI. PIGMENTS OF CHARACliQ

No. of retinal loci sampled

Calculated Locality at which caught

Per cent ,I,,,,,, Al pigment

r.m.s. error

4 3

519.8 511.3

26.2 66.1

565

5

511.2

66.2

0.012 Castahna 0014 Riserva Ducke 0059 Castahna

590 568 580 585

4 4 3 3

527.0 513.2 526.2 527-o

5:.(: *

Length km)

Max. o.d.

h,,

25.4 12-7

o-40 O-55

575 565

15.0

0.49

15.3 7-5 15-5 17-S

0.29 0.76 0.22 0.48

l-072 O-019 O-046 1,068

Castahna Castahna PeixeBoi Peixe Boi

W. R. A. Muxrz

2246

Only four speices of catfish were studied, firstly because few were caught, and secondly because in many cases the eyes were too small to work with. The results are summarized in Table 4 and Fig. 12. In all four cases there was a pronounced maximum at short wavelengths (between 420 and 440 nm), which was photostable and did not contribute to difference spectra. It has recently been reported (NICOL, ~RNOTT and BEST, 1973) that Siluriformes often have tan coloured tapeta, and some, including a species of Pimelodella, also have numerous yellow

!: -SOL ‘.

..i

‘.a. _ .b--

”

WAVELENGTH

FIG. 12. Retinal absorption in Siluriformes. The retinal absorptions of different species are shown by the tilled circles and continuous line (Oxydorus niger), vertical crosses and dashed line (Pimel~~&~iacristata), diagonal crosses and continuous fine ~Auchen~~~fjc~~hyslongimanus), and empty circles ~~~~j~~~jjchrhy~ @pus). The filled ci&s ami dashed l&e show a difftrence spectrum (tungsten light bleach) for Uxy&w.s niger. The vertical axis shows the density or density change as a percentage of the photosensitive maximum.

Length (cm)

Max. o.d.

Aso

of retinal loci sampled

25.4 28-O 23.0 12.5

0*50 042 o-35 O-37

590 595 593 590

1 1 4 4

Castahna Castahna Castahna Peixe Eioi

12.5

o-32

555

1

Local dealer

No.

Locality at ahich caught

Yellow Filters and the Absorption of Light

2247

spherules in the pigment epithelium processes. This may be the explanation of the present short wavelength maxima. At longer wavelengths the retinal absorption was chiefly due to photosensitive pigment, but the presence of the photostable pigment made it impossible to characterize this accurately. Nevertheless it is clear that, in contrast to the cichlids and characins, there was very little variation between species. (v) Pigments of the freshwater sting-ray, Paratrygon

motor0

Two specimens of Paratrygon motoro, a freshwater sting-ray, were obtained from a local dealer, and one was caught in the Lago do Peixe Boi. The results are summarized in Table 4. By comparison with the other bottom living species (catfish) studied, the pigment absorbs at short wavelengths (h,, about 510 nm). The sting-ray had, however, a tapetum that was golden in colour, which will tend to shift the sensitivity towards longer wavelengths. This tapetal colour disappeared on the addition of glycerol which suggests that, as with other chondrichthyes, it is an interference phenomenon (DENTONand NICOL, 1964; DENTONand LAND, 1967).

DISCUSSION

(a) Yellow intra-ocular filters Many of the cichhds studied had photostable yellow pigments in the cornea, lens and retina. The cornea1 pigment showed three maxima, at about 425, 450 and 480 nm, which agree with the maxima previously reported by MORELAN- and LYTHGOE(1968) and by BRIDGES(1969) for the corneas of various other fishes. Yellow pigments with maxima at these positions are common in the eyes of other animals as well [e.g. the oil droplets of the frog pigment epithelium, some of the yellow oil droplets of sea turtle (Chelonia) cones (LIEBMAN,1972), and the macular pigment of man (BROWNand WALD, 1963)], and their absorption spectrum is typical of a carotenoid, such as p-carotene. It was not possible to identify the yellow pigments found in the lenses and retinas, though it is clear that they are different from each other and from the cornea1 pigment. The cichlids as a group are highly diurnal, and at night remain immobile, usually pressed up against the bank or some other submerged object (LOWE-MCCONNELL,1969). They are also probably highly visual. Thus Astronotus ocellatus has been shown by behavioural tests to be able to resolve a visual angle of 5.3’ (WEILER, 1966) and Aequidens portalegrensis a visual angle of 5.8’ (BAERJZNJX et al., 1960). These results may be contrasted, for example, with the minimum separable angle of the minnow (Phoxinus laevis), which behavioural tests have shown to be about 10.8’ (BRUNNER,1934). Although in the present study yellow lenses and corneas were only found in cichlids, they are in fact common in other groups as well (see Mmrz, 1972 for summary). They are especially common in two other perciform groups, the wrasses (Labridae) and parrot fish (Scaridae) (MORELANDand LYTHGOE, 1968 and Table 5), and these two families are also highly diurnal with many species showing specialized methods of passing the night inactive; either by lying on their sides and burying themselves in the sand (Labridae), or by building a cocoon of mucus (Scaridae). Yellow intraocular filters would thus appear to be especially characteristic of highly diurnal teleosts, which avoid the consequences of the inevitable loss of sensitivity by remaining inactive at night.

W. R. A. Muxrz

2218

TABLE 5. TELEOST sPECrE.sFROM ALDABRA ISLAND (Imm4 OCEAN) T-T HAVE YELLOW CORNEAS OR LENSES (J.N.LY~HGoE AKD W. R. A. MUXZ, UNPUBLISHED OBSERVATIOXS~. A PLUS SIGN NDICATES THE PRESESCE OF CLEARLY VISIBLE YELLOW COLORATIOX FOR THE COR&ZAA SINGLE PLUS SIGN SHOWS THAT THE co~oua wti RESTRICTED TO THE TOP OF TH?Z CORNEA,A>D

A DOUBLE

EXTE?*DEDFURTHER DOWN,PASTTHEDORSAL

PLUS SIGN SHOWS

MARGINOFT?lE

Cornea Wrasse, Labridae Gompkosus caeruleus Lac&&de Lep~~~~o~s axiflaris Berm Lep~dapto~ kirsutus Lac&de Coris formosu Benn Coris frerei Gunther Co& angtiialu La&p&de Halichoeres centriquudrus (La&&de) Halichoeres scapuluris (Berm) Anampses caeruleopunctatus Riippell Cheilinus diagrammus (La&p&de) Thalassoma hebraicum (La&p&de) Parrot fish, Scaridae Scarus sordidus Shultz CaUyodon urbanus Smith Caliyodon viridifucatus Smith Xanothon bipallidus Smith

THAT

Lens

i-f

L

1-4 + (pale) ++ ii ff

L

++ -f-t +.f

0 0 t ;: L +

c-t+ +

-!+

0 -IO

+ + + +

4

Trigger fish, Balistidae Rhjneeanthus aculeutus (Linn.) ~inecantkus rectangulus (Schneider) Balistupus undiftutus (Mung0 Park)

+t

0 0 0

Puffer fish, Tetraodontidae Arothron citrinellus (Gunther) Arothron meleagris (Shaw)

-t-i-i-+

0 0

Sharp-nosed puffers, Canthigasteridae Cuntkiguster valentini (Bleeker)

-I-+

0

-I--i-

IT

PUPIL

It could be held that the yellow pigments of these fishes are not functional, but are merely a consequence of high light intensities causing some change in the cornea and lens. For example, the yellow pigmentation of the human lens, which increases with age, is apparently due to degradation products of proteins (McEw, 1959), and it is difficult to believe that this has any functional significance. It might be suggested that intense light causes similar changes in the eyes of fishes. On this view yellow pigments would be more common in diurnal fishes simply because such fishes are exposed to higher light intensities, and the greater depth of pigmentation at the top of the cornea would be similarly explained. The presence of three distinct yellow pigments in cichlid eyes makes this idea unlikely, however, and it is also striking that the yellow lenses start to absorb at exactly those wavelengths where the corneas starts to transmit again {e.g. Fig. 4), so that together they filter out all the short wavelength light. It is also difficult on this view to see why a fish such as Crenicichla should have a high degree of retinal pigmentation, but little colour in the lens or cornea. Finally, ChIo~o~hth~l~us agafsizi has a strongly yellow lens, but lives in the sea at depths over 400 ft, where light levels are extremely low (DEWON, 1956). It

Yellow Filters and the Absorption

of Light

2249

seems more likely therefore that the yellow pi-merits found in the eyes of fishes are functional, though it is not clear at present what this function may be. Various possibilities have been discussed at length elsewhere (e.g WALLS and JUDD, 1933; WALLS, 1942; MUNTZ, 1972). One obvious one is that they reduce the chromatic aberration of the eye, although it has been stated that this is not a serious problem in fish lenses (PUMPHREY,1961). Another possibility is that the yellow filters reduce the amount of scattered light reaching the retina. In clear water scattering is largely molecular in origin, and so would occur more strong!y at short wavelengths (Rayleigh scattering). Yellow filters, by removing much of this scattered light, would decrease the rate at which contrast decreases with distance, and hence improve the range at which objects could be seen under water. When scattering is due to particulate matter, however, it is spectrally neutral, and it is not known to what extent this is the case in the natural bodies of water inhabited by these fishes. The presence of yellow intra-ocular filters makes any attempt to correlate the visual pigments of such fishes with the spectral characteristics of the environment extremely difficult. Not only does the optical density of the yellow pigments vary between species, and probably between individuals of the same species; it also varies at different points on the cornea. The effective absorption of the visual pigments must also vary with these factors. Any attempt to correlate the spectral absorbance of fish visual pigments with the environment can therefore only be successful if it is known which fishes have yellow filters. A list of such species is given in MUNTZ (1972), and Table 5 gives a supplementary list of coral fishes, collected at Aldabra Island in the Indian Ocean, that also have yellow lenses or corneas (LYTHGOEand MUNTZ, unpublished observations). Satisfactory correlations between the visual pigments and the environment of fishes such as these will only be possible when the characteristics of their lenses and corneas are accurately known. Various kinds of yellow intra-ocular filters are found in many animals apart from fishes (MUNTZ, 1972), and similar considerations apply in these cases also. (b) Visual pigments and the environment All the results of the present study are summarized in Fig. 13. Although the visual pigments of the different species show considerable variation, there is no clear difference between fishes caught at different locations. This is perhaps not surprising, for both “white” waters and “black” waters show considerable variability in their colour, and the Lago do Peixe Boi may have had little more colour than the Lago Castanha at the time that the fish were caught. What is clear is that Amazonian fishes may be subjected to wide variations in the spectral quality of their environment. In the Rio Negro system the colour of the water varies considerably at different localities, and it may also vary greatly with time: SANTOS, SANTOSand BRINKMANN (1971), for example, report that in the Rio Preto da Eva, a “black” water tributary just below Manaus, the colour varied between 230 and 65 platinum units over a period of 8 months. In general, the presence of dissolved and suspended material in water both decreases the amount of light transmitted and results in its being of relatively longer wavelength (e.g. JAMESand BIRGE, 1938; JERLOV,1968; TALLING, 1971). Since daylength is almost constant throughout the year in these regions, this means that the amount of light at a given depth should correlate well with its spectral composition. The presence of two pigments may therefore allow fish to adapt rapidly to changes in water colour, for increasing the relative proportion of the AZ-based pigment in response to a decrease in the amount of light would Y.R.13/12--e

2250

W.

R. A.

Mm72

ELASMOBRANCHII t / 18, I $2 0 0 0

SILURIFORMES

0 , I ,:,o,:,o,

CHARACIOAE ,

0

Fro. 13. Summary of resuhs for different groups of fishes. The horizontal axis shows the wavelength of the XJO,and each symbol represents a different specimen. Square symbols show fishes with yellow corneas, and circular symbois fishes with no visible colour to the cornea. Specimens caught at the Lago Castahna shown by empty symbols, at the Lago do Peixe Boi by half-filled symbols, at Riserva Ducke by symbols containing a diagonal cross, at Manaus by the filled symbol, and from a local dealer by the symbol containing a horizontal line.

increase the relative sensitivity to long wavelengths. Although the photosensitivity of A,based pigments is onIy 70 per cent of that of A,-based pigments (DARTNALL,1968), this loss is probably considerably less than the gain resulting from the change in the pigment’s spectral absorbance. For example RELJTER (1969) has calculated that VP5232 would be 1.43 times as effective as VP502, in the clear, but highly coloured, fakes of Finland, in spite of its lower photosensitivity. It therefore seems probable that increasing the proportion of Atbased pigments in response to a decrease in light is straightforwardly adaptive in tropical waters. Such an effect of light on visual pigments is well known in a variety of fishes, appears to be possible for Aequidens tetramerus, and may well be widespread among Amazonian species. In northern and southern latitudes the daylength is not constant, and the relationship between the amount of light underwater and its spectral composition is more complicated. Increasing daylength may cut down the proportion of light reaching a given depth, as well as aiter its spectral composition, by increasing the growth of phytoplankton. Longer daylengths may then be associated with transmitted light of relatively longer wavelengths, as has been reported, for example for Lake Maggiore ~~LLEN~~ER, 1961), and if a fish in this case responded to Ionger daylengths by increasing the proportion of A,-based pigment this would appear to be, from the point of view of sensitivity, maladaptive. The possession of paired pigments may therefore confer less advantage (or even a disadvantage) on fishes from high latitudes, which may explain why they appear to be more common in tropical than temperate zone fishes (SCWWANZARA, 1967). There is no evidence that the visual pigments vary with retinal locus in any of the fishes used in the present experiments. This is unlike the situation in the rudd (Scardinius erythrophthalmus) and trout (Salmo fario and S. gairdneri), where the proportion of AZ-based pigment is greater in the ventral half of the eye, resulting in the absorption of relatively less short wavelength light by this part of the retina. In many cichlids the presence of a yellow cornea, in which the colour is denser in the dorsal half, would have a similar effect, but there is too little i~ormation at present to know whether the functional significance is the same in the two cases.

Yellow Filters and the Absorption

of Light

2251

In comparison to the cichlids and characins, the retinas of siluriforms show little variability and absorb at longer wavelengths, and it seems likely that a single pigment is involved in each case. Both SCHWANZARA (1967) and MUNTZ and NORTHMORE (1972) noted that species living deep in the water tended to be single pigment species. The freshwater sting-ray (Purutrygon ~otoro) also lives on the bottom, but the retina absorbs at much shorter wavelengths than do those of the siluriforms, though this difference may be reduced to some extent by the presence of a golden tapetum. Acknowledgemenfs-I thank the Director and Staff of the Instituto National de Pesquisas da Amazonia (INPA) at Manaus for the help and facilities they provided, DRS. ELIWBETH HONDA, HERALDSA. Barrstu and RCXXMARY LOWE-MCCONNELLfor help in identifying the fishes, and D. P. M. NORT~ORE for collaboration in the design and building of the spectrophotometer. The visit would not have been possible without the help and encouragement of Paorrsso~ EDUARD~ OSWALDCXRU~, and was supported by the Conseiho Nacionai de Pesquisas, the Royal Society and the Medical Research Council, to ail of whom I am very grateful, REFERENCES BAERENDS,G. P., BENNEMA,B. E. and V~CXLZANG,A. A. (1960). f.&r die Anderung der Sehscharfe mit dem Wachstum bei Aequidensportalegrensis (Hensel) (Pi&s, Cichlidae). 2001. Jber. Neapel. 88,67-78. BRIDGES.C. D. B. (1969). Yellow corneas in fishes. vision Res.. 9.435-436. BRIDGES;C. D. B.‘(1972). The rhodopsin-porphyropsin visual system. In Handbook of Sensory Physiology, vol. VII/l, Photochemistry of Vision (edited by DARTNALL,H. J. A.), Springer-Verlag. Berlin. BROWN, P. K. and WALD, G. (1963). Visual pigments in human and monkey retinas. Nature, Land. 200, 37-43.

BRUNNER,G. (1934). tiber die Sehscharfe der Elritze (Phoxinus Zaevis) bei venchiedenen Helligkeiten. Z. vergl. Physiol., 21,296-3 16. CRESCITELLI,F. (1972). The visual cells and visual pigments of the vertebrate eye. In Handbook of Sensory Physiology, Vol. VII/l, Photochemistry of Vision (edited by DARTNALL,H. J. A.). Springer-Verlag, Berlin. DARTNALL,H. J. A. (1952). Visual pigment 467, a photosensitive pigment present in tenth retinae. J. Physiol., Lond. 116.257-289. DARTNALL,H. J. A. (1961). Visual pigments before and after extraction from visual cells. Proc. R. Sot. B., 154,25&266.

DARTNALL.H. J. A. (1968). The photosensitivities of visual pigments in the presence of hydroxyiamine. Vision Res., 8,339-3X DARTNALL,H. J. A., LANDER,M. R. and MUNZ, F. W. (1961). Periodic changes in the visual pigment of a fish. In Progress in Photobiofogy (edited by CHRISTENSEN, B. C., and BUCHMANN,B.), Elsevier, Amsterdam. DENTON, E. J. (1955). Absorption du cristailin de Rana esculenta et d’Anguilla vulgaris. Bull. Mus. natn. Hist. nat., Paris 2’ s&ie, 27.41 S-425. DE&TON, E. J. (1956). Recherches sur l’absorption de la lumiere par le cristallin des poissons. Bull. Inst. oceanogr. Monaco 1071,1-10. DENTON, E. J. (1959) The contributions of the orientated photosensitive and other molecules to the absorption of whole retina. Proc. R. Sot. B. W&78-94. DENTON, E. J. and LAND M. F. (1967). Optical properties of the lamellae causing interference colours in animal reflectors.J. PhysioZ.,Lond. 191,23P. DENTON, E. J. and NICOL, J. A. C. (1964). The choroidal tapeta of some cartilagenous tishes (Chondrichthyes). J. mar. biol. Ass. U.K., 44,219-258. DENTON,E. J. and WARREN,F. J. (1956). Visual pigments of deep sea fish. Nature, Lond. 178,1059. DENTON.E. J. and WARREN,F. J. (1957). The photosensitive pigments in the retinae of deep-sea fish. J. mar. biol. Ass. U.K., 36,651-662. DENTON, E. J., MUNTZ. W. R. A. and NORTHMORE,D. P. M. (1971). The distribution of visual pigment within the retina in two teleosts. J. mar. biol. Ass. U.K. 51,9OS-915. G0s.s~. J.-P. (1963). Description de deux cichlides nouveaux de la region Amazonienne. Inst. R. SC. Nat. Belg.-Bull. T. 39,1-7. J~IES, H. R. and BRIDGE, E. A. (1938). A laboratory study of the absorption of light by lake waters. Trans. Wis. Acad. Sci. Arts Lett. 31, l-154. JERLOV,N. G. (1968). Optical Oceanography, Elsevier, Amsterdam. KNBPPEL,H.-A. (1970). Food of central Amazonian tishes. Contribution to the nutrient-ecology of Amazonian rain-forest-streams. Amazonia 2.257-352. LIEBMAN,P. A. (1972). Microspectrophotometry of photoreceptors. In Handbook of Sensory Physiology, Vol. VII/l, Photochemistry of Vision (edited by DARTNALL,H. J. A.), Springer-Verlag. Berlin.

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2252

LOW-E-MCCO~ELL, R. H. (1969). The chichlid fishes of Guyana, S. America, with notes on their ecology and breeding behaviour. J. Zool. Linn. Sot. 48,255-301. MCEWEX, W. K. (1959). The yellow pigment of human lenses. Am. J. Ophthal. 47, Pzo. 5 pt. II, l-11-146. MORELAND,J. D. and LYTHGOE,J. N. (1968). Yellow corneas in fishes. Vision Res. 8,1377-1380. Mu~rrz, W. R. A. (1972). Inert reflecting and absorbing pigments. In Handbook of Sensory Physiology, Vol. VII/l, Photochemistry of Vision, (edited by DARTNUL, H. J. A.), Springer-Verlag. Berlin. MUXTZ, W. R. A. and NOR-ORE, D. P. M. (1971). Visual pigments from different parts of the retina in rudd and trout. Vision Res. l&551-561. MUXZ, F. W. (1957). Photosensitive pigments from retinas of deep-sea fishes. Science, N. Y. 12.5,1142-1143. MYERS,G. S. (1947). The Amazon and its fishes. Part 1. The River. Aquar. J. l&4-9. Nrcot, J. A. C., ARZ(OTT,H. J. and BEST, A. C. G. (1973). Tapeta lucida in bony fishes (Actinopterygii): a survey. Can. J. Zool.51,69-8 1. RJMPHREY, R. J. (1961). Concerning vision. In The cell and the organism, (edited by RA~EY, I. A. and WIGGLESWORTH, V. B.), Cambridge University Press. RECTER, R. (1969). Visual pigments and ganglion ceil activity in the retinae of tadpoles and adult frogs (Rana temporuriu L.) Actu zool. Fenn. 122,1-64. REIJTER,T., WHITE, R. H. and WALD, G. (1971). Rhodopsin and porphyropsin fields in the adult bullfrog retina. J. gen. Physiol. 58,351-371. SWTOS, U. DE M., SANTOS,A. and BRINKMANN,W. L. F. (1971). A composiclo quimica do Rio P&o da Eva-Amazonia. Estudo preliminar. CiZnciu y Culturu 23.643-646. SCHWANZARA, S. A. (1967). The visual pigments of freshwater fishes. Vision Res. 7,121-148. SIOLI,H. (1967). Studies in Amazonian waters. Atlus do Simpdsio sBbre a Biotu Amuz&ticu 3,9-50. TALLMG, J. F. (1971). The underwater light climate as a controlling factor in the production ecology of freshwater phytoplankton. Mitt. Internut. Verein. theor. engew. Limnol. 19,214-243. VILLERMET,G. M. and WEALE, R. A. (1972). Age, the crystalline lens of the rudd and visual pigments. Nuture, Lond. 238,345-346.

VOLLENWEIDER, R. A. (1961). Photometric studies in inland waters: I. Relations existing in the spectral extinction of light in water. Memorie Ist. ital. Idrobiol. 13,87-l 13. WALD. G.. BROWN.P. K. and BROWN.P. S. (1957). Visual nizrnents and denths of habitat of marine fishes. Nuiure; Land. liO,969-971. . _WALLS,G. L. (1942). The Vertebrate Eye und its Adaptice Radiation. Cranbrook Inst. of Science, Michigan. WALLS,G. L. and JUDD, H. D. (1933). The intra-ocular colour-filters of vertebrates. Br. J. Ophrhul. 17,641675

; 705-725.

WEILER, I. V. (1966). Restoration of visual acuity after optic nerve section in Astronotus ocelfatus. Exp. Neural., 15,377-386. WELCH,P. S. (1948). Limnologicul methods. McGraw-Hill, New York.

Abstract-The visual pigments of twenty four species of Amazonian fishes have been studied by measuring the spectral absorbance of small areas of intact isolated retinas. In some species the absorbance of the corneas and lenses was measured as well. In 15 species measurements were made at more than one retinal location, and in no case was there any evidence that the visual pigments were different in different parts of the retina. The spectral absorbance of the retinas of cichlids and characins varied considerably between species, but these variations were not related in any clear way to the spectral characteristics of the different bodies of water from which they came. In one species (Aequidens tetramerus) however, thre specimens caught in highly coloured, moderately coloured and very clear wa:er respectively, had retinas absorbing at progressively shorter wavelengths. Evidence was obtained suggesting that this variation was due to alterations in the relative proportion of Al-based and Al-based visual pigments in the retina, in response to changes in light intensity. This effect, which has been described previously for several other species, may well be common in Amazonian fishes, and could allow rapid adaptive alterations in spectral sensitivity in response to changes in the light environment. Many of the cichlids studied had yellow corneas. The depth of the pigmentation increased towards the dorsal edge of the cornea, and varied greatly between species. Those species that had yellow corneas frequently also had yellow pigments in the lens and retina. Such yellow filtering pigments, which occur in many other fishes apart from cichlids, and also in many other groups of animals as well, make any attempt to correlate the spectral absorbance of visual pigments with the spectral characteristics of the environment very difficult. In contrast to the cichlids and characins, the retinas of siluriform fishes showed little interspecific variation. The retina of a freshwater sting-ray (Purutrygon motoro) absorbed at short

Yellow Filters and the Absorption

of Light

wavelengths compared to those of other bottom living fishes, but the tapetum of this species was golden in colour, which will shift the effective absorption of the retina to longer wavelengths.

R&sum&On etudie les pigments viseuls de 24 especes de poissons de I’Amazone en mesurant I’absorption spectrale de petites aims de rttines isol&s et intactes. Pour certaines ap&ces, on mesure aussi l’absorption de la co&e et du cristallin. Pour 15 esptces les mesures portent sur difTerentes parts de la retine et dans aucun cas les pigments visuels ne semblent differer. L’absorption spectrale des r&tines de cichlidQ et de characinidts varie beaucoup dune esp&e a I’autre sans qu’on constate de relation Claire entre ces caracteres spectraux et les eaux d’oh viennent les animaux. Dans une esp&ce cependant (Aequidens fefrumerus), trois specimens pris respectivement dam des eaux fortement color+, mod&rement color&es et tr&s claims, ont des &tines qui absorbent progressivement vers les courtes longueurs d’onde. On pense que cette variation provient d’alttrations dans les proportions relatives de pigments ayant pour bases A, et AZ, en reponse a des changements d’intensite lumineuse. Cet dfet a et6 deja decrit pour plusieun autres esp&ces, il se peut qu’il soit commun chez les poissons de l’Amazone, ce qui permettrait des adaptations rapides de la sensibilitt spectrale en reponse a des changements dans I’environnement lumineux. Beaucoup des cichlides etudib ont des corn&s jaunes. La profondeur de la pigmentation augmente vers le bord dorsal de la corn&s et varie beaucoup selon les especes. Les especesacomCes jaunes ont souvent aussi des pigments jaunes dans le cristallin et la rttine. De tels pigments jaunes filtrants, presents chez beaucoup d’autres poissons que les cichlides, et aussi dans beaucoup d’autres groupes animaux, rendent t&s difficile tout essai pour Ctablir des correlations entre I’absorption spectrale des pigments et les caracteristiques spectrales de l’environnement. Contrairement aux cichlides et aux characinidb, les poissons siluriformes presentent peu de variations dans leur rttine selon I’esp&ce. La retine dune raie d’eau deuce (Purutrygon motoro) absorbe plus aux courtes longueun d’onde par rapport a d’autres poissons vivant au fond, mais le tapis de cette esp&ce est dune couleur do&e, cequi deplace labsorption effective de la retine vers les grandes longueurs d’onde.

Zusamtnenfassung-Die Sehpigmente von 24 Amazonas-Fischarten wurden untenucht, idnem die spektrale Absorption kleiner FlZichen der intakten, isolienen Retina gemessen wurde. Dariiberhinaus wurde bei einigen Arten die Absorption von Homhaut und Linse gemessen. Bei 15 Arten wurde an mehr als einem Netzhautort gemessen. Dabei ergab sich kein Hinweis darauf, dass Sehpigmente venchiedener Netzhautorte untenchiedlich sind. Die spektrale Absorption von Buntbarschen (cichlidae) und Salmlern (characidae) lnderte sich auffallend von Art zu Art; doch hingen diese Abweichungen nicht klar erkennbar von den spektralen Eigenschaften des jeweiligen Wassers ab, in dem die Arten lebten. Eine Art, die Grlinglanzbuntbarsche(Aequidens tetramerus), die in stark gefarbtem, mittel geflarbtem bzw. sehr klaren Wasser gefangen w-urden, zeigte eine retinale Absorption, die zu ktirzeren Wellenllngen hin zunahm. Es wurde der Nachweis geftihrt, dass diese Variation aufdie d;nderung irn delativen Anteil an At-und A,-Sehpigment der Netzhaut zuriickzufiihren war, die sich als Reaktion auf die wechselnde Lichtintensitlt einstellte. Dieser Effrekt, der schon friiher fur andere Arten beschrieben wurde, dlirfte bei Amazonasfischen normal sein und eine rasche Anpassung auf unterschiedlich stark einfallendes Licht erlauben. Viele Buntbarsche besassen eine gelbe Hornhaut. Die Tiefe der Pigmentierung nahm zum dorsalen Ende der Homhaut hin zu und war bei den einzelnen Arten sehr unterschiedlich. Diejenigen .&ten, die eine gelbe Homhaut besassen, hatten h&fig such gelbe Pigmente in Lime und Netzhaut. Diese gelbtiltemden Pigmente, die such bei vielen anderen Fischen unabh;ingig von den Buntbarschen genau wie bei anderen Lebewesen auftreten, machen es sehr schwer, die spektrale Absorption da Sehpigmentes mit spektralen Eigenschaften der Umgebung zu korrelieren. Im Gegensatz zu den Buntbarschen und Salmlem zeigte die Netzhaut der welsartigen Fische nur geringe Unterschiede zwischen den einzelnen Arten. Die Netzhaut des SugwasserStechrochens absorbierte bei ktirzeren Wellenl~gen als die anderer auf dem Grund Iebender Fische, aber das Tapetum dieser Art war goldfarben, so dass sich die effektive Absorption der Netzhaut zu lringeren Wellenllngen hin verschob.

2253

2254

W. R. A. MUNTZ

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