VISUAL PIGMENTS IN A TROPICAL FRESHWATER FISH ETROPLUS MACULATUS (TELEOSTEI)

Transcription

1 J. Exp. Biol. (1967), 47, With 1 text-figure Printed in Great Britain VISUAL PIGMENTS IN A TROPICAL FRESHWATER FISH ETROPLUS MACULATUS (TELEOSTEI) BY V. VIRABHADRACHARI, R. V. KRISHNAMOORTHY AND V. PARVATHESWARARAO Department of Zoology, Sri Venkateswara University, Tirupati, India (Received 10 April 1967) INTRODUCTION The occurrence of porphyropsin as the rod pigment in the freshwater fish and rhodopsin in the marine ones and a shift in the predominance from porphyropsin to rhodopsin in a freshwater fish on migration into a marine environment and from rhodopsin to porphyropsin in a marine fish on migration into a freshwater environment have been known for a long time (Wald, 1958, 1960, 1963). While this is so in natural phenomena like migration, it is not yet known whether a similar shift in the pigment predominance would occur when a strictly freshwater euryhaline teleost, belonging to the primary category of Darlington (1957), is acclimated to a saline medium in the laboratory. Hence it was considered desirable to undertake a study on these lines. Etroplus maculatus is a very common freshwater teleost occurring in and around Tirupati, and it fits into the primary category of Darlington (1957). Its ability to withstand transfer and metabolic acclimation to saline media with considerable ease has already been demonstrated (Pampapathi Rao, 1958; Parvatheswararao, 1967a). Consequently this fish was considered a suitable material for the present study. MATERIAL AND METHODS The procurement and stocking of the fish was as described by Parvatheswararao (1965). The fish were acclimated to 100% sea water in the laboratory as per the procedure described by Virabhadrachari (1961). They were fed daily with cooked rice and pieces of earthworm. The methods suggested by Crescitelli (1956a, b) were employed for the extraction and analysis of the retinal pigments.the fish were darkadapted for at least 3 hr. prior to the extraction of the pigment. Six to eight retinae were pooled for each estimation. The eyes of the fish were dissected out and kept in 4 % potassium alum solution for min. Later the cornea of each eyeball was punctured so as to expose the retina to the alum solution and the eye was left in this condition in the alum solution overnight. The thus hardened retinae were washed repeatedly in distilled water and borate-kcl buffer at ph The pigment was then extracted into 2 % digitonin in alkaline borate-kcl buffer using a glass homogenizer 0-5 ml. digitonin solution was used for each retina. The homogenate was centrifuged at 4000 rev./min. for about 30 min. The optical density of these unbleached digitonin extracts was measured in a Hilger-Watt UVISPEK spectrophotometer using a micro-

2 V. VlRABHADRACHARI AND OTHERS cell. The entire procedure was carried out in a dark room under deep red light. Simultaneously the optical density of the bleached retinal extracts was also measured to ascertain its photosensitive property. RESULTS The retinal extract from the fish maintained in the freshwater medium showed an absorption maximum at 515 mp, and that from the fish acclimated to 100% sea water, m//(100% sea water) 515 mft (fresh water) 014 a 1 a, O Wavelength (m/*) Fig. 1. Absorption curves of the visual pigment of E. maculatus from fresh water and from 100% sea water., Freshwater medium; O O, 100% sea water medium. a maximum at 495 m/i (Fig. 1), Hence it is concluded that the visual pigment was porphyropsin in fish from fresh water and rhodopsin in fish acclimated to 100 % sea water. According to the suggested nomenclature of Munz & Beatty (1965) these pigments are VP 5152 and VP 495 x respectively. The retinal extract was found to be photosensitive. In a bleached condition its absorption was found to decrease. Further, the absorption decreased with increased time of bleaching (Table 1). Hence it is concluded that the retinal extract contained visual pigment.

3 Visual pigments 309 In fish acclimated to 50 % sea water the absorption maximum was highly variable and ranged between 495 and 515 m/i. This might represent a transition between the freshwater phase and the 100% sea water phase in the visual pigment predominance (Table 2). Table 1. Photosensitivity of the visual pigment extracted from the dark-adapted freshwater Etroplus maculatus. (The extracts from 12 retinae were pooled.) Time (in min.) of bleaching in 100W. source 0 S Optical density of the extract at 515 m/» OIIO Table 2. Absorption maxima of the visual pigment 0/Etroplus maculatus, acclimated to media of different concentrations Absorption maximum Acclimation medium No. of estimations (m/t) Freshwater % sea water % sea water DISCUSSION The controversy over the nomenclature of the visual pigments is well known. Porphyropsin and rhodopsin were suggested earlier as appropriate names to the visual pigments, believed to originate from the retinal rods (Wald, 1939a, b; 1955). This, however, has been disputed in recent times on highly convincing grounds and a less questionable scheme of nomenclature, based on criteria like the characteristic absorption maximum of the pigment and its chromophore constituent, has been suggested (Munz & Beatty, 1965; Beatty, 1966). The visual pigment of E. maculatus was VP 5152 in fresh water and changed to VP 495i on acclimation to 100% sea water and thus this forms an interesting record to demonstrate that in the strictly freshwater euryhaline teleosts a change in the visual pigment occurs on laboratory acclimation to osmotic stress exactly as it happens in the migratory euryhaline teleosts in the course of their natural migrations. The photosensitive nature of the substance involved in this change clearly indicates that it is a visual pigment. It appears that E. maculatus has a mixture of the two pigments, VP 5154 and VP 495 Xl in both media, VP 5152 predominating over VP 495! in fresh water and the other way about in 100% sea water. This shift in the predominance, consequent upon a change in the medium, is evidenced by the variable but transitory nature of the absorption maxima of the retinal extract from fish acclimated to 50 % sea water. This is comparable to the transitory phase in the visual pigment system noticed in the metamorphosing tadpole of the bullfrog, Rana catesbiana (Wald, 1946). The generalization

4 3IO V. VlRABHADRACHARI AND OTHERS of Wald (1953) that euryhaline fishes usually have mixtures of porphyropsin and rhodopsin, the predominant pigment being the one characteristic of the spawning environment is thus in a sense strengthened by the present results. It is of interest that the shift in the pigment predominance, known to occur in the natural cyclic phenomena like migration of the migratory euryhaline teleosts, should also occur in a nonmigratory euryhaline strictly freshwater teleost like E. maculatus, when acclimated to 100 % sea water in the laboratory, despite the fact that it is never exposed to saline media under natural conditions and thus fits into the primary category of Darlington (1957). However, it is not surprising in the lightof recorded exceptions that changes in the visual pigment are not necessarily restricted to the migratory or metamorphosing species only but can as well occur in others (Beatty, 1966). Further, many strictly freshwater species are known to possess mixtures of porphyropsin and rhodopsin, the latter even predominating. Instead of regarding these results as mere exceptions, it is more appropriate to incorporate them in a reconsideration of Wald's theory of visual pigment distribution in the teleosts, which suggests a correlation between the habitat salinity and visual pigment. Perhaps habitat salinity per se may not be the exclusive criterion in deciding the type of visual pigment a fish should possess (see also Schwanzara, 1967). It has actually been shown in a freshwater teleost. Scardinius erythropthalmus, that manipulations of the photoperiod in the laboratory brought about changes in the relative proportions of the visual pigments. (Dartnall, 1962; Beatty, 1966). This problem, as related to the teleost fishes, is discussed elsewhere (Parvatheswararao, 19676). SUMMARY 1. In its natural habitat (fresh water) E. maculatus had VP 5152 (porphyropsin) as the predominant visual pigment. 2. On acclimation to 100% sea water VP495 X (rhodopsin) was found to be predominant. 3. On acclimation to 50 % sea water the visual pigment system of the fish presented a transitory phase. 4. The significance and implications of these results in relation to the earlier work is discussed. We are beholden to Dr F. W. Munz of the Biology Department, University of Oregon at Eugene, for his critical comments, and to Dr W. S. Hoar of the Zoology Department, University of British Columbia at Vancouver, for his encouraging interest in the work. REFERENCES BEATTY, D. D. (1966). A study of the succession of visual pigments in pacific salmon (Oncorkynchus). Can. J. ZooL 44, CRESCTTELIJ, F. (1956a). The nature of lamprey visual pigment. J. gen. Pkytiol. 39, CBESCITELIJ, F. (19564). The nature of gecko visual pigment. J. gen. PhytioL 40, DARLINGTON, P. J. (1957). Zoogeography: the Geographic Distribution of Animals. New York: John Wiley and Son. DABTNALL, H. J. A. (1962). The photobiology of visual processes. In The Eye, vol. 11, pp Ed. H. Davson. New York: Academic press. MUNZ, F. W. & BEATTY, D. D. (1965). A critical analysis of the visual pigments of salmon and trout. Vision Res. 5, 1-17.

5 Visual pigments 311 PAMPAPATHI RAO, K. (1958). Salinity tolerance of Etroplus maculatus (Block). Curr. Set. 37, 99. PARVATHESWAHAHAO, V. (1965). Influence of different temperature-salinity combinations on the oxygen consumption in the fresh water fish, Etroplus maculatus (Teleostei). Helgolander toiss. Meeresunters. 13, PARVATHESWARARAO, V. (1967 a). Some mechanisms underlying salinity acclimation in a freshwater fish, Etroplus maculatus (Teleostei). Marine Biology (in the Press). PARVATHESWARARAO, V. (19676). On the problem of distribution of visual pigments in teleost fishes a new approach. Natunoissenschaften. (Communicated.) SCHWANZARA, S. A. (1967). Visual pigments of tropical freshwater fishes. Life Sciences 6, VrRABHADRACHARi, V. (1961). Structural changes in the gills, intestine and kidney of Etroplus maculatus (Teleosti), adapted to different salinities. Q. Jl. microsc. Sci. 103, WALD, G. (1939a). On the distribution of vitamins A t and A,. J. gen. Pkysiol. 33, WALD, G. (19396). The porphyropain visual system. J. gen. Pkysiol. 33, WALD, G. (1946). Evolution of visual pigments. J. gen. Pkysiol. 30, WALD, G. (1953). The biochemistry of vision. Ann. Rev. Biochem. 33, WALD, G. (1955). The photoreceptor process m vision. Am. J. Ophthalmol. 40, WALD, G. (1958). The significance of vertebrate metamorphosis. Science, N.Y. 138, WALD, G. (i960). The distribution and evolution of visual systems. In Comparative Biochemistry, vol. I, ed. M. Florkin and M. S. Mason. New York and London; Academic Press. WALD, G. (1963). Phylogeny and ontogeny at the molecular level. In Evolutionary Biochemistry, ed. A. I. Oparin. London: Pergamon.