タイ産カタクチイワシの網膜反射板 Retinal Tapetum

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1 タイ産カタクチイワシの網膜反射板 Retinal Tapetum 誌名 水産増殖 = The aquiculture ISSN 著者 Awaiwanont, K. 巻 / 号 49 巻 1 号 掲載ページ p 発行年月 2001 年 3 月 農林水産省農林水産技術会議事務局筑波産学連携支援センター Tsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research Council Secretariat

2 SUISANZOSHOKU Retinal Tapetum in Three Anchovies (Encrasicholina heteroloba, E. devisi and E. punctifer) from Thai Waters Kamonpan AWAIWANONT 1 Accepted February 8, 2001 Abstract: The histology of retinas of three anchovies, namely shorthead anchovy (Encrasicholina heteroloba), Devis anchovy (E. devisi), and Buccaneer anchovy (E. punctifer) from Thai waters, were examined under light microscopy and scanning electron microscopy. Their retinas were from a light-adapted state. The visual cell layers contained cones and rods as well as apical processes of the pigment epithelium cells. The single layer of cones was arranged in parallel rows. The outer segments of the cone cells were enrolled by the retinal tapetum. Rods were stacked and densely covered by the retinal tapetum of pigment epithelial cells. Three types of tapetum could be recognized; platelets, needles, and crystallites. This retinal tapetum should subserve their visual systems, to become highly adapted for the dim light environment. Key Words: Retinal tapetum; Visual cells; Anchovy; Dim light; Thai waters Introduction The retinal tapetum is a reflecting layer which returns light transmitted through the retina to the visual cells, and provides these photoreceptor cells with a second opportunity for stimulation, thereby enhancing retinal sensitivity 1-3). The tapeta of most teleosts are found in the pigment epithelial cells possessing long processes, which extend vitread and contain reflecting materials 3). Among anchovies the retinal photoreceptor cells and tapeta were described in the Bay anchovy (Anchoa mitchilli), the Striped anchovy (Anchoa hepsetus) 4-6), the Black Sea anchovy (Engraulis encrasicholus) 7), as well as in the Japanese anchovy (Engraulis japonica) 2. Fishes that possess the retinal tapetum are believed to inhabit a dimly lit environment 2). Anchovies are pelagic fish that generally inhabit the shallow water along the coastal area. In Thai waters, light fishing by purse seine, lift net and cover net are used for catching them. Nowadays, this kind of fishing gear has been in conflict with the other small scale fisheries and also has resulted in over fishing by unlimited light use, small mesh size of the net, and so on. This study was conducted to understand the visual system of the anchovies found in Thai waters. It might be applied to the improvement of the light fishing, such as control of the light source, avoidance of an incidental catch, etc. There is no detailed information on the retinal structure, particularly the retinal tapetum of those anchovies. Therefore, in the present study the retinas of three anchovies, Encrasicholina heteroloba, E. devisi and E. punctifer were examined with light and scanning electron microscopy. Materials and methods Three species of anchovies were collected at night with purse seines from Thai waters. From each species, 5 specimens were used for the light (LM) and scanning electron microscopy (SEM) observations. The eyes were dissected and fixed in Bouin s solution for LM. After dehydration in an alcohol series until 70%, the speci- 1 Faculty of Fisheries, Kagoshima University, Shimoarata, Kagoshima , Japan. 2 Awaiwanont, K., W. Gunarso, M. Sameshima, S. Hayashi, and G. Kawamura, unpublished data.

3 K. Awaiwanont mens were preserved and brought to Japan. Following further dehydration, the specimens were embedded in paraffin, sectioned into 4 and 6 m thick slices, and stained with Mayer s haematoxylin-eosin. For SEM, the enucleated eyes were fixed in a 2.5% glutaraldehyde-0.1m cacodylate buffer at ph 7.4, postfixed in 1% osmium tetroxide-0.1m cacodylate buffer, and then dehydrated in an alcohol series. The specimens were dried later by a critical point dryer. The retina was then fractured into small pieces, fixed vertically on a sample stage with platinum iridium paste, and sputter-coated with Pt/Pd using a Hitachi E ion sputter. Finally, the specimens were examined with a scanning electron microscope, Hitachi S-430, at 15 KV. The shorthead anchovy, the Devis anchovy, and the Buccaneer anchovy are pelagic fishes, which generally reside along the coastal area (Fig. 1). The pattern of their retinal structure is similar to that found in the other teleost species. Where the photoreceptor cells, cones and rods are distributed throughout the retina as well as the pigment epithelial cells. The retinal specimen of the shorthead anchovy was from a light-adapted state. The outer limiting membrane was lined with a long single layer of cones. Above this layer of cones, the rods were stacked and they were densely covered by the retinal tapetum of pigment epithelial cells. They were difficult to observe in the photographs for this reason (Figs. 2a, 3a). The reflecting particles in the processes of the pigment epithelial cells were oriented and arranged in orderly rows (Figs. 3a, 3b). They always enrolled the outer segment of the cone cells. This retinal tapetum was found in three distinct forms: platelets, crystallites, and needles (Fig. 3b). The platelet tapetum (about 0.9 m in width) was confined to the vitread region of the pigment epithelial cells, forming two clearly defined stacks arranged in the form of a V inserted into the truncate areas, where they fitted between the parallel cone units. The crystallite tapetum (about 0.25 m in width) was the smallest tapetal component and the most variable in shape; it spread over the pigment epithelial cells. The needle tapetum (about 0.3 m in width) occurred mostly in the vitread region of the pigment epithelial cell. The retinal structure of the Devis anchovy was similar to that of the shorthead anchovy. The retinal specimen was obtained from a lightadapted fish (Fig. 2b). The long single layer of cones was located close to the outer limiting membrane. The rods were stacked above the cones, covered densely by the retinal tapetum and the pigment epithelial cells. The reflecting Results Anchovies from Thai waters, the shorthead anchovy (Encrasicholina heteroloba), the Devis anchovy (E. devisi), and the Buccaneer anchovy (E. punctifer).

4 Transverse section of the retina of shorthead anchovy ( ), Devis anchovy ( ), and Buccaneer anchovy ( ). choroid, cone, rod, retinal tapetum, outer limiting membrane 20. particles, which enrolled the outer segment of the cone cells, were oriented and arranged in orderly rows. Three distinct types of retinal tapetum forms were observed: platelets, crystallites, and needles (Fig. 4a). The retinal specimen of the Buccaneer anchovy was from a light-adapted fish. The structure of the retina observed was also similar to those of the other two species previously examined, with a single layer of cones located close to the outer limiting membrane (Fig. 2c). The rods, including ellipsoides and outer segments, extended above the cones, and were densely covered by the retinal tapetum and pigment epithelial cells. The outer segments of the cone cells were oriented and arranged in orderly rows, which were enrolled by the reflecting particles (Fig. 4b). These were also identified as platelets, crystallites, and needles (Fig. 4c). Discussion In the present study the author investigated the retinal characteristic, particularly the retinal tapetum of three anchovies from Thai waters. These retinas showed some unusual features compared with those of the coastal pelagic fishes. Generally, the photoreceptor cells of other coastal pelagic fishes are composed of cone and rod cells 2). It is unlikely for them to present the tapetum while light is available. Unusual features or specialization of the retinas that enhance scotopic vision have been found in the Scanning electron micrograph showing the parallel arrangement of the cones ( ) and the tapetal materials ( ) of the shorthead anchovy. cone, cone outer segment, outer limiting membrane, retinal tapetum. platelet type, crystallite type, needle type.

5 K. Awaiwanont Scanning electron micrograph showing the tapetal materials of the Devis anchovy ( ) the arrangement of cone cells ( ) and the tapetal materials ( ) of the Buccaneer anchovy. cone, cone outer segment, retinal tapetum, platelet type, crystallite type, needle type. retinal tapetum 8). The retinal tapetum increases retinal sensitivity by reflecting light back onto the photoreceptors 2,9). Some teleost species, inhabiting either turbid or deep-sea waters where the level of illumination is low, possess a prominent retinal tapetum 10). The retinal tapetum, or eyeshine, lies in the choroid or in processes of the pigment epithelial cells and occurs in many animals such as in some reptiles and bats 11-12). It occurs also in fishes inhabiting three different environments: deep oceanic waters, coastal waters, and fresh water rivers and ponds, which are places of low levels of illumination, particularly in the coastal and fresh waters of warm and tropical regions 13). Among anchovies, the structure and function of the cones was investigated in the Black Sea anchovy, Engraulis encrasicholus. The structure of their outer segment was a specific characteristic common to three species of the Engraulidae 7), and it is believed that they can perceive differences in the direction of polarized light as well as differences in color. In the Japanese anchovy, Engraulis japonica, the grouped-stacked rods and retinal tapetum were observed 2. The structure and arrangement were similar to those of other anchovies, such as E. mordax, Anchoa compressa, A. mitchilli, and A. hepsetus 4-6,14-15). Interestingly, pure-rod retinae and grouped-stacked rods are more commonly found in deep-sea fishes, which include about 28 families of meso-pelagic and deep-sea fishes 2,16). It is probable that the presence of the grouped rods in these fish is associated with the subdued light environment. Based on Munz s 17) investigation of rod absorption spectra, O Connell 15) suggested that such morphology of the rods is adapted for vision in the dim light of turbid coastal waters rather than the dim light of deeper, but clear, oceanic water. A. compressa is one of several teleosts that has the rod pigments indicating absorption maxima near the green-yellow portion. This spectrum corresponds to the range of maximal light transmission in turbid water. In contrast, a number of deep-sea fish have an absorption maximum in the blue portion, which is known to be the range of maximal light transmission in clear oceanic water 18). Three types of tapetal components occurred in the pigment epithelial cells of the anchovies from Thai waters. They were classified as platelets, needles, and crystallites according to Fineran and Nicol 5). The main function of the

6 tapetum is thought to be the provision of a light reflecting system. The reflected light will provide the light sensitive cells with a second opportunity, thereby enhancing retinal sensitivity 1,5). This second passage through the photoreceptor cells allows more light to be absorbed and seems to be effective in promoting vision in dim light conditions. The crystallite and needle types of tapetum probably serve as diffuse reflectors, whereas the platelet type acts more as spectral reflectors. From the association of tapetal platelets with the outer segment of cone cells (Figs. 3, 4), it is presumed that the main function of their stacks is to provide a reflector system to direct light back into the cones, while shielding the highly sensitive rods from the strong light. The shorthead anchovy, the Devis anchovy, and the Buccaneer anchovy are pelagic fish from Thai waters. Light fishing by purse seine, lift net and cover net are used for their capture. From this study, it is obviously that their visual systems are highly adapted for the dim environment. Therefore, these findings might be applied for the conservative management of Thailand s coastal light fishing. Acknowledgement The author would like to thank Dr. Munefumi Sameshima, School of Medicine, Kagoshima University, Dr. Gunzo Kawamura and Dr. Miguel Federico Vazquez Archdale, Faculty of Fisheries, Kagoshima University, for critically reading the manuscript. This research was funded by The Ministry of Education, Culture, Sports, Science and Technology of Japan. References 1 Duke-Elder, S. S. (1958): The eyes of fishes. In: The Eye in Evolution. Henry Kimpton, London, pp Walls, G. L. (1967): The vertebrate retina. In: The Vertebrate Eye and Its Adaptive Radiation. Hafner, New York London, pp Zyznar, E. (1975): Tapeta lucida and the organization of visual cells in teleosts. In: Vision in Fishes. Plenum, New York London, pp Feneran, B. A. and J. A. C. Nicol (1976): Novel cones in the retina of the anchovy (Anchoa). J. Ultrastruct. Res.,, Fineran, B. A. and J. A. C. Nicol (1977): Studies on the eyes of anchovies Anchoa mitchilli and A. hepsetus with particular reference to the pigment epithelium. Phil. Trans. R. Soc. Lond. B, Fineran, B. A. and J. A. C. Nicol (1978): Studies on the photoreceptors of Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the cones. Phil. Trans. R. Soc. Lond. B, Zueva, L. V. (1981): Retinal cones of the Black Sea anchovy Engraulis encrasicholus. An analyzer of polarized light in vertebrates. J. Evol. Biochem. Physiol.,, Vandenbyllaardt, L., F. J. Ward, C. R. Braekevelt, and D. B. Mcintyre (1991): Relationships between turbidity, piscivory, and development of the retina in juvenile walleyes. Trans. Am. Fisher. Soc.,, Braekevelt, C. R. (1982): Fine structure of the retinal epithelium and retinal tapetum lucidum of the goldeye (Hiodon alosoides). Anatomy and Embryology,, Braekevelt, C. R., D. B. McIntyre, and F. J. Ward. (1989): Development of the retinal tapetum lucidum of the walleye (Stizostedion vitreum vitreum). Histology and Histopathology,, Walls, G. L. (1967): The tapetum lucidum. In: The Vertebrate Eye and Its Adaptive Radiation. Hafner, New York London, pp Pirie, A. (1966): The chemistry and structure of the tapetum lucidum in animals. In: Graham-Jones, O. (ed.) Aspects of comparative ophthalmology. Pergamon Press, Oxford, pp Nicol, J. A. C., H. J. Arnott, and A. C. G. Best (1973): Tapeta lucida in bony fishes (Actinopterygii); a survey. Can. J. Zool.,, Moore, G. A. (1944): The retinae of two North American teleosts with special reference to their tapeta lucida. J. Comp. Neurol,, O Connell, C. P. (1963): The structure of the eye of Sardinops caerulea, Engraulis mordax and four other pelagic marine teleosts. J. Morph,, Ali, M. A. and M. Anctil (1976): Retina of Fishes. Springer, Berlin Heiderberg New York, pp Munz, F. W. (1958): Photosensitive pigments from the retinae of certain deep-sea fishes. J. Gen Physiol. (Lond.),, Kobayashi, H. (1962): A comparative study on electroretinograms in fish, with special reference to ecological aspects. J. Shimonoseki Coll. Fish,,

7 K. Awaiwanont 3 Encrasicholina heteroloba E. devisi E. punctifer retinal tapetumretinal tapetum platelet needle crystallite 3 retinal tapetum