- Email: [email protected]
Cones in the retina of the Lemon Shark (Negaprion brevirostris)
Vision
Rex.
Vol.
3, pp. 397-399.
CONES
Pergamon Press1963. Primed
IN THE
RETINA
(NEGAPRION
in Great
Britain
OF THE
LEMON
SHARK’
BREVIROSTRIS) S. H. GRUBER~
Institute of Marine Science, University of Miami, Florida
D. H. HAMASAKI~ and
C. D. B. BRIDGES~
Bascom Palmer Eye Institute, University of Miami, Florida (Received
31 Auglrsf 1963)
Abstract-Sections of the retinae from six Lemon Sharks, Negaprion bresirosfrir, have been ‘examined. These shpw that the Lemon Shark retina contains rods and cones, the former preponderating in the ratio of abbut 12 : 1. R&m&-On a examink les r&ines de six Requins Citrons. Negaprion brevirostris. Ces sections montrent ti la fois des bltonnets et des cbnes, les premiers ttant les plus nombreux dans le rapport de 12 : 1 environ. Zusammenfassung-Schnitte der Retina von sechs Zitronenhaifischen (Negaprion breviroswi.0 wurden untersucht. Sie zeigen, dass die Netzhaut des Zitronenhailisches Stsbchen und Zapfen enthAt, wobei die ersteren im VerhPltnis von etwa 12 : I iiberwiegen.
that several elasmobranch genera (Mustefus, Myliobntis, possess cones, it is recognized that a very large number of elasmobranchs appear only to possess rods (ROCHON-DUVIGNEAUD, 1943). WALLS (1942) considers that the elasmobranch retina is characteristically pure rod, a point of view which has been recently strengthened by GILBERT’S (1961) finding that cones were absent in fifteen species of sharks. Amongst the sharks investigated by Gilbert were specimens of the genus Negaprion. At the beginning of an investigation concerning vision in Negaprion brevirostris, the present authors found it necessary to make a histological examination of its retina. As a result of this work, it is concluded that the Lemon Shark retina contains cones as well as rods, the latter preponderating in the ratio of about 12: 1. Retinae from five young Lemon Sharks (about 0.5-l-O m in length) and one older shark (3-O m in length) were studied in the present work. Three sharks were light-adapted (two of these were kept for 8 hr in a sunlit glass aquarium and one was taken from the sea during daytime): these were enucleated in daylight. Two others were dark-adapted for 9 hr and then enucleated under the illumination of a dim red photographic safelight. The remaining shark was killed (in daylight) and enucleated 2-3 hr later. Subsequently all the eyes were treated in an essentially similar fashion; i.e. they were placed in Kolmer’s fluid (POLYAK, 1941) for 1-2 hr, then the anterior segment and lens were removed and the opened optic cups ALTHOUGH it has been reported
Squatina and Lamna)
1 Contribution No. 292 was suooorted in .oart bv- the Office of Naval Research Contract No. 840 .. (19 quote sharks). 2 Supported in part by Research to Prevent Blindness, Inc.; National Council to Combat Blindness, Inc.; and the National Institutes of Health. National Institute of Neurological Diseases and Blindness (&2B-5277C3), Department of Health, Education and Welfare, Bethesda, Maryland. 3 Supported in part by National Council to Combat Blindness, Inc. (#G-268), and National Science Foundation (#GB-186). 397
398
S.
H.
GRUBER.
D. H. HAMASAKIAND C. D.
B. BRIDGES
were allowed to l%xfor at least 24 hr in fresh Kolmer’s fluid. Routine paraffin sections of 4-j ,Dthickness were prepared, some cut vertically (i.e. plane-parallel to the antero-posterior axis of the eye) and others tangentially to the surface of the retina. Although several stains were used, it was found that Mallory’s triple stain gave the best differentiation of the visual cells. Two types of visual cell were identified in all the retinae examined. These differed in structure, shape and staining properties. Fig. la illustrates a typical vertical section. The more numerous visual cells, considered here to be rods, were tall and cylindrical, extending from the external limiting membrane into the pigment epithelium. The average length of the rods was 3 1 p, their uniformly staining yellow-orange outer segments contributing about 16 1~ to that figure. The diameters averaged 2.6 I(: for the inner segment and 2.3 ,u for the outer segment (these figures are the mean of fifty determinations on separate receptors). Except in the region of the characteristically strongly-staining ellipsoid, the inner segment was pale blue in color and finely granular in texture. In contrast to the rods, the other visual cells, designated here as the cones, were short and pyramidal in shape. In length, these cells averaged 19 p. Unlike those of the rods, the non-staining, tapering cone outer segments did not extend as far as the pigment epithelium, and were only 8 ,D in length. Their diameters were 1.5 ,u at their midpoints. The inner segments, which terminated at the level of the inner margin of the rod ellipsoids, consisted of the outer strongly-staining ellipsoids and the inner poorly-staining and coarsely granular paraboloids (Fig. 1a). The staining properties of the paraboloid conferred a characteristic appearance on the inner segments of the cones, thus making them easily distinguishable from the rods. At the widest point, the cone inner segment measured 4.7 ,D in diameter. Owing to the different dimensions of the rod and cone inner segments, a tangential section of the retina at the level of the cone ellipsoids would be expected to cut the rods at the pale-staining region of their inner segments. Fig. 1b illustrates such a section. In this figure, the larger heavily-stained structures represent cross-sections of the cone ellipsoids; they are about 1.5 times the diameter of the paler structures which represent the lower region of the rod inner segments. From sections such as this the rod : cone ratio may be easily calculated. Thus, in those sections of the retina obtained from the posterior pole of the eye twelve rods were found for each cone present. In one important respect the cones of the Lemon Shark differed from most other vertebrate cones: their nuclei were indistinguishable from those of the rods. In this connection the cones resemble those of another shark, Lamna (ROCHON-DUVIGNEAUD, 1943). No Comparison of the retinae from lightoil-droplets were seen in either rods or cones. adapted and dark-adapted sharks revealed no evidence of photomechanical movements, neither did there appear to be any difference in staining properties corresponding to these different states of adaptation. Apart from obvious post mortem changes, the retinae from the shark which had been dead for 2-3 hr prior to enucleation appeared similar to those of the other five sharks. The rod : cone ratio was about the same as for the other retinae, indicating that the cones of the present work cannot be identified as post mortem artefacts. In those elasmobranchs which possess cones, the difference between the rods and cones may be more or less pronounced from species to species. Thus, according to WALLS (1942), although the cones are distinct from the rods in Mustelus canis, they are more so in M. mustelus and completely cone-like in Myliobatis. Walls considers that modem elasmobranch cones are “new”, secondary derivatives of rods. If this view is correct, examination of the
la. Vertical section of the Lemon Shark retina (oil-immersion. Maltory’s triple stain). Inset, left, represents tine tracings of a typicrtl rod and cone triken from this photograph. p.e.-pigment e~~t~leli~~~~,l.r.f.---fayer of rods and cones, c.l.m--eternal Iimiting membrxnc, FIG.
FIG;.
I b.
o.n.l.-outer
wclear
Tangentin
section
law +
of the
. r.--rod
Lemon
(oil-immersion,
1 c .-cone,
Shark Mallory’s
e.--ellipsoid.
retina triple
at the stain).
p.-paraboloid.
level of the cone ellipsoids
Cones in the Retina of the Lemon Shark
399
eyes from many elasmobranch species might reveal the presence of cones in various stages of transition. Work at present continuing shows that at least three other sharks (Carcharhinus springeri, C. _falciformis and Sph~rna nrokarran) possess cones which are morphologically similar to those of the Lemon Shark. On the other hand, the cones of the sting ray, Da.syatis americana, also examined in the present work, are typically cone-like in appearance. In conclusion, the authors would like to emphasize that the above findings are morphological in nature. It is not permissible to suppose that these cones differ functionally from the rods. Similarly, these findings cannot be used to assume the existence of color vision in the Lemon Shark. Acknowle&emenrs.-Acknowledgemenr is due to Dr. W. \VISBY for his encouragement: also to Mrs. A. of the Bascom Palmer Eye Institute for her valuable assistance in the histological preparations; also to the Seaquarium, Miami, for supplying a large specimen of the Lemon Shark. IOANNIDES
REFERENCES GILBERT,
P. (1961). Abstracts of symposium, papers, Tenth Pacific Science Congress, Honolulu, Hawaii, pp. 176177. POLYAK, S. L. (1941). The R&m. University of Chicago Press, Chicago. ROCHON-DUVIGNEAUD, A. (1933). Les _veuxet la vision cfes wrre%rPs. Masson, Paris. WALLS, G. L. (1942). The verrebrute eye and ifs adaptive mdiurion. Cranbrook Press, IMichigan.
Rex.
Vol.
3, pp. 397-399.
CONES
Pergamon Press1963. Primed
IN THE
RETINA
(NEGAPRION
in Great
Britain
OF THE
LEMON
SHARK’
BREVIROSTRIS) S. H. GRUBER~
Institute of Marine Science, University of Miami, Florida
D. H. HAMASAKI~ and
C. D. B. BRIDGES~
Bascom Palmer Eye Institute, University of Miami, Florida (Received
31 Auglrsf 1963)
Abstract-Sections of the retinae from six Lemon Sharks, Negaprion bresirosfrir, have been ‘examined. These shpw that the Lemon Shark retina contains rods and cones, the former preponderating in the ratio of abbut 12 : 1. R&m&-On a examink les r&ines de six Requins Citrons. Negaprion brevirostris. Ces sections montrent ti la fois des bltonnets et des cbnes, les premiers ttant les plus nombreux dans le rapport de 12 : 1 environ. Zusammenfassung-Schnitte der Retina von sechs Zitronenhaifischen (Negaprion breviroswi.0 wurden untersucht. Sie zeigen, dass die Netzhaut des Zitronenhailisches Stsbchen und Zapfen enthAt, wobei die ersteren im VerhPltnis von etwa 12 : I iiberwiegen.
that several elasmobranch genera (Mustefus, Myliobntis, possess cones, it is recognized that a very large number of elasmobranchs appear only to possess rods (ROCHON-DUVIGNEAUD, 1943). WALLS (1942) considers that the elasmobranch retina is characteristically pure rod, a point of view which has been recently strengthened by GILBERT’S (1961) finding that cones were absent in fifteen species of sharks. Amongst the sharks investigated by Gilbert were specimens of the genus Negaprion. At the beginning of an investigation concerning vision in Negaprion brevirostris, the present authors found it necessary to make a histological examination of its retina. As a result of this work, it is concluded that the Lemon Shark retina contains cones as well as rods, the latter preponderating in the ratio of about 12: 1. Retinae from five young Lemon Sharks (about 0.5-l-O m in length) and one older shark (3-O m in length) were studied in the present work. Three sharks were light-adapted (two of these were kept for 8 hr in a sunlit glass aquarium and one was taken from the sea during daytime): these were enucleated in daylight. Two others were dark-adapted for 9 hr and then enucleated under the illumination of a dim red photographic safelight. The remaining shark was killed (in daylight) and enucleated 2-3 hr later. Subsequently all the eyes were treated in an essentially similar fashion; i.e. they were placed in Kolmer’s fluid (POLYAK, 1941) for 1-2 hr, then the anterior segment and lens were removed and the opened optic cups ALTHOUGH it has been reported
Squatina and Lamna)
1 Contribution No. 292 was suooorted in .oart bv- the Office of Naval Research Contract No. 840 .. (19 quote sharks). 2 Supported in part by Research to Prevent Blindness, Inc.; National Council to Combat Blindness, Inc.; and the National Institutes of Health. National Institute of Neurological Diseases and Blindness (&2B-5277C3), Department of Health, Education and Welfare, Bethesda, Maryland. 3 Supported in part by National Council to Combat Blindness, Inc. (#G-268), and National Science Foundation (#GB-186). 397
398
S.
H.
GRUBER.
D. H. HAMASAKIAND C. D.
B. BRIDGES
were allowed to l%xfor at least 24 hr in fresh Kolmer’s fluid. Routine paraffin sections of 4-j ,Dthickness were prepared, some cut vertically (i.e. plane-parallel to the antero-posterior axis of the eye) and others tangentially to the surface of the retina. Although several stains were used, it was found that Mallory’s triple stain gave the best differentiation of the visual cells. Two types of visual cell were identified in all the retinae examined. These differed in structure, shape and staining properties. Fig. la illustrates a typical vertical section. The more numerous visual cells, considered here to be rods, were tall and cylindrical, extending from the external limiting membrane into the pigment epithelium. The average length of the rods was 3 1 p, their uniformly staining yellow-orange outer segments contributing about 16 1~ to that figure. The diameters averaged 2.6 I(: for the inner segment and 2.3 ,u for the outer segment (these figures are the mean of fifty determinations on separate receptors). Except in the region of the characteristically strongly-staining ellipsoid, the inner segment was pale blue in color and finely granular in texture. In contrast to the rods, the other visual cells, designated here as the cones, were short and pyramidal in shape. In length, these cells averaged 19 p. Unlike those of the rods, the non-staining, tapering cone outer segments did not extend as far as the pigment epithelium, and were only 8 ,D in length. Their diameters were 1.5 ,u at their midpoints. The inner segments, which terminated at the level of the inner margin of the rod ellipsoids, consisted of the outer strongly-staining ellipsoids and the inner poorly-staining and coarsely granular paraboloids (Fig. 1a). The staining properties of the paraboloid conferred a characteristic appearance on the inner segments of the cones, thus making them easily distinguishable from the rods. At the widest point, the cone inner segment measured 4.7 ,D in diameter. Owing to the different dimensions of the rod and cone inner segments, a tangential section of the retina at the level of the cone ellipsoids would be expected to cut the rods at the pale-staining region of their inner segments. Fig. 1b illustrates such a section. In this figure, the larger heavily-stained structures represent cross-sections of the cone ellipsoids; they are about 1.5 times the diameter of the paler structures which represent the lower region of the rod inner segments. From sections such as this the rod : cone ratio may be easily calculated. Thus, in those sections of the retina obtained from the posterior pole of the eye twelve rods were found for each cone present. In one important respect the cones of the Lemon Shark differed from most other vertebrate cones: their nuclei were indistinguishable from those of the rods. In this connection the cones resemble those of another shark, Lamna (ROCHON-DUVIGNEAUD, 1943). No Comparison of the retinae from lightoil-droplets were seen in either rods or cones. adapted and dark-adapted sharks revealed no evidence of photomechanical movements, neither did there appear to be any difference in staining properties corresponding to these different states of adaptation. Apart from obvious post mortem changes, the retinae from the shark which had been dead for 2-3 hr prior to enucleation appeared similar to those of the other five sharks. The rod : cone ratio was about the same as for the other retinae, indicating that the cones of the present work cannot be identified as post mortem artefacts. In those elasmobranchs which possess cones, the difference between the rods and cones may be more or less pronounced from species to species. Thus, according to WALLS (1942), although the cones are distinct from the rods in Mustelus canis, they are more so in M. mustelus and completely cone-like in Myliobatis. Walls considers that modem elasmobranch cones are “new”, secondary derivatives of rods. If this view is correct, examination of the
la. Vertical section of the Lemon Shark retina (oil-immersion. Maltory’s triple stain). Inset, left, represents tine tracings of a typicrtl rod and cone triken from this photograph. p.e.-pigment e~~t~leli~~~~,l.r.f.---fayer of rods and cones, c.l.m--eternal Iimiting membrxnc, FIG.
FIG;.
I b.
o.n.l.-outer
wclear
Tangentin
section
law +
of the
. r.--rod
Lemon
(oil-immersion,
1 c .-cone,
Shark Mallory’s
e.--ellipsoid.
retina triple
at the stain).
p.-paraboloid.
level of the cone ellipsoids
Cones in the Retina of the Lemon Shark
399
eyes from many elasmobranch species might reveal the presence of cones in various stages of transition. Work at present continuing shows that at least three other sharks (Carcharhinus springeri, C. _falciformis and Sph~rna nrokarran) possess cones which are morphologically similar to those of the Lemon Shark. On the other hand, the cones of the sting ray, Da.syatis americana, also examined in the present work, are typically cone-like in appearance. In conclusion, the authors would like to emphasize that the above findings are morphological in nature. It is not permissible to suppose that these cones differ functionally from the rods. Similarly, these findings cannot be used to assume the existence of color vision in the Lemon Shark. Acknowle&emenrs.-Acknowledgemenr is due to Dr. W. \VISBY for his encouragement: also to Mrs. A. of the Bascom Palmer Eye Institute for her valuable assistance in the histological preparations; also to the Seaquarium, Miami, for supplying a large specimen of the Lemon Shark. IOANNIDES
REFERENCES GILBERT,
P. (1961). Abstracts of symposium, papers, Tenth Pacific Science Congress, Honolulu, Hawaii, pp. 176177. POLYAK, S. L. (1941). The R&m. University of Chicago Press, Chicago. ROCHON-DUVIGNEAUD, A. (1933). Les _veuxet la vision cfes wrre%rPs. Masson, Paris. WALLS, G. L. (1942). The verrebrute eye and ifs adaptive mdiurion. Cranbrook Press, IMichigan.