VISION IN ELASMOBRANCHS:
HISTOLOGY OF THE RETINA AND ERG SPECTRAL SENSITIVITY
by
Joel L. Cohen f
A thesis
submitted in partial
fulfillment of the requirements for the degree of
Master of Arts in the Department of Biology
California State University, Fresno
August, 1972
I1U LWOU'l IA LULLI CP LIE KMC TABLE OF CONTENTS
PAGE
LIST OF TABLES vi '
LIST OF FIGURES vi ' '
INTRODUCTION 1 I Purpose
Function of the Retina 5
EIasmobranch Visual Cells
METHODS AND MATERIALS ... 6 Animals
Optics and Stimulation 9 Ca I ibration 9 Procedure
u. , . 10 Histology
RESULTS 12 I 2 Hexanchus griseus
a. , 12 Histology I 5 Electrophysiology
Triakis semifasciata ^
4- , 20 Histology 20 Electrophysiology 25 Rhinobatos productus
Histology 2^ 32 Electrophysiology V i
TABLE OF CONTENTS (Continued)
PAGE
DISCUSSION 35
Histology 33
Electrophysiology 3^
CONCLUSIONS 42
LITERATURE CITED 43 V i i
LIST OF TABLES
TABLE PAGE
1. Comparison of The Thicknesses of each layer for The
fundus and side of the retina 18
2. Comparison of cell ratios 19 V i i i
LIST OF FIGURES
FIGURE PAGE
1. Optical system number one 7
2. Optical system number two 8
3. Cross section of the retina of Flexanchus griseus
stained with hematoxylin and eosin 13
4. Spectral sensitivity curve of Hexanchus griseus 16
5. Amp Iitude-i ntensity curve of Hexanchus griseus 17
6. Cross section of the retina of Triakis semifasciata
stained with hematoxylin and eosin 21
7. Spectral sensitivity curve of Triakis semifasciata 23
8. Amp Iitude-intensity curve of Triakis semifasciata 24
9. Cross section of the retina of Rhinobatos productus
stained with PTAH 26
10. Rods and cones in the retina of Rhinobatos productus
stained with hematoxylin and eosin 28
11. Horizontal section of the retina of Rhinobatos productus
at the level of the cone el Iipsoids stained with
PTAH 30
12. Spectral sensitivity curve of Rhinobatos productus 33
13. Amplitude intensity curve of Rhinobatos productus 34
14. Relative spectral distribution of solar energy in
, XQ different types of sea water INTRODUCTION
Purpose
The eyes of vertebrates have adapted to their habitat to such an extent that Detweiler (1947) stated "So closely correlated is the mode of life of the animal with the structure of the retina, that, from a histological section, one can predict something of the habits of the animal, as well as its visual acuity."
Vision plays an important role in the life of a shark. Experimen tal evidence shows that although olfactory cues initiates an advance toward an object, the final phase of the approach is controlled by vision (Hobson, 1963).
McLaughlin and O'Gower (1971) have shown that the heterodont shark, Heterodontus portusjacksoni, gives evidence of a homing ability.
They suggested this homing ability probably is dependent on vision.
The early studies on elasmobranch vision were primarily histologi cal in approach (Franz, 1905, 1913; Rochon-Duvigneaud, 1943; Verrier,
1929, 1930). Although most recent studies are physiologically oriented, the need for continued histological studies is evidenced by the appear ance of articles in the literature which refute the older findings
(Gruber et al. 1963, Plamaski and Gruber, 1965; Stell, 1972).
Many of the recent physiological studies have concentrated on determining the spectral sensitivity of the retina, using electro
physiological techniques.
Kobayashi (1962) was the first to study electrophysiological re
sponse in the retina of eIasmobranchs. He used eyecup preparations and
oriented his findings to ecological questions. Using the dogfish 2
i-iuste I us manazo and the batoid rays and skates PI atyrh ina, Dasyat i s, Ra ja,
Na rka, and Ho I orhinus, he determined their respective spectral sensitivity and the properties of their eIectroretinograms. From these data, he con cluded that the dogfish eye was well adapted for an open ocean existence and the eye of rays and skates for a bottom dwelling habit.
O'Gower and Mathewson (1967) using living intact animals determined the spectral sensitivity of the lemon shark Negaprion brevirostris.
Hamasaki et al. (1967), also using intact living animals, determined the
properties of the e I ectroreti nogram of the lemon shark, Negaprion
brevirostris, the nurse shark, GingIymostoma cirratum, and the stingray,
Dasyatis sayi.
Using microeIectrodes, Tamura et al. (1966) and Tamura and Niwa
(1967) determined the spectral sensitivity of Dasyatis akajei and
Heterodontus japonicus.
To interpret the findings of physiologists, behavioral experiments
must be done to determine the actual visual capabilities of elasmobranchs.
At present few behavioral studies have been performed (Clark, 1961;
Tester and Kato, 1966; and Gruber, 1969).
This thesis is concerned with the retinal histology and scotopic
(dark adapted) spectral sensitivity of three species of elasmobranch as
determined by electroreti nography. The cellular make up of the retinas
of each species of elasmobranch will be discussed as well as a deter
mination of rod to bipolar cell and rod to ganglion cell ratios. The
purpose will be to determine whether the retinal histology and spectral
response appears adaptive to the species' habitat. 3
The three species studied are the sixgill shark, Hexanchus griseus
(Bonnaterre), the leopard shark, Triakis semifasciata Girard, and the shove I nose guitarfish, Rhinobatos productus (Ayres). Hexanchus griseus
is a primitive deep sea shark found in depths ranging from 75 fathoms to as deep as 3430 fathoms (Bigelow and Schroeder, 1948). I have caught specimens in depths ranging from 70 to 150 fathoms. Triakis semifasciata
is found in rather shallow waters and in sloughs (Roedel, 1950).
Rhinobatos productus inhabits the shal low waters of shores and bays
(Barnhart, 1936).
Function of the Retina
The retina has evolved to perform one of two functions, acuity and sensitivity (Detwiler, 1943). Sensitivity of an eye means its ability to respond to weak stimuli. By acuity we mean the ability to continue
to see separately and unblurred the details of the visual object as those details are made smaller and closer. Summation as discussed here is the
gathering together of many rods to form bipolar cells and ganglion cells.
This causes the response to stimulus to be additive (WalIs, 1942).
Acuity in the retina is governed by three factors, the slenderness of
the visual cells, their closeness of spacing and the number connected
with one optic nerve fiber (Walls, 1942).
Diurnal vertebrates have retinas designed for acuity. Their visual
cells are slender and closely spaced. There are a great number of cone,
bipolar cells and ganglion cells, so that in some cases, one cone is
attached to one bipolar cell (Walls, 1942).
On the other hand, nocturnal animals have evolved retinal character
istics that enable them to have very sensitive eyes including enlargement 4 of the rod outer segment and summation. These animals have very long rod outer segments which enable them to contain more photosensitive material than a short one would allow. They have a large number of rods and fewer number of bipolar and ganglion cells, so that summation is extensive (Walls, 1942).
In nocturnal animals, however, there is an area of the retina in which mostly cones are present and the number of secondary and tertiary neurons increases. This area is called the area centralis and gives an area of acuity in an eye designed for sensitivity (Walls, 1942).
EIasmobranchs are considered to be nocturnal animals (Walls, 1942).
As such they would be expected to have a retina that is designed for sensitivity. Verrier (1924), Franz (in Walls, 1942) and Kato (1962) have found mostly rod retinas and a high degree of summation in sharks.
It should be expected that this summation would vary according to the
habitat of the animal. Verrier (1929) found that ScyIIium had a rod:
gang I ion ceII ratioof 8:1 and Acanthias 5:1. Franz (in Walls, 1942)
found that the ratio in Etmopterus wa s 147:1 and in various sma II sha rks
20:1. In the blacktip shark, Carcharhinus melanopterus this ratio was
found to be 77:1 and in the white tip shark, Triaenodon obesus it was
82:1 (Kato, 1962). Such large variations in these ratios point out the
difficulty in microtechnique and counting methods.
Elasmobranch Visual Cells
It is generally thought that eIasmobranchs possess pure rot retinas.
Flowever, cones have been found in Squatina and Muste I us (Franz 1905,
1913), in My Iiobatis (Verrier 1930), and in Lamna (Rochon-Duvignon,
1943). More recently Gilbert (1961) studied sixteen species of 5 elasmobranch and found cones only in Muste I us. Some of his findings were reversed when Gruber et al. (1963) found cones in the lemon shark,
Negaprion brevirostris and in three other sharks (Carcharhinus springeri,
C. falciformis and Sphyrna mokarran). Hamasaki and Gruber (1965) also found cones in the nurse shark, GingIymostoma cirratum and the stingray
Dasyatis sayi . Gruber (1969), however, did not find cones in Mustel us sp. Stell (1972) recently found cones in SquaI us which was thought to have only rods. METHODS AND MATERIALS
Anima I s
Shove I nose guitarfish and leopard sharks were caught with a gill net in Elkhorn Slough, Monterey County, California. The Slough is about 20 feet deep at high tide. Sixgill sharks were caught in Monterey Bay,
California, using large mesh gill net and long lines set in depths of
70 to 150 fathoms. Animals were placed in a large outdoor aquarium until needed. This varied from 24 hours to 4 weeks. The variation in time between capture and use did not seem to affect the results.
Optics and Stimulation
A six volt automobile spotlight (Westi nghouse) was used as the light source. The lamp was powered by two 12 volt automobile storage batteries connected in series, and underrun at 2.5 amps. Current was continually monitored by an ammeter and controlled by a variable resistor.
Two optical systems were used. The first (Fig. I), was a single channel system which used a combination of lens and shutter to colli- mate the light used on the interference and neutral density filters.
This system focused the image of the filament of the lamp upon the retina.
The second system (Fig. 2) was also a single channel system, but it had a shutter at the focal point of the first lens and produced a homogeneous circle of light.
Monochromatic light was obtained by using interference filters of approximately 10 nm half bandwidth. The interference filters covered the
visible spectrum ranging from 400 nm to 731.7 nm in 30 nm intervals
(Baird Atomic, Oriel Optics and IR products). SHIELD
Fig. I. Optica I system number one. SHIELD
Fig. 2. Optical system number two.
CD 9
Intensity was controlled by using Kodak Wratten number 96 neutral density fiIters.
CaIibration
The transmission properties of the interference filters and neutral density filters were calibrated on a Cary 14 recording spectrophotometer.
The output of the light source was obtained by using a selenium cell
(Edmund Scientific) which was calibrated against an EC&G model 580 radiometer. Each interference filter was placed in the system and the selenium cell put in place of an eye. Voltages were read off a Tektronix
422 OsciIloscope. This value was multiplied by the appropriate constants and a reading in photons was obtained. To find the energy at each neutral density value used in an experiment, the energy output obtained with the interference filter was multiplied by the % transmission of the neutral density filter at that wavelength.
Procedure
Animals were stunned by a sharp blow on the head. An eye was removed and a wad of cotton placed in the empty socket. The sharks usually recovered and displayed no ill effects afterwards. This procedure a I lowed for the future use of the other eye. At that time, the animal was pithed.
The anterior portion of the eye was removed with a razor blade. The lens and vitreous humor were subsequently removed. This was important to allow an adequate flow of oxygen to reach the retina. The preparation was placed in an electrical ly shielded Faraday Cage.
A stream of oxygen, which was bubbled through water, was allowed to flow over the eye. 10
The ERG was recorded using a cotton wick electrode in a U shaped tube fiI led with Ringer's solution. The cotton wick was placed directly on the retina. The eye rested on a Ringer's soaked cotton pad which served as the indifferent electrode. A siiver-siIver chloride rod con nected the wick electrode to an AC coupled preamplifier (Tektronix 122) and displayed on a Tektronix 422 Oscilloscope. Images were recorded using a standard 35 mm camera. The high frequency response of the preamp was set at 50 cycles, while the low frequency response was set at .2 cycles.
The time constant was one second.
Before each experiment, eyes were dark adapted for between 30 and 60 minutes. The stimulus duration for the leopard sharks was 40 mi Mi- seconds. For a I I other anima Is, the stimuI us was 20 mi IIiseconds.
The light intensity required to evoke a standard ERG response for each wavelength of light was determined. For the leopard sharks, the criterion was a just perceivable ERG. For the sixgi I I shark and shovel- nose guitarfish, the criteria was 10-12 microvolts amplitude of the b-wave.
Histology
Eyes were enucleated from freshly caught specimens. Slits were made with a scapel in the pupil to insure complete fixation. The eyes were placed in Susa's and picric acid for eight hours and washed overnight in running water. They were then placed in 40$ ethyl alcohol and trans
ferred every eight hours through serial concentrations of alcohol until they reached 70$ ethyl alcohol, where they remained until infiltration and embedding. The procedure for infiltration and embedding is as follows: I) 1:1 mixture of 95I ethyl alcohol and dioxane—I hour, 2) 100% dioxane-2
hours, 3) 100% dioxane-2 hours, 4) saturated solution of paraffin and
dioxane-2 hours, 5) Paraffin (Bioloidin) m.p. 56-58° C-4 hours, 6) Para
ffin (Bioloidin) m.p. 56-58° C-4 hours, 7) Paraffin (Tissuemat) m.p.
58° C-5 hours, 8) embed in paraffin (Tissuemat).
Sections were cut parallel to the receptors using an American
Optical number 820 rotary microtome set at 5 microns. Two stains were
used, Delafield's hematoxylin and eosin, and Phosphotungstic acid
hematoxylin (PTAH) method for central nervous system tissue (Luna, 1968).
Ratios of rod to bipolar cell and rod to ganglion cell were made
according to the method of Trowell and Westgarth (1959). RESULTS
Hexanchus griseus
Histology: The pigment epithelium of the sixgill shark is without
pigment granules. In cross section, each cell is rectangular with a
large nucleus. Sectioned horizontally, these cells are hexagonal.
Vertical sections through the layer of rods shows them to be long and
slender (Fig. 3). The outer segment is easily broken in sectioning. The outer segment measured approximately 26.4 microns long, the inner segment
12.0 microns. The outer segment width was 2.0 microns while the inner segment was 3.0 microns wide. The total length of the rods averaged
38.4 microns.
The outer limiting membrane appeared as a solid line dividing the
layer of rods from the outer nuclear layer.
The cells of the outer nuclear layer, which consist of the nuclei of the rods were arranged in 3 or 4 rows. The shape of the nuclei ranged from round to tear drop. The nuclei, when stained with hematoxylin and eosin appeared granular.
The outer pI exiform layer consists of the fibers of the visual ce I Is and bipolar cells. This layer was very thin and almost impossible to differentiate from the inner nuclear layer.
The inner nuclear layer consists of the nuclei of the horizontal cells, bipolar cells, amacrine cells and the Muller fibers. The hori
zontal cells are massive cells resting in the outer portion of this layer.
They are two layers thick, with the lower iayer (inner) much narrower than the outer layer. Below this are found the remaining cells. The bipolar cells are large, and granular in appearance. They are found throughout the layer, but serm to predominate in the basal portion. The amacrine cens were sma 1 I er and more numerous. They stained much darker
Than The bipolar eel Is. The Mu I ler cei I s are found at the base of the
MuIIer ,ibers and are cylindrical in shape and darkly stained.
ihe inner plexiform layer contains the synaptic junctions of the ganglion cells and bipolar cells. Many fibers ran through this layer.
Some nuclei, probably displaced gang I ion eel Is were also found.
The ganglion cells were found just below the inner plexiform layer, bordering the nerve fibers.
The nerve fibers appeared as thin lines running parallel to the retina.
The rat io of rod to b i po I ar cell s was 5:1. The rod to gang I ion ce I ratio was 24:1. (See Table I )
The thicknesses of each layer can be found in Table 2.
Electrophysiology: The ERG of Hexanchus griseus showed a large positive b-wave in the dark adapted state. A slight or no a-wave was al
seen. No off-response was observed.
The spectral sensitivity curve (Fig. 4) had a peak at 460 nm. The
eye barely perceived light at 580 nm and did not respond to light of
wavelengths longer than 580 nm.
Figure 5 indicates the relationship between amplitude and the log
intensity at different wavelengths. The amplitude intensity curve
appeared slightly S-shaped. 16
>
111 > 2.0 - > {— 1~ < 2 J LLJ LU W o:
CD 3 O i- -J < o
WAVELENGTH (nrn)-
Fig. 4. Spectral sensitivity curve of Hexanchus griseus. 460 nm
490 nm
520 nm
53Own
10 20 30
LOG INTENSITY
' 5. Cvfv® o* » TABLE I. Comparison of cell ratios Species Rod:bipoIar ceI I Rod:gangI ion eel Hexanchus griseus 5:1 24: Triakis semifascia+a 3.7: i 11.2: Rhinobatos productus 2.7: I 7.8: I TABLE 2. Comparison of the thicknesses of each layer for the fundus and side of the retina Entire Pig. Visual Cel Out. Nuc, Out. PI ex. n. Nuc. n. PI ex. Retina Epith. I ayer I ayer layer I ayer layer Hexanchus 206.64 8.33 48.29 29.64 9.99 49.95 49.95 fundus 233.10 6.66 49.95 29.97 9.99 46.62 89.91 side Tri aki s 174.90 6.66 29.97 19.98 3.33 53.28 53.28 fundus 189.81 6.66 33.33 26.64 6.66 46.62 62.27 side Rhinobatos 153.18 6.66 36.63 13.32 9.99 36.63 46.62 fundus 156.51 6.66 33.30 16.65 3.33 46.62 49.95 side 20 Triakis semifasciata Histology: The cells of the pigment epithelium of the leopard shark viewed in vertical cross section are rectangular with a large, centrally located nucleus. There are no pigment granules present. In horizontal section, these cells appear hexagonal. The visual cell layer consists of long slender cells which are the rods. When stained with hematoxylin and eosin, both the inner and outer segments stain the same. However, when stained with PTAH, some cells stained much darker blue than other morphologically similar cells (Fig. 6). The cells of the outer nuclear layer are round in appearance and lie in 3 or 4 layers. Some nuclei stain darker than others with hematoxylin and eosin. The horizontal cells are arranged in three layers. The outermost layer contains horizontal cells larger than cells of the remaining two layers. These large cells are rectangular in shape, while the other cells are more flattened. The bipolar cells seem to be situated in a single layer on the innermost portion of this layer. Nuclei, possibly misplaced ganglion cells were found in the inner plexiform layer. The ratio of rod to bipolar cells was 3.7:1. The rod to ganglion cell ratio was 11.2:1. (See Table 2) Electrophysiology: An a-wave and positive b-wave was seen in the dark adapted ERG of Triakis semifasciata. Maximum sensitivity was found at 520 nm. The leopard shark perceived light up to 640 nm (Fig. 7). An amplitude intensity curve (Fig. 8) showed the response to be approximately S-shaped. 23 Hg. 7. Spectral sensitivity curve of Triakjs serjiiasciata. Fig. 8. Amplitude-intensity curve of Triakis semifasciata. 25 Rhinobatos productus HlStol°9y: The ceMs in +he Pigment epithelium, as in the other elasmobranchs studied, are rectangular in shape and have a large nucleus. In tangential section they are hexagonal. They are without pigment granules. Two types of cell can be seen in the visual cell layer. These differ in size, shape and staining abilities. The rods are long and slender. Their outer segment is about 12.6 microns long and 1.5 microns wide. The inner segment is 12.0 microns long and 2.5 microns wide (Fig. 9). The cones look I ike typical teleost cones (Fig. 10). They are short and pyrimidal in shape and have an el I ipsoid and parabaloid region. The ellipsoid region stains darker than other parts of the cell with hematoxy lin and eosin and PTAH. No oi I droplets could be seen. The outer segment was 8.1 microns long and 1.8 microns wide. The inner segment of the cones was 12.6 microns long and 4.5 microns wide. The nucleus was situated on the receptor side of the external limiting membrane, although sometimes it was seen to overlap the membrane. Because of the difference in diameter and staining abilities of the el I ipsoid region of the cones and rods, a tangential section was made. This preparation allowed enumeration of rods and cones present in the ratio 5:1 (Fig. I I ). The horizontal cells of the inner nuclear layer were arranged in Three rows. The cells of the outermost layer were rectangular in shape and two to three times as wide as the ce I I s of the inner layer Ce 1 Is appeared more compressed. Generally +he bipolar cells were °n +he basal portion of this layer. Mu I ler fibers were also seen to run Through this I aver. 32 The inner plexiform layer, ganglion cell layer and nerve eel I layer appeared as in the other eIasmobranchs studied except for their widths. The rod to bipolar ratio was 2.7:1. The rod to ganglion cell ratio was 7.8:1. EI ectrophys io I ogy : A prominant a-wave and positive b-wave was seen in the dark adapted ERG of Rhinobatos product!s. Maximum sensitivity was at 490 nm (Fig. 12). The spectral sensitivity curve was very broad. The eye responded to light up to 700 nm. The size of the b-wave increased as the intensity of the stimulus was increased. This is shown by the amplitude-intensity curve (Fig. 13). With a full strength stimulus, a large a-wave and small b-wave appeared. As the intensity was decreased, the a-wave decreased in size and the b-wave increased. 1 33 Fig. 12. Spectral sensitivity curve of RMnobatos i^fductus. 490 nm 40 5 50 nm > LlJ 30 Q "D III _J 0. S 20 < 670 nm 700 nm 10 1.0 2.0 3.0 LOG INTENSITY Fig. 13. Amplitude intensity curve of Rhinobatos productus. DISCUSSION Histology The structure and physiology of the retina of Qi k ar 'na of eIasmobranchs reflect adaptations to different habitats. No pigment was found in the pigment epithelium in any of the three elasmobranchs studied. This is consistent with the findings of others (Franz, 1905, 1913, Verrier, 1930). The horizontal cells of each elasmobranch examined are massive cells lying in two or three rows in the outer part of the internal nuclear layer. Walls (1942) believed their function to be support in elasmobranch retinas. Recent evidence shows, however, that they are connected electrically to one another and probably play an important role in the visual process (Kaneko, 1971, Naka and Witkovsky,. 1972). Walls (1942) stated that one of the factors which would give greater sensitivity to the retina would be long rod outer segments. The rod °uier segment is where the photosensitive material rhodopsin is found, heater amounts of rhodopsin would confer greater sensitivity to the retina. A group comparison t-test showed that there was a highly significant Aviation (P < .01 ) between the lengths of the rod outer segments of jjgxanchus griseus and both Triakis semifasciata and Rhinobatos productus, d^.nchus oeing longer. Summation is another factor which increases sensitivity (Walls, l942). If many rods are connected to few bipolar cells and ganglion Ce|ls> the effect of the light hitting the rods is summated. Here again, ratio of rods to ganglion cells is greatest for Hexanchus (see Tab! e 2 ). In both Hexanchus and Triakis I only found rods. However, there was some differential staining of thevisuai cell nuclei in Triakis. with taitoxylin and eosin, some nuclei staining darker and some appearing grainier than others. When stained with PTAH, some visual cells stained much darxer than others. From the above information, I suspect that Jriakis has cones, but am not sure. If it does have cones, they would appear rod-1 ike as in F.uste I us (Walls, 1942). Further work will have to be Gone 10 elucidate the true nature of the photoreceptor eel Is in Triakis. Rhinobatos productus had cones that were quite different from the rods and appeared similar to teleost cones. Cones have not been previ ously reported for Rhinobatos. Cones were found in the ratio of 5 rods to one cone. The length of the one outer segment was 8.1 microns, its width was 1.8 microns. The inner segment length and width was 12.6 herons and 4.5 microns respectively, the overall length being 20.7 herons. These compare favorably to the sharks Neqaprion brev i rostr i s, G'ncjlymostoma cirratum, and the ray Dasyatis sayi which had cone outer segment lengths of 8, 8, and 9 microns respectively with widths of 1.5 microns. The inner segment length varies from 10 microns in G_. £jj*raturn to 14 microns in D. say i, while the diameters vary herons for N. brev i rostr is to 8 microns in D. say^ (Hamasaki and Gruber, 1965). Gruber et al. (1963) found a rod to cone ratio of 12.1 _ rostr is. Hamasaki and Gruber (1965) found the rod *n 6. cirratum to be 7:1 and 5:1 in D. sayi- These compare favorably the rod to cone ratio I found in R_. productu— 37 The cone nuciei of Kninobatos were very prominent. They seemed to be situated on the receptor side of the external limiting membrane. However, some nuclei were observed to overlap the membrane. This situation is the same as that repoi bed for the ray, Dasyat i s sayi by Hamasaki and Gruber (1965). According to Walls (1942), elasmobranch cones are "new", secondary derivatives of rods. The presence of such cones in Rhinobatos may reflect the evolution of the guitarfish. Bigelow and Schroeder (1948) placed the guitarfishes intermediate between the sharks and the more highly specialized rays. The cones in Rh i nobatos and the rays Dasyatis and My I iobatis are typically cone like. The cones previously described in sharks range from rod I i ke to almost cone I ike. Walls' (1942) statement that elasmobranch cones are secondary derivatives of rod seems reasonable since rays are considered more specialized (advanced) than sharks. One can infer from this that guitarfish which look like a cross between sharks and rays are closer to the rays than to the sharks on the basis of retinal hisiology. The presence of cones in Rhinobatos is puzzling, especial y a member of a supposedly nocturnal group. However, a number tions suggest Rhi nobatos is active during the day. During 1972 shark derbies at Elkhorn Slough, Monterey County, Ca I i forma, guitarfish were caught using hook and line during daylight. Hera I stated that in previous years guitarfish made up large percentages of i m elasmobranchs caught in the derbies. In the present study, around the holding tank actively during the day. 38 Elec+rophysiology The point of maximum sensitivity for Rhinoh.w y Tor Kn1nobatos was found to be at 497 nm. Tnis agrees closely with the finding by Crescitel I i (|969) of a photopigmenr ar 497 nm based on retinine|. Rhinobatos had the broad- ;^t sensitivity curve of the three e I asmobranchs and perceived light into the red part of the spectrum. Rhi r,os,r:os sroductus is found in shallow waters and bays (Barnhart, 1936). 7he presence of cone vision in a member of a supposedly nocturnal group has been documented here. However, a number of observa tions suggest Rhinobatos is active during the day. Because both rods and cones are present day and night vision should be possible. The finding of the broadly shaped spectral sensitivity curve would allow this animal to make use of the light in shallow waters, which transmit light max ima I I y in the green part of the spectrum (Jerlov, 1951). See Fig. 14. The point of maximum sensitivity for the sixgiI I shark, Hexanchus 3Dseus, was found to be at 460 nm. This peak is similar to that found for the visual pigments of other deep sea sharks. Denton and Warren "956) found in deep sea fish a golden colored pigment which had its naximum wavelength displaced 20 nm toward the blue end of the sp from visual purple, which absorbs around 500 nm. The pigment was named chryopsin (Denton and Shaw 1963). Denton and Shaw (1963) found a pigment wh'ch absorbed maximally at 472 nm in an e I asmobranch, Centrpscymonus ggjalepsjs. Denton and Nicol (1964) found a retinal pigment absorbing a+4?7 nm for Hydrolagus affinis, a deep sea chimaeroid. 39 WAVELENGTH i n f'9. 14. Relative spectral distribution of solar energy different types of sea water from Munz (1964). 40 The actual peak found for H._ b'qriseus ^eus in thicTnis studyc+1,Hw may be somewhat higher than 460 nm, since I was using interference filters with fixed wavelengths of 460 nm and 490 nm, each with a bandwidth of 10 nm; i.e. the 460 nm filter transmitted light from 455 to 465 nm. The shift in sensitivi ry to shorter wavelengths may be a function of at least three phenomena. Jerlov (1951) found that the radiation maximum in open oceans was about 475 nm. I|Kampa and Boden (1956) found that the spectral sensitivity of organisms in the sonic scattering layer was maximal at 478 nm. In laboratory studies they found that the primary peak in the luminescence of Euphausia pacifica, a euphausiid that is commonly found in the sonic scatterine layer, was 476 nm. So it seems likely that the retina of Hexanchus has evolved to make maximum use of the light that is present at depth. Barlow (1956) has shown that the sensitivity of the retina could be limited by its own intraretinal noise. This noise would be caused by th thermal decomposition of the rhodopsin molecule. if the ra i e o decomposition could be decreased the sensitivity of the eye wo creased. Barlow (1957) following deVries (1949) has suggested that shift toward the blue end of the spectrum would decrease the rate decomposition. Thus another reason exists for maximum absorpt • _i0 +han that found in fish shorter wavelengths to evolve in deep sea anima living closer to the surface. • , . n xnr +he leopard shark, Triads The wavelength of maximum absorption i-,- ( i Qfi?) found a maxima ^lilasciata was found to be at 520 nm. Kobayashi of 525 nm for the batoid rays Dasyatis and Holmtimus. Tamura et ^ (l9*6) using intracellular electrodes found the maximum to be at 52 nm for Dasya~ i s also, i nese rays, like Triple _L ,, / » - — are shallow water inhabitants. Triakis inhabits shallow waters and sloughs (Roede!, 1950). The coastal waters transmit light between 520 and 570 nm. Thus the spectral characteristics o: the ;eopard shark retina make it suitable to live in the shallow waters of sloughs and oceans. CONCLUSIONS | The study of the histology and eIectrophysioIogy of elasmobranch retinas show that e I asmobranchs have specific visual adaptations for the environments in which they are normally found. Thus, not only does the deep sea sixgill shark have an extensively summated retina and very long rod outer segments to give it maximum sensitivity, but it is most sensitive to that wavelength of light which predominates in the deep sea. 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