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Citation: Douglas, R. H., Collin, S. P. and Corrigan, J. (2002). The eyes of suckermouth
armoured catfish (Loricariidae, subfamily Hypostomus): pupil response, lenticular
longitudinal spherical aberration and retinal topography. The Journal of Experimental Biology
(JEB), 205(22), pp. 3425-3433.
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The pupil of the majority of teleost fish, unlike in most other
vertebrates, does not change in size in response to variation in
ambient illumination, and the iris is generally assumed to be
immobile. However, a review of earlier literature shows that
some teleosts are, in fact, able to change the size of their pupil
quite considerably, and recently we provided the first
quantitative assessment of such changes in fish, describing
pupillary mobility in the marine plainfin midshipman
Porichthys notatus that was as rapid and extensive as that in
humans (Douglas et al., 1998).
The aim of the present study was to quantify the pupil
responses of another group of teleosts; the suckermouth
armoured catfish, which belong to the Loricariid family of
Siluriform catfish resident in the freshwaters of Panama and
South America. Specifically, we investigated members of the
subfamily Hypostomus, which, along with some other
members of the Loricariidae, are popularly referred to by the
generic name ‘Pleco’ or ‘Plecostomus’, but, in fact, comprise
over 100 species in approximately 18 genera (Nelson, 1994).
This group was chosen for study because Walls (1963)
suggested that they may show extensive pupil mobility,
although no details of the extent and speed of migration were
provided. Clearly, however, the pupil responses of these catfish
will be very different to those of P. notatus, because although
the fully dilated pupil is, as in most other vertebrates, round,
when constricted it takes on the shape of a ‘crescent moon’
owing to the presence of a dorsal iris operculum.
Iris opercula are common among some elasmobranch fish,
such as skates and batoid rays (Beer, 1894; Franz, 1905, 1931;
Young, 1933; Walls, 1963; Kuchnow and Martin, 1970;
Kuchnow, 1971; Gruber and Cohen, 1978; Nicol, 1978; Collin,
1988), and also occur in some bottom-dwelling teleosts
(Bateson, 1890; Beer, 1894; Walls, 1963; Munk, 1970; Collin
and Pettigrew, 1988a; Nicol, 1989). Despite the relatively
widespread occurrence of such asymmetric pupils, their
function remains uncertain. One explanation is that they may
reduce the effect of any longitudinal spherical aberration the
lens might possess by restricting light to the lens periphery
(Murphy and Howland, 1991). Most fish suffer little from
longitudinal spherical aberration owing to a refractive index
gradient within the lens (see Sivak, 1990 for a review).
However, if crescent-shaped pupils do function primarily to
reduce such aberration, one might expect the lenses of animals
with these pupils to be poorly corrected for it. We therefore
determined the longitudinal spherical aberration of the lens in
a species of suckermouth armoured catfish.
3425
The Journal of Experimental Biology 205, 3425–3433 (2002)
Printed in Great Britain © The Company of Biologists Limited 2002
JEB4345
The dilated, round pupils of a species of suckermouth
armoured catfish (Liposarcus pardalis) constrict slowly on
illumination (over 35–40min) to form crescent-shaped
apertures. Ray tracing of He–Ne laser beams shows
that the lenses of a related species (Pterygoplichthys
etentaculus), which also has a crescent-shaped pupil, are
well corrected for longitudinal spherical aberration,
suggesting that the primary purpose of the irregular pupil
in armoured catfish is not to correct such aberration. It is
suggested that the iris operculum may serve to camouflage
the pupil of these substrate-dwelling species. An
examination of the catfish retina shows the photoreceptors
to be exclusively single cones interspersed with elongate
rods and demonstrates the presence of multiple optic
nerve head papillae. Two areas of high ganglion cell
density, each side of a vertically oriented falciform
process, provide increased spatial resolving power along
the axes examining the substrate in front of and behind
the animal.
Key words: catfish, pupil, retina, iris, aberration, Liposarcus
pardalis, Pterygoplichthys etentaculus.
Summary
Introduction
The eyes of suckermouth armoured catfish (Loricariidae, subfamily
Hypostomus): pupil response, lenticular longitudinal spherical aberration and
retinal topography
Ron H. Douglas
1,
*, Shaun P. Collin
2,3
and Julie Corrigan
1
1
Applied Vision Research Centre, Department of Optometry & Visual Science, City University, Northampton Square,
London EC1V 0HB, UK,
2
Department of Anatomy & Developmental Science, School of Biomedical Science,
The University of Queensland, Brisbane 4072, Queensland, Australia and
3
Anatomisches Institut,
Universität Tübingen, Österbergstrasse 3, Tübingen, 72074, Germany
*Author for correspondence (e-mail: r.h.douglas@city.ac.uk)
Accepted 8 August 2002
3426
Finally, the retinal ganglion cell topography of these catfish
was assessed. Although few studies have investigated the
density of retinal ganglion cells with crescent-shaped
pupils, the crescent-shaped iso-density contours in retinal
wholemounts of the sharp-nosed weaver Parapercis cylindrica
(Teleostei; Collin and Pettigrew, 1988a) and the shovel-nosed
ray Rhinobatos batillum (Elasmobranchii; Collin, 1988) in
conjunction with a similarly shaped pupil suggest that a
relationship may exist.
Materials and methods
Animals
All animals were obtained from local aquarium suppliers in
London, UK and Tübingen, Germany and identified by the
British Museum of Natural History, London.
Measurement of pupil response
Three individual Liposarcus pardalis Castelnau 1855
[standard lengths (SL) 140–150mm] were held in a 12h:12h
L:D cycle for at least 3months. To examine their pupil
response, they were removed from their home tanks during the
dark phase of their L:D cycle using a dim red torch and placed
in a small aquarium. Following 1h of acclimation to this tank,
pupillary responses were filmed for 60min using infrared
illumination during continual exposure to one of 13 intensities
of white light. Each fish was examined at the same time each
day to avoid any circadian influences on the pupil response,
receiving only one light exposure per day. As these animals
naturally tend to stay motionless in the light, the only form of
restraint necessary during filming was a Perspex ‘tent’ placed
over them. Stimuli were delivered from directly overhead, via
a shutter-controlled opening and a mirror, from a Kodak
projector located in an adjacent room, which also housed
all recording apparatus. The intensity of illumination was
controlled by neutral density filters. One eye of each animal
was videotaped using an infrared-sensitive camera (Cohu, San
Diego, USA) positioned in a plane parallel to the cornea. Pupil
area was subsequently determined from individual video
frames using NIH-image. To facilitate comparison among
individuals of different eye size, all measurements were
expressed relative to the fully dilated pupil area of each animal
just prior to experimental light exposure.
Determination of lens longitudinal spherical aberration
Following immersion in a lethal dose of methane tricaine
sulfonate salt (MS 222), both lenses were removed from a
single Pterygoplichthys etentaculus Spix and Agassiz 1829
(SL 200mm). P. etentaculus, whose pupil is also crescent
shaped when constricted, was preferred to L. pardalis for this
part of the study owing to its larger size. The lenses of P.
etentaculus, which, like those of other suckermouth catfish (L.
pardalis, Glyptoperichthys lituratus and Sturisomatichthys sp.;
R. H. Douglas and S. P. Collin, unpublished data), are antero-
posteriorly flattened, were glued to the tip of a fine pipette,
placed in a small glass tank and immersed in a teleost Ringer
solution containing small amounts of scatter liquid concentrate
(Edmund Scientific, Barrington, USA). The beam of an He–Ne
laser (emission maximum 632.8nm) was then passed through
the lens along the antero-posterior axes. A camera positioned
at the side of the lens was used to ensure that the laser beam
passed through its vertical midpoint. This was achieved by
adjusting the height of the laser beam until it was not deflected
by the lens from the horizontal plane. The laser beam was then
filmed from above while traversing the lens in the horizontal
plane. Individual video frames were analysed using software
to determine the back vertex distance (BVD) for a number of
beam entry positions. A non-linear regression was applied to
the data using the following equation for back vertex distance
(x): x=a+by
2
+cy
4
, where a represents back vertex focal length,
b represents the 3rd order spherical aberration, c represents the
5th order spherical aberration, and y represents the normalised
beam entry position. This equation represents the longitudinal
ray aberration of a rotationally symmetric optical system on
axis.
Retinal structure and ganglion cell topography
The eyes of a single L. pardalis (SL 220mm) were
embedded in resin for light microscopy, and two interrupted
series of 0.5µm sections were cut on an ultramicrotome and
stained with Toluidine Blue. One eye was serially sectioned in
the dorso-ventral axis and the other along the naso-temporal
axis.
Both eyes of two individuals of Liposarcus multiradiatus
Hancock, 1828 (SL 88mm) and one L. pardalis individual
(SL 158mm) were used for topographic analyses of retinal
ganglion cell distribution. After 3h of dark adaptation and
immersion in a lethal dose of MS 222, eyes were enucleated
with at least 2–3mm of the optic nerve still attached.
Extraocular muscle tissue was removed, with care taken not to
puncture the eyecup. While immersed in oxygenated teleost
Ringer solution, a further 1–2mm of the optic nerve was
removed, and the remaining nerve stump swabbed dry with
a tissue wick. Crystals of fluorescein-conjugated dextran
(3000MW, anionic, lysine-fixable; Molecular Probes, Leiden,
The Netherlands) were then applied to the entire diameter of
the lesioned optic nerve. After approximately 5min, to allow
uptake of the dextran label, the whole eye was re-immersed in
oxygenated Ringer solution for 24h at 21°C.
Following incubation, the limbus of each eye was pierced
and the cornea and lens were removed. A small dorsal incision
was made in the retina for orientation. The eyecup was
immersion fixed in 4% paraformaldehyde in 0.1moll
–1
phosphate buffer (pH7.4) for 40min before dissection in
0.1moll
–1
phosphate buffer. Each retina was removed from its
scleral eyecup and wholemounted (ganglion cell layer
uppermost) on a subbed slide (double-dipped in 5% gelatin),
covered in either Fluoromount (Calbiochem, San Diego, USA)
or 0.1moll
–1
phosphate buffer and coverslipped. Dehydration
was inhibited by sealing the edges of the coverslip with nail
polish. One retina of L. multiradiatus was wholemounted and
prepared as above but left overnight in a humidified Petri dish
R. H. Douglas, S. P. Collin and J. Corrigan
3427The eyes of catfish
before being stained for Nissl substance with 0.05% Cresyl
Violet for 3min. The wholemount was then dehydrated in
a series of alcohols and mounted in DPX to reveal the
distribution of glial cells.
Topographic analysis of the retrogradely labelled ganglion
cells was carried out following the protocol of Collin and
Northcutt (1993), where up to 200 regions per retina (50–60%
of the retinal area) were sampled, allowing small fluctuations
in density to be noted. Iso-density contours were constructed
by joining areas of similar cell density. Retinal shrinkage was
assumed to be minimal because all retinae were examined
while hydrated. The total number of labelled ganglion cells
was calculated by multiplying the average cell density between
each iso-density contour by its area. Area measurements were
calculated by scanning the topography map into a PC
and analysing the area of each contour using NIH-Image.
Retrogradely labelled ganglion cells were viewed and
photographed on a Zeiss Axiophot 135M
fluorescence microscope (Jena, Germany) fitted
with a fluorescein filter block using Kodak Tmax
400 ASA film.
Results
Pupil response
Fig. 1 shows the pupil response of a single L.
pardalis individual to six different intensities of
illumination. The maximum amount of pupil
constriction as a function of light intensity for all
three L. pardalis examined is shown in Fig. 2.
The threshold to elicit a measurable pupil
response lies between a corneal irradiance of
2.9×10
–2
µWcm
–2
and 8.4×10
–2
µWcm
–2
for all
animals. Overall, the response of the pupil to light
was very slow. The time taken for half maximal
contraction (t
0.5max
) was in the order of 1–8min,
with full constriction usually taking 35–45min.
Once constricted, the pupil remained so for the duration of the
experiment, showing no signs of re-dilating in continual
illumination (Fig. 1).
Pupillary constriction in L. pardalis consists of two
components; a general reduction in the diameter of the pupil
and the outgrowth of an operculum from the dorsal margin of
the iris (Fig. 3). Consequently, while the fully dilated pupil of
L. pardalis is more or less round (Fig. 4A), when constricted
it appears as a ‘crescent moon’ with an irideal flap obscuring
the central pupil (Fig. 4B). In general, especially at higher light
levels, the decrease in overall pupil diameter occurs more
rapidly than the increase in opercular area (Fig. 3).
Longitudinal spherical aberration of the lens
The lenses of P. etentaculus are well corrected for
longitudinal spherical aberration, showing only relatively
small differences in back vertex distance (BVD) for laser
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Time (min)
Relative pupil area
0 20 40 60 80
Complete darkness
8.4×10
–2
µW cm
–2
1.37
µW cm
–2
4.6×10
1
µW cm
–2
2.65×10
2
µW cm
–2
5.6×10
3
µW cm
–2
0.2
0.4
0.6
0.8
1.0
1.00E–02 1.00E+00 1.00E+02 1.00E+04
Minimum relative pupil area
log corneal irradiance
Fig. 1. Pupil response of a Liposarcus pardalis individual during
60min exposure to different intensities of white light.
Fig. 2. Minimum pupil area in response to different intensities of
white light. Corneal irradiance is measured in µWcm
–2
. The
different symbols represent data from three individual Liposarcus
pardalis.
0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized dimension
Time exposed to light (min)
0 10 20 30 40 50 60 70
Fig. 3. Pupil response of a Liposarcus pardalis individual to 60min exposure to
white light at an intensity of 5.6×10
3
µWcm
–2
. The three curves represent
normalized lines for area of irideal operculum (squares), pupil area (crosses) and
horizontal pupil diameter at its widest point (triangles).