Bull Mar Sci. 93(2):339–353. 2017
https://doi.org/10.5343/bms.2016.1044
research paper
Species-specific development of retinal architecture in
elopomorph fishes: adaptations for harvesting light in the dark
1
Department of Biological
Sciences, Florida Institute of
Technology, 150 West University
Blvd., Melbourne, Florida 32901.
2
Department of Biology,
University of West Florida, 11000
University Pkwy, Pensacola,
Florida 32514.
* Corresponding author email:
<[email protected]>, phone: 321674-8194.
Date Submitted: 2 March, 2016.
Date Accepted: 26 January, 2017.
Available Online: 22 February, 2017.
Michael S Grace 1 *
Scott M Taylor 2
ABSTRACT.—Elopomorph fishes are distinguished in
part by a shared leptocephalus larval form. These glassclear, ribbon-like larvae are remarkably similar across the
elopomorpha, yet they mature to radically different forms as
adults. Along the timecourse of development, they transition
through a variety of spectrally distinct habitats, occupying
very different temporal niches as adults—some diurnal
predators in high-light environments, others crepuscular,
and others nocturnal. We found that in concert with these
changes, the retinas of a variety of elopomorph fish change
in similarly dramatic ways over the course of development.
While the mature bonefish [Albula vulpes (Linnaeus,
1758)] retina is specialized for visual tasks in a high-light
environment, the Atlantic tarpon (Megalops atlanticus
Valenciennes, 1847) and ladyfish (Elops saurus Linnaeus,
1766) exhibit multiple specializations for function at night or
otherwise in very dim light conditions. These include stacked
rod photoreceptors that are gathered into massive bundles,
retinomotor movement of photoreceptor outer segments, and
a highly reflective tapetum. In the adult form, the speckled
worm eel (Myrophis punctatus Lütken, 1852) maintains a
retina with nearly all rods. The dramatic divergence among
taxa over the course of development produces species with
distinctly specialized visual capabilities. Moreover, the
ability to change over the course of development may underlie
the capacity for resilience in the face of anthropogenic
insults, including light pollution in increasingly developed
coastal zones. On the other hand, if light history can drive
retinal change, exposure to artificial light at night may be
detrimental to the survival of individuals that move between
light-polluted and naturally dark locations.
Because there is substantially less light available for vision, life at night poses
unique challenges for the visual system of animals, particularly for species that are
heavily reliant upon vision for predation, predator avoidance, and/or conspecific
interactions including courtship and reproduction. The difference in light levels
between day and night may span more than 10 orders of magnitude, which is far
greater than the sensitivity range of a given class of vertebrate retinal photoreceptor
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the University of Miami
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cells. In fact, even as fish move between light and deep shadow in the daytime, or
between darkness and locations illuminated by artificial light at night, the changes in
illumination level may require adjustments of the retina to maintain visual function.
The outer retinas of vertebrate animals contain two morphologically and
functionally distinct types of light-detecting photoreceptor cells. Rod photoreceptors
are highly-sensitive light detectors that allow monochromatic vision in low-light
conditions. Cone photoreceptors have much lower absolute sensitivity to light such
that they are functional only at higher light conditions, and because many species
possess multiple types of cones with distinct spectral sensitivities, cones often
provide color vision. Therefore, a highly rod-dominated retina, that is, a retina with
a high rod:cone ratio, is better adapted for low-light or nocturnal vision than a retina
with a lower rod:cone ratio. The rod:cone ratio is only one of a variety of vertebrate
retinal features that may be subject to selection pressure, allowing different species
(or different life stages within a species) to effectively occupy distinct temporal and
light-level niches.
The superorder Elopomorpha comprises a group of bony fishes that possess
an unusual larval form known as the leptocephalus. These highly elongated and
laterally-flattened larvae have very small heads relative to the much larger, glassclear, ribbon-like body, yet their heads contain well-developed and highly-pigmented
eyes that are highly rod dominated for vision under dim light conditions (Taylor
and Grace 2005, Taylor et al. 2011a,b, 2015), compared with the larval-stage
retinas of most other teleost species, which contain only cones (Evans and Fernald
1993, Negishi and Wagner 1995, Shand 1997, Helvik et al. 2001, Lara 2001). In a
comprehensive analysis of leptocephalus visual development, Taylor, Grace and
colleagues discovered that as elopomorphs develop through a series of life stages,
their retinas change in remarkable ways, and the retinal development trajectory can
be dramatically different among elopomorph taxa (Taylor and Grace 2005, Taylor et
al. 2011a,b, 2015). For example, in their adult forms, the Atlantic tarpon (Megalops
atlanticus Valenciennes, 1847) and ladyfish (Elops saurus Linnaeus, 1766) are large,
fast-swimming piscivorous fishes that are frequently observed feeding at or near the
water surface during the day, during crepuscular phases, and at night. However, their
retinas change dramatically from the larval stage to the adult stage, and they change
in ways that are very different from that of the diurnal, high-light dwelling bonefish
[Albula vulpes (Linnaeus, 1758)].
The purpose of the work presented here is to assess some of the post-larval
specializations that develop in the retinas of different members of the Elopomorpha
with respect to their abilities to efficiently detect light for visual processes under
different lighting conditions. This work is discussed in the context of fish behavior
and the conservation of species that have significant ecological and economic values.
Materials and Methods
Animals and Tissue Preparation.—All animal procedures were approved by
the Institutional Animal Care and Use Committee (IACUC) of the Florida Institute
of Technology. Settlement-stage M. atlanticus, E. saurus, and A. vulpes were captured by plankton net during night-time flood tides at Sebastian Inlet, Florida, as
described previously (Shenker et al. 1993, Taylor and Grace 2005), immediately
euthanized in 100 mg L−1 tricane methanesulphonate (MS-222), and stored in
Grace and Taylor: Adaptations of the elopomorph retina
341
paraformaldehyde-based fixative (see Taylor and Grace 2005). For light-adapted juvenile E. saurus and M. atlanticus, fish were collected during bright daylight hours
from impounded mangrove marshes adjacent to the Indian River Lagoon in Brevard
County, Florida, by either seine net or hook-and-line fishing. Fish were immediately
euthanized with 500 mg L−1 tricane methanesulphonate (MS-222) and eyes removed
and placed in fixative solution. For dark-adapted juvenile E. saurus and M. atlanticus, fish were collected at dusk and held for 2 hrs in complete darkness prior to
enucleation under dim red light. Juvenile A. vulpes were collected during bright daylight hours by seine net from 1-m depth clear water at Key Biscayne and Naples,
Florida. Adult M. atlanticus and E. saurus were collected at night by hook-and-line
at Sebastian Inlet, Florida. Adult A. vulpes were collected by hook-and-line in waters
<2 m depth off Key Biscayne, Florida.
Specimen preparation and histological methods were as described (Taylor et al.
2011a,b). Briefly, tissues were embedded in Durcupan ACM epoxy resin (Electron
Microscopy Sciences, Hatfield, PA) for light microscopy (LM) and transmission electron microscopy (TEM). Sections were cut at 70 nm (TEM) or 1.0 µm (LM) on a Leica
Ultracut UC6 ultramicrotome (Leica Biosystems, Buffalo Grove, Illinois). For opsin immunofluorescence, tissues were infiltrated in 20% sucrose, embedded in OCT
compound, and frozen sections cut at 16–18 µm on a Leica CM1850 cryostat (Leica
Biosystems, Buffalo Grove, Illinois).
Immunofluorescence.—Frozen sections were incubated for 8 hrs at room temperature with the mouse monoclonal anti-rhodopsin MAB5316 (Millipore) at 1:500
dilution. Antibody dilutions and labeling procedures were performed as previously
described (Taylor et al. 2011a). This antibody (RRID AB_2156055) was raised by
hyper-immunization of mice with rat rod outer segments; it specifically labels rod
photoreceptor outer segments in mammals (Hicks and Barnstable 1987, Xu and Tian
2008) and fish (Taylor et al. 2011a). Primary antibody was omitted as a control (which
produced no labeling).
Transmission Electron and Light Microscopy.—TEM and light microscopy
were performed as previously described (Taylor et al. 2011a). Briefly, for TEM, sections were mounted on formvar-coated copper mesh grids and stained in a CO2-free
environment for 1 min with 0.5% aqueous lead citrate, rinsed with 0.01 m NaOH and
then stained for 1–2 min in 2% aqueous uranyl acetate. To enhance contrast, the tissue was rinsed again with CO2-free water and then stained for one additional minute
in fresh 0.5% lead citrate. Imaging was performed on a Zeiss EM900 TEM (Carl Zeiss
Inc., Peabody, Massachusetts) at 80 kV. Photoreceptor outer segments lengths and
widths were digitally measured from the longest/widest profiles in a single section
using AnalySIS 5/iTEM imaging software (Olympus Soft Imaging Solutions Corp,
Münster, Germany). Individual photoreceptors were randomly selected for measurement and sampled throughout all retinal regions (dorsal, ventral, nasal, temporal).
Obviously oblique OS were excluded from analysis and lengths were measured only
when the entire outer segment length was visible. Photoreceptor cell counts were
performed as previously described (Taylor et al. 2011a) and mean rod:cone ratios calculated across the retina and for specific retinal regions (dorsal, ventral, nasal, temporal). Cells were quantified in 1.0 μm–thick radial cross-sections using standard
light microscopy and digital imaging. Cells were quantified in the dorsal, ventral,
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nasal, and temporal regions, within four 0.05-mm strips in each region. Standard
errors were calculated for all cells quantified across these retinal regions.
Results
Rod Densities and Rod-Cone Ratios.—Photoreceptor densities and rod:cone
ratios were calculated from data in previously published papers (Taylor et al. 2011b,
2015). The adult speckled worm eel, Myrophis punctatus Lütken, 1852, a strictly nocturnal species, has a rod density of 105 rods 0.05 mm−1 and a rod:cone ratio of 28:1
(Taylor et al. 2015). In comparison, adult M. atlanticus has a rod density of 152 rods
0.05 mm−1 and rod:cone ratio of 21:1, while adult E. saurus has 100 rods 0.05 mm−1
and a rod:cone ratio of 17:1. Both of these species are often observed feeding both
diurnally and nocturnally, as well as during crepuscular periods (SM Taylor pers
obs). Finally, adult A. vulpes, primarily a diurnal feeder in very shallow water, has the
lowest rod density (76 rods 0.05 mm−1) and the lowest rod:cone ratio (13:1). Results
are summarized for the entire retina in Table 1 and rod:cone ratios for specific retinal
regions (dorsal, ventral, nasal, temporal) are summarized in Table 2.
Functional Morphology of Photoreceptor Outer Segments.—
Morphological analysis of the photoreceptor layer in adult M. atlanticus reveals a
retina specialized for light capture in low-light environments (Fig. 1). By both light
and transmission electron microscopy, it is readily apparent that rod outer segments
are highly organized into bundles. These bundled outer segments are stacked, but
parallel with each other, and appear to exclude other cellular components (i.e., cone
outer segments and retinal pigmented epithelium processes). By contrast, the diurnal A. vulpes, which occupies brightly lit, shallow coastal waters, lacks rod bundles.
Albula rods are thin, highly elongated, and arranged in multiple, overlapping layers
(Fig. 1).
Light Reflecting Tapetum Lucidum.—At the earliest stages of development,
the retinas of all elopomorphs studied lack visible evidence of a light-reflecting tapetum lucidum (M. atlanticus, E. saurus, A. vulpes, and a variety of eel species from
five different families; Taylor et al. 2011b). However, by 65 d after settlement in inshore juvenile habitat, E. saurus exhibits the development of a retinal tapetum in
conjunction with changes in the structure and organization of rod outer segments
(Fig. 2). By adulthood, both M. atlanticus (Figs. 1A–C, 3) and E. saurus exhibit a welldeveloped tapetum lucidum that completely surrounds rod outer segment bundles in
both light-adapted and dark-adapted conditions, and surrounds extended cone outer
segments in the dark. In contrast, the diurnal bonefish (A. vulpes) retina completely
lacks the tapetum lucidum throughout all developmental stages (adult shown in Fig.
1D–F).
Development of Retinal Architecture.—The light-reflecting tapetal layer
begins to develop during metamorphosis of the leptocephalus larvae. In early postmetamorphic E. saurus, there is a significant amount of melanin beyond and between
photoreceptor outer segments (Fig. 2), but the tapetum is poorly, if at all, developed.
However, within the first 2 mo beyond the start of metamorphosis, melanin levels
appear significantly reduced, and by the same time the formation of the tapetum has
begun (Fig. 2).
Megalops atlanticus
Elops saurus
Albula vulpes
Myrophis punctatus
Nocturnal/diurnal
Both
Both
Diurnal
Nocturnal
Rod density
129 (3.3)
100 (4.3)
76 (2.3)
105 (4.2)
Rod:cone
21:1
17:1
13:1
28:1
Rod OS length
5.96 (0.14)
5.83 (0.21)
21.12 (2.27)
16.65 (0.37)
Rod OS width
2.20 (0.66)
1.90 (0.43)
2.68 (0.23)
2.95 (0.25)
Tapetum
Y
Y
N
N
Grouped rods
Y
Y
N
N
Stacked rods
Y
Y
Y
Y
n
1
1
1
3
Table 1. Retinal specializations for low-light vision in four ecologically-distinct elopomorph species at the adult stage. Rod densities are measured in cells per
0.05 mm and rod measurements are in μm (n ≥ 3 cells per retina). See Taylor et al. (2011b, 2015) for detailed methods and additional information. Errors represent
standard error of the mean (in parentheses) for all cells quantified across all retinal regions. OS = outer segment.
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Table 2. Comparison among species of rod:cone ratios across different retinal regions at the adult
stage. See Taylor et al. (2011b, 2015) for detailed methods and additional information.
Megalops atlanticus
Elops saurus
Albula vulpes
Myrophis punctatus
Rod:cone
dorsal
18:1
22:1
12:1
35:1
Rod:cone
ventral
24:1
21:1
14:1
31:1
Rod:cone
nasal
18:1
17:1
14:1
28:1
Rod:cone
temporal
25:1
17:1
14:1
30:1
n
1
1
1
3
Photoreceptor outer segment morphology begins to change over the same course
of time. Early in the metamorphic process, rods are shorter and thicker compared
to 2 mo later when they are relatively longer and thinner, and appear to already be
grouping into bundles separated by tapetal material (Fig. 2). In M. atlanticus and E.
saurus, rod outer segments become smaller over the course of development, during
the same time period when rod densities dramatically increase (Fig. 2). At settlement
(metamorphosis), M. atlanticus rod OS are 18.3 mm in length and 4.9 mm in diameter,
Figure 1. Retinal specializations in elopomorph fishes associated with light habitat. (A–C) In
the diurnally/nocturnally active elopomorph M. atlanticus, rod outer segments (arrowheads) are
small in size, clustered into tight bundles and stacked into multiple layers, evident with (A)
anti-rhodopsin immunohistochemistry, (B) transmission light microscopy, and (C) transmission
electron microscopy; also, light colored, light-reflecting tapetal crystals (t) are clearly evident between the rod bundles. In comparison, (D–F) in the diurnal species A. vulpes, rod outer segments
(arrowheads) are much larger, are not bundled into groups and there is only melanin pigment (m)
but no tapetum. However, as in all elopomorphs studied so far, rods are stacked into multiple
layers. Scale bars: A, B, D, E = 25 μm; C, F = 2.5 μm.
Grace and Taylor: Adaptations of the elopomorph retina
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Figure 2. Ontogenic development of retinal specializations for low-light vision. (A) In the diurnally/nocturnally active elopomorph E. saurus, early post-metamorphic fish have a single layer
of cones (c) and rods (r) and only have melanin pigment (m) with no reflective tapetum. (B)
However, 65 d later, rods (r) have become smaller and more numerous, are starting to form distinct groups, are surrounded by lighter-appearing tapetal material (t), and are extended farther
out (sclerally) beyond the cones (c). Scale bar = 25 μm.
but at the juvenile and adult stages, these have been reduced to 9.9 mm length, 2.5 mm
diameter, and then 6.0 mm length, 2.2 mm diameter, respectively (Taylor et al. 2011a).
Using identical methods, rod OS lengths and diameters were measured here in E.
saurus, A. vulpes, and M. punctatus. From the settlement to juvenile to adult stage
in E. saurus, rod OS lengths decrease from 15.1 to 11.6 to 5.8 mm, respectively, and
OS diameters changed from 4.4 to 1.6 to 1.9 mm, respectively. In comparison, rod
outer segments in the diurnally-active species A. vulpes remain about the same size
throughout development (mean length 21.6 mm, diameter 2.6 mm), while in the nocturnal worm eel, M. punctatus, rod OS size only decreases moderately from settlement (length 23.7 mm, diameter 3.7 mm) to adult (length 16.7 mm, diameter 3.1 mm).
Retinomotor Movement.—In larval and early post-metamorphic elopomorphs,
rod and cone photoreceptor outer segments occupy the same positions in the retina
throughout the daily light-dark cycle (data not shown), but later in the juvenile stage,
substantial retinomotor movements develop in M. atlanticus (Fig. 3) and E. saurus.
Under light-adapted conditions (i.e., daytime; Fig. 3A–C), cone outer segments were
observed retracted toward the inner margin of the retina, nearer the incoming light,
while rod bundles were extended distal to the photoreceptor soma, farther from the
incoming light. Conversely, in the dark-adapted retina (i.e., nighttime; Fig. 3D–F),
cone outer segments were extended distally toward the back of the eye, while rod
outer segment bundles were observed retracted toward the inner margin of the retina, closer to the incoming light. It should be noted that melanin granules within the
retinal pigmented epithelium, which are known to exhibit retinomotor movement in
other species (see Burnside et al. 1982, Burnside and Basinger 1983), appeared not to
move according to lighting conditions in the species studied here. In both M. atlanticus and E. saurus, melanin granules form aggregations between or “capping” cone
outer segments in the light-adapted retina, and formed dense aggregations between
neighboring rod outer segment bundles in the dark-adapted retina (Fig. 3).
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Figure 3. Specialized retinomotor movements in juvenile M. atlanticus and E. saurus. (M. atlanticus shown; E. saurus is identical with respect to the results shown). (A–C) In the light-adapted
retina, entire groups of rods (r) extend sclerally and are completely surrounded by reflective
tapetal material (t); cones are retracted vitreally, exposed to incoming bright light, surrounded
on the scleral side by melanin pigment (m) and tapetum. Rods and cones are shown at higher
magnification in (B) and (C), respectively. (D–F) In the dark-adapted retina, cones extend sclerally and are surrounded by reflective tapetal material (t); Entire groups of stacked rods (r) are
retracted vitreally, exposed to incoming dim light, surrounded on the scleral side by reflective
tapetum and flanked by melanin pigment (m). Rods and cones are shown at higher magnification
in (E) and (F), respectively. Scale bars: A,B,C,D,E = 25 μm; F = 10 μm.
Discussion
Rod and Cone Cell Populations.—The larvae of elopomorph fishes (leptocephali) are highly rod-dominated (Taylor et al. 2011a,b, 2015), in stark contrast to the
pure-cone or highly cone-dominated retina in the larvae of most teleost fish species.
This suggests that elopomorph leptocephalus larvae are adapted for vision in lowlight conditions. This observation is consistent with the diurnal vertical migrations
of leptocephalus larvae, deeper during the day and shallower at night (Miller 2009),
that result in a perpetual dim-light existence prior to settlement. As leptocephali undergo settlement, generally moving into shallower coastal or estuarine habitats, they
undergo metamorphosis into the juvenile stage. During metamorphosis, both rod
and cone photoreceptors are rapidly added to the retina (Taylor et al. 2011a,b, 2015).
The retinas of all elopomorphs remain rod-dominated into adulthood, but rod
densities and rod:cone ratios can be excellent predictors of the degrees of diurnality
Grace and Taylor: Adaptations of the elopomorph retina
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and nocturnality exhibited by particular species (Pankhurst 1989, Fishelson et al.
2004). The results presented here clearly distinguish the benthic, nocturnal speckled
worm eel (M. punctatus), and the crepuscular and nocturnal tarpon (M. atlanticus) and ladyfish (E. saurus) from the strictly diurnal bonefish (A. vulpes). The former three species all have greater rod densities and higher rod:cone ratios than A.
vulpes, consistent with effective visual function in very distinct light environments.
Interestingly, M. atlanticus rod density was higher than even the benthic M. punctatus, but dramatic difference in eye size alone suggests that the Atlantic tarpon relies
much more heavily upon vision than does the speckled worm eel, which may rely
more heavily upon chemical cues for guidance of behavior (Fishelson 1995, Barreto
et al. 2010). Comparing rod:cone ratios among different retinal regions can also suggest differences in specific visual abilities and behaviors. For example, in M. atlanticus, substantially higher rod:cone ratios in the ventral and temporal regions suggest
better low-light sensitivity in the forward and upward-looking directions in this surface-feeding, nocturnal predator. In comparison, the comparatively high rod:cone
ratio in the dorsal retina of M. punctatus suggests greater low-light sensitivity in the
downward-looking direction in this nocturnal bottom-feeder.
Tapetum Lucidum and Photon Harvesting.—The tapetum lucidum (TL) is a
layer in the back of the eye that reflects light back onto the photoreceptors, increasing
the amount of light available for vision and thus increasing low-light visual sensitivity (Wagner et al. 1998, Schwab and others 2002, Ollivier et al. 2004, de Busserolles
et al. 2014, Francke et al. 2014). A reflective TL can be detected in a particular species by shining a flashlight into the eye and observing that the eye “shines” from
light reflected back through the pupil. Many species of fishes possess a TL, which is
strongly associated with nocturnality and with species living in deep water or high
turbidity (Somiya 1980, Takei and Somiya 2002, Francke et al. 2014). Fishes may possess either a choroidal TL with reflective material behind the retina, or a retinal TL
in which reflective material—often guanine crystals—is present in the cytoplasm
of retinal pigmented epithelial (RPE) cells (Schwab et al. 2002, Ollivier et al. 2004).
RPE cells create a layer behind (and sometimes extending between) the photoreceptors that can contain light-absorbing melanin pigment and/or light-reflecting tapetal
material.
Both M. atlanticus and E. saurus have a substantial retinal TL (Figs. 1, 2; see also
Taylor et al. 2011a, 2015). In E. saurus, the TL develops shortly after metamorphosis, around the time that feeding behavior begins (Fig. 2; Taylor and Grace 2005).
The common snook, Centropomus undecimalis (Bloch, 1792), is not closely related
but is ecologically similar to M. atlanticus and E. saurus, and also possesses a TL
(Eckelbarger et al. 1980), suggesting that the TL is ecologically beneficial for large,
crepuscular/nocturnal predatory fishes. In contrast, the diurnal, shallow-water elopomorph A. vulpes (Fig. 1D–F) and the strictly nocturnal worm eel M. punctatus do
not have a TL (Taylor et al. 2015). Instead, the RPE contains high amounts of melanin
pigment, indicating that it absorbs light rather than reflecting it back onto the photoreceptors. Diurnal species typically lack a TL because reflection and scattering of
light can decrease daytime visual acuity, resulting in a blurred image (Beynon 1985).
This further supports the conclusion that the A. vulpes retina is specialized primarily
for daytime vision. Conversely, M. punctatus, like many other anguilliform eel species (Elopomorpha: Anguilliformes), is a nocturnal species that burrows in sediment
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during the day. There are also no tapeta lucida in the few other eel species in which
retinal morphology has been studied (Braekevelt 1988a,b, Wang et al. 2011), suggesting that anguilliform eels generally may lack the TL, possibly because many rely on
olfaction rather than vision to detect food, predators, and conspecifics (Fishelson
1995, Barreto et al. 2010).
Rod Photoreceptors: Size Trade-Off, Stacking, and Bundling.—The sensitivity of individual photoreceptors has been modeled to increase along with increasing outer segment dimensions (de Busserolles et al. 2014). Therefore, one strategy for
increasing visual sensitivity in dim light is to increase OS length and diameter. This
is readily apparent in some deepwater fishes (de Busserolles et al. 2014). There is a
trade-off, however, because increasing the size of individual photoreceptors decreases photoreceptor density within the same lateral space. Thus, while larger photoreceptors are individually more likely to be activated by photons, fewer photoreceptors
will be activated per unit retinal area. This will result in reduced convergence of light
information onto higher order neurons and, therefore, decreased image brightness.
In M. atlanticus and E. saurus, rod photoreceptor outer segments become smaller
over the course of development while their densities dramatically increase. This suggests that in these species, convergence of light information may increase over the
course of development, along with image brightness and low-light visual sensitivity.
Several ecologically distinct elopomorph species have a multibank retina, meaning
that rod inner and outer segments are stacked into multiple layers. Examples include
the large predators M. atlanticus (Taylor and Grace 2005, Taylor et al. 2011a), E. saurus (Taylor and Grace 2005, Taylor et al. 2015), and A. vulpes (Taylor and Grace 2005,
Taylor et al. 2015), the nocturnal worm eel M. punctatus (Taylor et al. 2015) and the
catadromous Japanese eel, Anguilla japonica Temminck and Schlegel, 1846 (Omura
et al. 2003). A multibank retina is common in deepwater fish species that live in a
constant dim-light environment (Wagner et al. 1998, de Busserolles et al. 2014), but
is uncommon in coastal/shallow water species, such as the elopomorphs described
here. The stacking of rods, in theory, increases visual sensitivity by allowing a higher
rod density to be maintained in the retina, increasing convergence of light information onto higher order neurons and increasing the likelihood that photons will
stimulate rods before passing completely through the retina.
M. atlanticus and E. saurus rod inner and outer segments are grouped into tight
clusters in addition to being stacked (Fig. 1) (Taylor and Grace 2005, Taylor et al.
2011a, 2015). This specialization is thought to pool light information from many rods
to increase visual sensitivity in low-light conditions, and may be a specialization to
enhance contrast and movement detection in water with little light or poor optical
quality (Pusch et al. 2013, Francke et al. 2014). In most cases, grouped photoreceptors
are associated with a reflective tapetum as seen here, where photoreceptor bundles
are interdigitated with wedges of RPE containing reflective tapetal crystals (Somiya
1980, Francke et al. 2014). This arrangement effectively produces reflective “cups”
around the rod clusters, trapping and scattering the light within the cups and dramatically increasing the amount of light available for absorption by the rods (Francke
et al. 2014).
Together, the presence of stacked and bundled rod outer segments, high rod densities, high rod:cone ratios, and the presence of a well-developed tapetum likely provide M. atlanticus and E. saurus with exceptionally good low-light/nighttime visual
Grace and Taylor: Adaptations of the elopomorph retina
349
sensitivity compared to many other coastal teleost species that lack this combination
of features. A. vulpes—another elopomorph—provides an excellent contrast. This
shallow-water, high-light, diurnal species lacks all of these adaptations for efficient
low-light visual function.
Retinomotor Movements.—If too much light falls on the retinal photoreceptors, particularly the highly sensitive rod cells, they quickly become overstimulated
and nonfunctional. The mammalian eye adjusts the amount of light falling on the
retina by constricting or dilating the pupil, but aside from a few known exceptions
(Douglas et al. 1998), pupil diameter is fixed in teleost eyes (Schmitz and Wainwright
2011). To adjust light capture by photoreceptors, the retinas of fish (and some other
taxa) may utilize retinomotor movements in which photoreceptor outer segments
and melanin pigment granules move within the retina to optimally position according to time of day (Burnside et al. 1982, Burnside and Basinger 1983, Taylor and
Grace 2005). In bright light, cone outer segments and melanin pigment granules of
some species move vitreally within the retina, toward the front of the eye. At the
same time, rod outer segments move toward the sclera at the rear of the eye, behind
the melanin, which decreases the amount of light reaching rods (Fig. 2B). This places
cones, which function optimally in bright light, directly in the light path while protecting rods from over-stimulation. Opposite movements occur in the dark: rods
move vitreally, while cones and melanin pigment move sclerally, allowing rods maximum exposure to available light. Retinomotor movements, therefore, modulate retinal architecture for optimal photon capture according to lighting conditions.
The capacity for retinomotor movement changes over the course of development.
In at least some species, the retina becomes capable of retinomotor movements either
during or shortly after metamorphosis from the larval to the juvenile stage (present
study, Taylor and Grace 2005, Mukai et al. 2012). In E. saurus, the distance the rods
extend (in bright light) increases more than 2-fold within the first 65 d after metamorphosis (Fig. 2) and continues to increase through the juvenile and adult stages
(Taylor and Grace 2005). These observations suggest that, in at least some species,
the ability of the retina to adjust to light level increases through ontogeny.
While retinomotor movements have been identified in many teleost species, interspecific comparison of the characteristics and extent of these movements is lacking.
In the elopomorphs M. atlanticus and E. saurus (but not A. vulpes), dense groups
containing exclusively rods or cones are maintained in both light and dark conditions, and these entire clusters undergo retinomotor movements (this report; Taylor
and Grace 2005). In bright light, cone clusters surrounded by RPE cells containing
reflective tapetal material contract vitreally toward the outer limiting membrane.
This results in the formation of tapetal “cups,” where reflective tapetal material surrounds the cone clusters except on the side facing the incoming light. Meanwhile, rod
bundles extend sclerally and are completely surrounded by RPE/tapetum, preventing
bright light from reaching rod outer segments. In the dark, the opposite movements
occur, causing rod clusters to be exposed to incoming light, surrounded by reflective
tapetum on the remaining sides (Taylor and Grace 2005). Similar exclusive photoreceptor groupings and associated retinomotor movements have been described in
a small number of species, the majority of which live in low-light conditions or high
turbidity (Francke et al. 2014) and where increased light sensitivity and/or contrast
detection should be important.
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Bulletin of Marine Science. Vol 93, No 2. 2017
Implications for Conservation.—Anthropogenic disturbances including
coastal construction runoff, waste and nutrient influx, and other forms of pollution
can substantially increase turbidity and eutrophication, significantly altering optical
qualities of coastal and estuarine waters (Horodysky et al. 2010). When these influences are combined with biological, environmental, and meteorological events, optical quality can change abruptly and dramatically (Koltes and Opishinski 2009). Both
experimental and theoretical studies indicate that increased turbidity can harm
feeding abilities in some fishes (e.g., large, visually-guided predators), while enhancing feeding abilities in others (e.g., small fish larvae) (Utne-Palm 2002, Horodysky
et al. 2010, Ranåker et al. 2012). These differential effects might exert selective pressures on community structures. How particular species and/or developmental stages
are affected by changes in optical qualities of the water depends upon specific feeding behaviors and visual capabilities, dictated in part by retinal morphology and
function.
Turbidity and eutrophication reduce light transmission through the water, resulting in reduction of the total amount of light available for vision (Omar and Matjafri
2009). Overall light intensity is also reduced if there is a reduction in light availability or an increase in water depth. It is therefore common for species that live in
high turbidity, are nocturnal, or that live in deep water to have retinal specializations that improve low-light visual sensitivity. Consistent with this, the elopomorph
species M. atlanticus and E. saurus, both of which live at least part of their lives in
highly-turbid estuaries and are both nocturnally and diurnally active, have exceptionally high rod:cone ratios, a tapetum lucidum, grouped/stacked rods, and specialized retinomotor movements. All of these characteristics have also been described
in deepwater species (Wagner et al. 1998), and in the nocturnally active mormyrid
fish, Gnathonemus petersii (Günther, 1862) that inhabits highly turbid rivers and
streams (Pusch et al. 2013, Francke et al. 2014). It therefore might be expected that
in nocturnal species with retinae adapted for low-light visual sensitivity (e.g., M. atlanticus), vision will be less affected by changes in water clarity than in species that
have retinas adapted for high-light/daytime vision (e.g., A. vulpes). However, given
the low light levels and challenges that nighttime imposes on the visual system, it is
unknown how further reductions in available light will affect the abilities of nocturnal species to feed, find predators and locate mates.
In summary, the extremely low availability of light at night makes vision challenging for any nocturnal animal. The challenge is even more important in water, where
clear water by itself reflects, scatters, and absorbs light, and dissolved and particulate
matter reduce light availability even more. Nocturnally active fishes can have a variety of retinal specializations that improve sensitivity to light. Comparative studies
utilizing elopomorph fishes, which begin life as similar larvae with similar retinas
and later diverge into radically different forms, have improved our understanding of
retinal specializations for nocturnal vision. The retina, however, is only one part—
the starting point—of the visual system, and many of the functional attributes of
the retina and brain associated with nocturnal vision are still poorly understood.
For example, modifications of the biochemical phototransduction apparatus in photoreceptor outer segments, and neural processing within the retina and in visual
centers in the brain could each (or all) be modified to affect visual sensitivity. To fully
understand adaptations of visual systems to life at night, future studies must focus
Grace and Taylor: Adaptations of the elopomorph retina
351
also on photoreceptor biochemistry and neural information processing. Ultimately,
increasing our understanding of retinal adaptations will better define how fishes interact with their environments and how they might be affected by human-induced
and natural habitat disturbance.
Acknowledgments
The authors thank M Scripter, Florida Institute of Technology, and M Larkin, University of
Miami RSMAS, for help with collection of juvenile and adult specimens.
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