Abstract
The purpose of my experiments was to test the behavioral response and capabilities of
the cubomedusan Tripedalia Cystophora to various visual stimuli; My hypothesis was
that the cubomedusae would respond more strongly (be more attracted to) color toward
the blue end of the visual spectrum as well as to reflective small objects within the water
that mimic their food sources. Lab Experiments showed that T. Cystophora did indeed
favor blue light (440 nanometers, 480 nanometers) over red light (700 nanometers) when
a tight cone of light of a given color was placed at a the end of a rectangular tank. Green
light was intermediatly favored (515 nanometers, 540 nanometers) as was violet light
(385) nanometers. Experiments also showed that T. Cystophora were attracted to long
wave Ultra Violet Light (365 nanometers) but not significantly to short wave Ultra Violet
(254). Also, in experiments testing preference for texture of the light cone, T. Cystophora
favored reflective bodies within the light cone (chemically neutral brine shrimp, small T.
Cystophora) over only the light cone. A uniform rising movement was also noticed upon
the onset of darkness.
Introduction
The Cnidarian medusae Tripedalia Cystophora, found in the waters of Puerto Rico,
is a member of class Cubozoa. Cubomedusae are gelatinous organisms capable of bell¬
pulsing locomotion but are unable to swim against strong currents. Members of class
Cubozoa (formerly included in the Scyphozoa) differ in several unique characteristics,
including their box shaped exumbrella, a more potent nematocyst toxin, and rhopalial
sensory stalks having complex eyes with cornea, spherical lenses and retinal cells
connected to nerve fibers (Yomatsu & Yoshida, 1976; Laska and Hundgen, 1982). T.
Cystophora is a smaller member of the class, having a noticeably cube shaped bell of
about one cm in diameter. Their rhopalial stalks are positioned on each "wall" of their
cubic bell, with the complex eyes facing inward, one upward and one downward. Each
rhopalial stalk contains two complex eyes, two eyes slits with rudimentary enlongated
lenses, and two simple ocelli (Stewart 1995). As their sixteen ocelli (light sensitive
organs) are adequate for directional light sensing by themselves, this study attempts to
provide stimuli that mimic images of potential behavioral importance in order to
determine the role of their apparently camera-type eyes. Given the elaborate design of
their complex eyes and their apparent high level of visually organized behavior
(Matsumoto, 1995, and Stewart, 1995), it remains to be determined whether or not
Cubozoans have more than mere directional light sensitivity, or in fact have image
forming eyes.
There has been much interest among researchers as to the presence of a complex eye
and no apparent brain or centralized nervous system; debate remains as to if and how the
medusae’s ganglionic nerve ring processes and responds to images so as to coordinate
image oriented behavior. Field studies of T. Cystophora in its natural mangrove habitat
have shown the medusae feeding by making "passes" through the light shafts caused by
the overhanging foliage and mangrove roots (Stewart 1995). The theoretical interest of
this study is the relationship between the behavior of T. Cystophora and the dynamics of
these light shafts, with particular interest as to the capabilities of their visual system.
Behavioral studies have shown that Cubozoans are attracted by light and bright objects,
and are repelled by darkness and dark objects, such as divers, seagrass beds, and wharf
pilings (Matsumoto, 1995 and Page, 19
Methods-
My Apparatus was designed to mimic a light shaft as it would occur in a mangrove
swamp. I placed a focused illuminator at one end of the long axis of a 60 * 30 * 30 cm
rectangular tank, one sixth of the way down the tank, as shown in Fig. 4. It produced a
tight cone of light, 2 cm in diameter at the top and 4 cm at the bottom, much like a light
shaft in sea or lagoon water. In all of my experiments, I videotaped the tank with a low
light camera as I turned the light column on and off; I used two diffuse red lamps
suspended high above the tank to provide ambient light. The lights were sufficiently
diffüse so that the medusae showed no sign of their characteristic phototactic response.
The tank was surrounded by neutral gray paper.
My method of observing preference was to plot the horizontal locations of the jellyfish
at regular intervals during a given trial, and then take the mean horizontal location for a
given time and compare the means over the time of the experiment. I did so by analyzing
the videotape of the experiment and plotting the positions off of the tape, using arbitrary
units of measurement such that the tank measured 300 of these units in length, so each
unit of displacement was 0.2 cm of the actual tank. As my goal was to link changes in
light to changes in behavior, I always had 3 minutes of darkness before 2 minutes of the
light cone followed by another 3 minutes of darkness; this was to allow them to
randomize, which they do quite significantly at the onset of darkness.
Methods: Color Preference
Firstly 1 tested the reactions of the cubomedusae to light shafts of different colors,
spanning from red to ultraviolet. To test the animals’ response to a given color, I would
place one of six 2"*2" filters on a small, transparent plexi-glass suspension bridge over
the tank such that the entire beam of the illuminator passed through the filter into the
water. The filters were of the following wavelengths: 380 nm (violet), 440 nm (blue),
480 nm (blue), 515 nm (green), 540 nm (green), and 700 nm (red) and all of the same
optical density, allowing 40% of the light to pass through. Then, I used a Mineralight UV
lamp to repeat the experiment with 254 nm and 365 nm UV light. The Mineralight UV
lamp was significantly more diffuse than the beam of the illuminator, so it was not
compared to the other colors. The Color experiments were all done in succession, on the
same hour in the same day.
Methods: Light Texture
My second experimental series studied the changes in the behavior of T.
Cystophora in response to changes in the texture of the light shaft; I am defining texture
in this case as local variations within the visual field at a given time. To test this, I used
the same apparatus as in the color preference experiment, yet this time, altering the
texture of the illuminator’s light column so as to provide analogs to images and textures
from the cubomedusae’ s natural habitat. Firstly, I applied the cone of light alone (3 min
of darkness, 2 min of light cone, 3 min of darkness), as shown in Fig. 4. This was to
simulate a simple light shaft. Secondly, I placed a 30 cm * 3 cm diameter cylindrical
glass tube, capped at the bottom, densely filled with brine shrimp, in the path of the light
shaft, so that the light shaft was contained within the cylinder, as shown in Fig. 5. The
same 3/2/3 conditions were applied to this setup. This setup was an attempt to mimic the
rapid swimming motions of their copepod prey, Oithona nana (Stewart, 1995). Lastly, I
removed the shrimp column and subjected the cubomedusae to 3 min of darkness; upon
turning on the light, I injected directly down into the light column roughly 25 young
cubomedusae, the largest no greater than 2 mm in bell diameter, as shown in Fig. 6. I left
the light shaft on for 2 min, then subjected the cubomedusae to 3 min of darkness. The
young cubomedusae are highly phototactic and the illumator'’s light column acted as a
sort of trap, keeping the young cubomedusae within in for the length of the illumination.
The purpose of using young cubomedusae was to place bright reflective moving specks in
the light column, in order to simulate their copepod prey, potential mates, or simply other
feeding cubomedusae.
Results
For each of the six filter colors, T. Cystophora exhibited its characteristic
phototactic behavior, shown in Fig. 1. All such reactions (changes in mean horizontal
location) were statistically significant over no light (all p«0.01). As far as preference for
colors, the reaction was strongest for blue at 480 nm, followed by blue at 440 nm, violet
at 380 nm, green at 540, and red at 700 nm. Only blue at 480 nm was significant over
the other colors, p=0.034, and blue at 440 nm was almost so, p-0.052, given a double
tailed t-test with a alpha of 0.05. The mean horizontal displacement in response to the
short wave ultraviolet (254 nm) did not significantly change. The long wave UV (365
nm), however, elicited a change in mean horizontal displacement from 142 to 118 while
the light was on, with the lamp at approx. 70 (again, tank length 300), as shown in Fig. 2.
The light from the UV Lamp was significantly more diffuse than that from the
illuminator; it was not so much of a shaft of light as it was simply shining on that quarter
of the tank.
For the texture experiments, with the light at 50, the mean horizontal displacement of
the cubomedusae reached 108 for normal white light (Fig. 3). For the tube of Brine
Shrimp, the mean displacement reached 96. For the small cubozoans within the light
column, the mean displacement reached 69. The mean displacement without light was
141.
Discussion: Color Preference
Given the significant preference for blue at 480 nm, it seems that color of light
can influence the cubomedusan behavior. This may be due to blue shifted opsins; 480
nm is the wavelength of light which most readily penetrates sea water. This finding
would support the Sensitivity theory of vision, which says that an individual sees best in
what wavelengths are most available in a given visual system (Lythgoe, 1979). Given the
nature of the visual system of T. Cystophora, this could account for increased sensitivity
to the location of light shafts at distances greater than one meter, through horizontally
scattered light which appear blue (Loew and Lythgoe, 1985). Further support for these
conclusions will require increased knowledge of the opsins of the cubomedusan eye.
Futher experiments in which order of light exposure is varied may show even more
conclusive behavioral preference, perhaps mimicking natural enviromental cycles and
wavelength distributions.
Again defining texture as localized differences in the visual field at a given time,
my texture experiments were tailored to examine degree of change in behavior to change
in texture. Although both the shrimp tube trial and the young cubomedusan light trap trial
were statistically significant over white light shaft alone, the nature of the increased
response is unclear; the stimuli clearly cause changes in mean position, but it could be
due to the texture of the light shaft or simply to increased intensity of horizontally
scattered light. The observed preferences may be representative of secondary
characteristics of the eyes, i.e. a movement response mechanism, image response
mechanism, etc., but better controlled stimuli will be necessary, with more clearly
defined visual signal characteristics. Further experiments which compare chemically
neutral objects that are moving within the light column and scatter as much horizontal
light as a more intense control light column might show actual preference.
There has been doubt that image formation is possible with the application of
optical theory to the cubomedusan complex eye; the spherical lens of T. Cystophora
would have to have a refractive index greater than that of any known biological lens
substance in order to form images given its distance from the retina (Land, 1981 and
Nilsson, 1989). However, the inferometric placement of their sixteen ocelli being
sufficient to detect directional light sources, the presence of their complex eye system
implies higher function other than directional phototaxis. Given inward position of their
complex eyes, it is possible that by looking through their bell they create an effective
compound lens; although it is highly transparent, it may have regions with sufficiently
high refractive properties to accomplish this. Analysis of this could be conducted with by
testing the index of refraction of the regions of the bell; the use of a laser pointer was
suggested by Christian Reilly. If the eyes are not image forming, however, then perhaps
they are light detection mechanisms of a greater quality than the ocelli can provide,
perhaps infering distance to target quantities from inferometry. Given their apparently
heightened blue sensitivity, long distance light shaft detection (hence blue light) might
explain the need for complex eyes. Experiments to test their ability to sense shafts of
different colors at long distances (or at short distances and of low densities) might shed
light on this possibility.
Conclusions:
T. Cystophora demonstrated preference to blue light at 480 nm over several other
colors, but without knowledge of the opsins of the cubomedusan eye, the presence of
color vision or simply blue shifted opsins isn’t conclusive. Also, T. Cystophora’s
preference for visually “loaded" textural schemes of an analog of a mangrove light shaft
versus an empty light shaft may display image processing and response, or possibly only
an direct relationship between light intensity and strength of phototactic response.
Acknowledgements:
1 would like to thank in the extreme Dr. Stuart Thompson, Melissa Coates, and
Christian Reilly for their invaluable guidance and assistance during this study, as well as
for their tutelage in the field of Visual Ecology. I would also like to thank Dr. James
Watanabe for his prolific assistance, Chris Patton for his assistance and patience, and Dr.
George Matsumoto for his experimental expertise.
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Horizontal Response to Colored Light Column vs. Time (3 min dark, 2
min Light, 3 min Dark)
160
140
——blue 480
—— blue 440
-red 700
——violet 380
-green 540
——Control
50 100 150 200 250 300 350 400 450
Time (seconds)
Figure 1. The above graph shows the mean horizontal position of the cubomedusae vs. time, with
vertical lines at 180 sec and 300 sec to show when the light went on (180) and off (300). The spectra of
the filter are listed in the legend and representative lines are color matched to their respective wavelength.
The light was located at 50, represented by the horizontal line.
Horizontal Response to Ultra Violet Light (Diffuse) vs. Time
180
160

310
3 120

100
——UV Short (254nm)
—UV Long (365 nm)
100
20 Ime fseconds)
40
500 600
Figure 2. This graph displays the mean horizontal position of the cubomedusae vs time for the UV
spectral response experiment. The vertical lines represent when the light went on (180 sec) and off (360
sec). The UV lamp was located at 70, represented by the horizontal line.
160
140
120
100
—e—ight
—shrimp
—cubes
50 100 150 200 250 300 350 400 450 500
Figure 3. This graph displays the mean horizontal position of the cubomedusae vs. time for the light
texture experiment. The vertical lines represent when the light came on (180 sec) and went off (300 sec)
The light was located at 50, represented by the horizontal line.
Figure 4. The light shaft created by the illuminator alone.
Figure 5. The light shaft with the tube of brine shrimp in it.
Figure 6. The light shaft with the small cubomedu.
Filter Spectra
Violet (2380
2.000
ABS
———————————— —— — —
E
-0.100
500
600
400
nm
Blue (2440 nm
2.000
ABS

-- - - — -
— — — — —
— —

-0.100
500
600
400
nm
Blue (Q480 nm
— —— —
— ——
700
700