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DISPATCH 8: - 9.20.2006 | Tammy Frank et al. Well, it's time to give a wrap up of some of the research that was going on this cruise. In addition to telling you about the research my lab conducted, I've also asked the other researchers on board to give a summary of their work. 
In my lab, we study the visual physiology of deep-sea animals. My graduate student, Beth Whitehill, and I are
studying whether there are differences in the visual physiology of shallow living (175- 250 m) juveniles and deeper
living (600-800 m) adults of the lophogastrid Gnathophausia ingens. I've been examining the spectral sensitivity and response characteristics, while Beth has been fixing eyes for structural studies, to determine whether there are differences in eye function that help them see well in both light environments, i.e. the relatively bright light in the shallow water environment, and the relatively dim light in the deep environment. The eyes of the juveniles are obviously smaller, which would make them less sensitive to light, but Beth is also finding structural differences between the eyes of adults and juveniles that would also serve to reduce the sensitivity of the juveniles. I'm finding that the juveniles are more sensitive to the shorter wavelengths than the adults, which could be due to the presence of a short wavelength pigment that is lost in the adults once they migrate into deeper waters. In addition, I've also determined that temperature has a significant effect on the response speed of the eye, i.e. warmer temperatures significantly increase the response speed of the eyes of both juveniles and adults. This is easier to understand if you think of the speed of the eye in terms of shutter speed of a camera. In dim light, you need a very slow shutter speed in order to get any picture at all, and chances are, the image will be blurred, but at least you know that something is there. In brighter light, you can use a faster shutter speed and the image will be clearer. For the juveniles, living in brighter, warmer surface waters, this increase in response speed helps them see their predators and prey more clearly. This means that there's a decrease in sensitivity, but since they live in relatively bright shallow waters, they can afford to sacrifice sensitive for a clearer image. In the colder darker depths where the adults live, high sensitivity is the name of the game, so the colder temperatures found at these depths are actually beneficial to the adults in that it helps increase the sensitivity of their eyes.
Beth and I are also studying a phenomenon known as eye glow area, which results due to the presence of
a reflecting tapetum or mirror behind the retina. If photons of light pass through the retina without
being absorbed by the visual pigment, they hit the "mirror" behind the retina and bounce back into the
retina, giving the visual pigment a second chance to collect light, and essentially doubling the sensitivity of the eye. This is present in a lot of night active mammals, like dogs and cats, which is why they can see so much better in the dark than we can. Shine a flashlight at your pet's eyes tonight, and see them appear to glow - this is known as eye glow.
A lot of deep-sea shrimp also have reflecting tapeta in their eyes, and the size of the eye glow area can give us an estimate of the aperture of the eye, i.e. how big their equivalent of a pupil is. In order to do this, a light is shined on the eye, and the resulting eye glow is photographed and stored for later analysis. Since these are very light sensitive species, this has to be done under dim red light because we don't want to destroy their eyes (and hence disrupt the eye glow) with white light, and my physiological experiments have demonstrated that their sensitivity to red light is very low. Unfortunately, our sensitivity to dim red light isn't that great either, and Beth has been seriously straining her eyes in order to make these measurements (but then, it wouldn't be the true graduate student experience without some suffering). We are getting some interesting results, in that the dorsal (back side up) eye glow area is substantially smaller than the lateral (end on) or ventral (stomach side up) eye glow area 
We'll be looking at some of the juveniles when we return to the lab (we hope to cart three ice chests of live animals back with us to Florida), and then compare these results with my electrophysiological and Beth's histological results to get a comprehensive picture of the visual systems of both the juveniles and adults.
Copepods are tiny crustaceans (shrimp and lobsters are crustaceans as well), ranging from microscopic in size to almost a centimeter long. Jon Cohen, a Harbor Branch post-doctoral fellow in my lab has been working on the visual system of a deep-sea copepod, quite a remarkable feat since these animals are about the size of a grain of rice. Gaussia is a large (relatively speaking - shown in picture next to a dime) black copepod that's very common at this trawl site. Gaussia Visual System: - Jon Cohen The eye of the copepod Gaussia princeps has a very unusual structure, consisting of 3 small eyes (~ 6 cells each) fused together (at arrow in photo), with each positioned behind a reflector. In other copepods with similar eyes the reflectors are positioned behind the eyes, not in front as in Gaussia. This unusual configuration in Gaussia creates a small area in front of the animal where light can reach the eye, while light is blocked from other angles. Why? Perhaps these reflectors serve to block light created by this copepod (it is brilliantly luminescent) from blinding its own eye. The verdict is still out on this. 
The Gaussia eye is most sensitive to blue light, which makes sense, since blue light is pretty much the
only wavelength left at their daytime depths. It has a relatively slow eye, and temperature affects the speed
of the eye. This is of interest for Gaussia as this animal migrates from colder water during the day
(~500 m) to warmer water during the day (~300 m). The speed of Gaussia's eye more than doubles with every
10 °C increase in temperature. This is similar to what one would expect based on how the speed of biochemical reactions changes with temperature, and vision is a biochemical reaction. Gaussia may be able to take advantage of this temperature-induced increase in the speed of its vision. Gaussia feeds at night, and very fast luminescent flashes of its prey that would be too short for it to see in cold water may be visible in warmer water.
Hans-Joachim Wagner is the head of the Max Planck Research School for Neural and Behavioral Sciences in Tubingen, Germany, as well as the head of the anatomy department at University Tubingen. He joined us on this cruise to continue his studies on the visual systems of deep-sea fish. During the more than 20 trawls, I had the opportunity to collect material from more than 250 specimens. These covered a wide range of species, with special emphasis on four species of hatchetfish, several stomiids, bigscales, snipe eels and gulper eels. For my projects I mainly collected isolated pineal organs, brains and eyes. 1. Visual system of mesopelagic fish In a comparative study of the sensory systems of mesopelagic fish (Wagner, H.-J. (2001) Sensory brain areas in mesopelagic fishes. Brain, Behavior and Evolution 57, 117-133) I have shown that the great majority of species relies primarily on vision. However, while the spectral sensitivity and the visual pigments have been extensively characterized, the neural basis of vision is poorly understood. This is all the more surprising as deep-sea fish, while being highly specialized for maximizing sensitivity, have a retina which is less complex than in surface fish. They have no cones, hence lack color vision, and therefore their retina contains significantly fewer types of nerve cells. Therefore deep-sea fish present an interesting model system for the study of basic neural mechanisms in a monochromatic visual system. With the material collected on this cruise we want to study the retina-tectal system of ganglion cells. For this purpose several experiments were performed in which fluorescent tracer molecules were introduced either into the eye or the optic tectum. Isolated brain-retina preparations were then cultured for 2-3 days in order to allow the tracer to be transported to the target area. Analysis of brain sections and retinal wholemounts with a confocal microsope will allow us to identify the population of retinotectal ganglion cells on the one hand, and to study the target layers and cells in the optic tectum. An additional aspect of this approach consists in a comparative study of the neural organization of the optic tectum in various species of mesopelagic fish with different sensory preferences, with highly visual species such as hatchet fish on the one hand, and non-visual species such as melamphaeids or gulper eels on the other. While previous studies had focused on the sheer volume on the various sensory areas, the present material will allow us to study cell types and transmitter systems. 2. Biological rhythms in mesopelagic fish Many mesopelagic fish undertake diel vertical migrations (meaning they migrate from deep dark depths into surface waters at night to feed, and then back down to the depths during the day to hide from their predators) and are thus strongly coupled to the solar cycle. This is in contrast to deep-sea fish from the seafloor of the abyssal plains which live outside the reach of sunlight. Since melatonin, primarily released by the pineal gland, serves as a signal molecule to synchronize the various rhythms of individual organ systems, we have studied the melatonin content of pineal glands at different times of the 24h cycle. In a number of demersal (living just above the bottom) species, no coupling to the solar cycle was observed. By contrast, preliminary findings from last year's New Horizon cruise showed that for mesopelagic fish night-time values were consistently higher than day-time values, suggesting the presence of a circadian rhythm in these fish similar to most surface fish. The material from the present cruise will allow us to confirm the previous observations. In a second approach to the study of endogenous rhythms in mesopelagic fish I collected isolated pineal glands, brains, retinae, liver, and muscle tissue preserved in "RNA later" for the study of the molecular organization of the endogenous clock. In collaboration with Dr. R. Lucas (University of Manchester) we want to isolate the oscillator genes and to compare them to known sequences from the zebrafish. Finally I collected isolated dark adapted retinae from Aristostomias grimaldi and several other mesopelagic species for experiments on the properties of visual pigments to be carried out by Prof. R. Douglas (City University, London) and Dr. J. Partridge (Bristol University) Jim Bishop, a senior staff scientist at Lawrence Berkeley National Laboratory, contacted me to find out if there was any room for him on my cruise, as he needed to test a new piece of equipment. I hadn't planned on trawling from around midnight until 7 a.m., and offered him that time, with the disclaimer that if we were having trouble catching the animals we needed, we would be going to 24 hour ops. We were both fairly confident that he would get a chance to deploy his instrument, so he and Todd Wood, a member of his group, came along on this cruise as well. Shiptime is very expensive and difficult to come by, so most of us welcome the opportunity to share any available space or time with other scientists not directly involved with the funded research to maximize the cost benefits. CO2 AND THE OCEAN BIOLOGICAL CARBON PUMP The biological systems within the ocean play a key role in the global cycle of carbon and thus in regulation of atmospheric CO2. Every marine plant is eaten once per week (on average) by the zooplankton and fish. Sinking detritus from 'dinner leavings' and fecal matter transports a huge about of carbon downwards each year. The rate is ~10 Pg C/yr (1 Pg = 1015 grams). This carbon redistribution in the sea, commonly called the 'biological carbon pump', is very fast, highly variable, and very hard to observe from ships. A lack of good observations of the ocean biological carbon pump means that it is not possible to predict with confidence the consequences of anthropogenic CO2 induced warming and ocean acidification on the life processes that sequester carbon in the sea. Aboard WECOMA, we are testing new robotic observing technology that will overcome observing limitations in two ways. CARBON FLUX EXPLORER First, we had hoped to launch for the first time a totally free and autonomous vehicle called the Carbon Flux Explorer. The CFE is designed to follow the daily variations of carbon sedimentation for seasons at various depths in the ocean and to report findings via real time satellite link to shore. The CFE vehicle is based on the ocean profiling Sounding Oceanographic Lagrangian Explorer (SOLO) developed at Scripps Institution of Oceanography by Russ Davis and the engineers of the Instrument Development Group. SIO has modified the SOLO so that it can carry our carbon flux measuring instrument. Our instrument is designed to passively observe the sedimentation of sinking carbon particles. Its components include an imaging system, external and internal lights, computer, and controller and about 2.5 kgs (5 lbs) of lithium batteries. The SOLO and our flux instrument (the payload of the SOLO) were ready for first launch into the ocean on Sept 15th. There was a long check list of items prior to launch and each one must pass in order to proceed to the next step. Our instrument and SOLO were operational, mission commands programmed; however, at step #23 of the launch sequence (think space shuttle count down) we lost communication with the SOLO's computer. Attempts to revive it failed. The CFE will not see the ocean until engineers at SIO understand the cause of the failure. Failures are an expected result of developing ocean instruments. BUOY TESTING We have a second program to support the CFE development. We have twin carbon flux measuring instruments operating at 250 m below a surface BUOY. These instruments are in all ways identical to that of the Explorer. Both instruments should 'get' the same result and thus give us confidence that the technology is robust. These twin systems are also outfitted to collect samples which will be analyzed for calibration purposes. The aim is to gain days of experience with the instruments under water. The BUOY has been deployed and recovered several times on this cruise, and is now currently deployed again, waiting to be recovered on Thursday. We know where the BUOY is since it gives us GPS (global positioning system) every half hour. By the time WECOMA docks Sept 22nd, we expect to have ~10 days of data to analyze ashore. |
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