September 7, 2012
How Do Crabs See Food on the Ocean Floor?
A few years ago, when Tamara Frank, Sönke Johnsen and Thomas Cronin, a team of marine biologists, descended nearly half a mile to the ocean floor near the Bahamas in a tiny submersible*, they were fairly stunned by what they saw: close to nothing. “We were surprised by how little bioluminescence is down there,” Frank told LiveScience. In one of the world’s first explorations of bioluminescence on the deep ocean floor, they found that, unlike in the open ocean, where scientists estimate that 90 percent of organisms produce bioluminescent light, just 10 to 20 percent of the creatures at the bottom of the ocean (mainly plankton) were capable of glowing.
When the team parked the submersible, shut the lights off and simply observed, though, they were amazed. “If you sit there with the lights out, you’ll see this little light show as plankton run into different habitats,” Johnsen said. “There is no substitute for actually being in that habitat to understand what it’s like to be those animals.” Over time, they identified several organisms that no one expected to glow that were generating light, including coral, starfish, sea cucumbers and the first-ever bioluminescent sea anemone, as described in a study published yesterday in The Journal of Experimental Biology.
They also discovered that the several species of crabs inhabiting the ocean floor had a very unusual characteristic: As described in a concurrent paper published in the same journal, they found the first crabs ever identified as capable of seeing ultraviolet (UV) light.
While measuring the wavelengths of light produced by each of the organisms, the team noticed in particular the crabs’ skill at grasping plankton and other food to eat. “They just hang out in these plantlike things, and every so often—they have these amazingly long claws—they reach over and they’re clearly picking something off and bringing it to their mouths,” Frank said.
Intrigued, they tested the crabs’ vision for themselves. Using special equipment on the submersible, they suctioned the creatures into light-tight containers and brought them to the surface, then conducted an experiment aboard their ship. Flashing various colors and intensities of light at the crabs while using electrodes to monitor their eye movement, Frank discovered that all seven species tested were capable of seeing blue light. This wasn’t particularly surprising, as blue is the only color of light that can naturally penetrate down to the ocean floor as all other colors are filtered out by the water.
The second part of the experiment, though, was rather surprising. Two of the crab species they found,Eumunida picta and Gastroptychus spinifer, also moved their eyes in a way that indicated they could see green and ultraviolet light.
This raised an immediate question. “There is absolutely no UV and violet light coming down at that depth; it’s long gone,” said Johnsen. In that case, why on earth would the crabs have evolved to be capable of seeing it? Scientists have long assumed that organisms living on the nearly pitch black sea floor were colorblind, since there is so little color to be seen.
Their answer, for now, is only a hypothesis—but an extremely compelling one. “Call it color-coding your food,” said Johnsen. If the creatures can see green, blue and ultraviolet light, they might be capable of distinguishing between UV-emitting anemones and green-glowing toxic corals (which are not safe to eat) and blue-glowing plankton (which are the crabs’ primary food source).
“It is only a hypothesis. We could be wrong,” Johnsen said. “But we can’t think of another reason why an animal would use this ability to see UV and violet light because there isn’t solar light left.”
Part of the reason we know so little about the seafloor environment, he says, is because of the difficulty in getting the funding for and access to a submersible necessary to conduct these sorts of observations. The researchers, though, say that learning about this habitat is a crucial first step in building support to protect it.
“The sea floor is three quarters of the earth’s area and the water column is over 99 percent of the earth’s liveable space, yet we know less about it than the surface of the moon,” Johnsen told the BBC. “I think people will only protect what they love, and they’ll only love what they know. So part of our job is to show people what’s down there.”
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RESEARCH ARTICLE (requires subscription)
Light and vision in the deep-sea benthos: I. Bioluminescence at 500–1000 m depth in the Bahamian Islands
1Biology Department, Duke University, Durham, NC 27708, USA
2Nova Southeastern Oceanographic Center, Dania, FL 33004, USA
3Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA
4Ocean Research and Conservation Association, Fort Pierce, FL 34949, USA
- Received February 29, 2012.
- Accepted July 1, 2012.
Bioluminescence is common and well studied in mesopelagic species. However, the extent of bioluminescence in benthic sites of similar depths is far less studied, although the relatively large eyes of benthic fish, crustaceans and cephalopods at bathyal depths suggest the presence of significant biogenic light. Using the Johnson-Sea-Link submersible, we collected numerous species of cnidarians, echinoderms, crustaceans, cephalopods and sponges, as well as one annelid from three sites in the northern Bahamas (500–1000 m depth). Using mechanical and chemical stimulation, we tested the collected species for light emission, and photographed and measured the spectra of the emitted light. In addition, in situ intensified video and still photos were taken of different benthic habitats. Surprisingly, bioluminescence in benthic animals at these sites was far less common than in mesopelagic animals from similar depths, with less than 20% of the collected species emitting light. Bioluminescent taxa comprised two species of anemone (Actinaria), a new genus and species of flabellate Parazoanthidae (formerly Gerardia sp.) (Zoanthidea), three sea pens (Pennatulacea), three bamboo corals (Alcyonacea), the chrysogorgiid coralChrysogorgia desbonni (Alcyonacea), the caridean shrimp Parapandalus sp. andHeterocarpus ensifer (Decapoda), two holothuroids (Elasipodida and Aspidochirota) and the ophiuroid Ophiochiton ternispinus (Ophiurida). Except for the ophiuroid and the two shrimp, which emitted blue light (peak wavelengths 470 and 455 nm), all the species produced greener light than that measured in most mesopelagic taxa, with the emissions of the pennatulaceans being strongly shifted towards longer wavelengths. In situ observations suggested that bioluminescence associated with these sites was due primarily to light emitted by bioluminescent planktonic species as they struck filter feeders that extended into the water column.
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Johnson Sea Link
Johnson Sea Link was the name of a deep-sea scientific research submersible built by Edwin Albert Link in 1971. Link built the submersible at the request of his friend Seward Johnson, founder of the Harbor Branch Oceanographic Institution. It was the successor to Link’s previous submersible, Deep Diver, which had been determined to be unsafe for use at great depths or in extremely cold temperatures.
In 1973 the craft was trapped for over 24 hours in the wreckage of the sunken destroyer USS Fred T. Berry. Whilst the craft was eventually recovered by the rescue vessel A. B. Wood, two of the four occupants perished: 31-year-old Edwin Clayton Link, the son of Edwin Link, and 51-year-old diver Albert Dennison Stover. The submersible’s pilot, Archibald “Jock” Menzies, and ichthyologist Robert Meek survived.
In 1975, a second Johnson Sea Link was constructed by the Harbor Branch Oceanographic Institution.
In 1977, the JSLs were used to examine the wreckage of the ironclad Civil War battleship, USS Monitor.
The submersible and its research program were featured in a Voice of America story in 2005
Deep-sea crabs have colour vision
By Ella DaviesReporter, BBC Nature
Investigating the “murky depths”, US-based researchers recorded the glow of tiny bioluminescent species using a submersible vehicle.
The team also studied how crustaceans react to this light, and found previously unknown sensitivities to blue and ultra violet wavelengths. They suggest crabs use their colour vision to discern edible food.
The research, conducted in the Bahamas, is published in two papers in the Journal of Experimental Biology and was undertaken with Professor Sonke Johnsen from Duke University in North Carolina, US, Dr Tamara Frank from Nova Southeastern University in Florida, US and colleagues.
Bioluminescent species are known to be common higher in the water column. But because it is much harder to access few studies have been made of the animals living on the sea bed.
“It’s very, very hard to get this sort of data,” said Prof Johnsen, citing access to a submarine, safely collecting specimens and recording the light sensitivities on a rolling ship as considerable challenges.
Descending to sites between 600 and 1000m down, the scientists observed flashes of bioluminescence where plankton collided with boulders and corals.
But they found that only 20% of the species they collected emitted light; a much smaller percentage than further up in the water column.
The team recorded the different glowing colours of the species they encountered to begin to understand the relationships between organisms living at these depths.
Grab a crab
To test how larger species perceive their environment despite the lack of sunlight, the researchers used a specialist suction arm on the submarine to carefully collect crustaceans living at the sites. Of the eight species studied by the team, all were sensitive to blue light and two also reacted to ultra violet (UV) wavelengths.
“I personally think it’s fascinating that there are animals that see UV in one of the most UV-poor habitats on the planet,” said Prof Johnsen.
According to the marine expert, the species with the ability to detect two channels of colour could be using this to tell the difference between the green-glowing, often toxic, corals they live on and the blue-hued plankton they eat.
“The idea that they may be using it to colour-code their food is exciting, but of course still in the hypothesis stage,” he said.
Part of the deep-sea mission’s original aim was to confirm whether the sea floor was carpeted in a glowing mat where sunken debris from higher up in the water column was decomposed by bioluminescent bacteria.
But conditions in the largely unexplored depths denied the scientists any evidence to support the theory.
“The currents were too high on the bottom to allow the pile up of any mats, so this is something that has to be left for future expeditions,” Dr Frank told BBC Nature.
“Wouldn’t that be something if the ocean floor was covered with mats of glowing corpses and poo?”
She also theorised that the crabs’ slow sight could be explained by the presence of glowing patches.
“I was surprised at how slow the isopod eyes are,” she said, “By slow, I mean that… they basically operate like a camera with the shutter held open for such a long time that anything moving is blurred.”
“Since the [crustaceans] are scavengers, known to specialize on decaying dead stuff, I thought that maybe this very slow shutter speed might give them the sensitivity to see these dim glowing mats.”
Prof Johnsen agreed that further exploration of the depths is needed to appreciate the complex systems at work.
“The sea floor is three quarters of the earth’s area and the water column is over 99% of the earth’s liveable space, yet we know less about it than the surface of the moon,” he said.
“I think people will only protect what they love, and they’ll only love what they know. So part of our job is to show people what’s down there.”
RESEARCH ARTICLE (Open Access)
Light and vision in the deep-sea benthos: II. Vision in deep-sea crustaceans
1Oceanographic Center, Nova Southeastern University, Dania Beach, FL 33004, USA
2Biology Department, Duke University, Durham, NC 27708, USA
3Department of Biological Sciences, University of Maryland Baltimore County,Baltimore, MD 21250, USA
- Received March 1, 2012.
- Accepted June 18, 2012.
Using new collecting techniques with the Johnson-Sea-Link submersible, eight species of deep-sea benthic crustaceans were collected with intact visual systems. Their spectral sensitivities and temporal resolutions were determined shipboard using electroretinography. Useable spectral sensitivity data were obtained from seven species, and in the dark-adapted eyes, the spectral sensitivity peaks were in the blue region of the visible spectrum, ranging from 470 to 497 nm. Under blue chromatic adaptation, a secondary sensitivity peak in the UV portion of the spectrum appeared for two species of anomuran crabs: Eumunida picta (λmax 363 nm) andGastroptychus spinifer (λmax 383 nm). Wavelength-specific differences in response waveforms under blue chromatic adaptation in these two species suggest that two populations of photoreceptor cells are present. Temporal resolution was determined in all eight species using the maximum critical flicker frequency (CFFmax). The CFFmaxfor the isopod Booralana tricarinata of 4 Hz proved to be the lowest ever measured using this technique, and suggests that this species is not able to track even slow-moving prey. Both the putative dual visual pigment system in the crabs and the extremely slow eye of the isopod may be adaptations for seeing bioluminescence in the benthic environment.
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Pdf file full text Light and vision in the deep-sea benthos- II. Vision in deep-sea crustaceans
Slideshow: Deep-sea, bottom-dwellers give off their own glow
Critters living on the sea floor give off less of their own light than creatures floating in the open ocean at the same depths.
August 29, 2012 | Photos by Sönke Johnsen, Duke University.
In the deep sea, creatures give off their own light. But bottom-dwelling animals don’t bioluminesce, or give off as much light as creatures floating in the water column at similar depths, according to a new study.
Duke biologist Sonke Johnsen and his collaborators collected species of starfish, sea cucumbers, brittle stars, sea lilies, crustaceans, octopi, coral, sea pens, anemones, sponges, and one sea worm from three ocean-bottom sites near the Bahamas using the Johnson-Sea-Link submersible during a 2009 research cruise.
On board a ship, the scientists stimulated the animals with lights and chemicals to see which ones gave off their own light and photographed the results. Surprisingly, less than 20 percent of the collected creatures glowed, and most gave off light in greener wavelengths, compared to species that live at the same depths but donât live on the sea bottom.
The observations, which the team describes in the Sept. 6 Journal of Experimental Biology, also suggest that the bioluminescence happening at these sites comes primarily from plankton that glow after bumping into filter feeders protruding into their path.
Citation: Light and vision in the deep-sea benthos II: Bioluminescence at 500-1000 m depth in the Bahamian Islands. Johnsen, S., et. al. (2012). Journal of Experimental Biology.
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Deep-Sea Crabs Grab Grub Using UV Vision
Some crabs on the sea floor can see UV light and use the ability to select healthy food
September 6, 2012 Ashley Yeager
Crabs living half-a-mile down in the ocean, beyond the reach of sunlight, have a sort of color vision combining sensitivity to blue and ultraviolet light. Their detection of shorter wavelengths may give the crabs a way to ensure they grab healthy grub, not poison.
“Call it color-coding your food,” said Duke biologist Sonke Johnsen. He explained that the animals might be using their ultraviolet and blue-light sensitivity to “sort out the likely toxic corals they’re sitting on, which glow, or bioluminesce, blue-green and green, from the plankton they eat, which glow blue.”
The discovery explains what some deep-sea animals use their eyes for and how their sensitivity to light shapes their interactions with their environment. “Sometimes these discoveries can also lead to novel and useful innovations years later,” like the X-ray telescope based on lobster eyes, said Tamara Frank, a biologist at Nova Southeastern University.
She and her collaborators report the findings online Sept. 6 in the Journal of Experimental Biology.
Frank, who led the study, has previously shown that certain deep-sea creatures can see ultraviolet wavelengths, despite living at lightless depths. Experiments to test deep-sea creatures’ sensitivity to light have only been done on animals that live in the water column at these depths. The new study is one of the first to test how bottom-dwelling animals respond to light.
The scientists studied three ocean-bottom sites near the Bahamas. They took video and images of the regions, recording how crustaceans ate and the wavelengths of light, or color, at which neighboring animals glowed by bioluminescence. The scientists also captured and examined the eyes of eight crustaceans found at the sites and several other sites on earlier cruises.
To capture the crustaceans, the team used the Johnson-Sea-Link submersible. During the dive, crustaceans were gently suctioned into light-tight, temperature-insulated containers. They were brought to the surface, where Frank placed them in holders in her shipboard lab and attached a microelectrode to each of their eyes.
She then flashed different colors and intensities of light at the crustaceans and recorded their eye response with the electrode. From the tests, she discovered that all of the species were extremely sensitive to blue light and two of them were extremely sensitive to both blue and ultraviolet light. The two species sensitive to blue and UV light also used two separate light-sensing channels to make the distinction between the different colors. It’s the separate channels that would allow the animals to have a form of color vision, Johnsen said.
During a sub dive, he used a small, digital camera to capture one of the first true-color images of the bioluminescence of the coral and plankton at the sites. In this “remarkable” image, the coral glows greenish, and the plankton, which is blurred because it’s drifting by as it hits the coral, glows blue, Frank said.
That “one-in-a-million shot” from the sub “looks a little funky,” Johnsen noted. But it, along with video of the crabs placidly sitting on a sea pen, and periodically picking something off it and putting it in their mouths and the data showing the crabs’ sensitivity to blue and UV light, suggests that they have a basic color code for their food. The idea is “still very much in the hypothesis stage, but it’s a good idea,” Johnsen said.
To further test the hypothesis, the scientists need to collect more crabs and test the animals’ sensitivity to even shorter wavelengths of light. That might be possible, but the team will have to use a different sub, since the Johnson-Sea-Link is no longer available.
Another challenge is to know whether the way the crabs are acting in the video is natural. “Our subs, nets and ROVs greatly disturb the animals, and we’re likely mostly getting video footage of stark terror,” Johnsen said. “So we’re stuck with what I call forensic biology. We collect information about the animals and the environment and then try to piece together the most likely story of what happened.”
Here, the story looks like the crabs are color-coding their food, he said.
Light and vision in the deep-sea benthos I: Vision in Deep-sea Crustaceans. Frank, T., Johnsen, S. and Cronin, T. (2012). Journal of Experimental Biology.
Light and vision in the deep-sea benthos II: Bioluminescence at 500-1000 m depth in the Bahamian Islands. Johnsen, S., et. al. (2012). Journal of Experimental Biology.
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The Johnsen Lab
Sensory Biology at Duke University
The Effect of Ultraviolet Vision on Predation
The issue of ultraviolet transparency is particularly intriguing. Recent research has shown that UV radiation is more abundant in near-surface ecosystems than previously supposed. This presents two problems for zooplankton that have evolved a concealment strategy based on transparency.
The first involves UV vision among potential predators and prey. UV vision has been demonstrated in many marine species and it has been conservatively estimated that there is sufficient UV light for vision down to 200 m in clear ocean water. Visual pigments with UV sensitivity (though not necessarily peaking in the UV) have been found in the Atlantic halibut, 22 (out of 41 examined) species of coral reef fish, juvenile salmonids, and decapod and stomatopod crustaceans. Among freshwater teleosts UV vision appears to be fairly widespread. Several researchers have hypothesized that UV vision is primarily used to improve detection of planktonic prey, and some have shown that the presence of UV light improves the search behavior of certain UV-sensitive zooplanktivorous fish. The presence of UV sensitivity in planktivorous but not in non-planktivorous life stages of salmonids, the correlation between UV vision and planktivory in coral reef fish, and the correlation between ocular UV transparency and planktivory all suggest that UV vision is often used to increase the contrast of planktonic prey.
The second problem related to UV radiation is potential radiation damage. Numerous studies have shown that pelagic organisms are damaged by UV radiation in various ways, including deleterious effects on DNA, proteins, tissue, activity, growth, reproduction, and chemical defenses. Differential levels of these effects have been shown to influence biomass, sex ratios, and species compositions of both terrestrial and marine ecosystems. These effects are primarily due to UV-B radiation (280-320 nm), and in clear polar water have been observed to depths of 20-25 m. At lower latitudes, where surface UV irradiance is higher, these effects are likely to be observed even deeper.
One of the primary defensive mechanisms against UV radiation damage is the use of UV absorbing pigments. However, because UV-protective pigments must attenuate UV light to be effective, their presence reduces an organism’s transparency in the UV, and thus increases its sighting distance for predators and prey with UV vision. This presents a potential dilemma for transparent epipelagic zooplankton: protection or concealment. This conflict is particularly difficult to resolve in clear, oceanic waters where UV radiation levels are high, and camouflage is especially challenging. Reports of decreasing ozone levels at polar, temperate, and tropical latitudes create an additional complication, because transparent zooplankton may face concomitant increases in UV radiation. A responsive increase in UV-protective pigmentation (at either an individual or population level) increases UV visibility, resulting in potentially increased predation or decreased feeding success. A responsive increase in depth may decrease access to prey, phytoplankton, or warm water.
We have investigated this topic in several ways. First, we have measured the UV and visible transparency of zooplankton and the waters they inhabit, and then modeled the effect of the measured UV absorption on radiation protection and visibility to UV-visual predators. Second, we have measured the effects of UV radiation on the sighting distance of zooplankton when viewed by the Bluegill in a laminar flow tank.
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Vision on the ocean floor