Science Metropolis – Boston

Black Sea Bass became Pavlov’s Fish in an experiment conducted at the Marine Biological Laboratory

– by Joseph Caputo and Nuño Dominguez

Fish aren’t known to be exciting. They can’t roll over, shake hands or play dead. But when two Marine Biological Laboratory scientists train fish to catch themselves, a fish school takes on a whole new meaning.

Richard Chappell uses eye cups from the skate, Raja erinacea, to study the relationship between zinc and glutamate. (Credit: Joseph Caputo/MBL)

Paradoxically, the photoreceptor cells in our retinas release more of their neurotransmitter, glutamate, in the dark, when there is nothing to see, than they do in the light. This is doubly surprising since although glutamate is a major signaling molecule in the retina and throughout the central nervous system, it is also a potent cytotoxin that, in large doses, can kill nearby cells. What keeps our retinas from disintegrating each night as glutamate continues to be released is unknown, but growing evidence suggests our molecular protector may be zinc, a metal abundant in tissues throughout the body.

Zinc’s relationship to vision was first recognized when it was found that night blindness is associated with zinc deficiency, and recent studies have shown that a diet supplemented with this trace metal can reduce the progression of one form of age-related blindness. But despite its apparent benefits, not much is known about the relationship between zinc and the eye.

Richard Chappell, a professor of biological sciences at Hunter College, is at the MBL this summer with doctoral student Ivan Anastassov and Harris Ripps, a senior research scientist at MBL and emeritus professor of ophthalmology at the UIC College of Medicine in Chicago, to investigate how zinc may control the wily glutamate. Using the retina of the skate, a cartilaginous fish resembling a manta ray, they record electroretinograms (ERGs) to measure how retinal neurons respond to light stimuli in the presence and absence of normal levels of zinc. Their preliminary results indicate that ionic zinc (Zn2+) is co-released with glutamate from skate rods, and feeds back onto the photoreceptor terminals to suppress the release of glutamate, thus providing an automatic gain control mechanism that reduces the risk of glutamate toxicity.

Demonstrating the role of Zn2+ in the regulation of glutamate release from skate rods is still a long way from fully understanding its potential use in therapy for human diseases where glutamate toxicity may be involved, but its ubiquity among vertebrates shows promise. The presence of available Zn2+ and/or its transporters has been observed in the photoreceptor region of salamanders, zebrafish, mice, and skates, but “The question is whether this is an integral part of the physiology of the retina,” says Ripps. “Once you understand the normal retina, you can determine the basis of retinal disorders.”

The warty comb jelly, Mnemiopsis ledyi, is a voracious carnivore, competing with fish for small crustaceans and zooplankton in the European seas. (Credit: Lars Johan Hansson)

In the waters surrounding Woods Hole, Massachusetts, the warty comb jelly, Mnemiopsis ledyi, lives out its days, bumping against eel grass and collecting small crustaceans with its sticky tentacles. The delicate creature, which resembles a small jellyfish without the stinger, is just another member of the food web here on the Western Atlantic coast.

Across the ocean is a different story. Accidentally introduced to the Black Sea in the early 1980s, the warty comb jelly spread rapidly through the Caspian Sea in the 1990s and has most recently invaded the Baltic Sea. In Europe, M. ledyi is considered a voracious predator, easily snatching dinner from local fish. Countries surrounding the Baltic Sea are now concerned what’s going to happen to their waters.

“Their impact seems to be increasing and that’s been tied to warming water temperatures, giving them an ecological advantage,” says Sean Colin, assistant professor of biology at Roger Williams University. He and John Costello, professor of biology at Providence College, are at the Marine Biological Laboratory (MBL) this summer to determine who and how much M. ledyi eats.

Comb jellies are unique in how they process food. Eight rows of brush-like cilia beat against the water, creating a current that brings prey closer to the mouth. Using high-speed video, the team is observing their feeding behavior, predator and prey interactions, as well as the hydrodynamics of how they swim. “This will help us to understand on which types of ecosystems they might have a large impact or small impact and under which conditions they are going to be able to thrive,” Colin says.

Comb jellies aren’t all bad news. Also at the MBL this summer is Anthony Moss, an associate professor of biology at Auburn University, who is studying the ability of M. ledyi to quickly repair itself – a few minutes to a few hours depending on the injury – without scarring. The jellies have exceptional regenerative powers, capable of repairing up to 50 percent of their bodies. He hopes to apply his observations to wound healing across all organisms.

To see videos of the comb jelly eat its prey, visit the MBL Website.

The cylindrical sea squirt, Ciona intestinalis, also known as the sea vase, can regenerate any part of its body, including its brain. (Credit: Joseph Caputo/MBL)

Cut off one finger from a salamander and one will grow back. Cut off two and two will grow back. It sounds logical, but how the salamander always regenerates the right number of fingers is still a biological mystery.

The salamander isn’t the only animal with this regenerative ability. Take the sea squirt, Ciona intestinalis, a cylindrical marine creature about the size of a small cucumber that regularly loses its siphons, or feeding tubes, to hungry predators. At the base of each siphon are eight photoreceptors, cells used to detect light. Whenever the sea squirt experiences a violent loss at the siphon base, the number of photoreceptors that grow back is always eight.

Understanding the molecular pathway responsible for this phenomenon is a research objective for MBL investigator William R. Jeffery, a former director of the MBL Embryology course and professor of biology at the University of Maryland. “The question I’m interested in is not only what mechanisms are involved in regeneration, but how exact [photoreceptor] patterns are formed,” Jeffery says.

Following up on previous research, in which he experimentally induced variations in the number of photoreceptors that regenerate by manipulating the siphon’s diameter, this summer Jeffery will test the role of the Notch signaling pathway, a highly conserved molecular cascade that determines how an embryo forms. If Jeffery is on the right track, not only will he develop a model of regeneration in sea squirts, but in salamanders as well. Basic research on animal regeneration is a foundation for a major goal in medicine: Learning how to guide human stem cells to regenerate new tissues or organs.

Evolution guarantees this sea anemone doesn’t need a stress test. Credit: Norbert Bieberstein/istockphoto.com

Though the starlet sea anemone, a translucent marine creature as long as a credit card, may appear helpless, years of evolution have prepared it for any attack nature or humans have in store. Rather than spikes, teeth or claws, the soft anemone, a native to the coasts of New England, defends itself with its genes.

While humans stress about bees and mortgage payments, the anemone’s anxieties concern starvation, suffocation, pollution and coastal development.  Despite the laundry list, it is a thriving family of creatures. Cousins of the starlet sea anemone can be found over a range of temperatures and conditions. Their secret is a wide variety of stress-response genes, which define immediately against toxins, osmotic shock, illness and physical wounds.

This knowledge of the creature’s biology didn’t emerge through observational studies. Instead, it was made possible by the recently-acquired ability to compare genomes. By plugging DNA sequences from the sea anemone into a genetic database, John R. Finnerty, a biology professor at Boston University, compared genes known to have a role in stress-response with the genomes of related creatures. With this information he now has a few guesses as to how sea anemone’s evolved to be so resilient.

“[The starlet sea anemone] is known to harbor extensive genetic variation,” Finnerty writes in the paper. “This suggests that the natural dispersal ability of the animals may be quite limited, that local adaptation may be driving genetic differentiation, or a combination of both.”

Being that the anemone can survive most conditions, especially at the local level, Finnerty sees the creatures’ as canaries in the coal mine for salt marshes.  Meaning that if the anemones start to go, the area is in serious trouble.

His findings were pulished in the June issue of The Biological Bulletin, located at the Marine Biological Laboratory in Woods Hole, Massachusetts.

This figure shows the likely habitats of vibrio bacteria found near Plum Island, Mass. Dot colors indicate the predicted habitat of the bacteria (red are believed to attach to zooplankton, yellow to large organic particles, green to small organic particles, and blue are free-floating). The outer ring indicates the microbe’s preference for warm weather (gray) or cold (black). The inner ring shows where the microbes were found (attached or free-floating). The 25 shaded bands within show the ecological populations based on habitat and genetic similarity. Credit: Lawrence David and Dana Hunt, MIT

For centuries, a species was defined by observation; you can see the similarities between wolves and dogs or cats and tigers. With the recent ability to compare genomes and trace evolutionary lineages, what makes a species is further defined by genetic code. But when it comes to bacteria, who share genes like humans share chips at a party, our concept of species is thrown out the window.

Researchers at MIT may have found a better way to classify the millions of bacteria that inhabit our oceans, bodies and homes. Using samples obtained from the waters off Plum Island, Massachusetts, they found that bacteria organize themselves into “professions” or “lifestyle groups,” which live off of distinct ecological niches. One example is V. splendidus, a common marine bacteria. Some of its members live off zooplankton, while others survive by attaching to small organic molecules.

“Most methods in use either over or underestimate greatly the number of microbial populations in a sample, leading either to a confusing array of populations, or a few large, but extremely diverse groups,” said Martin Polz, a microbiologist in MIT’s Department of Civil and Environmental Engineering in a press release .

Polz and his colleague Eric Alm, an MIT biological engineering professor, published their results in the May 23, 2008 edition of Science Magazine.

To categorize the bacteria in their samples, they compared a protein-coding gene (hp60) that is quick to mutate under various environmental conditions. That allowed them to catch V. splendidus switching ecological niches, perhaps on its way to evolving into a new “profession.”

“What is really new about our approach is that we were able to combine both molecular data (DNA sequences) with ecological data in a single mathematical framework,” said Alm. “This allowed us to solve the inverse problem of taking samples of organisms from different environments and figuring out their underlying habitats. In essence, we modeled the evolution of a microbe’s lifestyle over millions of years.”