The fish that puts the "ugh" in ugly is increasingly popular with biologists 

The pop-eyed creature is covered with goo. Gobs of fish flesh dangle like drool from its enormous lower jaw. Shaped like a colossal pollywog, Opsanus has an embarrassingly big mouth and a ridiculously little tail. While other fish dart gracefully through the water, Opsanus wallows in the mud. And let's not forget its signature sound: where other marine denizens sing ethereal songs, the toadfish grunts. 

Charitable as you might be, there's just no getting around it. The toadfish puts the"ugh" in ugly. But since the early 1980s, biologists from various universities (and several medical schools) have been coming to the MBL to study the funny-looking fish. One or another aspect of Opsanus' anatomy makes it a good model for research into insulin secretion and diabetes, hearing and dizziness, nausea and motion sickness, drug metabolism and pollution, and toxicology. 

Last summer a team of muscle physiologists showed that the animal's swim bladder muscles are the fastest twitching muscles in the vertebrate world - a nice irony given that the sluggish toadfish is not exactly the cheetah of the seas. The explanation of this apparent contradiction is simple: the toadfish doesn't use its remarkably fast muscles for chasing prey or fleeing predators. Instead, the swim bladder muscles twitch away 200 times per second to vibrate the bladder and create a grunting sound that plays a role in courtship. 

"The male toadfish builds a nest, and he doesn't want to go out and leave the nest vacant," says Larry Rome, a University of Pennsylvania muscle physiologist. So the male fish makes a grunting whistle to let any nearby females know he is ready and willing (See "He's Whistling, But is She Listening" in this issue of LabNotes). 

A summer investigator at the MBL, Rome studies fish and terrestrial animals in an attempt to understand how muscles work. He and his collaborators at the University of Pennsylvania Medical School are tackling the question at the level of molecules, cells, and whole animals. 

"Knowing how a healthy motor system works will make it much easier to identify the mechanisms for muscle pathologies," Rome says. "For instance, some types of heart disease involve slowing of the relaxation rate of cardiac muscle. Our research sheds light on the mechanisms that can cause that slowing. Further, we are using, and thus characterizing, artificial compounds that may be able to 'rescue' muscles that are relaxing too slowly." 

Rome began studying toadfish in 1995 because it had been reported as far back as 1962 that the swim bladder muscle vibrates at remarkable speed. 

"We were interested in it because it breaks all the rules," Rome said. The next fastest vertebrate muscle, found in the rattlesnake's tail, shakes away at a mere 90 hertz (vibrations per second). Hummingbirds beat their wings 50-60 times per second, while the leg muscles of Olympic gold medal sprinter Michael Johnson operate at about 5 hertz. 

What, Rome wondered, is Opsanus' secret? 

Muscle cells do their work by alternately contracting and relaxing; if the cell is to operate at high speeds, both chores -- contracting and relaxing --  must be carried out quickly. A cell that contracts without having finished relaxing remains locked in a contracted state (called tetany), unable to do any work. 

Over the summer, Rome and colleagues working at the MBL identified three mechanisms, which, combined with two neurological adaptations previously identified by other investigators working at the MBL, make it possible for the swim bladder muscle to operate at a record-setting pace. 

One is some additional molecular machinery that helps muscle fibers relax. Most muscle cells have molecular pumps to move calcium (the trigger for contraction) back into its storage area after each contraction. The swim bladder muscle is jam-packed with pumps, allowing it to move calcium 50 times faster than other muscle cells. Another adaptation involves troponin, the molecule that calcium attaches to when it signals the fiber to contract. In the swim bladder muscle, troponin is modified to let go of calcium faster when it is time to relax. And third, the parts of the muscle fiber that slide over each other, producing force, are designed to let go of each other 100 times faster than in locomotory muscles. Rome and colleagues described all three molecular mechanisms in August's Proceedings of the National Academy of Sciences. 

Knowing how evolution favors good ideas, you might wonder why these marvelous mechanisms haven't been built into locomotory muscle. Why must human sprinters creep along at 5 hertz or less, when the toadfish swim bladder knows how to perk along at 200 cycles per second? 

"The swim bladder muscle doesn't generate a lot of force," Rome explains. "It can do some mechanical work  -- enough to vibrate the swim bladder, but not enough to move an animal." In a speed-for-power trade-off, the molecular pumps that allow the swim bladder's sonic muscle to relax 200 times per second take up space that would be filled with force-generating fibers in locomotory muscles. 

Rome is continuing to work on toadfish this winter in Philadelphia. He plans to study Opsanus again next summer at the MBL, where he'll have a lot of company, as a growing cadre of researchers turns to the bizarre-looking fish that is becoming one of biology's most popular marine models. Not bad for a creature that would never find its way into anyone's lab, if looks mattered at all.