Genetically modified fish that glow in the tank are being credited with reviving sales of aquatic pets in the US.
But the arguments over these engineered curiosities have highlighted the potential for the GM animal debate to eclipse even that of biotech plants.
In certain lighting conditions, the pet zebrafish will fluoresce green or red. The greens have a gene added from jellyfish; the reds contain a gene found in sea anemones.
Sold under the trademark GloFish, they first went into the aquarium market in the US in January. And they have been selling successfully in many states. Only the State of California currently prohibits their sale.
"They're selling very fast," trader Tom Synitico told BBC World Service's Analysis programme.
"They've got great colour. People are tired of seeing the same thing in the pet store all the time. It's adding a bit of excitement to the industry."
The fish are the most visible examples of what are called transgenic animals - ones which have had their genes artificially altered in some way.
When California's Fish and Game commissioners took their decision to hold up sales, there were, according to vice president Sam Schuchat, serious ethical considerations at stake.
"The majority voters were people who, when they thought about it, were simply uncomfortable about the idea of taking a gene from one species, putting it into another species, with no other reason than to create a novelty pet," he said.
"If you're doing it in a way that's going to help people, OK; but if you're doing it just so you can go, 'oh, cool, a fish that glows in the dark'... that is an overly selfish stance to take towards nature."
But the commissioners' position is now to be reviewed. Indeed, the state's Department of Fish and Game has recommended the pets' sale be permitted.
In Washington, the Food and Drug Administration has found no reason to ban sales. It has decided the fish are ornaments to be looked at - and not eaten. It has stated the animals pose no threat to the food supply.
This decision worries the Center For Food Safety, a lobby group, which has been seeking a retail ban.
Its legal director, Joe Mendelssohn, argued this made it possible in the future for a manufacturer to create a GM foodstuff fish, such as carp, and push through the sale of the animal simply because it said the product was for purely ornamental purposes.
"That really is a serious safety risk," he added. "It really is a loophole and our food and drug administration has created it. The consequences could be tremendous."
Professor Scott Angle, an ecologist at University of Maryland, told Analysis there was a need for strong regulation.
He said he was worried a GloFish could get out of an aquarium and into warm waters, and breed with other closely related species of fish.
"That's a very remote possibility, but it's one potential hazard that has to be considered," he argued.
"So what we need to do is take a group of people who understand the GloFish and are experts in warm, tropical fish species, and assess whether the potential for breeding with indigenous species in the wild is a concern."
But Yorktown Technologies, the company behind the GloFish, denies this is the case.
"The safety of our fish has been reviewed for over two years, by some of the world's leading experts in environmental risk assessment," Alan Blake, the firm's CEO, told BBC News Online.
"As noted in the California Fish and Game Department risk assessment, there exists a unanimous conclusion from these scientists that GloFish pose no greater risk than wild-type zebrafish.
"The primary reason for this is that fluorescent zebrafish have consistently demonstrated reduced environmental fitness when compared to wild-type zebrafish, meaning they are less able to survive in the wild."
He also stressed the FDA found the GM zebrafish posed no greater risk to the environment than their "natural" counterparts which have been sold in the US for many years.
The zebrafish are not the only transgenic fish to become involved in the GM debate.
A Canadian company, Aqua Bounty, has created modified salmon which grow at twice the speed - and keep growing when normally they would pause.
Aqua Bounty is currently seeking regulatory approval for its salmon. If passed as safe, these fish will head for dinner plates.
"The faster production process means you can do more with less space, so that you begin to reduce the pressure by fish farms for coastal locations," stated Aqua Bounty's Joe McGonagall.
"Secondly, it reduces the amount of waste discharge."
Environmentalists oppose this technology for much the same reason they object to the GloFish: studies have suggested escaped GM fish could breed with and damage wild populations.
The GM companies stress what they believe are the potential benefits to the environment through modified animals.
Yorktown Technologies says its GloFish were originally developed to be "swimming canaries", to detect toxic substances in the water supply. The fish would only fluoresce when the toxins were present.
A range of animals is now being modified to improve yield or to make them produce valuable drug compounds in their milk.
For example, pigs can be made that excrete fewer pollutants into the water table, and chickens have been created that produce anti-cancer agents in their eggs.
At a farm at the University of California, Professor James Murray has developed goats with a human gene inserted to alter the character of their milk to make it anti-microbial, to prevent it from going off.
Environmentalists fear GM salmon could breed with wild ones
"The advantage of transgenic products is we're going to be able to create new products that people haven't been able to get before," he said.
"We're going to be able to extend the kinds and types of cheeses that you can make from goats, and therefore you're going to have more variety."
He told Analysis he was hopeful cows would be adapted for this purpose, too.
"The other thing is of course that if the milk's anti-microbial, and we have it in dairy cows, you could argue that it's going to be healthier for a child to drink that milk."