Mudpuppy: a salamander that keeps its gills for life

In many salamanders, growing up means losing the gills. The mudpuppy (Necturus maculosus) skips that step. A 2006 paper in the International Journal of Developmental Biology calls it an obligatory paedomorph: metamorphosis, the big makeover from larva to adult, never occurs spontaneously and cannot be induced by drugs. In plain English, you cannot talk it into growing up, and researchers have tried chemistry.

The result is an animal that looks like an oversized larva: a flat head, a short flattened tail and big, red, feathery gills that never go away. It lives in lakes, rivers and ponds from southeastern Manitoba and southern Quebec to northern Georgia, according to the Animal Diversity Web. It sounds like a cartoon sidekick, and with a maroon feather boa of gills it looks a bit like one.

This article covers what staying young means, how the gills work, what scientists have tested, a mudpuppy’s year, and how the animal is doing. It also clears up a belief that sometimes gets mudpuppies killed. The axolotl has a related trick and its own article.

Meet the mudpuppy

The Animal Diversity Web (ADW), a University of Michigan database, describes the mudpuppy as gray or rusty brown to nearly black, with black or blue-black spots. Young animals are black with yellow stripes. It has four toes on each of its four limbs, and the Missouri Department of Conservation (MDC) adds small eyes without eyelids. Ontario Nature lists another common name, waterdog.

It lives in rivers, weedy ponds, large lakes and permanent streams, hiding under logs, rocks or weeds by day. The ADW says it has been reported down to 30 meters (about 100 feet), and Ontario Nature mentions that depth in Lake Michigan. In the Detroit River, a team led by Jaquelyn Craig collected 411 mudpuppies, 37 to 392 millimeters long, from water 3.5 to 15.1 meters deep. The ADW calls the animal an opportunistic carnivore, with crayfish a major food, plus insect larvae, small fish, fish eggs, worms, snails and other amphibians. The MDC says it feeds at night and finds fresh carrion by smell.

What staying young means

Most salamanders begin as larvae in water, with external gills and a fin on the tail. Metamorphosis turns that larva into the adult body. Paedomorphosis, “child form,” means keeping larval traits as an adult. The ADW calls the mudpuppy neotenic, a permanent larva that keeps large maroon external gills throughout its life. Ontario Nature says it does not metamorphose and stays aquatic for life.

“Larva” describes the body plan here, not the age. The ADW and the MDC say mudpuppies mature at about 4 to 6 years, and they breed. A mudpuppy with gills is an adult.

Other salamanders handle the same choice differently. Purdue University says hellbender larvae lose their gills at 1 to 2 years and adults breathe through the skin, as the hellbender article explains. The axolotl keeps the larval look, but the ADW says thyroid hormone injections can push it to metamorphose, though wild ones rarely, if ever, do, and that most lab animals die shortly after.

Gills that follow the oxygen

The mudpuppy’s gills are not fixed in size. The ADW says cold, well-oxygenated water produces shorter gills. The MDC puts it as larger gills in slow, warm, low-oxygen water and smaller gills in faster, cooler, higher-oxygen water. A larger gill gives more surface for the same job, which is a reasonable reading, although the sources do not spell it out.

So the gills that look like a baby’s are working hardware, and they are adjustable. They are not leftovers.

The metamorphosis mystery

Why does the mudpuppy never grow up? Metamorphosis in amphibians depends on tissues responding to thyroid hormone. An obvious idea was that the mudpuppy’s tissues cannot respond. In 2006 Virginie Vlaeminck-Guillem and colleagues tested it. They found thyroid hormone receptors, the molecules that catch the hormone’s signal, in the central nervous system, the lining cells of digestive and urinary organs, heart muscle and skeletal muscle, and a second receptor type in the brain. Signals in other tissues were too weak to judge.

They concluded that, contrary to earlier hypotheses, mudpuppy tissues are responsive to thyroid hormone. That moves the puzzle without solving it. The abstract we read does not say what blocks the change, so the honest answer is that it is still open.

What this looks like in practice: a mudpuppy’s year

Here is the breeding calendar as the ADW describes it, with other sources noted.

Mudpuppies also make use of human-built structures. In the Detroit River, Craig and colleagues found that spawning females used cement structures for egg laying and that larvae found refuge in eggmats, which are artificial spawning habitats.

Common mistakes, myths and mix-ups about the mudpuppy

Myth: it harms game fish, so it is bad news. The ADW says some anglers kill mudpuppies in the mistaken belief that they affect game fish populations, and that there is no evidence they do. Mudpuppies do eat small fish and fish eggs, according to the ADW, but the same source finds no evidence that this affects game fish populations. The MDC calls the animal harmless to natural fish populations, and the ADW calls it completely harmless.

Mix-up: a mudpuppy is a baby of something else. It is an adult. It has gills and a larval look, and it breeds.

Mix-up: it is the same as an axolotl. Both keep larval traits, but the axolotl can be pushed to metamorphose with thyroid hormone, according to the ADW, and the mudpuppy cannot, according to the 2006 paper.

Mistake: its larval traits mean it never changes. The gills grow and shrink with the oxygen in the water.

Conservation status

The mudpuppy is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

Least Concern is not the same as no problems. The ADW calls the species locally common but says some populations are declining, and names siltation, pollution and habitat loss from development. It adds that the animal’s sensitive skin makes it vulnerable to toxins in the water, and lists it as endangered in Iowa and a species of special concern in Maryland and North Carolina. Ontario Nature lists poor water quality, dredging, fishing nets, boat propellers and accidental catches by anglers, and reports that over 60% of the mudpuppies examined at one high-PCB site on the St. Lawrence River had limb deformities. That figure is from one source. The 2015 Detroit River paper also notes declines in the Great Lakes region. We have no confirmed overall population trend.

The MDC points out one more stake: it says the mudpuppy is the only host of the salamander mussel, which it calls an endangered species in Missouri. We found no second source for the host claim.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.