Amphibians
Siren
Sirenidae · also called mud eel
A siren looks like a mistake — an eel with two tiny arms and a mop of feathery gills behind its head. It is in fact a salamander that never grew up, spending its whole life in the larval body, in the warm weedy swamps of the American Southeast. When the swamp dries, it wraps itself in a cocoon of its own skin and waits.
Overview
Sirens are the family Sirenidae, around 5 species, in the southeastern United States and a corner of northeastern Mexico. They are long, eel-shaped aquatic salamanders with a finned tail, external gills, a pair of small front legs, and no hind legs or pelvis at all. They are mostly carnivores of the pond bottom, though sirens eat more plant matter than almost any other salamander.
Never growing up
Sirens are neotenic — they keep their gills and larval form for life while still breeding. Unlike the axolotl, they seem to have lost the ability to metamorphose entirely, with no response to the hormones that drive the change in other salamanders. They also have lungs and gulp air when the water runs low on oxygen.
Drought cocoon
When its wetland dries, a siren burrows into the mud and secretes a cocoon of dried skin and mucus around itself, clear only at the mouth. Sealed in, it can survive many months — possibly over a year — until the rains return.
Odd for a salamander
Sirens are among the few salamanders that make sounds, clicking and squealing when handled. And the reticulated siren, or leopard eel, reaching about 60 cm, was only formally described in 2018.
Status
Sirens as a family are Not Evaluated. The widespread greater and lesser sirens are common across the Southeast, but the recently described narrow-range species are poorly known, and all depend on shallow, warm, weedy wetlands that are widely drained, filled, or polluted.
Siren: common questions
Is a siren a fish, an eel, or a salamander?
A siren is a salamander, and therefore an amphibian, despite looking very much like an eel. The confusion is understandable, since a siren has a long scaleless body, a tail fin, no hind legs, and external gills, and it lives permanently in water. But it has true amphibian features, including a pair of clawless front legs with distinct toes, lungs as well as gills, moist glandular skin, and amphibian eggs laid in water. Eels are bony fish with fins, scales, and a completely different anatomy. The name siren comes from Greek mythology and was applied because early naturalists were unsure what the animal was.
Why does a siren keep its gills its whole life?
Sirens are neotenic, meaning they reach adulthood while retaining the body form of a larva, including the feathery external gills. For an animal that never leaves the water, metamorphosing into a land-adapted adult would bring no benefit and would cost the gills that let it breathe efficiently underwater. Sirens also have lungs and can gulp air when the water is low in oxygen, so they keep both systems. Unlike the axolotl, which can be forced to metamorphose with hormone treatment, sirens appear to have lost the ability to transform at all, having no functional response to the thyroid hormones that trigger the change in other salamanders.
How does a siren survive when its pond dries out?
It goes dormant in the mud, a strategy called estivation. As the water disappears, the siren digs down into the soft bottom and its skin glands produce a layer of mucus that dries into a tough, parchment-like cocoon enclosing the whole body except for an opening at the mouth. Sealed inside, the animal slows its metabolism dramatically, stops feeding, and lives on stored fat and on the breakdown of its own muscle. It can hold out like this for many months, and there are reports of sirens surviving more than a year buried in dried mud, then reviving when the wetland refills.
Are sirens dangerous or venomous?
No. Sirens have no venom and are not aggressive toward people. A large siren that is grabbed may thrash powerfully, produce clicking or squealing sounds, and can give a surprisingly hard nip with its horny jaw sheaths, which are used for crushing snails and can pinch a finger, but there is no toxin involved and the animal would much rather escape into the mud. Some sirens do secrete mildly irritating skin compounds, common in salamanders, so it is sensible to wash your hands after handling one, but they pose no real threat.