Sirenidae, the sirens, is a family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of aquatic salamander

Salamander

Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s. Family members have very small forelimbs, and lack hind limbs altogether. In one species, the skeleton

Skeleton

The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 in their forelimbs is made of only cartilage

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

. In contrast to most other salamanders, they have external gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s bunched together on the neck

Neck

The neck is the part of the body, on many terrestrial or secondarily aquatic vertebrates, that distinguishes the head from the torso or trunk. The adjective signifying "of the neck" is cervical .-Boner anatomy: The cervical spine:The cervical portion of the human spine comprises seven boney...

 in both larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

l and adult states. Sirens are found only in the southeastern United States

Southeastern United States

The Southeastern United States, colloquially referred to as the Southeast, is the eastern portion of the Southern United States. It is one of the most populous regions in the United States of America....

 and northern Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

.

Description

Sirens are quite distinct from other salamanders, hence they form their own suborder, Sirenoidea. Sometimes they are even referred as a completely distinct order (Meantes or Trachystomata). Genetic analysis confirms that sirens are not closely related to any other salamander group. Many of their unique characteristics seem to be partly primitive and partly derivative.

Sirens are generally eel

Eel

Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-like in form, with two tiny, but otherwise fully developed, forelimbs. They range from 25–95 cm (9.8–37.4 in) in length. They are neotenic

Neoteny

Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

, although the larval gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s are small and functionless at first, and only adults have fully developed gills. Because of this, it is most likely sirens have evolved from a terrestrial ancestor that still had an aquatic larval stage. Like amphiuma

Amphiuma

Amphiuma is a genus of aquatic salamanders, the only extant genus within the family Amphiumidae . They are also known to fishermen as "conger eels" or "congo snakes", which are zoologically incorrect designations...

s, they are able to cross land on rainy nights.

Except for some patches of small teeth on their palate

Palate

The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...

 and on the splenial

Splenial

The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....

 bone on the inner side of their lower jaw, their mouth has lost all dentition and has been replaced with a horny sheath that resembles a beak. Sirens feed mainly on worm

Worm

The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

s, small snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, shrimp

Shrimp

Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...

s, and filamentous algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

.

If the conditions of a water source are unsuitable, a larva will shrink its gills to mere stumps, and may not function at all. They are also able to burrow into mud of drying pond

Pond

A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens, water features and koi ponds; all designed for aesthetic ornamentation as landscape or architectural...

s and encase themselves with a cocoon of mucus

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 to survive periods of drought. During such periods, they breathe with their small but functional lung

Lung

The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s.

Unlike other salamanders, an interventricular septum

Interventricular septum

Interventricular septum , abbreviated IVS, is the stout wall separating the lower chambers of the heart from one another....

 is present in the heart. At least two of the species can produce vocalizations. The structure of sirens' reproductive systems suggests that they employ external fertilization

External fertilization

External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....

.

The combined biomass

Biomass

Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 of Siren intermedia

Lesser Siren

The Lesser Siren is a species of aquatic salamander native to the eastern United States and northern Mexico. They are referred by numerous common names, including Two-legged Eel, Dwarf Siren, and Mud Eel...

species in a Texas pond exceeded the total biomass of the pond's seven species of fish.

Taxonomy

The siren family (Sirenidae) is subdivided into two genera, with two species each:

Family SIRENIDAE

• Genus Pseudobranchus (Gray, 1825) – Dwarf siren
  Dwarf Siren
  Dwarf sirens are eel-like salamanders of the genus Pseudobranchus. Dwarf sirens possess external gills throughout adulthood and lack hind legs. Dwarf sirens can be distinguished from members of the genus Siren in that dwarf sirens have three toes on each foot rather than four...
  
  s
  • Pseudobranchus axanthus (Netting
    M. Graham Netting
    Maurice Graham Netting — known as M. Graham Netting — was a herpetologist, an early participant in the conservation and environmental movement, and a director of the Carnegie Museum in Pittsburgh.-Biography:...
    
     & Goin, 1942) – Southern Dwarf Siren
    Southern Dwarf Siren
    The Southern Dwarf Siren is a perennibranch salamander lacking hind legs. Found exclusively in Florida, it is one of two currently recognized species of dwarf sirens. Two subspecies are currently recognized; P. a. axanthus, the narrow-striped dwarf siren, and P. a...
    
    
  • Pseudobranchus striatus (LeConte, 1824) – Northern Dwarf Siren
    Northern Dwarf Siren
    The Northern Dwarf Siren, Pseudobranchus striatus is a perennibranch salamander lacking hind legs. Found in the south-eastern United States, it one of two currently recognized species of dwarf sirens. Three subspecies are currently recognized; P. s. striatus, the Broad-striped Dwarf Siren, P. s....
    
    
• Genus Siren (Österdam, 1766) – Sirens
  Siren (genus)
  Siren is a genus of aquatic salamanders of the family Sirenidae. The genus consists of two living species, along with one extinct species from the Eocene Epoch and three from the Miocene...
  
  
  • Siren intermedia (Barnes, 1826) – Lesser Siren
    Lesser Siren
    The Lesser Siren is a species of aquatic salamander native to the eastern United States and northern Mexico. They are referred by numerous common names, including Two-legged Eel, Dwarf Siren, and Mud Eel...
    
    
  • Siren lacertina (Linnaeus, 1766) – Greater Siren
    Greater Siren
    The greater siren is an eel-like amphibian. The largest of the Sirens, they can grow from to in length. They range in color from black to brown, and have a lighter gray or yellow underbelly....