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Sirenidae

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Sirenidae is a family of aquatic salamanders known as sirens. These amphibians are highly distinct from most other salamanders. They are characterized by their eel-like body shapes. Most sirens possess two very small forelimbs. They lack hind limbs entirely. In one specific species, the skeleton within the forelimbs consists only of cartilage. This unique anatomy makes them a fascinating subject for biological study.

Sirens are neotenic organisms. Neoteny is a process where an animal reaches adulthood while keeping physical traits from its larval stage. For sirens, this means they retain external gills bunched on their necks. While larval gills may initially be small and functionless, adults possess fully developed gills. However, these gills are considered inefficient for breathing. Because of this, sirens are obligate air-breathers. They rely on well-developed lungs to breathe air. This suggests they likely evolved from ancestors that lived on land but had aquatic larval stages.

These animals have a unique diet compared to other amphibians. Most salamanders are strictly carnivorous. Sirens are omnivorous, meaning they eat both animals and plants. They consume worms, small snails, and shrimp. They are also the only salamanders observed eating filamentous algae. Their jaw structure is specialized for this diet. They possess sharp-edged, toothless ridges made of keratin. Additionally, the coronoid bone in the lower jaw and the vomer and palatine bones in the upper jaw contain patches of small, unbladed teeth.

Survival for a siren requires specific adaptations to changing environments. If a water source becomes unsuitable, a larva can shrink its gills into mere stumps. During droughts, sirens can burrow into the mud of drying ponds. They encase themselves in a cocoon made of mucus to survive. While in this state, they use their lungs to breathe. Their internal anatomy supports this life cycle. For example, their hearts contain an interventricular septum, which is a wall that divides the heart chambers.

Reproduction in the Sirenidae family involves complex social behaviors. Research indicates that sirens use external fertilization. In captive breeding experiments, scientists observed specific courtship rituals. In the species Siren intermedia, males circle around females. They may rub or bite the female's flank region. Both the male and female then flip onto their backs. It is believed the female spawns while the male fertilizes the eggs. Following this, the female leaves the area. The male then provides paternal care by guarding the nest.

Taxonomy helps scientists organize the many types of sirens. The family is divided into five genera. Three of these genera are extinct, while two are extant, or still living. The extant genera include Pseudobranchus, known as dwarf sirens, and Siren. The genus Pseudobranchus contains species like the northern dwarf siren, P. striatus. The genus Siren includes the greater siren, S. lacertina, and the western siren, S. nettingi. One notable observation occurred in a Texas pond. In that location, the total biomass of Siren intermedia exceeded the combined biomass of seven different fish species.

Understanding the history of sirens is vital for studying amphibian evolution. Genetic analysis provides different views on their placement in the tree of life. Some studies place them as the sister group to other Salamandroidea. Other analyses suggest they are the sister group to all other salamanders. This distinction is important because of how they reproduce. While Salamandroidea use internal fertilization, sirens use external fertilization. This reproductive method suggests that sirens may belong to one of the oldest groups of salamanders.

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