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First functional fish head joint discovered in deep-sea dragonfishes in museum collections


Scientists with the Smithsonian's National Museum of Natural History and the French Museum national d'Histoire naturelle are the first to describe and illustrate an evolutionary novelty among fishes—a unique, flexible connection between the skull and vertebral column in barbeled dragonfishes, a group of closely related deep-sea predatory fishes. The description details the first and only example of a complex, flexible head joint among fishes known to science.

First functional fish head joint discovered in deep-sea dragonfishes in museum collections
This image shows the skeleton and anatomy of a barbeled dragonfish with a true functional head joint (A) in contrast to the 
relatively inflexible connection usually found in fishes (e.g., a young Mediterranean sea bass (B)). The bone is stained red 
and cartilage is stained blue. Some barbeled dragonfishes have reduced the size or completely lost several of the vertebrae 
just behind the head, while five groups exhibit a true functional head joint like the one pictured above (A). Both adaptations
 allow a considerable degree of head motion that is significantly expanded in the groups that have a functional head joint 
[Credit: Nalani Schnell, Museum national d'Histoire naturelle]
The description and illustrations, reportedin the journal PLOS ONE, present a detailed analysis of the functional head joint and includes a number of striking images and figures to show the unique anatomy of these fishes. The scientists described five groups of barbeled dragonfishes with a true functional head joint. Other groups of barbeled dragonfishes have evolved more limited degrees of head flexibility by reducing the size or number of vertebra in their head and neck anatomy.

video
Scientists demonstrate the anatomy of a barbeled dragonfish's jaws as they are fully opened, 
including the elevation of the head. At the connection between the head and flexible part of 
the vertebral column, the functional head joint is exposed by the movement and notably 
enhances the degree of cranial elevation [Credit: Nalani Schnell, 
Muséum national d'Histoire naturelle]

"The arsenal of specialized traits that barbeled dragonfishes have evolved as deep-sea predators—huge mouths with dagger-like teeth, distensible stomachs, snake like, black bodies with light producing organs and elaborate chin barbels with bioluminescent tissue—make them ferocious and voracious ambush predators, thus the name dragonfishes," said Smithsonian scientist Dave Johnson. "We suspect that the head joint adaptation helps these fishes engulf their prey items, since the added flexibility allows them to open their mouths up to 120 degrees wide—an angle that is unmatched in any other group of fishes."

First functional fish head joint discovered in deep-sea dragonfishes in museum collections
The photo and the x-ray image show the external and internal morphology of a barbeled dragonfish that lives in the deep 
sea and attracts prey with its luminescent chin barbel. The x-ray image shows a barbeled dragonfish that has eaten
 a large lanternfish whole. This and other stomach content analyses support the hypothesis that the functional head 
joint greatly reinforces the maneuverability of the head in order to ingest large prey items whole 
[Credit: Nalani Schnell, Museum national d'Histoire naturelle]
Scientists studied barbeled dragonfish specimens from the collections of the National Museum of Natural History and five other natural history collections around the world. Barbeled dragonfishes are a dominant group of predatory fishes that live mainly between 650 to 3,300 feet deep in the ocean's mesopelagic zone, otherwise known as the twilight zone, so museum collections provide an invaluable resource for scientists seeking to access and study these otherwise difficult-to-reach deep-sea fishes.

Source: Smithsonian [February 01, 2017]
TANN

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