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Smithsonian National Museum of Natural History
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Stomiid
Stomiids are a group of deep sea fishes that have peculiar anatomical characteristics to help them survive in their extreme environment. Click on thumbnails below to see a compilation of barbeled dragonfish specimens used in an analysis of backbone anatomy by Johnson and colleagues (Photo Credit: Nalani Schnell).

Stomiid slide Stomiid Stomiid Stomiid Stomiid

An international team of scientists including Nalani Schnell (Germany), Ralf Britz (England), and Smithsonian ichthyologist Dave Johnson, has been awarded the Reinhard Rieger Award for excellence in research in zoomorphology for their 2010 publication in the Journal of Morphology, New Insights into the Complex Structure and Ontogeny of the Occipito-Vertebral Gap in Barbeled Dragonfishes (Stomiidae, Teleostei).

Barbeled dragonfishes are members of the deep-sea Family Stomiidae. There are 27 genera (groups) and 270 species, which, like other deep sea creatures, exhibit bizarre behaviors and peculiar anatomical features to survive in their extreme environment.

Stomiids have a long body that typically lacks scales, a series of light-emitting organs called photophores along their body, and a photophore-containing string-like structure called a barbel hanging from their chin whose light may be used to attract prey toward the dragonfish’s very toothy mouth.

Occipito-vertebral gap
The occipito-vertebral gap in the anterior region of the stomiid backbone, where part of the backbone (C1) meets the base of the skull (Oc), is a region that noticeably lacks vertebrae. In this region, the notochord (Nch) is markedly still present and may give the fish a wide range of flexibility for consuming prey in the ocean depths (Photo Credit: Nalani Schnell).

The most puzzling characteristic of these fishes is found where their backbone (or vertebral column) approaches the back of their skull. Here in the anterior region of the backbone, stomiids have a noticeable space between the skull and what appears to be the first vertebra. This space, or occipito-vertebral gap, has always been presumed to increase head flexibility in dragonfishes, which would facilitate ingestion of larger prey, though this theory has not been proven. In the early 20th century scientists suggested that this boneless gap was the result of an evolutionary loss of one to ten vertebrae.

Johnson, along with colleagues Ralf Britz of The Natural History Museum in London and PhD candidate Nalani Schnell from the University of Tubingen in Germany, sought to explore the occipito-vertebral gap more closely, and from a developmental perspective. The team used a comprehensive approach for their investigation, which involved staining their fish specimens so that bone, cartilage and nerves were seen in different colors. Furthermore, they paid particular attention to the attachment sites of muscle segments to individual components of the skeleton.

Flagellostomias bureei and Malacosteus niger
In other stomiids, like Flagellostomias bureei (top) and Malacosteus niger (bottom), Johnson and his colleagues determined that the presence of the notochord is intriguingly due to a physical extension of the notochord. (Photo Credit: Nalani Schnell).

Their results confirmed that for two of the 26 genera of stomiids that they studied (Chauliodus and Eustomias) and for the species Leptostomias gladiator, the original theory of evolutionary loss of vertebrae is correct. In those fishes, their results suggest that 2-7 vertebral centra (the central bony part of the backbone that surrounds and protects the spinal cord) can be missing from the occipito-vertebral gap. The persistence of neural arches (components of the backbone located adjacent to each vertebral centra) where each of the lost vertebrae should be assisted their conclusion.

However, they discovered that the remaining 24 genera have no missing vertebrae. Instead, the boneless gap behind the skull in these genera is the result of an extended portion of the flexible spinal precursor – the notochord.

Chauliodus
In some stomiids, like the Chauliodus species shown here, the presence of the notochord is apparently due to missing vertebral centra. In this image, 7 vertebral centra are missing, but their respective neural arches (NA1-7) persist (Photo Credit: Nalani Schnell).

The presence of an elongated notochord in adult fishes is unusual. In contrast, the research team concluded that the absence of vertebral centra in Chauliodus, Eustomias and Leptostomias gladiator might be facilitated by a distinctive developmental pattern of dragonfishes. During development of most vertebrates, the flexible notochord is incorporated into the bony vertebral column sequentially through the process of ossification of individual vertebrae. In most fishes, the sequence of ossification starts at the skull and proceeds to the tail. In dragonfishes, however, this sequence is reversed, proceeding from the tail region forward to the skull so that the last vertebrae to form are the most anterior ones. Apparently, the missing vertebral centra in a subset of dragonfishes are simply the result of their failure to ossify.

The current research by Schnell, Britz and Johnson, which represents part of the doctoral dissertation research of Nalani Schnell, a student at the University of Tuebingen, who has worked with both Johnson and Britz, has answered a question held in the field of deep sea ichthyology since the early 20th century. In the formal presentation of the Reinhard Rieger Award, the paper was acknowledged for its beauty and simplicity, acknowledging the groundbreaking nature of their work and their findings in barbeled dragonfish zoomorphology.

September 2010

Reference:

Schnell, Nalani K., Britz, Ralf and Johnson, G. David. 2010. New Insights into the Complex Structure and Ontogeny of the Occipito-Vertebral Gap in Barbeled Dragonfishes (Stomiidae, Teleostei). J. Morph., 271(8): 1006-1022.

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