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Smithsonian National Museum of Natural History
Drawing of a botanical specimen

Peer Through Specimens

Click the images of each fish below to peer through them with x-ray vision

Mene maculata (Bloch & Schneider, 1801)
size of specimen: 17 centimeters (6.7 inches)

Ogcocephalus corniger (Bradbury, 1980)
size of specimen: 16 centimeters (6.3 inches)

Raja montereyensis (Gilbert, 1915)
size of specimen: 19.5 centimeters (7.7 inches)

Whether at the doctor's office or in the museum lab, all x-ray images (also called radiographs) employ the same principle: a beam of x-rays is generated and focused on an object, which absorbs some of the rays. Bone is a dense tissue that absorbs most of the x-rays, whereas soft tissue such as the heart or gut absorbs fewer. The x-rays that pass through a fish are captured on film or a digital detector, which produces an image that makes the specimen appear transparent: bone is white, and soft tissue is grayishblack. The x-rays in this collection were taken at exposures of 30 to 70 kilovolts for 5 to 10 seconds, depending on the size and density of the fish.

X-rays help scientists understand theinternal biology of the specimens, shedding light on food preferences, growth patterns and evolutionary variations between species. Differences in habitat, size and adaptations can be observed through the skeleton, helping scientists determine how species survive in different environmental circumstances. These insights are important to understanding the impact of increasing environmental changes.

Before the discovery of the X-ray, scientists could only obtain these insights through dissection, which took time, energy, and was ultimately destructive to the specimen. X-rays give fish experts, also known as ichthyologists, a fast easy, and nondestructive way to enhance their research.

For many years, the production of x-ray images involved a chemical film developing process. Today, radiographer Sandra J. Raredon uses a digital radiographic machine. She places a specimen on a digital tablet and “hits” it with a beam of x-rays. The rays that pass through the fish are captured on the tablet and translated directly into an image on a computer monitor. No film is used. The image is recorded as a digital file, ready for immediate study, and is ultimately archived in the files of the National Collection of Fishes.

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