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The skeletal beauty and complexity of these creatures of the salt are revealed in X-ray images from the National Museum of Natural History

The Smithsonian Institution’s National Collection of Fishes is the largest and most diverse of its kind in the world. With an estimated 6.2 million specimens comprising some 75 percent of the world’s 32,000 species, it has unique value as an archive of the natural history of fish.

Click through the gallery below to see more of the truly beautiful x-ray images:

Most of the specimens in the collection were fixed in Formalin and transferred to alcohol for long-term storage, but the collection is anything but stagnant. Collectors add thousands of specimens a year to the ever-growing archive, which biologists worldwide use to describe and document the global diversity of fish.

In addition to notable external characteristics, such as color pattern and fin number and position, variations in the many bones of the skeleton have for centuries provided the data used to describe new fish species or infer the evolutionary relationships of one species to another.

Radiographs or X-rays allow us to study a fish skeleton without dissecting or in any way altering or destroying the specimen. Radiographs may be prepared of any fish specimen, often unique or rare specimens, or large samples in which a researcher may compare features among individuals.

“The first law of ecology is that everything is related to everything else.” — Barry Commoner

Although these X-rays were made for research purposes, the strikingly elegant images demonstrate the natural union of science and art. For example, the Lookdown (Selene vomer), with its sloped head and enlarged crest on its skull, appears to “look down” as it swims.

Radiographs are relatively quick and easy to prepare using modern digital X-ray equipment. Sandra Raredon is a master of the digital X-ray technique. She first selects the best specimens to X-ray — those that are relatively flat, not bent, and with complete, unbroken, fins and fin rays.

Larger specimens may reveal greater detail than smaller ones, but ultimately the specimens chosen are those of scientific interest. The arrangement of the three specimens of the Lookdown (in the gallery below) is deliberately artistic, which also makes it easy to compare one specimen to the other.

“Nature isto be found in her entirety nowhere more than in her smallest creatures.” — Pliny the Elder

The X-ray procedure is like what many of us have experienced in the doctor’s office. Specimens are placed on the X-ray machine platform. A beam of X-rays is generated and focused on the specimens, which absorb some of the rays. Bone is dense and absorbs most of the X-rays. Soft tissue, such as the heart or liver, absorbs fewer.

These specimens have not been eviscerated; the guts are still there but are rendered invisible. The X-rays that pass through the specimens are captured on a digital detector, which produces an image that makes the fish look transparent: Bone is white, soft tissue grayish-black. These X-rays were taken at exposures of 30 to 70 kilovolts for 5 to 10 seconds, varying with the size and density of the fish.

The images largely follow scientific, not artistic, conventions; there is one specimen per frame, and the fish faces left. They may be grouped to save time or to facilitate comparison. Characteristics of the skeleton are seen readily: the number of vertebrae, number of fin spines, configuration of the bones that support the caudal or tail skeleton, size and shape of the jawbones or the presence or absence of teeth in the jaws.

The results of such comparisons are published in the scientific literature on fish systematics and taxonomy, and the images become part of the archives of the National Collection of Fishes.

“It is notthe strongest of the species that survives, not the most intelligent that survives. It is the one that is the most adaptable to change.” — Charles Darwin

The Smithsonian Institution Traveling Exhibition Service produced the exhibit X-Ray Vision: Fish Inside Out, which began a tour in 2011 at Yale University’s Peabody Museum in New Haven, Connecticut, and is on view at the Legacy of the Lakes Museum in Alexandria, Minnesota, through Oct. 31. The exhibit features 40 black-and-white digital prints of X-rays or radiographs arranged in evolutionary sequence, allowing visitors to tour the long stream of fish evolution. X-rays in the exhibit also can be viewed on Flickr.

The exhibit was inspired by the book Ichthyo: The Architecture of Fish (Chronicle Books, 2008) by Stephanie Comer and Deborah Klochko, with essays by Jean-Michel Cousteau, Daniel Pauly and Lynne R. Parenti, and X-rays by Sandra J. Raredon.

Lynne R. Parenti and Sandra J. Raredon, curators of X-Ray Vision: Fish Inside Out, work at the National Collection of Fishes at the National Museum of Natural History in Washington, D.C. Raredon is the Fish Division’s radiologist and photographer; Parenti is a research curator who uses X-rays in her studies of the comparative anatomy of fish. Portions of this essay appeared in Ichthyo: The Architecture of Fish and on the website of the Australian National Maritime Museum in Sydney.




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