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Fiction writers have long fantasised about the concept of invisibility cloaks, but for many fish being completely transparent is a normal way of life. Neale Monks sheds some light on the phenomenon.

Hiding in plain sight is especially useful if you live in open water where there’s nothing to hide behind. Transparency has evolved many times among aquatic animals, not just among fish, but also things like jellyfish, crustaceans, even squids. Quite a few fish families include species that are transparent, in both freshwater and the sea. Among the best known to biologists are the larval forms of eels and their relatives. These are the famous leptocephali larvae, adapted to such a fine degree they do not even possess red blood cells until they are ready to metamorphose into their adult form!

Other species remain transparent throughout their lives, and some of these appear in the aquarium trade on a regular basis. The peculiar beauty of these strange fish has ensured their popularity over the years, with the aptly-named Glassfish (usually Parambassis species) and Glass Catfish (Kryptopterus spp) perhaps the best known. Several tetras are strikingly transparent, including the popular X-ray Tetra Pristella maxillaris, and occasionally you’ll see things like the Glass Knifefish, Eigenmannia virescens and the tiny Danionella translucida in the better aquarium shops.

How to be see through

But while it’s simple enough to see why being transparent is useful for fish that live in open water, understanding how they perform this anatomical trick is rather more difficult. After all, if being transparent was easy, then we’d expect it to be a common adaptation rather than a curiosity. Studies of eel larvae as the classic transparent fish have shown that much of their body is made of glycosaminoglycan ‘filler’ rather than muscle or bone. This gelatinous material contains a lot of water, making the fish more transparent, but is still relatively stiff. 

This is rather a clever trick: while a lot of marine invertebrates are transparent, they are often feeble swimmers at the mercy of ambient water currents. What eel larvae manage to do is use a gelatinous matrix in lieu of dense bones, giving their muscles something to work against, making these fish much more mobile, though still somewhat reliant on strong water currents to carry them long distances.

Tiny Danionella

Indeed, some of the ‘glassy’ fish you’ll come across in the hobby are essentially neotenic versions of their close relatives. Neoteny is the persistence of juvenile traits in sexually mature animals, and the glass danios, Danionella, show this really well. While closely related to Tiny Danionella Indeed, some of the ‘glassy’ fish you’ll come across in the hobby are essentially neotenic versions of their close relatives. Neoteny is the persistence of juvenile traits in sexually mature animals, and the glass danios, Danionella, show this really well. While closely related to things like Danio and  Microrasbora, they are much smaller and many parts of their skeleton are poorly developed, looking more similar to the skeletons of Danio fry than the adults. In fact, what seems to happen in Danionella development is that the later parts of skeletal development are arrested, and what you get is a small, but sexually mature, fish resembling (in some ways) a juvenile Danio. Besides a reduced skeleton, these fish lack scales, skin pigments, and even a lateral line system. While they might sound rather fragile (and imports are certainly patchy, at best) in a suitable tank with gentle filtration these species haven’t proven to be especially difficult to keep.

Read the rest of the feature in the April issue, available to read instantly on our digital edition HERE  or purchase the print edition HERE
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