Transmitted light, crossed polarizers with a selenite plate. The European eel, Anguilla anguilla, is currently red listed as a critically threatened species by the International Union for Conservation of Nature’s Threatened Species due to commercial over-harvesting.
According to A Manual of Microscopical Mounting, fish scales for polariscope must be balsam or fluid mounted (Martin 1872). He further recommends that all mucous must be removed when the scales have been freshly collected because fish-slime is extremely difficult to remove once it has dried.
In 1684, Antonie Leeuwenhoek (1632 – 1723), working with a single-lens microscope of his design, discovered scales hidden beneath the epidermal cells buried in the connective tissue layer of an eel’s skin. He drew a finely detailed diagram of the scales and published it, along with a short article, in the Royal Society of London Philosophical Transactions (1684). Leeuwenhoek’s pictures show incredibly accurate detail for having been viewed with such an early instrument. His drawings document the magnification and resolution obtainable with the simple microscopes he built.
Eel scales are of an anatomical type termed cycloid; a style associated with the more primitive branches on the boney-fish evolutionary tree. Eel scales show many concentric rings that Leeuwenhoek, in his article, correctly supposed were too abundant to be age-indicating growth rings. Finding the tiny scales embedded under the skin of what was believed to be a scaleless fish might seem to be an arcane discovery of interest only to ichthyologists. But this was not so. Publishing his finding that eels have scales ignited a controversy to be argued for centuries afterward. The deeper point of contention was “Are eels now kosher?”
During this historical period, eels were an important food-fish in Europe. Freshwater eels migrate from lakes and rivers into saltwater for spawning. This allowed them to be easily caught seasonally in European rivers but, being thought to be scaleless fish, they were not kosher. Jews represented a significant part of Europe’s population that did not eat eels. Leeuwenhoek’s finding drew considerable interest from seafood merchants as well. Hebrew scholars had to reassess the edibility status of eels regarding dietary laws. In pondering the question, they reasoned that the fish scales referred to in the Torah were the type that could be flaked from a fish while leaving its skin behind. To completely remove an eel’s scales would require skinning the fish; therefore their flesh remained non-kosher.
Eels comprise a large order within the class of bony fish with only the freshwater eel family (North American and European) having microscopic scales. All other eel types (moray, conger, electric) totally lack scales. In the freshwater eels, the scales only start forming after a fish has grown to more than eight inches in length. After that they are still difficult to find. The scales can only be uncovered by raising up the superficial layer of an eel’s skin and teasing away the tissue beneath. Using a dissecting scope viewing through crossed polarizers makes finding the tiny scales an easier task. While illuminated by crossed polarizers, eel scales are imparted with a characteristic light pattern called St. Andrews Cross. Unfortunately polarized light in microscopy was unknown to Leeuwenhoek.