Eel Scales – Leeuwenhoek said yes, but rabbis ruled “no way!”

Eel Scales, 50x magnification, transmitted light, crossed polarizers with a selenite plate.
Eel Scales, Preparer unknown. A paper wrapped cardboard slide. Early nineteenth century.
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.The listed reason is commercial overharvesting. In other words, we almost ate all of them.

According to A Manual of Microscopical Mounting, fish scales for polariscope must be balsam or fluid mounted (Martin 1872). The author additionally recommends that mucous must be removed from the scales when they have been freshly collected. This is because fish slime is extremely difficult to remove without damaging a scale after 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.

  Anatomically, eel scales show a growth pattern termed cycloid, a style associated with the more primitive relatives on the boney-fish evolutionary tree. Eel scales have concentric rings that Leeuwenhoek, in his article, correctly supposed circular lines were too abundant to be age-indicating growth rings, such as found in a tree trunk. The finding of tiny scales embedded into the skin but not visible on the surface might seem to be an arcane discovery of interest only to ichthyologists. But this was not so. When Leeuwenhoek published finding scales in eel skin, it ignited a controversy to be argued for centuries afterward. The contentious aspect of the discovery was “can eel now be considered a kosher food?”

   During Leeuwenhoek’s time, eels were a vital food-fish for Europeans. Once in a freshwater eel’s lifetime, they migrate from lakes and rivers into saltwater for spawning. The seasonal trip allows them to be easily netted from European rivers in large numbers. When in season, eels were an inexpensive source of protein. But since they were thought to be scaleless fish, eels were not kosher. Observant Jews, a significant part of Europe’s population, did not eat eels. Leeuwenhoek’s finding opened up a large market for finfish merchants. Hebrew scholars began reassessing the edibility status of eels regarding kosher dietary laws. After pondering the question, they reasoned that the fish scales the Torah referred to were of a type that could be flaked from a fish while leaving its skin behind. Since the complete removal of an eel’s scales required skinning the fish, its flesh remained non-kosher.

   About eight hundred species of eels dwell in the world’s oceans. They comprise a large taxonomic order within the class of bony fish. But only the freshwater eel family, those found in North American and European waters, possess microscopic scales. All other eel species (moray, conger, electric, etc.) lack scales. Additionally, those that develop scales only do so after the fish has grown eight inches in length. The scales can only be uncovered by raising up the outside epidermis of an eel’s skin and teasing through the dermal connective tissue beneath. Working with a dissecting scope employing crossed polarizers makes finding the tiny scales an easier task. Eel scales glow with a characteristic light pattern called St. Andrews Cross when viewed through crossed polarizers. Unfortunately, polarized light in microscopy was unknown to Leeuwenhoek, so his discovery is genuinely remarkable.

How to footnote this page.

Reiser, Frank W. (2022). Eel Scales - Leeuwenhoek said yes, but rabbis ruled “no way!” Searching an Invisible World for its Tiniest Things. Accessed at: