Fish: Al-Butas (البطس) — the Fish That Wrote Books in the Dark 🐟

The Week Fish Keep Surprising Us

2026 has been a strange year for fish. Scientists announced dozens of brand-new deep-sea species this month, and a completely new genus of blind eel was discovered because a TikTok video led researchers to a borewell in the Ganges basin. I mention that because today's entry — buried in the ب chapter of Hayat al-Hayawan — is about a fish that would have fit right in with those headlines. Medieval books had their own fish wonders, and this one is maybe the strangest stationery the natural world ever produced.

The Entire Entry, In One Sentence

Here is the whole thing. The book's entry on البطس reads:

أنواع من السمك لها مرارات يكتب بها الكتب فإذا جففت قرئت في الظلام كما نقرأ بالنهار في ضوء الشمس ذكر ذلك صاحب المعطار.

“Types of fish whose gallbladders are used to write books; when dried, the writing can be read in darkness just as it is read in the daytime in sunlight. The author of al-Mi'tar mentioned this.”

That's it. One sentence. No species name, no description of what the fish looks like, no habitat, no second witness. The word البطس doesn't even get a pronunciation lesson — the book just drops the claim and walks away. In a book that will spend pages arguing about a bird's name, this entry has the confidence of someone telling you something they assume everyone already knows.

Fish Ink You Can Read in the Dark

Let me unpack what's being claimed, because it's worth slowing down for. The gallbladder — المرارة — of these fish is used as ink. Someone, somewhere, cut open this fish, took the bile, and wrote books with it. And the written page, once the ink dried, stayed legible in total darkness — as easy to read as a page in bright daylight.

I have to be straight with you: this is the book's claim, reported from one source, and the entry gives us no species to check, no method to try, nothing to verify. That's the whole game with this series — the book says it, I tell you the book said it, and I don't pretend otherwise. Medieval compilers recorded wonders the way we share viral videos: quickly, excitedly, sometimes with one source and no fact-check.

“The Author of al-Mi'tar Said So”

What fascinates me most isn't even the glowing ink — it's the citation. Most entries in this book pile up authorities: al-Jawhari said this, Ibn Sida said that, Aristotle weighed in, a poet confirmed it. This entry leans on a single name: صاحب المعطار, “the author of al-Mi'tar.”

Who was he? The entry doesn't say, and I won't invent an answer. The honest reading is that al-Damiri is doing what every honest compiler does — telling you where he heard it. This is hearsay with a name attached. A thousand years later, that name is our only thread, and it's a thin one.

Why Ink Belongs in a Book About Animals

It feels odd at first — a fish filed between the duck and the mosquito (the entry sits right after the duck and just before البعوض) — until you remember what this book actually is. Hayat al-Hayawan isn't a modern field guide. It's a warehouse: what animals look like, what poets said about them, what scholars ruled about them, and what people used them for — remedies, tools, materials. Fish appear here as food, as omens, as medicine, and in this case, as stationery.

The butas is the animal-as-inkwell. Think about that for a second. Somewhere in the medieval world, someone may have been reading a book at night by the light of a fish.

The Ledger

The strict ledger, because this series does not invent: (1) البطس is “types of fish” — plural, unnamed species; (2) their gallbladders are used as ink for writing books; (3) once the ink dries, the writing can be read in darkness as clearly as it is read in daytime sunlight; (4) the entry's only source is “the author of al-Mi'tar.” Every fact here comes from that one sentence — and from nothing else.

Frequently Asked Questions

1. What is al-butas?

   Al-butas (البطس) is a type of fish in Hayat al-Hayawan, described in a single-sentence entry: its gallbladder is used as ink for writing books, and the dried writing can be read in darkness as easily as in daylight. The entry gives no species name, no description of the fish, and no second source — only the report of the author of al-Mi'tar.

2. Does fish-gall ink really glow in the dark?

   The book reports it as a historical claim from a single source, the author of al-Mi'tar, with no species name or method given. We present it as a medieval belief — a curiosity from a 14th-century book — not as a verified fact, and certainly not as instructions to try at home.

3. Why does a book about animals talk about writing ink?

   Because Hayat al-Hayawan collects everything about animals: their looks, the poetry written about them, the legal rulings on them, and what people used them for — remedies, tools, materials. The butas fish is the animal as stationery, sitting in the book right between the duck and the mosquito.