diff --git a/src/simulator/crates/mc-save/src/format.rs b/src/simulator/crates/mc-save/src/format.rs
index 949e39dc..7f1c3e48 100644
--- a/src/simulator/crates/mc-save/src/format.rs
+++ b/src/simulator/crates/mc-save/src/format.rs
@@ -64,6 +64,25 @@ pub struct SaveFile {
     /// is consistent).
     #[serde(default)]
     pub vision_state: Option<serde_json::Value>,
+
+    /// Worldsim continuous side-state (`mc_worldsim` / `mc_ecology`).
+    ///
+    /// Carried as opaque JSON so `mc-save` stays decoupled from the ecology
+    /// crates (mc-save depends only on mc-core). Producers (api-gdext /
+    /// api-wasm / mc-worldsim) serialize their worldsim side-state — the
+    /// per-tile eco-damage accumulator (`eco_map: BTreeMap<(u16,u16),
+    /// TileEcoState>`) and the live fauna population map
+    /// (`tile_populations: BTreeMap<(i32,i32), Vec<PopulationSlot>>`) — into a
+    /// single JSON object at save time and restore it on load.
+    ///
+    /// Absent in saves produced before this field was added — `#[serde(default)]`
+    /// reads `None`, in which case the loader rebuilds ecology from scratch via
+    /// emergence over the first few post-load turns (populations are lost but
+    /// the game state stays consistent — the same recovery path `EcologyState`
+    /// documented before round-trip existed). No `SAVE_FORMAT_VERSION` bump,
+    /// matching how `player_observations` and `vision_state` were added.
+    #[serde(default)]
+    pub worldsim_state: Option<serde_json::Value>,
 }
 
 /// Crate-version snapshot captured at save-time.
diff --git a/src/simulator/crates/mc-save/src/lib.rs b/src/simulator/crates/mc-save/src/lib.rs
index 0c88d41d..6f380b72 100644
--- a/src/simulator/crates/mc-save/src/lib.rs
+++ b/src/simulator/crates/mc-save/src/lib.rs
@@ -25,6 +25,7 @@
 //!     grid: GridState::new(4, 4),
 //!     player_observations: std::collections::HashMap::new(),
 //!     vision_state: None,
+//!     worldsim_state: None,
 //! };
 //! let bytes = save(&file).expect("serialise");
 //! let reloaded = load(&bytes).expect("deserialise");
diff --git a/src/simulator/crates/mc-save/tests/round_trip.rs b/src/simulator/crates/mc-save/tests/round_trip.rs
index d0d372b1..de2ad431 100644
--- a/src/simulator/crates/mc-save/tests/round_trip.rs
+++ b/src/simulator/crates/mc-save/tests/round_trip.rs
@@ -26,6 +26,7 @@ fn make_save(cols: i32, rows: i32) -> SaveFile {
         grid: GridState::new(cols, rows),
         player_observations: std::collections::HashMap::new(),
         vision_state: None,
+        worldsim_state: None,
     }
 }