In Bedrock Edition, nautilus entities currently spawn at any Y-level in ocean biomes, unlike Java Edition where they are properly restricted to Y-levels 38-58. This appears to be an implementation oversight rather than an intentional design difference between editions.
How to Reproduce:
In an ocean biome, place a 30×30 stone platform at any Y-level above 58
Fill the platform with two layers of water
Wait and observe nautilus spawning
Observed Result:
Expected Result:
Nautilus should only spawn between Y-levels 38-58 to match Java Edition's implementation
Note: This bug report addresses the height restriction issue specifically, splitting it from the broader behaviors described in MCPE-229487. While some cross-edition spawning differences may be intentional, the complete absence of height limits appears to be an oversight rather than by design.
It is not possible to match Java Edition aquatic spawning without completely replacing the Bedrock spawn algorithm. All environmental spawn attempts in Bedrock occur on top of a solid material block with full top surface (or ice or top slab). Underwater spawn attempts, therefore, occur on the floor of the ocean or river, or platform made by the player. Bedrock’s aquatic spawn algorithm approximates Java’s aquatic height restriction by teleporting the mobs up to about the 5th block below the water surface after they spawn. For naturally-generated water that’s Y = 58.
Since 1.18.30 Bedrock approximates Java’s ocean/river spawning height restriction by including the following data in the data-driven spawn rules of aquatic mobs:
(It is necessary for the “min” value to be well below the normal ocean floor to allow fish to spawn above ocean trenches--see MCPE-150191).
All ocean/river mobs except squid and drowned include the above height filter in their spawn rules. Nautilus probably should too.