Wondrous Creatures · Board game rules
Can I trigger a newly revealed habitat tile immediately after discovering it?
Quick answer
Yes—once a Discovery Habitat Tile is placed, it works exactly like any other Habitat, so if a Crew member of yours is already adjacent, you can use a Free Action (Net) on it that same turn, but the normal resource/card interaction only happens when a Crew is placed adjacent to it.
Details
When your Score Tracker reaches a Discovery icon, you immediately choose a Habitat Tile and place it on a Lake hex, and from that moment "all players can interact with the newly revealed Habitat Tiles in the same way as they would with existing Habitats" [p.11 — Discovery]. This means the tile is now a fully functional Habitat for everyone, including Special Effect Icons if present.
However, the standard way to gain a resource or Creature Card from a Habitat happens as step 3 of the "Place a Crew Member" Action: "The placed Crew member interacts with all adjacent Habitats" [p.9 — 1. Place a Crew Member]. This interaction is tied to the act of placing a Crew member, so if the Discovery reward is triggered by an action other than placing a Crew (e.g., completing an Achievement or an Instant Card effect), you cannot retroactively claim the resource/card bonus from the new tile unless you place a Crew member adjacent to it on a later turn.
If, however, one of your Crew members is already adjacent to the hex when the tile is revealed, you can immediately use the Free Action "Use a Butterfly Net" to either take an adjacent Egg or activate a Special Effect Icon on that new tile, since Free Actions "can be performed at any time during your turn" [p.19 — 2. Use a Butterfly Net].
Watch out: Don't confuse "interacting with the Habitat" (resource/card gain, tied only to Crew placement) with using a Net (a Free Action available any time). A newly revealed tile can be Net-activated the same turn if adjacent, but its normal Habitat bonus only triggers on a fresh Crew placement next to it.