Spore Mines are very basic, geno-modified organisms which the Tyranids use as 'living bombs', most commonly launched from living artillery pieces called Biovores. The mines are engorged sacs full of chemicals, viruses, toxins and poisons that drift slowly in the air currents, with nests of sensitive feelers dangling below questing for the heat and vibration of possible targets. Although it's possible to avoid the attention of a Spore Mine by moving carefully and quietly, weapons discharges and other violent noises will stimulate a detonation with often fatal results.
Due to their unique nature and mode of attack, Spore Mines use a number of special rules:
Movement: Spore Mines drift D6" in a random direction in each Tyranid Movement phase, as determined by rolling the Scatter dice. On a roll of a 'HIT' symbol on the Scatter dice, the Tyranid player may choose the direction of movement of the Spore Mine. Unlike most models, a Spore Mine can move within 1" of enemy models and vehicles during its movement and if it makes contact with an enemy model it detonates. Spore Mines, however, cannot make assault moves.
Detonation: If a Spore Mine makes contact with an enemy, or an enemy unit or vehicle shoots while within 2" of it, the mine explodes. Resolve the detonation once the unit/vehicle's firing is complete. Place the appropriate template or markers over the spot where the mine explodes and resolve the results according to its type as described in the army list. If a mine is reduced to 0 Wounds or otherwise killed, it will explode as detailed above. An enemy unit may divide its fire between all Spore Mine models within 6" of its members and treat them as a single target for shooting purposes. Spore Mines can also be detonated by synaptic control (see pg 9 for details).
Morale: Being essentially mindless a Spore Mine never has to fall back and is assumed to automatically pass any Morale checks it is required to take, even against attacks which normally force their target to fall back with no test taken. Spore Mines cannot be pinned. For the purposes of mission objectives, Spore Mines cannot capture table quarters, hold objectives or count as surviving troops in a Meat Grinder battle.
Preliminary Barrage: Clusters of Spore Mines are often scattered over an area in tens of thousands as a kind of Tyranid preliminary bombardment, drifting down from the air currents when attracted by weapons fire. To represent this, the Preliminary Barrage scenario special rule (see page 135 of the Warhammer 40,000 rulebook) is modified for Tyranids as follows.
Only roll to hit units, not obstacles. Any units 'hit' on a roll of 6 have attracted D3 Spore Mines. The Tyranid player places these within 1" of the unit and then rolls the Scatter dice for each and moves it 2D6" in the direction shown. Roll a D6 for each mine to randomly determine its type: 1-2 Explosive. 3-4 Poison. 5-6 Bio-acid.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): Under these revised Codex rules, Spore Mines can no longer crossfire an enemy unit that falls back into them, and correspondingly a Spore Mine within 6" does not prevent that unit from regrouping. None of the Tyranid weapons that use an Ordnance-style template (Spore Mines, the barbed strangler, etc) count as "real" Ordnance weapons — they don't get 2 dice for Armour Penetration against vehicles and don't use the Ordnance damage tables, they simply use the large blast marker. A Poison Spore Mine's auto-glance against an open-topped vehicle only scores a single hit, not the usual double-hit bonus template weapons get against open-topped vehicles — the rules explicitly say it scores a single hit. A Spore Mine marker may be deliberately placed in contact with an enemy model, so that if it fails to scatter it automatically detonates. Detonation by synaptic control cannot be used against Spore Mines that are locked in close combat.