If you manage to kill a synapse creature, the results can vary depending on proximity and strength of the synaptic bond, ranging anywhere from 'every Tyranid in the area suddenly has a stroke and keels over/their head asplode' to a sudden loss of coordination followed by subsidiary synapse creatures picking up the slack (though less effectively, resulting in worse aim, more simplistic tactics or the like) or every Nid in the immediate area panicking and dispersing like wild animals.
How that translates to space battles I'm not sure, though I would assume most of the generalities hold true
In space it is slightly different. Instinctual behavior is more complex, but Hive ships need to check Command each round for each ship to override it.
Instinctual behavior in space works like that:
1) Is ship going to collide with dangerous terrain (celestial phenomena and the like) if it continues normal movement?
If yes - "Burn retros" order(turn on the spot, less firepower) and turn away from danger. If no, go to 2
2) Is nearest enemy in front fire arc and no more than 15 cm(single move) away?
If yes - move into contact and board. (You may skip this instruction if you have bioplasma (15 cm range weapon)). If no, go to 3.
3) Is nearest enemy in front fire arc and more than 90 cm away?
If yes - "Full speed ahead" order(extra speed, less firepower). If no - go to 4.
4) Is nearest enemy in rear fire arc (only check for escorts)?
If yes - "Come to a new heading" order (extra maneuverability, less firepower). If no(or you are not an escort) go to 5.
5) Is enemy in front fire arc and in range of operational bio-weapons?
If yes - "Lock on" order (cannot turn, but reroll misses). If no - go to 6.
6) Does ordnance need reloading?
If yes - "Reload ordnance order" (reloads ordnance (torpedoes and attack craft), no drawbacks). If no - go to 7.
7) None of the above conditions apply? Than you must end your movement closer to the planet if you are able, otherwise move ahead at half speed.
Hive ship can roll command checks (Ld tests) for ships if it want them to do something different. It can do so unlimited times per turn, but only until first failure. Maximum Ld for Hive ships is 9, and that is 1/6 failure chance, so you can expect roughly 9(number of hive ships)*6(expected number of successful Ld tests per ship)=54 ships(maybe twice that number if fleet has all the fancy hive mind upgrades) of this fleet do something other than instinctual behavior each turn.
Actually, by crunch this fleet is highly irregular. Most Tyranids fleets in BFG have much higher Hive ship/cruiser ratio (0-2 cruisers and 6-12 escorts per hive ship). Maybe Durin just made it that way, but maybe it is the result of some plan#1-type action in the fleet's past(somebody successfully took out most of its Hive ships).
@durin - which is it? Do our admirals see any indications of such prior engagements?