The swimmer has a long and complicated ancestry. It began as a basic phago/flagello swimmer, evolved for the "Macrophages III" challenge. It's not too hard to evolve this kind of simple swimmer with "contaminate cells" plus some time with radiation, but getting one that is fairly efficient and doesn't waste much energy with unnecessary splits or extra modes can take a lot of patience.
I evolved several different swimmers in various attempts to solve "Macrophages IV" (which needs a super efficient swimmer) and "Macrophages V" (which needs a fairly efficient swimmer that goes in circles to avoid being killed by the edge).
For the "Countermeasure" challenge, there is already a simple swimmer living there, along with a devorocyte colony that feeds on them. The intended solution is a protected swimmer that can out-eat the other swimmers, and avoid being eaten by the devorocytes.
I started with my circling swimmer that won "Macrophages V". This swimmer looks fairly simple, though it uses 9 different modes. Several of these modes are almost identical: four of them are "egg" phagocytes (1,11,16,34), three are flagellocytes (10,12,35), and two are feeding/reproductive phagocytes (22,37).
This swimmer did very well in the challenge with no adjustments. It's so efficient, it outnumbers and starves the original purple swimmers in 15-20 seconds of incubation. This definitely qualifies for the "disrupt the ecosystem" part of the challenge, but without protection from the devorocytes, the swimmer can't quite reach the 420-cell goal.
So I began the laborious process of evolving keratin. I put the swimmer in an experimental plate with devorocytes, and let them mutate. Whenever keratin cells would appear, I would "cell boost" them and their offspring, or boost swimmers that showed keratin sometimes in their genomes, basically to reward the genomes that were on the right track.
This didn't work too well, but with patience I eventually got a swimmer with one (temporary) keratinocyte, which would reproduce once, then lose its phagocyte and die.
They could propagate indefinitely, but because each only had 1 child before dying, their population could never grow. But that's OK: by cloning this genome many times under radiation, some eventually began to reproduce more successfully, until I had one that could make one child, then develop keratin for a short time while making a second child. When the second child is born, the parent loses its phagocyte and dies.
I kept evolving these against a bombardment of devorocytes until something amazing happened: they became symmetrical, growing keratin on each side, alternating with reproduction.
Their life cycle is remarkably simple, using only 5 modes. Mode 18 is the initial (egg) cell.
- 18pg => 21pg + 3fl (propulsion)
- 21pg => 38pg + 18pg (reproduction)
- 38pg => 21pg + 2kt (protection)
If they live long enough, they go on to make more keratin on each side. Sometimes they end up with multiple keratin cells on each side, but usually the new keratin just replaces the old keratin. The flagellocyte is never replaced, so it eventually dies of old age, leaving behind the head with keratin still attached; these immobile cells can still make new swimmers if food appears under them, or if other swimmers push them around.
Because the swimmers have a brief unprotected phase in their life cycle, devorocytes can still coexist with them for a surprisingly long time, but just barely. Given enough time, they will probably go extinct.
Here is the "Countermeasure" substrate with the final twin-keratin swimmers. I changed their color to blue so they would look nicer, but otherwise their genome was 100% created by evolution: wap countermeasure twin keratin.substrate