japonica shrimp huddled in ball

[sloshy]

I got home this evening to discover all 8 amano shrimp huddled together on my filter intake, not doing much. I have never noticed this before, and having been keeping them for months now I figured I'd seen all their behaviors. I think it's either something to do with mating or possibly something to do with my water. I put three plants in last week and didn't rinse them beforehand, but I think they'd of shown signs of poison earlier, and none of my fish are acting strangely. I did my normal weekly water change Sunday, and other than the new plants everything has been routine, including water parameters. Has anyone seen this type of behavior before?

[Zen4t5]

My amano shrimp will also hang around my intake. I always assumed that they were just eating the debris that had gotten stuck on the intake.