This is apparently a Toad Ball.  What happens is, like, a female toad (or toadess, maybe...), can attract more than one male.  The result is a number of males attempting to spawn with one female.  Sometimes the female gets away, leaving a ball of males too busy fighting each other off to notice the toadess has swam off and picked up the quiet toad sitting at the other end of the pondweed.  Much like nightclubs in Luton in the early 90's if I recall...

I think there may be at least 4, if not a few more clamped together here.