Pink sponge

At high tide the other day, I found half a dozen torn sponges just above the water line. Most were bleached white already.

Fragment of sponge, on stones washed by waves. With kelp.

A larger piece,damaged, and with what looks like burn marks, but still retaining a bit of pink colour.

These sponges are tough! I brought a couple home and tried to tear apart the smallest, which was already shredded around the edges. I couldn't rip it at all. There was a stick inside it that I wanted to see, and I finally had to get the scissors and cut it apart. (It was just a big sliver of wood; I thought it might be something else.)

The other sponge I brought home was still pink, and looked healthy, so I dropped it into my aquarium. It spread out some, looking well.

Sponge, as found, inside the aquarium.

The hermits and crabs are scavengers. Any time I add dying material to the tank, they eat it immediately. The sponge, they have left alone. It's four days now. I added a blade of kelp with a bit of stipe at the same time; they've eaten all of it, but don't even nibble at the sponge. It still retains its pink colour.

Is it still alive? How can you tell?