Have hagfish predators

Underwater cobwebs made of slime?

That would be a pretty big spider, but if it lives mostly underwater, it's a little more workable.

It's probably easiest when it develops from existing water spiders.

These spiders bring air bubbles down so they can breathe. These bubbles can actually act as shiny bait for fish and larger marine predators. This means that no bioluminescence is required.

If the spider can create large enough bubbles, it may just need its prey to accidentally break through the bubble. The spider could then quickly seal the injury and the gilled creature would suffocate and weaken in the gas atmosphere of the bladder. Allows the spider to take them down easily.

Alternatively, the spider would simply create a web network with gaps large enough to pass through while a larger creature tangled itself up trying to follow. Once caught, the spider can finish off the prey and drag it into its bladder for consumption.

When the web is woven into a web, it appears to be slime to the uninitiated. However, it would have a much higher tensile strength than slime.


Another possibility is that the spider is actually tending a small farm of sessile nematocyst-wielding creatures, cnidarians. The cnidarians wove a structure on which the cnidarians could live, and sting and paralyze any large creatures that wandered into the web. The spider could then catch the prey and return its corpse to the cnidarians to feed them and keep things clean. This is similar to some small crabs that live in sea anemones.

Corsica

Thanks for the nightmares. I didn't want to sleep tonight anyway.

Samuel

@corsiKa If you are not sleeping in the sea you should be safe. Well, except for the teleporting spiders. They will most likely catch you. Worst of all, their teleportation sounds exactly like a building creaking and settling down at night.