So I've been playing Dragonborn and being the preeminent conjurer on these forums, I thought I'd enlighten you apprentices of the craft to a recent theory of mine on the nature of Lurkers. That is, that Lurkers are not daedra themselves, but mutated slaughterfish. This is most unfortunate, since I would have preferred them to be daedra. But alas, the evidence seems stacked against that theory. Here's why:

Now unless you've got the dual casting perk, casting a regular old Expel Daedra on daedra like Seekers and Storm Atronachs will only be met with the message "so-and-so is too powerful". Of course! The more powerful the daedroth, the more powerful of a spell is needed to expel them! Now, once you've picked up the dual casting perk, you can now expel these daedra like their weaker brethren.^1.

Well, what about Dremora Lords, you ask? Well unfortunately we do not have a spell powerful enough to expel these dangerous demons. Even a fully charged, dual cast Expel Daedra will not do the trick! Instead, we're greeted with the message "Dremora Lord is too powerful".

So with Lurkers we'd obviously expect the same, no? Lurkers are powerful Daedra. Perhaps the most powerful lesser daedra that we've encountered in Skyrim thus far. And yet, when casting "Expel Daedra" upon them, we receive the message "the Lurker resisted the spell's effects". Resisted its effects? How could this be?

As you'll see, only non-daedra can resist its effects. Just a quick test for you. Summon an Ash Spawn (a skill available even to one with rudimentary conjuring skills, such as yourself) and try expelling it with Expel Daedra. What message do you get? "Ash Spawn resisted its effects." Of course it did! Ash Spawn aren't daedra. They're much more akin to undead.

So what does this mean? Well, it could mean that Lurkers are not daedra at all, but something else. What else? Well, they could be daedric creatures - that is, mortal life living in Mora's realm of Oblivion, akin to Grummites - or they could be mutated Slaughterfish (or should I say, slaughterfish blessed by a touch of His noodly appendage?), given that you can find slaughterfish scales and slaughterfish eggs on their corpses with alarming regularity.

Thoughts?

1. Just note that using this spell on Seekers within Apocrypha will not result in expelling a daedroth to Oblivion, since you are in Oblivion already (fool)! Instead, it will prompt your tentacled foe to flee the scene, much as the useful "Turn Undead" spell functions on Nirn.

Now that is interesting. However, Slaughterfish Eggs/Scales are not the only organic material found on them: Horker Tusks and Netch Leather can also be obtained, says UESP. Both of these are (mostly) land-based creatures and thus probably aren't slaughterfish stomach contents, unless Lurkers are to be found around Tamriel. Perhaps the Lurker is formed from some twisted amalgamation of fish, Netch and Horker?

It is fitting that Herma-Mora should form his Lurkers from Slaughterfish, as they are widely known to be the most secretive of all aquatic creatures. The reasons for this are:

I think they just eat slaughterfish, since they also drop horker meat and netch leather.

This is most likely an oversight by the devs, anyway.

Neat! Another weird thing about lurkers is that sometimes they'll just rise up from Solstheim's coastal waters, and this is with no All-Maker stones in sight.

You raise very good points, but I think this is just an oversight.

To be sure one way or the other, we'd have to get a hold of some new lore in the next TES whatever, if it's there.
I'm going to assume oversight for now.