You cannot "harvest" horseshoe crabs.
However
You can purchase them quartered from a wholesale distributor and use those. Keep your receipts handy and readily available just in case, throw them in the glovebox while trapping.
That being said: We've always done well on squid and the leftovers from when we have steamers.
Since you will be feeding them (in your traps) make sure you give them 24-48 hours to purge before introducing them into the tank you will be keeping them in. So river-> quarantine 24-48 hours (cooler full of water whatever) -> tank you will be keeping them in.
If you're looking for skinning eels use the same tackle you would use for panfish and tip with a bit of squid
Where we go on moons is best and we put back those that arent casters but are legal length.