Generally, stiff neck hackles are used for tarpon flies. This is especially true for the ones that have splayed tails.
There are some that use saddle hackles. One of the noted guides in the Keys uses saddles and makes the fly about 6 inches long and it is his go to fly when all else fails.
I use strung hackles, both neck and saddle except for grizzly. It is very difficult to find a good "Saltwater" grizzly neck anymore but there are some out there that will work.
At one time, Bucky Metz talked to several of us about trying to breed a saltwater bird but that fell through.
From what I have seen, most of the saltwater grizzly necks these days are a spin off of a dry fly neck. Just my opinion.
It is probably too costly to really breed a chicken for saltwater grizzly necks because it would have to be an older bird so theat you get long, wide, stiff and webby hackles.
If I am not mistaken, Stu Apte originally used saddle hackles as did Winston Moore. Both of these tyer's patterns are still very effective today.
The Sea Ducer was originally called "The Hackle Fly" and was used in the Ozarks circa late 1800's to very early 1900's as a bass fly in rivers. Homer Rhodes took that style of fly and came up with his Homer Rhodes Tarpon Streamer.
You can still see the influence of "The Hackle Fly" style of tying on tarpon flies today.
The "Keys Style" of tarpon fly uses the splayed tail and the collar can be just about any material. It seems that palmered hackle was initially used and then other materials followed. Bob Kay told me that he used marabou on some of his tarpon flies becaue it absorbed water faster and had more movement in the water.
The idea or dynamics behind using a splayed tail and full collar on these types of tarpon flies is that it pushes more water causing more of a disturbance that the tarpon key in on.
Just a note on the Stu Apte tarpon flies that use squirrel tail for the collar. When I talked to Stu about his flies, he told me that prior to tying on the collar he wraps a small ball of thread and ties the collar (squirrel tail) DIRECTLY in front of the ball of thread and DOES NOT wrap the thred over the ball. This causes the squirrel tail hairs to flare instead of laying along the hook shank.
"The only stupid question is the one for which you seek no answer."