Note: If hog casing is unavailable, seven 5- by 10-inch pieces of kitchen parchment may be used instead. See end of recipe for related info.
Cut the fish into long strips; the shrimp and scallops can remain whole. Put the seafood, jalapenos and cilantro through a coarse blade of a meat grinder.
Combine the ground seafood with the egg whites, lemon juice, cumin, salt and pepper in a large bowl. Rinse the hog casing in cold water by letting the water run from the tap directly through it. (The casing should not have any holes.)
Gather the casing up onto the sauce stuffing funnel attachment on your meat grinder. Put the seafood mixture back in the grinder and begin to grind it through. When it begins to emerge from the funnel, tie the end of the casing in a knot, leaving 3 inches of it unfilled for expansion room.
Grind the mixture out, taking care not to overstuff the length of sausage; it should be about 1 inch wide. If necessary, squeeze it gently with your hands to make this width as you work.
Continue with the entire length; you will have about 4 feet of sausage. Remove the funnel and use your finger to press out the last remaining seafood mixture. Leave 3 inches for expansion and tie the end in a knot. Shape to make it even.
Coil it into a spiral and set it aside. Bring 1 inch of water to a boil in a large heavy skillet with a lid. Reduce the heat so that the water barely quivers. Prick the sausage with a sharp knife pint at 3-inch intervals.
Place it in the barely moving water, cover the pan and cook for 10 minutes. Turn off the heat and let the sausage rest for 10 minutes before serving.
NOTE: If you use kitchen parchment, butter the paper and place 1/2 cup of the mixture on each piece. Roll it up, twist the ends and tie them securely with string. Poach as directed.