Most Aplysia sea hares are found in shallow water.  They have several
months annually in whigh they mate.  In that case they are seen in lines
as they have one sex at the anterior end and the opposite at the
posterior end (I am told by references), favoring the line formation.
 
My wife saw hundreds at Pearl Harbor entrance several weeks agw in very
shallow reef flats that become exposed later in the tide.  One dive, I
saw a great number of the same species at 80 to 90 feet with the lines
covering at least a hundred feet in width and quite long.  All were
proceeding for deeper water, which would soon drop to several hundred
feet in a short distance.  Why, when they are shallow water molluscs?
 
I have an article in the next month (May)Internet HSN about this and an
Umbraculum.
 
Aloha,
Wes