Shrimps and oysters do have some EPA/DHA, but by far not as much
as the fatty fish, salmon/herring/mackerel...
Compared to brains - the only read paleo long chain w-3 sources they
are comparable.

You can look at:
http://www.geocities.com/paleolix/PG_weighted.html

This list is sorted by a value of weighted w3/w6 ratio, means
that the long chain FA's EPA/DHA are considered 10 times as efficient.
Or you can simply look at the EPA column (sorry no DHA column)

Amadeus