Surimi Substitution

crab can be substituted for surimi.  

Spiny Lobster Substitution

lobster (has claws)
or
Dublin Bay prawns (smaller)
or
large shrimp
or
monkfish (firmer texture) can be substituted for spiny lobster/

Soft Shell Clam Substitution

razor clams
or
littleneck clams
or
manila clams
or
mussels can be substituted for soft shell clams.

Shrimp Substitution

crayfish
or
lobster
or
scallops (more delicate texture)
or
crab
or 
imitation seafood (less expensive)
can be substituted for shrimp.

Sea Scallop Substitution

bay scallops
or
cod cheeks
or
monkfish
can be substituted for sea scallops.

Scallop Substitution

lobster
or
crab
or
shrimp (firmer texture)
can be substituted for scallops

Oyster Substitution

mussel
or 
littleneck (served raw)
or
cherrystone clam
or
soft-shell clam (deep fried)
can be substituted for oysters.