I had to use some additional debugging flags to identify the problem, as just -o sshfs_debug
didn't cut it:
-o sshfs_debug -o debug -o LogLevel=DEBUG3
This led me to the very verbose ouput that finally got me what I needed to know to solve my problem. The IdentityFile switch does not work out of the current directory, and I had to supply the entire path to the file thusly:
sshfs -o IdentityFile=/home/anna/keyfile.pem ec2-user@ecX-X-X-X.us-west-1.compute.amazonaws.com:/var/www sshfs/