Singular calamaro.
For most people the words calamari and calamaretti (baby squid) are interchangeable, although in fact they are different species (a young calamaro is a borsotto (plural borsotti) or a fischiono, plural fischioni).
Either way, they are usually fried in batter or flour, or put in fish soups. Calamaretti spillo are a common variety so small that they are eaten whole, either fried or on linguine.
Image by stu_spivack