Pencil squid are generally small cephalopods, those living inshore around 10 - 20 cm in length, though species living offshore can reach up to 40 cm long. Small groups of 5 - 10 individuals are often seen in the shallows while diving from Koh Lanta.
The body shape varies from short and stout to long and slender, and the eyes are covered with a transparent membrane.
The mouth is surrounded by 10 appendages, comprising eight arms and two tentacles. Tentacles are used to catch prey.
Arms have suckers along their entire length, while tentacles only have suckers at the tip. Octopuses have eight arms and no tentacles, while other cephalopods - such as cuttlefish and squids - have eight arms and two tentacles. Pencil squid arms are about half the length of the body.
Pencil squid are a range of colours, translucent to dense, bright scarlet (in some species with yellow and pink chromatophores) to dark brown (nearly black), darker dorsally, but highly variable depending on the behavioural situation.
All squid share the ability to squirt ink as a defensive measure.