Pliny the Elder [1st century CE] (Natural History, Book 9, 66): The genus shell-fish also includes the pinna. It occurs in marshy places, always in an upright position, and never without a companion which is called the pea-crab, or by others the sea-pen-protector: this is a small shrimp, elsewhere called a crab, its attendant at the feast. The pinna opens, presenting the dark inside of its body to the tiny fishes; these at once dart forward, and when their courage has grown by license, they fill up the pinna. Her marker having watched for this moment gives her a signal with a gentle nip. She by shutting up kills whatever she has enclosed, and bestows a share on her partner. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.63): The pina likewise belongs to the genus of shellfish, as Pliny says. It has a round body. Male and female travel together. The pina opens its shells to the moon and offers itself as if it is dead to the little fish. Those immediately rejoice, they are emboldened by this permission, and enter the pina's shell. As soon as the pina feels itself to be full, it takes hold of whatever it has enclosed, and gives a portion to his companion. This is a clear indication that there is a sense in the aquatics. - [Badke translation/paraphrase]