Feeding and external digestion in the Japanese pygmy squid Idiosepius paradoxus (Cephalopoda: Idiosepiidae)

Abstract

The ingestion of shrimps and fish in the Japanese pygmy squid Idiosepius paradoxus was examined. Idiosepius paradoxus paralysed shrimps within 1 min of seizure, extended the buccal mass into the exoskeleton of the shrimp without any biting action, and ingested the flesh. During ingestion of the shrimp flesh, the squids moved their beaks inside the buccal mass, but did not bite into the internal flesh. Idiosepius paradoxus appears to use external digestion by injecting digestive enzymes into the flesh and sucking up semi-digested flesh. The exoskeleton of the shrimp was never chipped or eaten, and the discarded remains of the shrimp contained virtually no flesh, resembling a moult with completely empty appendages. Idiosepius paradoxus was unable to paralyse captured fish. In the case of large fish, the squids left them or only partly ingested them. In the case of small fish, I. paradoxus held them within the arm crown after seizure. From a close inspection of discarded remains after feeding, it appeared that the bones of fish were not destroyed, but only the muscle had disappeared. When I. paradoxus fed on small fish, the discarded remains were intact body skeletons. The outer lip of I. paradoxus possesses a unique organ, the 'lip gland' that consists of goblet glandular cells. It seems likely that this organ produces a mucous secretion and appears to assist in external digestion.