Venous branching asymmetry in the pygmy squid Idiosepius (Cephalopoda: Idiosepiida) with reference to its phylogenetic position and functional significance

Abstract

The Japanese pygmy squid, Idiosepius paradoxus (Ortman, 1888), is one of the smallest cephalopods in the world. Their fully described developmental stages and the ready availability of eggs also make pygmy squids suitable material for cephalopod developmental studies. However, their phylogenetic position among the Decabrachia is unclear. We investigated the vascular anatomy of the Japanese pygmy squid using serial sections of the adult. Their venous system is different from that of teuthoids with respect to the presence of the vena cava sinistra. The branching pattern of the vena cava is similar to that of the sepiolids in having two pathways, but different in that it lacks a connection above the hind gut. The Idiosepius heart is also similar to that of sepiids in having the genital artery located dorsally. These features of Idiosepius agree with molecular phylogenies suggesting they are related to the sepiolids. We discuss the venous branching asymmetry with reference to its phylogenetic and functional significance.

Keywords:

• Idiosepiidae
• the pygmy squid
• vascular anatomy
• systematics
• haemodynamics

Acknowledgements

We thank Dr S. Shigeno of the University of Chicago for his kind suggestion and Prof. T. Sakamoto and Mr. W. Godo of Ushimado Marine Laboratory, Okayama University for their kind help in collecting specimens. We would also like to express our gratitude to Prof. H. Nishida and his colleagues of the Department of Biology, Osaka University, for kind help with the microinjection and microscopic methods.

This study was supported by the grants from the Research Institute of Marine Invertebrate Foundation and from the Japan Society for the Promotion of Science (Research grant 18570087).

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