The circulatory system of cuttlefish (Sepiidae) has been extensively described, yet no comprehensive reconstruction exists for bobtail squids (Sepiolidae), a family of ecological and evolutionary importance within Decapodiformes. To address this gap, we examined five cuttlefish species (Sepia lycidas, S. esculenta, S. tenuipes, S. japonica, and S. kobiensis) and two bobtail squids (Rossia bipapillata and Sepiolina neponensis). For each specimen, morphometric parameters (sex, total length, and mantle length) were recorded, and branchial hearts and renal appendages were processed for histological analysis using serial microtomy and a panel of stains (H&E, PAS, Masson’s trichrome, and Giemsa). This approach allowed us to trace vessel continuity, characterize tissue architecture, and identify sites of parasite colonization. From these observations, we established and illustrated the first complete circulatory system diagram for Sepiolidae, providing histological confirmation of vascular connections between gills, branchial hearts, systemic heart, and renal appendages. While the gross architecture parallels that of Sepiidae, sepiolids exhibited distinctive renal appendage histology, including denser epithelial folding and variable vascularization. Furthermore, renal tissues were screened for parasitic taxa, revealing the presence of dicyemids and Chromidina spp. in select individuals. Comparative histology has previously shown that the external surface of renal organs varies morphologically across cephalopods [1], and one dicyemid species was found in the branchial heart appendage of Rossia pacifica [2]. By combining morphometrics, histology, and parasitology, this study delivers the first validated account of sepiolid circulation, closes a major anatomical knowledge gap in Decapodiformes, and establishes a framework for future research on cephalopod physiology, host–parasite interactions, and evolutionary adaptations of the circulatory system.

References

[1] Furuya, H. (2004). Renal organs of cephalopods: A habitat for dicyemids and chromidinids. Journal of Morphology, 261(1), 1–12. https://doi.org/10.1002/jmor.10265

[2] Furuya, H. (2007). Redescription of two Dicyemennea (Phylum: Dicyemida) from Rossia pacifica (Mollusca: Cephalopoda). Journal of Parasitology, 93(5), 1064–1071. https://doi.org/10.1645/GE-1503.1