Cuttlebone morphology limits habitat depth in eleven species of Sepia (Cephalopoda : Sepiidae)

The cuttlebone is a rigid buoyancy tank that imposes a depth limit on Sepia , the only living speciose cephalopod genus with a chambered shell. Section s of 59 cuttlebones from a geographically diverse sample of 11 species were examined using confocal microscopy. Sepia species that live at greater dep ths had thicker septa and less space between pillars than did shallow speci es. A plate theory analysis of cuttlebone strength based on these two measu res predicted maximum capture depths accurately in most species. Thus cuttl ebone morphology confers differing degrees of strength against implosion fr om hydrostatic pressure, which increases with increasing habitat depth. Gre ater strength may come at the cost of increased cuttlebone density, which i mpinges on the cuttlebone's buoyancy function.