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. Sections of 59 cuttlebones from a geographically diverse sample of 11 species were examined using confocal microscopy. Sepia species that live at greater depths had thicker septa and less space between pillars than did shallow species. A plate theory analysis of cuttlebone strength based on these two measures predicted maximum capture depths accurately in most species. Thus cuttlebone morphology confers differing degrees of strength against implosion from hydrostatic pressure, which increases with increasing habitat depth. Greater strength may come at the cost of increased cuttlebone density, which impinges on the cuttlebone's buoyancy function.