The cuttlefish cephalopod, including some fossil ones

The modern cuttlefish, Sepia officials (Linnaeus, 1758), is not a fish at all. It is actually a small (about "hand size") coleoid cephalopod called a sepiid, whose closest relatives are spiraled colloids (see the August 8, 2019 post, which precedes this present one), squids, and octopods. The internal shell of S. officialis is the familiar "cuttlebone," which is used commercially as the source of lime for canaries and parakeets.

                      Rubber replica of a modern sepiid (8 cm length).

The animal is somewhat squid-like with with two long tentacles and eight short "arms" (note: only five are "visible" in the diagram above). The animal can be as large as 49 cm in length. Sepia officinalis is primarily a bottom dweller in sands and muds, and it swims rapidly (just like a squid) by jetting water. When pursued, it shoots forth a brownish ink like fluid called "sepia." Specimens  especially like to live in the Mediterranean Sea.

Internally, the gut of Sepia is long and mostly straight, except for an abrupt turn near its end, where it connects to the gills.

The cuttlebone, which is a specialized structure, called a  phragmocone, is an internal plate that overlies and shields the vital organs of the Sepia animal. The cuttlebone is located dorsally on the animal, thus allowing for neutral buoyancy in the horizontal position. The underside of the cuttlebone is very soft (easily scratched with a fingernail) chalky material (non-nacreous aragonite) that is like styrofoam, thus providing some degree of buoyancy for the animal.

Lower surface of a cuttlebone. This specimen is nearly 15 cm long and is 4.5 cm wide. Notice the pointed rostrum (very sharp) at one end of the cuttlebone.

                                        Upper surface of same cuttlebone.

Side view of same cuttlebone, 1.5 cm maximum thickness.

The cephalopod Spirula spirula and its interesting relatives

This post, and the following two posts, concern the coleoid cephalopods.

Cephalopods, which are the most specialized of the mollusks, are characterized by having a distinct head, arms, and tentacles. They are exclusively marine, and most are distinguished by their chambered shell although some of the more modern ones, however, have a reduced internal shell or no shell at all (octopus).

The main divisions of cephalopods are: ammonoids (including ammonites), nautiloids, and coleoids.

The main divisions of coleoids are belemnoids, spirulids (genus Spirula), sepiids (genus Sepia), squids, and octopods (octopus). In an earlier post, I focused on the subject of the belemnoids (an extinct group, which was dominant during the Jurassic and Cretaceous). The other coeloids are highly diversified and because they have a pair of gills are referred to as dibranchiates, which are mainly Cenozoic forms and their fossil records are poor. The following forms exemplify them.  


Spirula spirula (Linnaeus) is the only living member of this genus of spirulid coleoids. It lives as a free-drifting animal in deep seas of the world's warm oceans. Dead shells can be found on beaches. Spirula has a coiled chambered shell (phragmocone), but the walls of each coil do not touch the previous coils. The closely spaced chambers (septa) of the shell have a thin tube (siphuncle) extending through them via a ventral perforation. This tube allows for transport a gas used for buoyancy. The shell is carried internally, near the posterior end of the animal. Unlike the pearly Nautilius shell (see my August, 2016 post), there is no large, final body chamber.  

The internal chambered shell of Spirula spirula from Florida.  Maximum diameter of this shell is 2.3 cm.

Same shell as shown above;  interior of the last chamber (6 mm width), showing the perforation for the siphuncle. 

The fossil record of the early coleoids is sparse, therefore, there are many challenges in trying to unravel the evolution of this group because the degree of coiling differs dramatically.  Two very rare Eocene specimens are shown below: 

This first specimen consists of two fragments of the same specimen from the Llajas Formation in southern California is middle Eocene. The two fragments together are 23 mm in length.

This schematic diagram shows an inferred reconstruction of the Llajas Formation specimen shown above and its tapering but straight (orthoconic) phragmocone with chambers. For more information, see Squires, R.L., 1983. New mollusks from the lower middle Eocene Llajas Formation, southern California. Journal of Paleontology 57(2):354-362.

This other specimen is of late Eocene age from the Hoko River Formation in the extreme northwest corner of Washington State (in the Strait of Juan de Fuca. The image shows the dorsal view of the 20 mm length of the hoof-like shell. In its posterior area, the chambers of the phragmocone are much smaller and are tightly coiled, versus the later chambers.  For more information, see Squires, R.L. 1988. Cephalopods from the late Eocene Hoko River Formation, northwestern Washington. Journal of Paleontology 62(1):76-82.