Allonautilus scrobiculatus (Lightfoot, 1786) - crusty nautilus shell (BMSM 73300, Bailey-Matthews Shell Museum, Sanibel Island, Florida, USA)

This shell is from one of only 6 living species of an entire group of molluscs - the nautiloids. Nautiloids are basically a squid in a shell. Most modern squids do not have an external hard shell, but the crusty nautilus does. The shell is composed of aragonite (CaCO3 - calcium carbonate). In the fossil record, most nautiloids had a long, straight, slightly tapering shell. Some fossil forms had slightly curved or open coiled or tightly coiled shells. The modern crusty nautilus shell is tightly coiled. All nautiloid shells are/were hollow, but they were separated into many chambers by curvilinear walls. A tube of tissue called the siphuncle runs through the shell's chambers - it passively pumps gas into or out of the chambers. Increased gas content within the shell's chambers results in a low-buoyancy body, and the organism rises in the water column. As water is added to the chambers by the siphuncle, the organism's body becomes heavier and it sinks. These changes do not occur quickly.

Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Nautilidae

Locality: Solomon Islands, southwestern Pacific Basin

More info. at: en.wikipedia.org/wiki/Allonautilus and en.wikipedia.org/wiki/Chambered_nautilus and