Chambered Nautilus

The nautiluses and their relatives were once an extremely specious sub-class found throughout the world’s oceans. Today, living nautiluses are limited to a few species belonging to one family and two genera. Of the eleven named species, only four species are well established and easily distinguishable.

Nautilus are extremely different in many ways from all other living cephalopods in the sub-class Coleoidea. Studying these ‘living fossils’ however is useful in understanding the behaviour and evolution of ancestral cephalopods.

Identification

Among living cephalopods, only nautilus has an external chambered shell. This strongly coiled, pearly shell is divided into more than 30 chambers that are connected via a tube containing living tissue known as a siphuncle. The siphuncle is responsible for controlling the animal’s buoyancy by moving water in and out of the internal gas-filled chambers. The animal lives in the outermost chamber which can be closed for protection by a leathery hood known as an operculum.

The Nautilus, like other cephalopods, has arm-like appendages surrounding the mouth. However, these appendages are without suckers or hooks and number up to 47 pairs as opposed to the 8-10 appendages of other cephalopods.

Habitat

Nautilus inhabit continental shelf and slope waters associated with coral reefs, from the surface to great depths. It appears their depth distribution is limited by the strength of their shell against implosion. Experiments on living nautilus have found shells to implode at depths between 750 and 900m.

Distribution

Until recently the nautilus were thought to be rare animals occurring at only a few sites, but are now known to occur throughout most of the tropical Indo-Pacific region.

Other behaviours and adaptations

Nautiluses are primarily mobile benthic bottom dwellers. They move vertically through the water column by adjusting their buoyancy via water and gas levels in their chambers, and also swim using jet propulsion.

Unlike members of the Coleoidea sub-class, Nautilus eyes are simple with no lens or cornea, and chemical rather than visual cues are mostly used to locate food. Nonetheless, the eyes are large and although poorly developed by coleoid standards, they are well developed by the standards of other molluscs. It is unlikely nautilus uses its eye to hunt, but rather more likely to determine its movement relevant to bottom, orientating correctly to currents.

Shell colour patterns are variable within and between species - typically covering entire shells in juveniles, while restricted to the dorsal, chambered portion of the shell in adults leaving large white portions of the shell. In mature males the shell is broader and larger than in mature females. A nautilus shell is useful in helping to identify the age of the animal.

Predators of the nautilus include sharks and other fishes strong enough to break their shells. Captive turtles have also been observed to attack and consume these animals

Evolutionary relationships

Nautilus are modern descendents of the Nautiloids, which are the oldest group of cephalopods and date back to the Upper Cambrian period (500-550 million years ago). This group gave rise to all other cephalopods.

References

• Hanlon, R.T., & J.B. Messenger (1996) Nautilus. In Cephalopod Behaviour, Cambridge University Press, United Kingdom.
• Jereb, P., & Roper, C.F.E (eds). (2005). Cephalopods of the World: Chambered Nautiluses and Sepioids, Food & Agriculture Organization of the United Nations, Rome.
• Norman, M. (2000). Cephalopods- A World Guide, ConchBooks, Germany.
• Norman, M., & Reid, A. (2000). A Guide to Squid, Cuttlefish and Octopuses of Australasia, CSIRO Publishing, Victoria.
• Ward, P., Greenwald. L., & Rougerie. F. (1980). Shell implosion depth for living Nautilus macromphalus and shell strength of extinct cephalopods, Lethaia, 13(2):182.
• Wells, M. (1986) Legend of the living fossil, New Scientist, October, pp36-41.

Further reading

• Boyle. P., & P. Rodhouse. (2005). Cephalopods: Ecology and fisheries, Blackwell Science Ltd, Victoria.
• Saunders, W.B., & P.D.Ward. (eds) (1987). Nautilus, Plenum Publishing Corporation, Washington.