Family NAUTILIDAE

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Family NAUTILIDAE

Compiler and date details

1 November 2013 - Mark Norman, Museum Victoria, Carlton, Victoria, Australia

2 July 2001 - C.C. Lu, National Chung Hsing University, Taichung, Taiwan

Introduction

The only living genus in the family Nautilidae Blainville, 1825 is Nautilus, which occurs throughout much of the Indo-Pacific. Six nominal species are known, some of them with very restricted distributions; three are known from Australian waters. Two of these species, Nautilus stenomphalus and N. repertus, appear to be Australian endemics, although the validity of the latter is still regarded as questionable (Saunders 1987).

In nautilids, the external shell is coiled spirally and is partitioned by many septa which are perforated by a siphoncle tube. The many chambers formed by the septa are partially filled with fluid. Only the last coil (the living chamber) is visible. The umbilicus region may be closed or umbilicate or slightly perforate depending on the species. Externally the shell is sculptured with numerous growth lines. Many irregular red- to yellow-brown stripes radiate from the umbilicus. The inner surface of the shell is iridescent and pearly. The soft part of the animal is located in the last chamber attached to the septum. Nautilus regulates its buoyancy through the siphuncle by adjusting the fluid to gas ratio in the chambers. Nautilus is found from near-surface to about 500 m depth, the optimum depth is about 150-300 m.

Nautiluses are predatory. They live near the bottom not far off the coast, but rarely approach the shore. They are active nocturnally, when they feed on sedentary benthic prey, especially spiny lobsters, prawns and fishes, and carrion. Nautiluses find their prey mainly by smell. Their eggs are very large, up to 40 mm long with envelopes, and are shed singly, supposedly into shallow water at the bottom. A female lays only 10 eggs at intervals of nearly two weeks (Nesis 1982).

Diagnosis

Shell external, spirally coiled, only the last coil visible from outside. Umbilicus open or closed. Outer surface of shell shiny, porcellaneous white with curved radial reddish stripes. Inner surface iridescent and pearly. Shell consists of a spacious last (living) chamber that houses the mollusc body (soft part), and numerous uninhabited chambers, partly filled with fluid in life and serving as a hydrostatic apparatus. Partitions (septa) between these chambers are perforated medially by a continuous tube (siphuncle) passing through all chambers from the very first to the last. Head with large primitive eyes devoid of a lens and vitreous body, and with a small pupil. Mantle conical, slightly muscular, and devoid of chromatophores. Arms numerous, slender and suckerless, arranged in two circles, an outer circle of prehensile arms and an inner circle of oral arms. Bases of dorsal arms merge with a large, thick hood that closes the shell mouth when retracted. Males with lower part of inner arm circle modified on right or left side into a copulatory organ (spadix). Females with a seminal receptacle in a special sac under buccal cone.

Funnel not fused into a tube—its lower edges simply coiled one around the other. Funnel organ absent. Beak strong with calcite rostral tips. Radula with 11 longitudinal rows of teeth. An osphradium present. Two pairs of gills, 4 auricles and 4 renal sacs. Ink sac and luminous organs absent. Nervous system primitive, with the brain consisting of 3 pairs of ganglia. No stellate ganglion of giant nerve fibers. Nidamental and accessory glands present. Right oviduct only present. (Modified from Nesis 1982).