Nautilus belauensis

N. belauensis’ shell is similar to that of N. pompilius, but it is distinguished by its larger mean mature shell diameter and shell weight.^[5] Its shell characteristic pattern consists of bifurcating brown to red stripes that extend from the umbilicus to the venter without coalescing across the venter^[5] with delicate, longitudinally crenulated ridges^[2] that produce a distinctive, concentrically lirate pattern.^[2] It can also be distinguished from N. pompilius by its inwardly sloping umbilical walls and evenly rounded umbilical shoulder.^[6] The shell is also distinguishable by the presence of longitudinally crenulated shell sculpture, and a broadly triangular central rachidian radular tooth and a lack of umbilical callus.^[5] Fresh shells were also found to implode at 680-789m depth equivalent pressures.^[4]

At the immature/juvenile stage the Palau nautilus’ shell is covered in colored bands.^[3] Its body chamber is covered by a thick, gelatinous, and slippery periostracum.^[3] As it reaches the sub mature stage, most of its body chamber is white but with thin margins.^[3] However, it still lacks a black layer that develops upon maturity.^[3]

Maturity in the Palau nautilus is reached when there is a rapid decrease in growth rate until there is no additional growth.^[3] The body chamber shell wall thickens with deposition of a black layer along the apertural margin and the accentuation of the hyponomic and ocular sinuses.^[3] The exterior body chamber lacks colour banding.^[3] The last septum is thickened but has a reduced volume in the final chamber.^[3] There are usually around 35 septa in the shell.^[2] The maximum observed range in shell sizes extends from 180mm to 239mm in diameter^[5] with the umbilical diameter taking up 16% of the shell diameter.^[6] The Palau nautilus can live for 5-10 years after reaching maturity and have a life span that may range beyond 20 years.^[3]

The Palau nautilus is able to traverse across a wide range of temperatures and great lateral distances in short periods of time.^[7] It is also able to survive in warm water up to 30ºC for tens of hours up to several days.^[4] It typically migrates from deeper water into shallower water following sunset and returns to the deep before sunlight.^[7] It is also able to travel an average distance from 0.45km per day over 322 days up to 0.8km per day over 5 days.^[7]

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  N. belauensis seen from the front, showing the hyponome

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  Palau nautilus viewed from above

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  Diver with Palau nautilus showing size of a typical captured specimen

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  Close-up view of the eye of the Palau nautilus

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  View of a Palau nautilus from the side

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  A cage used to capture Palau nautiluses from a depth of approximately 30 m: note the five nautiluses and the mostly eaten chicken. The animals are released unharmed.