Species of cephalopods known as the Palau nautilus
The Palau nautilus, Nautilus belauensis, is mainly found in Palau in the Western Carolines[2] as its name suggests. It can be found on fore reef slopes[3] in depths ranging from 95m to 504m but typically prefers to remain within 150-300m where water temperatures range between 16.6 and 9.4 °C.[4] N. belauensis are highly mobile epibenthic scavenging herbalist and opportunistic predators that rely mostly on scent detection.[4] They are active both diurnally and nocturnally within the preferred depth range[4] although most shallow-water-incursions are usually nocturnal events that coincide with greatly diminished fish activities.[4]
Anatomy
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]
Development
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]
Behaviour
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 30 °C 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 0.45 km per day over 322 days up to 0.8 km per day over 5 days.[7]
