Crusty Nautilus
Encyclopedia
Allonautilus scrobiculatus, also known as the Crusty Nautilus, is a species of nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

 native to the waters around New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...

, specifically New Britain
New Britain
New Britain, or Niu Briten, is the largest island in the Bismarck Archipelago of Papua New Guinea. It is separated from the island of New Guinea by the Dampier and Vitiaz Straits and from New Ireland by St. George's Channel...

 and Milne Bay
Milne Bay
Milne Bay is a large bay in Milne Bay Province, southeastern Papua New Guinea. The bay is named after Sir Alexander Milne.The area was a site of the Battle of Milne Bay in 1942....

, and the Solomon Islands
Solomon Islands
Solomon Islands is a sovereign state in Oceania, east of Papua New Guinea, consisting of nearly one thousand islands. It covers a land mass of . The capital, Honiara, is located on the island of Guadalcanal...

. A. scrobiculatus is instantly recognisable by the large open umbilicus, which is around 20% of the shell diameter at its widest point. This species, along with the closely related A. perforatus
Allonautilus perforatus
Allonautilus perforatus is a species of nautilus native to the waters around Bali, Indonesia. It is known only from drifted shells and, as such, is the least studied of the six recognised nautilus species....

, were originally placed in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Nautilus
Nautilus (genus)
Nautilus is a genus of cephalopods in the family Nautilidae. Species in this genus differ significantly in terms of morphology from those placed in the sister taxon Allonautilus. The oldest fossils of the genus are known from the Late Eocene Hoko River Formation, in Washington State and from...

, but have recently been given their own genus on account of significant morphological differences. The most obvious are features of the shell, including crease and an encrusting layer (periostracum
Periostracum
The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...

) that covers most of the shell. Gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s and reproductive structures also differ significantly from members of the genus Nautilus. The shell is usually up to around 18 cm in diameter, although the largest specimen ever recorded measured 215 mm.

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