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The nautilus: unusual cephalopods

Nautilus are, probably, one of the least known cephalopods, because their mates the squids, the cuttlefishes and the octopuses are present in fish markets and supermarkets and because they can be seen easily diving or snorkelling. Here, we will focus on their biology and some curiosities.  

INTRODUCTION: THE CEPHALOPODS

Nautilus are a group of marine animals that are included in the Cephalopoda class, which, together with the bivalvia (clams, mussels…), gastropoda (snails, nudibranchs…) and other less-known groups, constitute the molluscs, with about 90,000 extant species (and other 70,000 fossil species). Cephalopods are marine and predator animals. Instead of having the typical foot of the molluscs, they have transformed it into a funnel that expels the water of the body (and, therefore, they can travel for the water by propulsion) and in a crown of arms. Cephalopods have male and female individuals. To reproduce, males introduce a capsule containing espermatozoa (spermatophore) inside the body of the female using a modified arm called hectocotylus.

THE NAUTILUS

The nautilus, or rather nautiloidea, are a subclass of cephalopods characterized by the presence of a coiled, pearly, external shell, which is punctuated with chambers, as a result of their growth. These chambers are connected by a tube tissue called siphuncle, which permit the regulation of the buoyancy of the animal by the entrance or the release of water and liquid.

Exemplar del gènere Nautilus (Foto: Servando Gion).
Individual of the genus Nautilus (Picture: Servando Gion).

Their body, placed in the outermost chamber with the body attached by muscles, present 47 pairs of tentacles, which lack suckers (but they produce adhesive substances), which take part in feeding and present several sense organs. Four of this tentacles, in the male are transformed into copulatory organs. The nervous system is diffused and they present a pair of eyes, which are relatively simple compared with other cephalopods. Like other cephalopods, the funnel permits to travel by propulsion. In case of threat, they can hide inside the shell thanks to the hood. For more anatomical details, watch the picture.

Anatomy of a nautiloid (Picture: Suggest Key Word).
Anatomy of a nautiloid (Picture: Suggest Key Word).

They are nocturnal animals, which feed on deep-water crustaceans and fishes. They live in the tropical region of the Indian and Pacific oceans, close to the bottom, from near the surface to about 500 m depth.

Despite they were abundant in the past, during the Paleozoic and the Mesozoic, nowadays there are just two genera,  Nautilus (with 4 species) and Allonautilus (with 2 species). To identify them, we have to focus on the size of umbilicus, the central part of the shell (outside view): while in Nautilus is small, so that measures between 5 and 16% of the shell; in Allonautilus is big, so that measures the 20% of the shell. Other intern traits, like gills and the reproductive system, let their differentiation. Despite there are differences between the species, they measure about 23 cm of diameter and weigh about 1,5 kg.

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Nautilus pompilius (left) and Allonautilus scrobiculatus (right) (Picture: Softpedia)

It is difficult to observe these animals. In fact, an Allonautilus scrobiculatus has been recently spotted after 30-year absence.

REFERENCES

  • Brusca & Brusca (2005). Invertebrados. Ed. McGraw Hill (2 ed).
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed)
  • Jereb, P.; Roper, C.F.E. (eds). Cephalopods of the world. An annotated and illustrated catalogue of cephalopod species known to date. Volume 1. Chambered nautiluses and sepioids (Nautilidae, Sepiidae, Sepiolidae, Sepiadariidae, Idiosepiidae and Spirulidae). FAO Species Catalogue for Fishery Purposes. No. 4, Vol. 1. Rome, FAO. 2005. 262p.
  • Malacologia.es: Biología de los moluscos

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