Cephalopod Shells (2):

Noury’s Argonaut, Argonauta nouryi. A representative Cephalopod from the Argonaut Shell or Argonautidae Family.

The Cephalopoda class of Cephalopods is one of ten classes within the superclass Aplacophora, and this superclass is the only superclass within the phylum Mollusca. This class is divided into two subclasses- Coleoidea and Nautioidea. Combined, these two subclasses contain two superorders, eight orders, thirteen superfamilies, and thirty-eight families. While sources differ considerably, most sources state that there are around seven hundred fifty to eight hundred extant species of Cephalopods.  Fossils indicate a possible ten thousand species have gone extinct.  Cephalopods (meaning head-footed ones) include squid, cuttlefish, octopuses, nautiluses, and argonauts. They are of great interest to the scientific community as they are believed to have appeared during the late Cambrian Period, more than 400 million years ago, which was before the first primitive fish began swimming in the ocean. This time period was also before the first mammals, before vertebrates invaded the land, and before there were upright plants on land. Cephalopods were once the dominant life form of the world’s oceans.

 Cephalopods are the most highly developed of the mollusks and, because of their many arms, amongst the most agile. Cephalopods are characterized by having a funnel (part of their “jet propulsion system”), a varying number of arms and tentacles surrounding their mouth, and a chitinous beak. All Cephalopods, except for octopuses, produce some form of shell, either internally or externally. The only Cephalopods that produce a true external calcareous shell are the Nautiluses (or Nautili) of the Nautiliidae family. These shells are tightly coiled “snail” shells. These shells have are smooth and lack the spire found in gastropod shells. They have a large aperture which opens to the body chamber where the animal lives. As the shell gets larger, the animal walls off the earlier parts of the spiral into gas chambers. The chambers help the animal maintain buoyancy. Nautiluses have between sixty and ninety tentacles. Similar in appearance to the Nautiluses are the Argonauts (also known as paper nautiluses). Argonauts are actually eight armed octopuses of the Argonautidae family. The females secrete a paper thin, non-calcareous, “shell”. The shell is tightly coiled and lacks a spire. It is not secreted by a mantle like a gastropod, but by two modified arms. The females trap surface air within the shell for buoyancy. The females use this shell primarily as a brooding chamber.

Both Nautiluses and Argonauts are free swimming. Argonauts are pelagic and seldom descend deeper than 30 m (100 feet). Nautiluses can be pelagic or found near benthic structures. They generally range deeper then Argonauts, 100 m (330 feet)  to 600 m (2000 feet)  during the day, and 30 m (100 feet) to 90 m (300 feet) at night to feed. Argonauts are found worldwide in subtropical to tropical seas. Nautiluses are limited to the Indo-Pacific region. Both families are carnivorous predators.

While Cephalopods in general have been a valuable food source to people around the world, the Nautiluses and Argonauts have not been widely consumed. Shells from both families are highly desired by shell collectors.

Two Cephalopod of the Argonaut or Argonautidae Family can be found in this website:

Greater Argonaut, Argonauta argo
Noury’s Argonaut, Argonauta nouryi