Pterotracheoidea
Encyclopedia
The Pterotracheoidea is, according to the Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

, a taxonomic  superfamily
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 of sea snails or sea slugs, marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod molluscs in the clade Littorinimorpha
Littorinimorpha
Littorinimorpha is a large clade of gastropods within Hypsogastropoda consisting primarily of marine species, but also aquatic and terrestrial species as well....

. They are commonly called heteropods or sea elephants.

Taxonomy

This superfamily comprises four families (classification based on Newman (superfamily Carinarioidea, pp. 804-808. In: Beesley et al., 1998), that is also used by the taxonomy of the Gastropoda by Bouchet & Rocroi (2005)
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

:
  • Family Pterotracheidae
    Pterotracheidae
    Pterotracheidae is a family of medium-sized to large floating sea snails, pelagic gastropod molluscs. They are in the superfamily Pterotracheoidea along with two other similar pelagic families, the Atlantidae and the Carinariidae....

     Rafinesque, 1814
  • Family Atlantidae
    Atlantidae
    Atlantidae is a family of sea snails, holoplanktonic gastropod molluscs in the clade Littorinimorpha.According to taxonomy of the Gastropoda by Bouchet & Rocroi the family Atlantidae has no subfamilies.- Description :...

     Rang, 1829
  • † Family Bellerophinidae
    Bellerophinidae
    Bellerophinidae is an extinct family of sea snails, marine gastropod molluscs in the clade Littorinimorpha.According to taxonomy of the Gastropoda by Bouchet & Rocroi the family Pterotracheidae has no subfamilies....

     Destombes, 1984
  • Family Carinariidae
    Carinariidae
    Carinariidae, known by the common name "heteropods" like their relatives in the Pterotracheoidea, is a taxonomic family of swimming or floating sea snails, pelagic marine gastropod molluscs in the clade Littorinimorpha.- Distribution :...

     Blainville 1818

Habitat

These holoplankton
Holoplankton
Holoplankton are organisms that are planktonic for their entire life cycle. Examples of holoplankton include some diatoms, radiolarians, some dinoflagellates, foraminifera, amphipods, krill, copepods, and salps.-Sources:Asexual Holoplankton:...

ic snails are found floating or swimming in tropical to subtropical open oceans and seas at a depth of maximum 200 to 300 m

Anatomy

These snails have adapted themselves to a pelagic  living :
  • a transparent body and shell
  • the foot has evolved into a swimming fin that produces motion through undulation.
  • the proboscis
    Proboscis
    A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...

     is mobile and can be extended giving it a trunk-like appearance (giving rise to their common name : sea elephants).


The taenioglossate radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

, situated at the tip of the proboscis, can be protruded to capture the prey.

They have paired, large spherical eyes, contained in a gelatinous mass, that they use to locate their prey. As the body is transparent, one can easily sea the internal organs and the statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

s with its statoliths (an organ that tells the animal whether it is upside down or not). The swimming fin of the foot has a ventral
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 or posteroventral sucker. This sucker has grown larger in the family Atlantidae where it serves to hold the prey. The fin sucker is only present in male snails of the Pterotracheidae.

The body size varies from microscopic (Atlantidae) to macroscopic (Carinariidae and Pterotracheidae). Fertilized eggs are laid in mucous strings that hatch after a few days into free swimming veliger
Veliger
A veliger is the planktonic larva of many kinds of marine and freshwater gastropod molluscs, as well as most bivalve mollusks.- Description :...

 larvae (except in Firoloida desmaresti where the eggs remain attached in a tubular filament to the female body).

All the heteropods float or swim with their ventral part upward. The atlantids are negatively buoyant, while the others have neutral buoyancy.

Shell

These snails all have a coiled shell present in their larval stage. But the shell is no longer present in the Pterotracheidae after metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...

. The keel (sharp ridge on a whorl of the shell) of the adult shell extends outwards in the Atlantidae and anteriorly in Carinariidae. The shell is calcareous in the Carinariidae. In the Atlantidae, however, the shell and the keel can be calcareous (genus Atlanta) or the shell calcareous and its keel composed of conchiolin
Conchiolin
Conchiolin and perlucin are complex proteins which are secreted by a mollusc's outer epithelium ....

(genus Protatlanta), or the shell composed exclusively of conchiolin (genus Oxygyrus). Only the Atlantidae can retract into their shell.
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