The Pterotracheoidea is, according to the

Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005) The taxonomy of the Gastropoda as it was revised in 2005 by Philippe Bouchet and Jean-Pierre Rocroi is a system for the scientific classification of gastropod mollusks (Gastropods are a taxonomic class of animals which consists of snails and sl ...

, a

taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...

superfamily of sea snails or sea slugs, marine

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s in the clade

Littorinimorpha Littorinimorpha is a large order of snails, gastropods, consisting primarily of sea snails ( marine species), but also including some freshwater snails ( limnic species) and land snails ( terrestrial species).Bouchet P. & Rocroi J.-P. (Ed.); Fr� ...

. They are commonly called heteropods or sea elephants.

Taxonomy

According to the World Registry of Marine Species, this superfamily comprises five families, two of which are extinct:

Habitat

These

holoplankton Holoplankton are organisms that are planktic (they live in the water column and cannot swim against a current) for their entire life cycle. Holoplankton can be contrasted with meroplankton, which are planktic organisms that spend part of their l ...

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 The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

living : * a transparent body and shell * the foot has evolved into a swimming fin that produces motion through undulation. * the

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular arthropod mouthparts, mouthparts used for feeding and sucking. In vertebrates, a pr ...

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

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

, 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 see the internal organs and the

statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, crustaceans, and gastropods, A similar structure is also found in '' Xenoturbella''. T ...

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 Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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 A veliger is the planktonic larva of many kinds of sea snails and freshwater snails, as well as most bivalve molluscs (clams) and tusk shells. Description The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxono ...

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 is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

. 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 Conchiolins (sometimes referred to as conchins) are complex proteins which are secreted by a mollusc's outer epithelium (the mantle). These proteins are part of a matrix of organic macromolecules, mainly proteins and polysaccharides, that ass ...

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

References

Seapy, Roger R. 2008. Pterotracheoidea Rafinesque, 1814. Heteropoda Lamarck, 1812, heteropods, sea elephants. Version 29 December 2008, The Tree of Life Web Project
{{Taxonbar, from=Q2116183 Littorinimorpha Taxa named by Constantine Samuel Rafinesque