Vetigastropoda

Vetigastropoda Temporal range: Ludlow–Recent[1] PreꞒ Ꞓ O S D C P T J K Pg N
Various shells of Vetigastropoda
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Clade:	Vetigastropoda Salvini-Plawen, 1989
Superfamilies
See text

The fossil vetigastropod Discohelix tunisiensis from the Matmor Formation (Middle Jurassic) of southern Israel.

Vetigastropoda is a major taxonomic group of sea snails, marine gastropod mollusks that form a very ancient lineage. Taxonomically the Vetigastropoda are sometimes treated as an order, although they are a clade in Bouchet and Rocroi, 2005.

Vetigastropods are considered to be among the most primitive living gastropods,^[2] and are widely distributed in all oceans of the world. Their habitats range from the deep sea to intertidal zones. Many have shells with slits or other secondary openings. One of their main characteristics is the presence of intersected crossed platy shell structure.^[3] Most vetigastropods have some bilateral asymmetry of their organ systems.^[4]

Description

Vetigastropods range in size from approximately 0.08 in (2 mm) long in the case of Scissurelloidea or Skeneoidea, to more than 11.8 in (300 mm) in length, as with the Haliotoidea. External colours and patterns are typically drab, but such groups as the Tricolioidea and some Trochoidea and Pleurotomarioidea have bright colours and glossy shells. The clade is characterized by having an intersected crossed platy shell structure.^[3]

Shells range from elongate turret-shaped structures, to near-spherical. Shell sculpture varies greatly from simple concentric growth lines, which may or may not be barely visible on the shell surface, to heavy radial and axial ribbing. The shell aperture is normally oval, and often tangential to the coiling axis. Most species have an operculum (a small lid-like organ). Within the shell, Vetigastropods have a single pair of cephalic tentacles, and a distinct snout containing the mouth. The lateral sides of the body typically have sensory epipodial tentacles.^[5]

Distribution

Vetigastropods are found throughout all oceans of the world, including tropical areas, temperate regions, and under polar ice.

Habitat

Vetigastropods are present in most marine environments from intertidal zones to the deep sea. They exist on rocky substrates, in soft sediments, and some have been found at deep-sea hydrothermal vents and cold seeps.

Behaviour

Most vetigastropods are dioecious, although some deep-sea varieties are hermaphrodites. Vetigastropods usually eject their gametes directly into the sea for fertilization, thus there is no courtship or mating between individuals for most species.

Diet

Vetigastropods typically feed on such organisms as bryozoans, tunicates, and sponges. Several species such as Haliotoidea and Trochoidea have evolved to feed directly on such plant material as algae and marine angiosperms. Deep-sea vetigastropods typically consume sediment.^[6]

Reproduction

Vetigastropods normally have very small eggs that produce lecithotrophic (yolk feeding) or non-feeding larvae. Many vetigastropods secrete egg envelopes and have glandular pallial structures that produce masses of jelly-coated eggs.

Larger species typically have yearly cycles of spawning, and produce millions of eggs per reproductive season. Smaller species produce fewer eggs, but can spawn year round.

Taxonomy

The Vetigastropoda have been referred to as a superorder as recently as at least 2007, by M. Harzhauser^[7] and in 2005 by D. Heidelberger and L. Koch^[8] following Ponder & Lindberg, 1997, although Bouchet & Rocroi, 2005 refer to this group simply as a clade, leaving taxonomic determination as a future option. The World Register of Marine Species (WoRMS) follows Bouchet & Rocroi regarding the taxonomic content of the Gastropoda but gives ranks to the higher taxa and defines Vetigastropoda as a subclass ^[9]

Bouchet & Rocroi, 2005 treats the Vetigastropoda as a major clade and as a sister clade to the Caenogastropoda but includes the Vetigastropoda in what are referred to as Basal taxa that are certainly Gastropoda. Ponder & Lindberg, 1997 previously assigned the Vetigastropoda, as a superorder, to the Subclass Orthogastropoda.

Phylogenetic analysis indicates that this taxon is one of the four natural groups within the Gastropoda: Vetigastropoda, Caenogastropoda, Patellogastropoda, and Heterobranchia. Research on the mitochondrial genome arrangement has shown that the Vetigastropoda (and Caenogastropoda) mostly retain the ancestral gene arrangement.^[10]

Superfamilies

Superfamilies within the Vetigastropoda include:

• Not assigned to a superfamily within Vetigastropoda are:
  • the families: Ataphridae Cossmann, 1915, Crosseolidae Hickman, 2013, Pendromidae Warén, 1991 (synonym: Trachysmatidae Thiele, 1925), † Schizogoniidae and Trochaclididae Thiele, 1928
  • the genera: † Discohelix Dunker, 1847 and Sahlingia Warén & Bouchet, 2001
• Superfamily Amberleyoidea
• Superfamily Angarioidea (created as a new superfamily by Williams et al. (2008).^[11])
• Superfamily Eotomarioidea
• Superfamily Fissurelloidea Flemming, 1822 - keyhole limpets
• Superfamily Haliotoidea Rafinesque, 1815 - abalones^[12]
• Superfamily Lepetelloidea
• Superfamily Lepetodriloidea McLean, 1988 - hydrothermal vent limpets
• Superfamily Murchisonioidea
• Superfamily Neomphaloidea
• Superfamily Phasianelloidea
• Superfamily Pleurotomarioidea Swainson, 1840 - slit snails
• Superfamily Porcellioidea
• Superfamily Scissurelloidea
• Superfamily Seguenzioidea Verrill, 1884
• Superfamily Trochoidea Rafinesque, 1815 - top snails
• Superfamily Turbinoidea - turban snails - In 2008 this superfamily has been split to Trochoidea and Phasianelloidea

References

External links

Wikispecies has information related to: Vetigastropoda

Wikimedia Commons has media related to Vetigastropoda.

• Aktipis, S. W.; Giribet, G. (2011). "Testing relationships among the vetigastropod taxa: A molecular approach". Journal of Molluscan Studies 78: 12. doi:10.1093/mollus/eyr023. 
• Daniel L. Geiger and Christine E. Thacker, Molecular phylogeny of Vetigastropoda reveals non-monophyletic Scissurellidae, Trochoidea, and Fissurelloidea; Molluscan Research 25(1): 47–55 , ISSN 1323-5818
• Vetigastropoda at the National Center for Biotechnology Information
• Gastropod taxonomy at Paleos
• Gastropod reproductive behavior
• Gastropod Neuroscience
• Reconstructions of fossil gastropods
• 2004 Linnean taxonomy of gastropods