Vetigastropoda

(Slit and top shells)

Phylum Mollusca

Class Gastropoda

Superorder Vetigastropoda

Number of families Approximately 12

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A large and diverse group of marine gastropods found from the intertidal zone to the deep sea. Most maintain some bilateral asymmetry in their organ systems; many have shells with slits or other secondary openings.

Evolution and systematics

The origin of the Vetigastropoda in the Paleozoic era (544–248 million years ago) is difficult to interpret because the fossil record contains such large and diverse gastropod clades (groups of organisms with features reflecting a common ancestor) as the Bellerophonta and Euomphalina. These clades may or may not be related to living vetigastropods. Morphological analysis of the fossil shells suggests that their coiling parameters (i.e., the way the shells were built) are different from those of extant vetigastropods. In addition, there is no consensus as to whether the Bellerophonta were gastropods at all or whether they were untorted (without twists) Monoplacophora. Traditionally, shells with slits or emarginations (notched margins) are common features of Palaeozoic gastropods and are often regarded as diagnostic of the most "primitive" vetigastropods. The earliest putative gastropod (Aldanella) completely lacks these structures, however, and it is possible that slits and emarginations were derived more than once from ancestors without slits. In addition, living representatives of some of the most basal vetigastropods have poorly developed shell elaborations or lack them altogether.

The vetigastropods represent a large subset of the snails that were once commonly known as the Archaeogastropoda. Around 1950 some researchers began to recognize that Archaeogastropoda was actually a collection of several distinct lineages, many of which were subsequently removed and given a higher taxonomic rank. Ultimately, most of the remaining "archaeogastropods" were found to possess a small sensory organ called a busicle at the base of their gills, and were grouped together as vetigastropods on the basis of this character. After the reclassification, researchers found that the species in this group shared other characters, many of them sensory structures.

Today the vetigastropods form a well demarcated group placed near the base of the gastropod tree. There are several major branches within the Vetigastropoda. One branch includes the Lepetelloidea or Pseudococculina, which were formerly groped together with the Cocculoidea in the Cocculiniformia. Another branch includes such vetigastropods with shell slits and emarginations as the Pleurotomarioidea, Fissurelloidea, and Haliotoidea. A third branch includes groups that have shells without secondary shell openings, such as the Trochoidea. The Sequenzoidea are also members of the Vetigastropoda. Neomphalida, a group of organisms that lives near deep-sea hot vents and has a unique combination of characters, may also belong to the Vetigastropoda.

Physical characteristics

Vetigastropod shells range from squat and globe-shaped to elongate turreted structures. Limpet morphology has evolved at least six times in the vetigastropods; in many examples a semi-coiled component is still present in the early shell. Shell sculpture varies widely from simple concentric growth lines, which may be barely visible on the shell surface, to heavy radial and axial ribbing as well as everything in between. The shell aperture, or opening, is typically oval and often tangential to the coiling axis. Most species have an operculum (small lidlike organ) that is used to cover the aperture after the head and foot have been pulled back into the shell. The animals are supple and have a single pair of cephalic tentacles as well as a distinct snout containing the mouth. The lateral sides of the animal typically bear sensory

epipodial tentacles. When present, copulatory organs are typically part of the right cephalic tentacle. The vetigastropod radula, or toothed ribbon that aids in feeding, is rhipdoglossate as in the Cocculiniformia and Neritopsina. The gills, kidneys, and hearts of many vetigastropods are bilaterally asymmetrical.

Vetigastropods range in size from the minute Scissurelloidea and Skeneoidea, which may be less than 0.08 in (2 mm) long, to members of the Haliotoidea, which may be more than 11.8 in (300 mm) in length. External color patterns are typically drab, but such groups as the Tricolioidea as well as some Trochoidea and Pleurotomarioidea have bright color markings and glossy shells. Reddish shades are the most common. Many vetigastropod shells are iridescent because of the presence of nacre or mother-of-pearl on their inner surfaces.

Distribution

Vetigastropods are distributed throughout all oceans of the world—in the tropics, temperate regions and even under polar ice.

Habitat

Vetigastropods are found in most marine habitats from the intertidal zone to the deep sea. They occur on rocky substrates; on and in soft sediments; on such exotic deep sea habitats as waterlogged food, the egg cases of sharks and skates, and plant debris; and in close association with other marine invertebrates. Some impressive radiations of vetigastropods have occurred at deep-sea hydrothermal vents and cold seeps. Many of the smallest taxa are found in interstitial (spaces between sand grains) habitats.

Behavior

Most vetigastropods are dioecious, although some deep-sea members of the group are hermaphrodites. Vetigastropods usually discharge their gametes directly into the sea for fertilization and development; thus there is no courtship or mating between individuals in the majority of species. Several vetigastropods have been shown to make escape responses in reaction to other predatory snails as well as sea stars. Escape responses include swaying and tilting the shell to avoid the sea star's tube feet, as well as short spurts of rapid movement away from potential predators after tissue contact.

Intertidal species are seldom active at low tide, to avoid physiological stress due to drying. Most subtidal and intertidal species are active at night when predators who rely on sight are less effective. Migratory movements of patellogastropods are limited to a general up shore pattern, from recruitment in the lower intertidal to life in higher intertidal zones.

Feeding ecology and diet

Most vetigastropods feed on such encrusting invertebrate organisms as sponges, bryozoans, and tunicates. Feeding directly on such plant material as algae and marine angiosperms has evolved in several groups, most notably the Haliotoidea and Trochoidea. Deep-sea vetigastropods typically ingest sediment; those associated with exotic substrates likely feed on microbes that are actively decomposing the substrate. Filter feeding has evolved in several vetigastropod species, including Umbonium in the Trochoidea.

Predators of vetigastropods include other predatory gastropods, sea stars, fishes, small mammals, and shore birds. Such shore birds as oystercatchers are especially voracious predators.

Reproductive biology

Vetigastropods generally have small eggs that produce lecithotrophic or nonfeeding larvae. Direct development has evolved in several species. Some brooding species use such features of the shell as the umbilicus or surface sculpture to hold the developing young. Unlike the patellogastropods, many vetigastropods secrete egg envelopes and have glandular pallial structures that produce masses of jelly-coated eggs on the sea bed. Early development may occur in some species within these masses of jelly containing the spawn. Copulatory organs in vetigastropods are often derived from cephalic and tentacular structures. Internal fertilization has evolved in several groups and is especially common in species living near deep-sea vents.

Larger species produce millions of eggs per reproductive season and typically have yearly cycles of spawning. Smaller species produce fewer eggs, but can be gravid and capable of spawning year round. Vetigastropod larvae pass through trochophore and veliger stages before settling and undergoing metamorphosis.

Conservation status

Most vetigastropods are relatively common, as one would expect of pelagic dispersing species. Endemic taxa are found among such usual biogeographical features as isolated islands in large oceanic basins and in such smaller seas as the Mediterranean. The only vetigastropods on the IUCN Red List are two members of the Skeneoidea, Teinsotoma fernandesi and Teinsotoma funiculatum from São Tomé and Principe in the Gulf of Guinea off western Africa. Both are Data Deficient. At more regional levels, several abalone species of the genus Haliotis are threatened both by overharvesting and by a pathogen in western North America that leads to a fatal wasting disease called withering syndrome. The overharvesting of large Fissurella species or lapas has also been problematic in Chile. In the tropical Pacific, the commercial harvest of Trochus species for the button industry began in the early twentieth century and has significantly reduced populations in some areas. Lastly, the shell trade puts tremendous value on some species of Pleurotomarioidea; collectors pay hundreds of dollars for exquisite specimens.

Significance to humans

The significance of vetigastropods to humans includes the use of their shells by inhabitants of the northern Pacific islands, southern Africa, and Australia and New Zealand to

make fishhooks, buttons and beads (especially nacreous groups), or bowls (large abalone species). The most common use of vetigastropods was and continues to be as food—for subsistence as well as haute cuisine (e.g., abalone steaks).

Species accounts

List of Species

Red abalone

Haliotis rufescens

order

Haliotoidea

family

Haliotidae

taxonomy

Haliotis rufescens Swainson, 1822.

other common names

French: Ormeau du Pacifique; German: Rote Seeohr; Italian: Abalone rosso.

physical characteristics

Shell is ear-shaped with a reduced posterior coil producing a flat bowl-shaped shell. Outer shell surface is red in color and sculpted with broad radial ribs. Interior of shell is iridescent mother-of-pearl. Grows as long as 11.8 in (300+ mm).

distribution

Western United States.

habitat

Intertidal and shallow subtidal zones, among or close to kelp forests.

behavior

H. rufescens larvae are sensitive to specific chemicals released by coralline algae and use these cues to begin settlement on the substrate.

feeding ecology and diet

Feeds primarily on large brown and red algae.

reproductive biology

Broadcast spawner; fertilization takes place externally. Has relatively short phase of pelagic development.

conservation status

Not listed by the IUCN. Overfished in much of its southern range; northern sport fisheries are regulated with size and catch limits.

significance to humans

Has been an important shoreline food item in California for over 13,000 years. Shell provides mother-of-pearl for jewelry and other forms of ornamentation; larger specimens serve as natural bowls.

No common name

Lepetodrilus elevatus

order

Lepetodriloidea

family

Lepetodrilidae

taxonomy

Lepetodrilus elevatus McLean, 1988, East Pacific Rise, Pacific Basin.

other common names

None known.

physical characteristics

Shell aperture is oval, with apex at posterior end and a deeply convex dorsal shell surface. Shell sculpture has concentric growth lines. Shell is whitish gray in color and covered by a greenish brown periostracum (exterior membrane). Interior of shell is whitish with a horseshoe-shaped muscle scar. Reaches 0.16–0.31 in (4–8 mm) in length.

distribution

East Pacific Rise and Galápagos Rift regions, Pacific Basin.

habitat

Deep-sea hydrothermal vents at a depth of 7,874–8,858 ft (2,400–2,700 m) associated with the tubeworm Riftia.

behavior

Nothing is known.

feeding ecology and diet

Grazes on Riftia tubes, ingesting bacteria and detritus.

reproductive biology

Dioecious; male penis is located near base of right cephalic tentacle.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Top shell

Trochus niloticus

order

Vetigastropoda

family

Trochidae

taxonomy

Trochus niloticus Linnaeus, 1758.

other common names

French: Troca; German: Kreiselschnecke; Local dialect: Lala, ammót, alileng, troka, susu bundar.

physical characteristics

A large conical shell with a broad expansive base. Shell marked with purple-pink wavy lines, often obscured by coralline algae. Aperture is ovoid with small open umbilicus. Up to 1.6–3.9 in (40–100 mm) in height.

distribution

Tropical Indo-Pacific waters.

habitat

Coral reefs and lagoons.

behavior

Primarily nocturnal; rests on coral heads and basalt substrates during daylight hours.

feeding ecology and diet

Feeds on filamentous algae and diatoms.

reproductive biology

Broadcast spawner; external fertilization and relatively short period of pelagic development.

conservation status

Not listed by the IUCN. Heavily overfished throughout its range for button blanks and food. Harvesting is now limited with size and bag limits; other regions are under full to partial closure. Aquaculture and restoration studies are being conducted to "reseed" reef flats.

significance to humans

Important source of food and decorative material from its nacreous shell.

Resources

Periodicals

Haszprunar, G. "Sententia: The Archaeogastropoda: A Clade, a Grade, or What Else?" American Malacological Union Bulletin 10 (1993): 165–177.

Hickman, C. S. "Archaeogastropod Evolution, Phylogeny and Systematics: A Re-Evaluation." Malacological Review Supplement 4 (1988): 17–34.

Salvini-Plawen, L., and G. Haszprunar. "The Vetigastropoda and the Systematics of Streptoneurous Gastropoda (Mollusca)." Journal of Zoology (London) 211 (1987): 747–770.

David Lindberg, PhD