Caenogastropoda

(Caenogastropods)

Phylum Mollusca

Class Gastropoda

Order Caenogastropoda

Number of families Approximately 100

Thumbnail description
The most diverse gastropod group living today; they include almost every gastropod shell form and are successful in every major habitat on Earth

Evolution and systematics

The earliest caenogastropods appear in late Silurian and early Devonian rocks from about 400 million years ago. From this initial appearance, the caenogastropods have radiated into the most diverse gastropod group living today. This diversity spans every aspect of biodiversity and includes morphology, habitat, behavior, and reproductive mode, among others. Partly because of this tremendous diversity, they were mistakenly divided into two groups: the Mesogastropoda and the Neogastropoda. However, today the term "mesogastropod" is understood to be just a level or grade of gastropod evolution, not an evolutionary lineage. In contrast, the Neogastropoda do represent a monophyletic group within the Caenogastropoda and first appear during the Cretaceous, although the apex of their diversification is in the Upper Cretaceous and during the Tertiary. Another recognized clade within the Caenogastropoda is the Architaenoglossa. Members of the Architaenoglossa were previously considered to be "lower mesogastropods," but are now recognized as the sister taxa of the Sorbeoconcha (which also includes the Neogastropoda). Thus, the Caenogastropoda are recognized as containing the Architaenoglossa and the Sorbeoconcha. The Architaenoglossa contain the freshwater group Ampullarioidea and the terrestrial group Cyclophoroidea. The Sorbeoconcha consist of all remaining caenogastropods. Finer level relationships within the Caenogastropoda remain problematic despite ongoing morphological and molecular studies. This lack of resolution based on morphological characters occurs in part because the earlier variation seen in organ systems and structures of other groups (e.g., the kidney/heart complex, radula) has become relatively fixed in caenogastropods (and especially in the Sorbeoconcha). Where novelty has evolved, it is often similar across taxa and confounds convergent evolution. In molecular studies, appropriate molecular markers for the divergence times within this taxon have yet to be identified.

The Caenogastropods are composed of the Architaenoglossa and the Sorbeoconcha. The Architaenoglossa include the Ampullarioidea and Cyclophoroidea, and the Sorbeoconcha consist of the Cerithiomorpha, Littorinimorpha, Ptenoglossa, and Neogastropoda. There are about 100 families within these smaller clades.

Physical characteristics

Caenogastropod shells are typically coiled and almost every shell form is found within this group, from flat squat shells to globose ones and even long, narrow, tightly coiled ones. A few are limpet-like, and one group, the Vermetidae, has shells that uncoil and look like worm-tubes; in a few caenogastropods, the shell is reduced to an internal remnant in the snail's body. The most pronounced morphological change is in groups such as the Entoconchidae, which are shell-less, worm-like internal parasites of echinoderms.

A major feature of the caenogastropods is modification to the pallial cavity, which contains the gill (or ctenidium) and associated sense organs and openings from the kidneys,

gonad, and intestine. In the Architaenoglossa, the terrestrial Cyclophoroidea have lost the ctenidium and osphradium, and the pallial cavity has been modified as a lung. In contrast, the freshwater Ampullarioidea have a single left ctenidium in addition to a new structure located on the right side of the pallial cavity that serves as a lung. In the Sorbeoconcha, pallial cavity water flow is driven by the large, ciliated osphradium, as well as by the ctenidium. There is also the formation of an anterior (or inhalant) notch, or siphon, in the shell and the mantle in many taxa. Some taxa also have a posterior notch in the shell that is associated with the exhalant current, and in some this resembles a shell-like tube.

The shell is never nacreous, and an operculum is present in most adults. Apart from members of the Neogastropoda, the radula usually has only seven teeth in each row. The radula of neogastropods has five teeth to one tooth in each row; it is altogether absent in some species.

The caenogastropods include some of the largest and smallest gastropods, ranging in adult size from 0.019–15.7 in (0.5–400 mm). The caenogastropods are also the most colorful of the gastropods and have the most elaborate markings and patterns.

Distribution

Caenogastropods occur worldwide in all marine, estuarine, freshwater, and terrestrial habitats.

Habitat

Caenogastropods occur in almost every habitat found on Earth. Most are found in the marine environment, where they extend from the high tide mark to the deepest oceans; several groups live in freshwater or terrestrial habitats. In the terrestrial realm, caneogastropods can be found in the wettest environments of tropical rainforests and in the driest deserts, where annual activity patterns may be measured in hours. Some of the smallest caenogastropods live below ground in the lightless world of aquifers and caves, and others interstitially in groundwater. Marine diversity is highest near shore and becomes reduced as depth increases beyond the shelf slope. Like many other organisms, caenogastropods reach their highest diversity in the tropical western Pacific and decrease in diversity toward the poles.

Behavior

Caenogastropod behavior can be characterized as feeding, fighting, fleeing, and mating, and most of these behaviors are primarily driven by chemoreception. Behavioral responses to visual cues are primarily limited to shadow responses and phototaxis, and because of the supposed lack of visual acuity, display behaviors are not known. However, salt marsh Littorina species have been shown to use vision to assess both shade intensity and shape orientation. In addition, in some Strombidae species, males sequester and guard females; fighting between males has been noted.

Like other gastropods that occur in the intertidal, caenogastropod activity and feeding behaviors vary with the tidal cycle: snails are inactive at low tide (except at night) and become active as the tide rises. In the subtidal, diurnal/nocturnal behaviors are important in avoiding predation; in the open ocean, vertical migrations of pteropods have been documented, with the snails moving to deeper water during daylight, but coming within 328 ft (100 m) of the surface at night. Migrations of upper and lower shore Littorina species have also been documented and may be associated with assortative mating.

Feeding ecology and diet

The majority of caenogastropods are carnivores, although herbivory has evolved in several groups, including the Littorinidae, Columbellidae, and Cypraeidae; many of these taxa also are capable of feeding on detritus as well as encrusting organisms such as tunicates, sponges, and bryozoans. Other caenogastropods have evolved filter-feeding behavior, such as the Clyptraeidae, and some occur as either endo- or ectoparasites in and on echinoderms (Endoconidae). Several large groups are scavengers (Nassariidae), feeding on dead fishes, crustaceans, and other organisms that come to rest on the bottom. Some neogastropods such as cone shells (Conidae) have poison glands and harpoon-like radular teeth that are used to actively hunt worms, other mollusks, and fishes. Off the bottom and up in the water column, members of the Janthinidae are drifting, pelagic carnivores feeding on jellyfishes, whereas the pelagic Heteropoda are active swimmers in search of other mollusks (including small cephalopods), crustaceans, and fishes.

Because of the diversity and abundance of caenogastropods in such a broad range of habitats, predators of caenogastropods include most marine, estuarine, freshwater, and terrestrial carnivores. These include fishes, crustaceans, sea stars, other mollusks, birds, and mammals.

Reproductive biology

Most caenogastropods have a penis to copulate or to exchange spermatophores. The caneogastropod reproductive systems are complex. The male system typically consists of the testes, a prostate gland, and a vas deferens that leads to the penis, which is typically located on the right side of the head. The female system typically has two glands following the ovary: the albumen gland and the capsule gland. Also, there is typically a bursa copulatrix that receives sperm during mating and a seminal receptacle where sperm is stored prior to being used to fertilize the eggs. The addition of the albumen gland and the capsule gland is of special significance because nutritional resources can now be encapsulated with the eggs in the capsule. Egg size is reflected in the initial size of the juvenile shell, or protoconch, and this feature has been useful in distinguishing feeding and non-feeding larvae in both recent and fossil taxa—non-feeding larvae tend to have larger eggs than feeding taxa. The first gastropod larval stage is typically a trochophore that transforms into a veliger and then settles and undergoes metamorphosis to form a juvenile snail. In some caenogastropods, the young hatch as trochophores, grow into veligers, feed on maternally provided albumen, grow, and ultimately break through the capsule and crawl away. In other taxa, they may be released from the capsule as veliger larvae and feed in the plankton before settling and metamorphosing into juvenile snails. Both simultaneous and protandric hermaphrodites are present in the caenogastropods, although the majority of species are dioecious. In freshwater and terrestrial caenogastropods, direct development with crawl-away larvae is the norm.

Conservation status

Marine species tend to be more common than freshwater and terrestrial species, and are often less restricted in their distribution and less susceptible to disturbance. Ten freshwater members of the Neritidae and 23 terrestrial snails (17 Helicinidae and six Hydrocenidae) are listed on the IUCN Red List. Many species are over-harvested for food, such as Strombus (queen conch in the Bahamas). Habitat destruction is the biggest threat to these species, with deforestation directly affecting terrestrial species and increased sedimentation load impacting freshwater species. Freshwater species are also affected by water diversion and reservoir projects, as well as pollution.

Significance to humans

Because of their tremendous diversity in form and color, caenogastropods have figured more prominently in human cultural than most other gastropod groups. The Phoenicians were famous throughout the ancient Mediterranean world for producing a royal purple dye made from organs found in members of the family Muricidae. Cowry shells were first used as money in China in 1200 b.c. So many cultures have used shells for money that it is the most widely and longest used currency in human history. Shell trumpets (constructed primarily of caenogastropod species such as Tonna, Bursa, Trophon, and Cassis) are found in many parts of the world (Polynesia, Japan, South America, and the Mediterranean), where they were used as both signaling and musical instruments. In India, the shanka has been used Hindu rituals for more than 1,000 years. In art, caenogastropod shells often decorate still-life paintings because of their brilliant colors and forms; one of the most famous shell paintings is undoubtedly Rembrandt's 1650 Cone Shell (Conus marmoreus).

Approximately 20 species of cone shells are known to be dangerous to humans, and stings from three species (Conus geographus, Conus textile, and Conus tulipus) have resulted in fatalities. However, cone shell venom is also being used to produce drugs for the control of pain. Lastly, shell collecting has been popular since the early nineteenth century and certain caenogastropod groups such as cone shells, cowries, and Murex have been among some of the most valuable and sought-after species.

Species accounts

List of Species

Apple snail

Ampullaria canaliculata

family

Ampullariidae

taxonomy

Ampullaria canaliculata Lamarck, 1819.

other common names

None known.

physical characteristics

Large globose shell, with whorls separated by deep channel; large, deep umbilicus. Shell smooth and yellow-green to brown in color, sometimes with darker spiral bands. Height 1.7–2.9 in (45–75 mm).

distribution

South America, but introduced throughout Asia for food. (Specific distribution map not available.)

habitat

Freshwater.

behavior

Upon sensing chemical cues of potential predators (e.g., turtles) or of crushed snails, it releases, drops to the bottom, and buries itself.

feeding ecology and diet

Omnivorous; feeds on algae, terrestrial plants, detritus, and animal matter.

reproductive biology

Internal fertilization; pink to white egg masses deposited above water line in aerial conditions.

conservation status

Not listed by the IUCN.

significance to humans

Intermediate host for trematodes, significant rice pest in Asia, and common in the aquarium trade.

Geography cone shell

Conus geographus

family

Conidae

taxonomy

Conus geographus Linne, 1758.

other common names

None known.

physical characteristics

Flat-spired shell, with knobby whorls; aperture elongate and slightly flared at anterior end. Shell marked with diffuse gold-brown tent markings, some densely grouped into bands. Length 2.3–4.7 in (60–120 mm).

distribution

Tropical Indo-Pacific. (Specific distribution map not available.)

habitat

Coral reef flats and fore-reef, in sand, and among coral rubble.

behavior

Feeds at night by capturing and anesthetizing small schools of fish within its rostrum before individually stinging them.

feeding ecology and diet

Feeds on worms, other snails, and fishes.

reproductive biology

Internal fertilization.

conservation status

Not listed by the IUCN.

significance to humans

Highly toxic venom; bites have been fatal.

Resources

Books

Graham, A. "Evolution within the Gastropoda: Prosobranchia." In The Mollusca. Vol. 10, Evolution, edited by E. R. Trueman and M. R. Clark. New York: Academic Press, 1985.

Ponder, W. F. "Superorder Caenogastropoda." In Mollusca: The Southern Synthesis. Part B. Fauna of Australia, vol. 5, edited by P. L. Beesley, G. J. B. Ross, and A. Wells. Melbourne, Australia: CSIRO Publishing, 1998.

Periodicals

Ponder, W. F., and Lindberg, D. R. "Towards a Phylogeny of Gastropod Mollusks: An Analysis Using Morphological Characters." Zoological Journal of the Linnaean Society, 119 (1997): 83–265.

David Lindberg, PhD