Adventist Youth Honors Answer Book/Nature/Land and Freshwater Mollusks

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Land and Freshwater Mollusks
South Pacific Division

Nature

Skill Level Unknown
Year of Introduction: Unknown


Limited Availability


1.Distinguish between sea snails, land snails, slugs, freshwater snails and freshwater bivalves.

Snails is a term applied to most of the members of the molluscan class Gastropoda that have a coiled shell that is large enough for the animal to retract completely into. Generally, the term "snail" means "land snail" but there are thousands of species of sea snails and freshwater snails.

Sea snails live in salt water/marine environments. Most have gills, though some species breath air and live in intertidal environments where they are in and out of the water regularly.

Land snails live on land. However, it is not always easy to say which species are terrestrial, because some are more or less amphibious between land and freshwater, and others are relatively amphibious between land and saltwater. The majority of land snails are pulmonates, i.e. they have a lung and breathe air. A minority have a gill and an operculum. Many of these operculate land snails live in habitats or microhabitats that are sometimes (or often) damp or wet, such as for example in moss. Land snails have a strong muscular foot; they use mucus to enable them to crawl over rough surfaces, and in order to keep their soft bodies from drying out.

Slugs are basically land snails without a visible shell. Sea slugs live in the ocean.

Freshwater snails live in wet or fresh water, but not marine (salt) water. Some groups of freshwater snails respire (breathe) using gills, whereas other groups need to reach the surface to breathe air. There are thought to be about 4,000 living species.

Freshwater bivalves live in freshwater, as opposed to saltwater, the main habitat type for bivalves. They come in two groups freshwater mussels and freshwater clams. They have a laterally compressed body enclosed by a shell consisting of two hinged parts connected by a flexible ligament. They have no head, and they also lack a radula. Freshwater bivalves live in many types of habitat, ranging from small ditches and ponds, to lakes, canals, and rivers. Species in the two groups vary greatly in size. Some of the pea clams (Pisidium species) have an adult size of only 3 mm. In contrast, one of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae; it can grow to a length of 20 cm, and usually lives in lakes or slow rivers. Freshwater pearl mussels are economically important as a source of freshwater pearls and mother of pearl.

2. Explain the basic structure and parts of the mollusk and its shell.

There are between 50,000 and 120,000 extant (living, not extinct) recognized species of mollusks across 8 families, so there is considerable variation. Limiting the scope to freshwater and land dwelling still leaves two families and thousands of species:

  • Gastropoda: land snails and slugs (land dwelling) and freshwater snails and freshwater limpets
  • Bivalvia: clams and mussels (freshwater)

The three most universal features defining molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the structure of the nervous system.

Other than these things, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small, and the main body cavity is a hemocoel through which blood circulates; their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults.

Archimollusc-en.svg

A striking feature of molluscs is the use of the same organ for multiple functions. For example, the heart and nephridia ("kidneys") are important parts of the reproductive system, as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity, which is important for excretion and reproduction. In reproduction, molluscs may change gender to accommodate the other breeding partner.

Slugs (and many sea dwelling mollusk) have no visible shells. For the snails, claims and mussels we can generalize about the shells.

Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides.) The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous (commonly known as mother of pearl) layer. The layers usually incorporate a substance called conchiolin, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone-tanned proteins.

The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle.

Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashells, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrroles and porphyrins.

The bivalvia shells are connected with a flexible ligament.

3. Explain the snail's method of eating, moving and reproducing.

We restrict the answer to land snails here, as they are the focus of the honor.

Eating: Most snails have thousands of microscopic tooth-like structures located on a ribbon-like tongue called a radula. The radula works like a file, ripping food into small pieces. Many snails are herbivorous, eating plants or rasping algae from surfaces with their radulae, though a few land species and many marine species are omnivores or predatory carnivores.

Moving: Land snails have a strong muscular foot; they use mucus to enable them to crawl over rough surfaces, and in order to keep their soft bodies from drying out.

Reproduction: The great majority of land snails are hermaphrodites with a full set of reproductive organs of both sexes. A few groups of land snails have separate sexes; they are male and female. The age of sexual maturity is variable depending on species of snail, ranging from as little as 6 weeks to 5 years.

Prior to reproduction, most pulmonate (air breathing) land snails perform courtship behaviors before mating, which may last between two and twelve hours. In a number of different families of land snails and slugs, prior to mating one or more love darts are fired into the body of the partner. This is thought by some scholars to be the inspiration for Cupid.

Prolific breeders, pulmonate land snails inseminate each other in pairs to internally fertilize their ova via a reproductive opening on one side of the body, near the front, through which the outer reproductive organs are extruded so that exchange of sperm can take place. Fertilization then occurs and the eggs develop. Each brood may consist of up to 100 eggs.

Garden snails bury their eggs in shallow topsoil primarily while the weather is warm and damp, usually 5 to 10 cm down, digging with their foot. Egg sizes differ between species, from a 3 mm diameter in the grove snail to a 6 mm diameter in the Giant African Land Snail. After 2 to 4 weeks of favorable weather, these eggs hatch and the young emerge. Snails may lay eggs as often as once a month.

The snail's shell develops while it is still an embryo; it is, however, very weak, and needs an immediate supply of calcium. Newly hatched snails obtain this by eating the egg from which they hatched. The cannibalization by baby snails of other eggs, even unhatched ones, has been recorded and is a problem in farming snails.

Promptly after they are finished ingesting their egg casings, they crawl upwards through the small tunnel in order to digest the egg. At this stage, the young are almost completely transparent and colorless. Their shell is usually slightly smaller than the egg they hatched from, but their length when out of their shell is slightly greater than the egg diameter. After a few weeks, the snails will begin to show their first tinge of color, usually slightly blue, before they turn their adult color. Roughly three months after they have hatched, they will look like miniature versions of their parents. They will continue to grow, usually for two to three years, until they reach adult size, although there have been confirmed recordings of snails growing amazingly fast - becoming even bigger than their parents in little more than a month.

4. Explain freshwater bivalves's methods of eating and moving.

Bivalves, (clams and mussels) are effective filter feeders and can have large effects on the water columns in which they occur. They remove plankton and organic particles from the water column and can significantly improve water quality and clarity.

Freshwater Mussels and clams have a foot that allows them to move around or anchor into the mud.

5. a. Describe the uses people have made of snails.

Food, notably by the French who call cooked snails escargot, but also in Spain, Philippines, Indonesia, Morocco, Nigeria, Algeria, Cameroon, Italy, Portugal, Greece, Belgium, Vietnam, Laos, Cambodia, Cyprus, Ghana, Malta, Terai of Nepal, southwestern China, Northeast India states such as Manipur, Tripura and parts of the USA. Sometimes snails have been eaten in times of famine by people who do not normally eat them. Cooked snail is chewy. More details here.
Cooked snails.JPG

The eggs of certain snail species are eaten in a fashion similar to the way caviar is eaten.

Note that snails are scavengers, not clean animals, and are not recommended as food for Pathfinders.

Pest control While most snails are considered agricultural pests, the decollate snail (Rumina decollata) will capture and eat garden snails, and therefore has sometimes been introduced as a biological pest control agent. However, this is not without problems, as the decollate snail is just as likely to attack and devour other gastropods that may represent a valuable part of the native fauna of the region.

Cosmetic skin creams derived from Helix aspersa snails are sold for use on wrinkles, scars, dry skin, and acne. A research study suggested that secretions produced under stress by Helix aspersa might facilitate regeneration of wounded tissue.

Snails are farm raised (Heliciculture) for food, snail slime, and for pest control purposes in some areas.

Symbolism Because of its slowness, the snail has traditionally been seen as a symbol of laziness. In Christian culture, it has been used as a symbol of the deadly sin of sloth. Psalms 58:8 uses snail slime as a metaphorical punishment. The terms "snail's pace" and "snail mail" suggest slowness.

Divination, with snails is an attempt to gain insight into a question or situation by way of an occultic, standardized process or ritual. Also not recommended for Pathfinders.

5. b. Describe the disease and pest problems associated with some land and freshwater snails.

Crop Damage: The same snails that some people raise or gather as food also are agricultural pests that cause considerable crop damage. Introduced slug and snail varieties tend to be worse pests than native species, probably due in part to the lack of natural controls. Snail pests attack crops ranging from leafy vegetables to fruits that grow near the ground, such as strawberries and tomatoes, to citrus fruits high up on trees.

The Federal Plant Pest Act defines a plant pest as "any living stage (including active and dormant forms) of insects, mites, nematodes, slugs, snails, protozoa, or other invertebrate animals, bacteria, fungi, other parasitic plants or reproductive parts thereof; viruses; or any organisms similar to or allied with any of the foregoing; or any infectious substances, which can directly or indirectly injure or cause disease or damage in or to any plants or parts thereof, or any processed, manufactured, or other products of plants..." The Animal and Plant Health Inspection Service (APHIS) categorizes giant African snails as a "quarantine significant plant pest." The United States does not allow live giant African snails into the country under any circumstances. It is illegal to own or to possess them. APHIS vigorously enforces this regulation and destroys or returns these snails to their country of origin.

Since large infestations of snails can do devastating damage, many states have quarantines against nursery products, and other products, from infested states. Further, it is illegal to import snails (or slugs) into the U.S. without permission from the Plant Protection and Quarantine Division (PPQ), Animal Plant Health and Inspection Service, U.S. Department of Agriculture. APHIS also oversees interstate transportation of snails. Anyone who plans to "import, release, or make interstate shipments of" snails, must complete APHIS's PPQ Form 526, Application and Permit to Move Live Plant Pests and Noxious Weeds. Submit the form to your State regulatory official. The state will process the request and make a recommendation to APHIS who will then make a decision.

Invasive Species and Ecosystem Damage: Accidental or deliberate introduction of various snail species into new territory has resulted in serious damage to some natural ecosystems.

Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via freshwater snail hosts. It is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease — 100 million in Africa alone." The worm is caught from contact with water that includes infested snails. The worms may infect the urinary tract or intestines. Symptoms may include abdominal pain, diarrhea, bloody stool, or blood in the urine. In those who have been infected a long time, liver damage, kidney failure, infertility, or bladder cancer may occur. In children it may cause poor growth and learning difficulty. It kills up to 200,000 people a year. Improving access to clean water and reducing the snail population reduces infection rates, as does the drug Praziquantel.

6. a. Explain how snails fit into an area's ecological balance.

Snails can be found in a very wide range of environments, including ditches, deserts, and the abyssal depths of the sea. Although land snails may be more familiar to people, marine snails constitute the majority of snail species, and have much greater diversity and a greater biomass. Numerous kinds of snail can also be found in fresh water. Many land snails are valuable because they can feed on a wide range of agricultural wastes, such as shed leaves in banana plantations. They are also considered pests because they eat live plants and fruit with commercial value. Snails are food for a wide range of different vertebrate and invertebrate animals.

6. b. Explain the need for conservation of natural areas as habitats for snails and other creatures.

Some animals are threatened from loss of natural habitat and pollution. Providing protected areas allows God's diversity to continue to exist and creates areas we can go see nature first hand.

Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, Villosa arkansasensis, is known only from the streams of the Ouachita Mountains in Arkansas and Oklahoma, and like several other freshwater mussel species from the southeastern USA, it is in danger of extinction.

Zebra mussel GLERL 4.jpg
Conservation also includes protecting from invasive species. Without their natural predators, introduced species can destroy the balance between native species. The golden mussel spread from south-east Asia to Argentina, where it has become an invasive species. Another well-travelled freshwater bivalve, the zebra mussel (Dreissena polymorpha) originated in southeastern Russia, but has been accidentally introduced to inland waterways in North America and Europe, where it damages water installations and disrupts local ecosystems.

7. Do either a or b:

a. Collect, photograph or sketch land snails, freshwater snails and/or freshwater bivalves from the local area (empty shells only).

Go forth and discover.

b. Keep and observe live snails or slugs in a suitable container.

Have fun with this.

References

The following Wikipedia and other articles were the source of this information and a good place to start learning more details about the facinating world of mollusks.

  1. The Living World of Molluscs
  2. Mollusca
  3. Slugs
  4. Snail
  5. Freshwater Snail]
  6. Freshwater Bivalve
  7. Freshwater Mollusc
  8. Schistosomiasis
  9. Oyster
  10. Freshwater Mussels