Digestive System of Pila; Apple Snail
• The digestive system of a mollusk contains the mouth, radula, stomach, intestine, visceral mass, esophagus and anus.
• The mouth and the esophagus is where the food is approached
• The digestive system of a mollusk is very simple.
• Food that a mollusk eats is gathered up by the cells in the glands from the stomach then passed to the blood.
• A mollusk also filters out algae and bacteria that may be gathered in the mass of water.
• Most mollusks are herbivores, carnivores, or filter feeders though there are some species that are detritus feeders or are parasitic.
• Many mollusks (ie. snails and slugs) feed through a tongue shaped structure called a radula.
• A radula is a layer of flexible skin with hundreds of tiny teeth making it appear like sandpaper.
• Inside the radula there is stiff rod of cartilage. It places the tip of the radula on the food and pulls the flexible area of skin back and forth over the cartilage.
• Herbivores use their radula to scrap algae off of surfaces in the water or to eat the buds and roots of flowers on land plants.
• Carnivores extend their radula into the shell of the prey and tear up and swallow the preys tissue.
• Mollusks have a very complex digestive tract. After being taken by the radula, the food goes to the digestive glands and then to its intestine. Wastes exit via the anus and food enters the mouth.
• In the class gastopods, the mouth and anus are at the same end but are different openings.
• In some mollusks like oysters, clams and scallops, they use there gills to sift through food in the water.
• There is a layer of sticky mucus on the gills that the food sticks to and then is pushed to the mouth by cilia on the gills as well.
Distribution of Mollusca in Nepal
Diversity of Mollusca: Nepal
Nepal—-rich diversity in the Himalaya—Mollusca≈200 sp. from low land to high land—-terrestrial —aquatic.
Studied —past 200 years—
No significant investigation
University researchers, Agro-economist, farmers—-Opportunistic collection of Mollusca
Identified as pest, vector, invasive species, endemic species–
Mollusca Diversity increases from West to East—-Rainfall (well represented group in East—Caenogastropoda)
What are the significance of Mollusca distribution?
Narrow range of ecological tolerance—–limited mobility—-highly sensitive to environmental changes
Indicator—–global change—– Shift—loss
Food values——
Role in ecosystem—–Treptaxids (Carnivore)—Primarily—-detritivores—-nutrients
Habitats
Small or medium– streams—organic matter & detritus also in small eutrophic ponds & paddy fields—- intensive agriculture and natural bodies of shallow water—warm in summer.
Dark black mud—-low in oxygen.
The microhabitat— fine mud & leaf litter in very shallow pools.
Spring, slowly running streams & rivers
Forest land, agriculture land, wetlands
Scientists worked on Mollusca in Nepal
Schileyko & Frank 1994
Kuznetsov 1996
Schileyko and Kuznetsov 1996, 1998
Subba 2003, Subba and Ghosh 1999, 2000, 2008
Thapa 2003
Khatiwara 2008, Khanal 2010, Devkota 2008, 2011, 2012
Budha 2005, 2007, 2008, 2010, Budha & Naggs 2008, Budha et al. 2012, Budha 2012
Nesemann et al. 2001, 2003, 2005a,b, Nesemann 2006, 2009
Taxonomic problems still exist
Kuznetsov and Schileyko 1997
Schileyko 1999
Wiktor 2001
Wiktor and Bossneck 2004
Kuzminykh and Schileyko 2005
Gerb er and Bossneck 2009
Irikov and Bekhem 2011
Nesemann et al. 2001, 2003, 2005a,b, Nesemann 2006, 2009
Mollusca in Nepal
Terrestrial Snails & Slugs
Family Genera Species
Prosobranchia snails 3 6 22
Pulmonata snails 23 40 87
Pulmonata slugs 5 5 8
Total 31 51 117#10
Freshwater Snails & Mussels
Families Genera Species
Bivalves 4 6 30
Gastropoda 10 26 49
Total 17 78
(Nesemann et al. 2001, 2007, Nesemann 2006, 2009, Irikov & Becham 2011)
Some species of Mollusca in certain locations of Nepal
Chitwan-Freshwater Snails
Bellamya bengalensis
Gabbia orcula
Gyraulus euphraticus Indoplanorbis exustus
Lymnaea luteola
Melanoides tuberculata
Pila globosa
Thiara granifera
T. lineata
Pea clams
(Mollusca: Bivalvia: Sphaeriidae) from Nepal
The pea clams (Sphaeriidae) of Nepal are represented by 11 taxa.
The highest diversity is found in the mid-altitudinal range from 795-1570 m.
Pea clams are poorly represented in the high Himalaya and the Terai.
Most pea clams are useful indicator species and, owing to their high abundance and long life span, they are useful in monitoring the water quality of streams.
Pea clams
(Mollusca: Bivalvia: Sphaeriidae) from Nepal
Musculium indicum*upper Bagmati Riverbasin, Kaski, Kavre
Musculium goshaitanensis*New to Nepal:Dhulikhel.
Pisidium (Euglesa) atkinsonianum* Kathmandu, Kavre
P. (E.) casertanum*New to the fauna of Nepal: Kavre District, Banepa
P. (Odhneripisidium) annandalei*southwest Kathmandu Valley
P. (O.) prasongi*W. reg., Kaski District, C. Reg. Kavre
P. (O.) kuiperi*upper Bhageri Khola at Godawari, Mustang
P. (Afropisidium) ellisi*New to the fauna of Nepal: Kavre District, Banepa
P. (A.) clarkeanum*Terai in Western, C. & E. regions
P. (A.) clarkeanum dhulikhelensis*Kavre District, endemic to the upper Punyamata Valley.
P. (A.) nevillianum *Terai in Central and Eastern regions, Midhills in Western region
Freshwater Mollusca: Rautahat: Mudbalwa VDC
Ilam, Panchthar, Taplejung, Sunsari, Dhankuta and Tehrathum, Dang, Banke, Kailali and Kanchanpur
Aegista (Plectotropis) tapeina confined to the altitude of 1700 m in the east which was covered with stones and decayed leaves, Their shells were found at wet and rocky places of the forest.
Macrochlamys lubrica,Rotungia williamsoni *Rocky places of K &K
Cryptaustenia ovum and Taphrespira compluvialis* low land and midland regions of east Nepal.
Laevicaulis alte (Black Slug)*Morang, Dang, B, K& K wet and moist areas in association with algal plants. Opeas sp.* Morang:outside the kitchen & climbing and approaching to the damp areas covered with vegetations like algae and mosses
Bacillium sp., Glessula tenuispira and Aegista (placetotropis) tapeina * stony beds of cardamom gardens of the P. & Ilam
Mollusca:Saptari (From Man made lakes)
• Subba and Pandey (2002) – described twenty-one species of mollusc of Jhapa district.
• Nesemann and Sharma (2003) – 45 species of aquatic mollusca (25 gastropods and 20 bivalves) from lowland (Terai) regions of Nepal; none of them are endemic.
Some freshwater Mollusca in Nepal
Economic Importance of Molluscs:
Some Mollusca are indirectly harmful to man but most of them are beneficial. The harmful molluscs are slugs and shipworms. Slugs are injurious in gardens and cultivations; they not only eat the leaves but also destroy plants by cutting up their roots and stems.
Teredo, the shipworm burrows into wooden structures immersed in the sea, it causes serious damage to wharves, piers and ships. But molluscs are a great source of human food in various parts of world, millions of maunds of clams, oysters, scallops and mussels are eaten in China, Japan, Malaya, Europe and America, oysters being regarded as a delicacy.
Other bivalves, octopuses and cuttlefishes furnish large quantities of food in Europe. Shells of freshwater mussels are used in the pearl button industry in all parts of the world, they are made from the nacreous layer of shells, no other material stands laundering as these buttons.
Shells of oysters are mixed with tar for making roads in America and lime from these shells is used in feeding poultry for formation of their egg shells. Lime is also used in buildings.
In many parts of the world molluscan shells are used for making ornaments and jewellery, in some parts shells of Cypraea (cowrie) are used as money and as ornaments.
Many freshwater clams and marine oysters produce pearls, but the most valuable natural pearls are produced by pearl oysters Pinctada margaritifera and Pinctada mertensi which inhabit the warmer parts of Indian and Pacific Oceans along the coasts of China, India, Sri Lanka and Japan. A pearl is made when a small foreign object, such as a particle of sand or a parasite, lodges between the shell and the mantle.
The foreign object becomes a nucleus around which concentric layers of nacreous are laid by the mantle, in this manner a pearl is formed. But pearls are also produced by most pelecypods including freshwater clams.
In Japan pearl culture is practiced by artificially introducing a small solid or liquid irritant below the mantle of the oyster, the resultant one year old pearl is then transplanted to another oyster, a pearl of good size is obtained in three years after transplanting.
Foot and Shell of Mollusca
Foot/podium in Mollusca
Characteristic—Muscular and Ventral
Creeping, Sucker adhere firmly to rock/ substances—rest, anchor the foot in sand, Burrowing, leaping, swimming.
Foot in different Mollusca
Monoplacophora
Primitive, center, Broad, circular and flat sole, animal creeps by muscular wave
Amphineura
Large, muscular, larger part on the ventral surface (Chiton) exceptionally in Chitononellus, Cryptoplax etc.
Vestigial foot-in the form of ciliated, longitudinal groove, mouth –cloaca (Neomenia), Absent (Chaetoderma)
Mucus & wave of muscular activity —propulsion Foot Fig. Chiton
Scaphopoda
Narrow—cylindrical and tongue shaped
Capable to protrude—oral aperture of Shell
Lower free end is conical—trilobed (Dentalium)
Digging— anchor
Gastropoda
Simple, elongated, undivided flat ventral creeping sole in creeping gastropods
Divisible —Propodium—mesopodium—metapodium
In Strombus the foot is greatly narrowed
in limpets and abalones– broadly expanded and serves as an adhesive disk
In pelagic gastropods, especially the heteropods and pteropods the foot is a swimming organ
Aquatic gastropods—-retain primitive foot
Act as holdfast organ—-attachment—movement—-adapted for clinging and moving—on rocky surfaces (Patella)
Modification
Anterior- foot—tactile papillae (Trochus)—Fleshy projection mentum (Turbonilla) & pedal tentacles—(Valvata, Aeolis)
Appendages
Epipodia—folds developed from side or base of the foot throughout the length
Parapodia—beset with papillae, tentacles—ringed lobes and sensory tentacles
In Fossorial gastropods—crawl —wet lands—propodium is separated from rest of the foot by constriction (Harpidae)—-transverse furrows (olividae)
Circular and disc-like (Oliva)
Propodium —powerful digging organs
In Fossorial gastropods—crawl —wet lands—propodium is separated from rest of the foot by constriction (Harpidae)—-transverse furrows (olividae)
Circular and disc-like (Oliva)
Propodium —powerful digging organs
Leaping gastropods
Prosobranchs—shell bearing gastropods (Strombidae)—–operculum—it forms sharp like dagger or claw—–
Sessile gastropods
Patella…. Foot reduced and ventral sole —sucker for attachment
In parasitic eg. thyca, Entoconcha—reduced or loss of foot
Pelecypoda
Cephalopoda
Shell in Mollusca
External, internal or absent.
Protective covering of the of the body —-secreted by the mantle.
Encloses the important parts of the body
Role in the species identification.
Snail shell varies in size, shapes and colors according to mode of life & habitat.
The Aplacophora or solenogasters are wormlike without shell which is replaced by a coat of calcareous spicules.
Opisthobranchs (e.g. Onchidiacea. Nudibranchs), pulmonates (terrestrial slugs are without or with reduced internal shell).
Shell in Mollusca
External, internal or absent.
Protective covering of the of the body —-secreted by the mantle.
Encloses the important parts of the body
Role in the species identification.
Snail shell varies in size, shapes and colors according to mode of life & habitat.
The Aplacophora or solenogasters are wormlike without shell which is replaced by a coat of calcareous spicules.
Opisthobranchs (e.g. Onchidiacea. Nudibranchs), pulmonates (terrestrial slugs are without or with reduced internal shell).
Shell in Mollusca
External, internal or absent.
Protective covering of the of the body —-secreted by the mantle.
Encloses the important parts of the body
Role in the species identification.
Snail shell varies in size, shapes and colors according to mode of life & habitat.
The Aplacophora or solenogasters are wormlike without shell which is replaced by a coat of calcareous spicules.
Opisthobranchs (e.g. Onchidiacea. Nudibranchs), pulmonates (terrestrial slugs are without or with reduced internal shell).
The shell consisting of outer organic and inner calcified layer
Organic layer —- sclerotized proteins which is known was periostracum —–absent in many marine species. Thickness and layers are varied in different species.
Thick in high altitude species and thin in warmer regions.
Periostracum is either homogenous, or ridged or sculptured, contains calcarious grannules, spicules and hair like structures
Calcified layer contains more than 90% calcium carbonate
Calcified layer is also classified into homogenous layer, prismatic layer, foliated layer, grained layer, crossed lamellar layer and complex layer.
Shell in Monoplacophora & Scaphopoda
Monoplacophora Scaphopoda
Shell in Polyplacophora
Measuring whorls
Direction of shell coiling
Planorbid
Shell in Bivalve
Shell composed of two parts joined by ligaments
Bivalves are very common in many kinds of saltwater habitats, but they are also found in brackish water and in freshwater.
Umbo—Protuberance at the terminal end of the valve, from which the shell grows
Abra alba
Shell in Cephalopoda
The external shell wall in Nautilus consists four layers; the periostracum, spherulitic prismatic layer, nacreous layer, and the prismatic layer.
The nacreous layer consists of numerous lamellae which are arranged concentrically.
Pila globosa
(The apple snail)
Reproductive System
The Pila is a dioecious animal with clear sexual dimorphism. Males have a smaller shell with less dilated body whorl with better developed and functional penis.
Male Reproductive System: The male reproductive system comprises of a testis, vas deferens, a copulatory organ and a hypobranchial gland.
(i) Testis: is a flat, cream-coloured, triangular organ, covered by a thin membrane and situated on the digestive gland (liver or hepatopancreas) in the upper part of the visceral mass.
• Vas Deferens: Fine tubules from the testis called vas efferentia, united to form a large Vas deferens which leaves the testis from its posterior end. It is differentiated into 3 regions – the proximal narrow tubular part, the middle small, sac like vasicula seminalis and the distal broad glandular part which runs forward along the left side of the rectum which ends in male genital aperture by a claw-like genital pepilla.
• Copulatory Organ: It is a long, stout, slightly curved tapering organ called Penis projecting from the mantle edge in front of the anus.
• Hypobranchial Gland: is and oval thickening with pleated surface at the base of penis sheaths of unknown function
Reproductive System
Female Reproductive System: The female reproductive system comprises an ovary, oviduct receptaculum seminis, uterus, vagina and copulatory organ or hypobrabchial gland.
(a) Ovary: is smaller than testis. It is flat dark, conical organ lying on the digestive gland in the upper part of the visceral mass. It is composed of a large number of rounded lobules or acini that unite to form small lobules, which in turn unite to form a large tube, the oviduct.
(b) Oviduct : It leaves the ovary near its middle. It descends along the inner border of the digestive gland and enters the posterior renal chamber and opens into receptaculum seminis.
(a) Receptaculum Seminis: is a smallest bean-shaped sac lying within the posterior renal chamber attached to the uterus. It serves to store spermatozoa received from male during copulation.
(b) Uterus: is a large, pyriform sac situated in the body whorl outside the body whorl outside the renal organ. It continued anteriorly into a tubular vagina.
(c) Vagina: The enters the branchial chamber of the mantle cavity and runs forward along the left side of rectum and opens by small slit like female genital aperture.
(d) Copulatory Organ: similar to the male, but highly reduced.
Breeding and development
Breeding in Pila takes place in the rainy season. It involves three processes: copulation, fertilization and laying.
Copulation: Copulation may occur in water on moist land. It takes about 3-4 hrs. The male and female snails come together facing one another with their right nuchal lobes lie opposite to one another. The penis of the male elongates along with its sheath and its proximal end comes in contact with the male genital aperture and the distal end passes into the mantle cavity of the female and finally to its genital aperture. Now the seminal fluid is shed into the vagina of the female where it migrates to be stored into the receptaculum seminis. After that, both separates.
Fertilization: Fertilization is internal in Pila. It takes place in the uterus, where both ova and spermatozoa pass, the former from the ovary and the latter from the receptaculum seminis.
Laying: Egg laying starts 2 or 3 days after copulation. The eggs are laid in sheltered places (holes and crevices) or moist land near water. A single female lays 200-800 eggs at a time. The eggs are rounded, about the size of the pea seeds and covered by whitish shell. They contain a good deal of food. No parental care.
• Development: The egg undergoes spiral cleavage, total and unequal, very similar in the early stages to that of Nereis. There is no larval stage. A tiny young snail similar to the adult in form, hatches from the egg.
Respiration in Gastropods
Respiration
Respiratory system
Cutaneous
Brachial and
Pulmonary
Aquatic habitat—-Gills or ctenidia
Gills are shifted to the front of the body with mantle cavity
It is elongated—right side of the mantle cavity, dorso-lateral wall of the brachial chamber
Main stem—-line of attachment of gills—-ctenidial axis
Lamellae—(thin traingular and ciliated leaflets)—-parallel in a single row—-at right angle to the axis of the gill
Monopectinate in Pila—-Bipectinate—Haliotis and Patella
Gills developed on the left side—monopectinate—with single row of leaflets on right side
Water current —mantle cavity on the left—sweeps over the ctenidium and leaves on the right
In opisthobranchs—tendency —loss of original gill—-secondary gills
In Pulmonata—terrestrial snails—-gills disappear & mantle forms a lung for aerial respiration
True ctenidium disappear—-mantle cavity is transferred into the pulmonary sac or lung for aerial respiration
Pulmonary sac is richly supplied with blood vescles
Muscular contraction and relaxation on mantle floor—-lower / raise—causing—-the air to rush in and out of the mantle
Air—enter—leave—the sac—through rounded pulmonary aperture (Pneumotostome) on the right side
A Monopectinate gills
B Water passage
C Blood flow
D Lamellae section
Reproduction in Gastropoda
Reproductive system in gastropods : slugs and snails —-varies greatly from one group to another within this very large and diverse taxonomic class of animals.
Marine—-gastropods—–separate sexes (male and female)–Patellogastropoda, Vetigastropoda, Cocculiniformia, Neritimorpha, and Caenogastropoda.
Terrestrial—-Hermaphrodite
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.
Adverse environmental conditions may delay the onset of sexual maturity in some snails
Prior to reproduction, most land snails perform a ritual courtship before mating. This may last anywhere between two and twelve hours. Prolific breeders, pulmonate land snails inseminate each other in pairs to internally fertilize their ova. Each brood may consist of up to 100 eggs.
Pulmonate land snails and slugs have 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. After this, fertilization occurs and the eggs develop.
Most land snails (Pulmonates)—- hermaphrodites
While land-dwelling Prosobranch snails are dioecious (separate sexes).
Pulmonate land gastropods simultaneous hermaphroditic with complex reproductive system.
It internal, except for the active protrusion (eversion) of the penis for copulation.
The opening genital pore close to the head of the animal.
This opening is virtually invisible however—unless— actively in use.
The love-dart (if available) is produced and stored in the stylophore (often called dart sac) and shot by a forceful eversion of this organ.
The mucus glands produce the mucus that is deposited on the dart before shooting.
The penis is intermitted to transfer the spermatophore.
The sperm container is formed in the epiphallus, while the spermatophore’s tail is formed by the flagellum.
When a bursa tract diverticulum is present, the spermatophore is received in this organ.
Together with the bursa tract and bursa copulatrix these form the spermatophore-receiving organ, which digest sperm and spermatophores.
Sperm swim out via the tail of the spermatophore to enter the female tract and reach the sperm storage organ (spermathecae) within the fertilization pouch-spermathecal complex.
Simplified diagram of the reproductive morphology of a pulmonate land snail with one love dart and a diverticulum.
There is a hermaprhodite gonads or ovotestis which leads into the hermaphrodite duct. There are separate male and female reproductive systems. The male system comprises a vas deferens with associated prostate like glands, penis and flagellum. The female system comprises oviduct, albumen gland, seminal receptacle, vagina bursa copulatrix which receive spermatophore at copulation.
In terrestrial pulmonates the sperms are transferred at copulation to the partner in the form of spermatophore which consists head, sperm sac and long tail.
Egg is surrounded by protienacious secretion of albumen gland.
Five phases in the reproductive cycle of gastropods: Courtship, copulation, nest building, egg laying and hatching of the young from egg.
Courtship: It takes long time. During this period a dart may be ejected by one of both partners, this penetrates either the foot or side of the body of the partner to stimulate. In slugs and some snails courtship begins with the animals crawling in circle with the mouth of the one animal pressed against the hind end of the other.
Copulation: It is the reciprocal transfer of the spermatophore into the vagina of the partner.
Oviposition: It is laborious and time consuming process. It is recorded that Helix pomatia took 6 to 12 hrs to make the nest hole in soil, 5 to 10 hrs for ensuing rest period and 30 hrs for egg laying.
Oviposition in Deroceras reticulatum takes 8 to 10 days after copulation.
Embryo develops in the veliger larva in marine gastropods. In pulmonates newly hatched animal is already in adult form.
Viviparity is seen in prosobranchia (family viviparidae) and stylommatophora (Veronicellidae, Vallonidae, Helicarionidae and Helicidae). In such cases embryonic development occurs in uterus and shells of the little snails can be seen through its wall.
Torsion in Gastropoda
In the larval development of gastropods, the viscero-pallium structures rotate anticlockwise through 180° from its initial position, so that the mantle cavity, with its pallial complex, is brought to the front of the body in the adult. As a results mantle cavity, gills etc. come to lie directly above the head and internal parts undergo rearrangements. This displacement referred to as torsion. or
• Torsion is defined as the translocation of the mantle cavity to an anterior position and the origin of streptoneury.
But head and the foot remains fixed.
Shell coiling and Torsion occurs in the neck region
Coiling of the shell and occurrence of torsion are more or less independent in ontogeny.
Coiling of the shell—–protection, strength, balance and continuous growth
Many workers have assumed that torsion came first in the limpet like common ancestor than afterward coiling took place.
Torsion is a much drastic even than the spiraling of the shell.
Theories of torsionEarly Recapitulationist theories: According to Haeckelian viewpoint “ ontogeny necessarily recapitulates phylogeny”. Followers of this theory Goette (1896) and Naef (1911, 1913) of the view that a transition of the pelagic larva to the benthic adult as seen as the repetition of the racial history.
Ontogeny (the development of embryos), scientists can learn about the evolutionary history of organisms. Ancestral characters are often, but not always, preserved in an organism’s development.
Theories influenced by development mechanism: Some theories based on embryological consideration.
In the late 1800s some scientists felt that ontogeny not only could reveal something about evolutionary history, but that it also preserved a step-by-step record of that history & claimed that ontogeny recapitulates phylogeny (ORP).
Means an organism’s development will take it through each of the adult stages of its evolutionary history, or its phylogeny. At the time, some scientists thought that evolution worked by adding new stages on to the end of an organism’s development. Thus its development would reiterate its evolutionary history—ontogeny recapitulating phylogeny.
Theories influenced by development mechanism: Some theories based on embryological consideration.
Process of torsion
Thomson 1958 recognized five processes of torsion;
180° rotation achieved in two stages, first 90° movement by contraction of larval retractor muscle and later a slower 90 ° rotation by different growth i.e Haliotes, Patella.
180° rotation by differential growth processes alone Vivipara
Rotation by differential growth processes, with anus coming to a position appropriate to the adult state e.g Aplysia.
Torsion no longer recognisable as a movement of viscero-pallium, the organs in the post torsional position from their first appearance e.g. Adalaria.
Effect of torsion
1. Displacement of mantle cavity: The mantle cavity which was primitively located posteriorly opens just behind the head and its associated parts are shifted forward after torsion.
2. Changes in relative positions: Before torsion, anus, ctenidia and renal orifices point backwards and the auricles lie behind the ventricle. After torsion, anus, ctenidia and renal orifices project forward and auricles lie in front of the ventricle.
3. Looping of alimentary canal: The alimentary canal which was straight with anterior mount and posterior anus, after torsion mouth and anus lie anterior due to the formation of loop.
4. Chiastoneury: Uncoiled nerves become twisted into a figure of 8 after torsion.
5. Endogastric coil: The coil of the visceral sac and the shell which was dorsal or exogastric becomes ventral or endogastric after torsion.
6. Loss of symmetry and atrophy: Previously bilateral symmetrical is replaced into asymmetry due to displacement of position of visceral mass.
Significance of torsion
Early theorists considered the adaptive value of torsion only for adult stages.
Lang (1891, 1900) viewed ancestral limpet in which mantle cavity and gills are posterior. He imagined this limpet as gradually undergoing a heightening of the shell so that ultimately it become tall and unstable. As a result of this instability, the snail would be unable to balance the shell when crawling and shell fell forward. The theory was criticized.
Naef (1911) improved theory of Lang assuming that ancestral pre-gastropod had a coiled shell, shaped like that of Nautilus with the spire of the shell projecting forward. The spire projecting over the head would interfere with locomotion by disturbing the balance of the body.
Morton (1958) suggests that perhaps respiratory and sensory advantages have had some effect in keeping the adult gastropod in condition of torsion and once the shell was coiled. He supported Naef’s explanation of advantages to adults.
Significance of torsion
Significance of the larval stages
According to Garstang (1928) torsion first occurred as a larval mutation of advantage to the larva adapted to pelagic life but of little direct use to the adult. This theory is supported by many authors. According this explanation, before torsion, the untwisted swimming larva fell an easy victim to its predators because posterior mantle cavity could receive the delicate head and velum only after the foot was already inside. After torsion, the mantle cavity became anterior so that sensitive parts like head, velum could withdraw first followed by the foot. The operculum sealed the aperture, the cilia of velum stopped beating, so that the larva could fall to the sea bottom and avoid its enemies swimming in the water.
The anus and nephridiopores are also brought into an anterior position where they discharge over the animals head.
In summary there are many other advantages torsion:
For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod.
In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position.
Another possible advantage for aquatic species is the osphradium (olfactory sense organs) are moved to an anterior position and are able to sample the water that the gastropod is entering rather than leaving; this may help the gastropod locate food or avoid predators
Detorsion
The changes occurring in torsion due to its reversion is known as detorsion. It is very characteristics of the whole group of the Euthyneura. In the least specialized Opisthobranchia and Pulmonata detorsion is not complete, so that visceral loop remains partly twisted and the anus and ctenidium are directed laterally, instead of anteriorly.
BMC Microbiology 