Skip to content

Mollusque Classification Essay

Les mollusques (Mollusca) sont un embranchement d'animauxlophozoaires. Ce sont des animaux non segmentés, à symétrie bilatérale quelquefois altérée. Leur corps se compose généralement d'une tête, d'une masse viscérale, et d'un pied. La masse viscérale est recouverte en tout ou partie par un manteau, qui sécrète une coquille calcaire. Le système nerveux comprend un double collier périœsophagien. La cavité générale est plus ou moins réduite au péricarde et aux néphridies. L'embranchement des mollusques (Mollusca) tire son nom du latinmollis, « mou ». La science consacrée à l'étude des mollusques est la malacologie[1], de l'équivalent grec malakos, « mou ».

Dans la classification phylogénétique, les mollusques sont des métazoairestriploblastiquescœlomates (les termes « cœlomate », « acœlomate » et « pseudocœlomate » ont récemment été retirés de la classification) bilatériensprotostomiens ; les synapomorphies notables de ce clade étant la présence d'une radula et d'un manteau[2].

L'embranchement contient plus de 130 000 espèces dont certaines sont très fréquemment consommées par l'Homme.

Certains mollusques peuvent secréter des perles en recouvrant de nacre les éléments irritants qui s'introduisent dans leur coquille.

Les mollusques ou leur coquille (et leurs perles parfois) ont fait l'objet de nombreux usages, dont alimentaires[3],[4] par l'Homme, depuis la préhistoire.

Morphologie et anatomie[modifier | modifier le code]

Caractères généraux[modifier | modifier le code]

Malgré la grande diversité de formes, plusieurs caractères se retrouvent chez tous les mollusques actuels[5],[6]. La partie dorsale du corps est un manteau qui secrète des spiculescalcaires, formant des plaques ou une coquille. Entre le manteau et la masse viscérale se trouve la cavité palléale au sein de laquelle débouchent l'anus et les conduits génitaux. Le système nerveux est constitué d'un anneau nerveux autour de l'œsophage avec au moins deux paires de cordons nerveux (trois chez les bivalves[7]).

La plupart des mollusques ont perdu toutes traces de métamérisation. Ils ont une symétrie bilatérale, mais qui peut être altérée par une torsion du corps (par exemple chez les Gastéropodes).

Leur tégument est mou. Il contient de nombreuses glandes qui sécrètent du mucus.

Les mollusques sont des cœlomates mais leur cœlome se limite à un péricarde, c'est-à-dire que le cœur est situé dans une cavité creusée dans du tissu d’origine mésodermique. La cavité générale des mollusques est plus ou moins oblitérée par du tissu conjonctif, à l'exception d'une partie qui enveloppe le cœur (péricarde) et d'une autre partie, en relation avec les deux autres, qui constitue les organes excréteurs (néphridies).

Anatomie générale[modifier | modifier le code]

Leur corps se subdivise en trois parties :

  • La tête qui contient les organes sensoriels et la bouche qui contient la radula. Elle est absente chez les bivalves.
  • Le pied ou sole pédieuse est un organe musculeux, typique des mollusques, destiné à la locomotion. Il revêt des formes très diverses suivant les espèces. Il est peu développé chez les Solénogastres et les Caudofovéates mais devient plus important chez les Eumollusques. Il forme la couronne de tentacules qui permet la prédation chez les Céphalopodes.
  • La masse viscérale, comme son nom l'indique, contient les viscères. Elle est contenue dans une mince tunique qu’on appelle le manteau. C’est le manteau qui sécrète la coquille de la plupart des mollusques, qui leur sert de protection et/ou de squelette et/ou de régulateur de la flottaison (exemple de la seiche).

Entre le manteau et la masse viscérale, le bourrelet palléal constitue une cavité palléale qui protège les organes respiratoires, et où débouchent les métanéphridies (organes excréteurs), l’intestin et les conduits génitaux.

Coquille[modifier | modifier le code]

Article détaillé : Coquille.

Des glandes du manteau des Eumollusques se regroupent et sécrètent généralement une coquille calcaire, qui comprend, de l'extérieur vers l'intérieur :

Cette couche interne, lorsque les lamelles sont suffisamment minces pour diffracter la lumière, constitue la nacre, et, indirectement, les perles fines.

Système nerveux[modifier | modifier le code]

Le système nerveux typique d'un mollusque comprend des ganglions cérébroïdes (qui peuvent fusionner pour former un cerveau) reliés d'une part à des ganglions pédieux, d'autre part à des ganglions viscéraux, par un double collier périœsophagien.

Appareil circulatoire[modifier | modifier le code]

La circulation est incomplète, lacunaire. Du cœur partent de courtes artères mais il n'y a ni veines, ni capillaires. Les Céphalopodes sont une exception parmi les mollusques et ont un système circulatoire clos avec un cœur systémique et deux cœurs branchiaux.

Le sang est incolore, ou légèrement coloré par de l'hémoglobine ou de l'hémocyanine dissoutes.

Cycle reproductif[modifier | modifier le code]

Les sexes sont généralement séparés. Quelques espèces courantes sont hermaphrodites comme l'escargot ou l'huître.

Les œufs sont plus ou moins riches en vitellus, et l'éclosion a lieu après un stade plus ou moins avancé de développement. Le début du développement embryonnaire est un clivage ou segmentation en spirale ce qui permet de classifier les Mollusques aux côtés des Annélides parmi les Spiralia.

Quand il y a larve libre (trochophore, véligère), celle-ci ressemble beaucoup à la trochophore des annélides.

Évolution[modifier | modifier le code]

Les mollusques descendraient d'une organisation de type « ver ». On pense qu'ils descendent d'animaux semblables à des Annélides de par les traces de métamérie découvertes chez les Monoplacophores. On estime leur apparition à au moins 500 millions d'années à partir d'un ancêtre commun (radiation adaptative).

La fonctionnalité qui semble avoir conditionné les mollusques primitifs paraît être la radula : un organe fonctionnant comme une râpe, sorte de langue porteuse de dents chitineuses, qui permet à l'animal de se nourrir plus efficacement. Par rapport aux "vermiformes" primitifs, qui ne peuvent que gober une nourriture fragmentaire, la radula donne un avantage adaptatif, dans la mesure où elle permet d'arracher de la nourriture sur des proies cohérentes (éponges, algues…). Les mollusques ont ainsi inventé l'art de brouter.

L'autre fonctionnalité caractéristique des mollusques est le blindage, permettant de se protéger de prédateurs actifs : l'acquisition de plaques calcaires protégeant le dos. Ces mollusques primitifs devaient donc ressembler à des polyplacophores (une sorte d'escargot qui peut se rouler en boule comme un hérisson ou un cloporte), mais ce type est à présent très marginal.

En s'adaptant à différentes formes de vie, ils ont progressivement conquis tous les types de milieu : surtout présents en milieu marin, les Gastéropodes et les Bivalves ont ensuite réussi à s'adapter à l'eau douce. Dans leur radiation adaptative, les mollusques ont donné naissance aux classes importantes suivantes :

  • Les gastéropodes (escargots, limaces, patelles…) continuent à ramper, et se caractérisent par une céphalisation plus avancée. La seule innovation que leur a apportée l'évolution est que cette reptation se fait sur un organe spécialisé, le pied. Les plaques calcaires de la carapace primitive se sont simplifiées au fil du temps, ce qui a conduit à ces coquillages généralement spiralés. Les premiers gastéropodes à respiration pulmonaire ont conquis les milieux terrestres au cours du Carbonifère. Mais les escargots modernes, du genre Helix ne sont apparus qu'au Crétacé.
  • Les bivalves (moules, huîtres…) sont devenus sédentaires et ont misé sur la protection que leur apporte la coquille calcaire, au point de ne pratiquement plus se déplacer. Leur mode de vie se rapproche de celui des anémones, voire des éponges, consistant à filtrer l'eau ambiante. Dans cette évolution, ils ont perdu leur tête, devenue inutile, et les yeux ne sont plus présents que sous forme dégénérée, dans quelques espèces. Les bivalves constituent un cas intéressant où une régression fonctionnelle (perte du déplacement propre aux structures vermiformes) se traduit par un succès évolutif. Les bivalves ont perdu leur radula, caractère qui avait été la cause de l'explosion radiative initiale des mollusques.
  • Les céphalopodes (poulpes, calmars, seiches…) ont appris à nager, et sont des prédateurs. La capacité d'attraper des proies qui peuvent chercher à s'échapper met une contrainte évolutive forte sur ce qui caractérise ce groupe : de bons yeux, et un cerveau performant capable de coordonner les mouvements de chasse. La coquille commune des invertébrés, que l'on retrouve chez l'argonaute, tend à se profiler en pointe, se réduire comme chez la seiche, voire disparaître totalement comme chez le poulpe.

Classification[modifier | modifier le code]

Le phylum des mollusques a été créé par Georges Cuvier (1769-1832) en 1795.

Selon World Register of Marine Species(11 avril 2016)[8], il y a actuellement huit classes de mollusques :

  • classe BivalviaLinnaeus, 1758 -- Bivalves (12 000 espèces vivant dans toutes les mers du monde et en eau douce)
  • classe CaudofoveataC. R. Boettger, 1956 -- Caudofovéates (100 espèces connues vivant dans toutes les mers du globe)
  • classe CephalopodaCuvier, 1795 -- Céphalopodes (786 espèces connues, toutes marines vivant dans toutes les mers sauf la Mer Noire)
  • classe GastropodaCuvier, 1795 -- Gastéropodes (103 000 espèces connues ayant une répartition mondiale)
  • classe MonoplacophoraOdhner, 1940 -- Monoplacophores (15 espèces connues vivant dans les fosses océaniques)
  • classe PolyplacophoraGray, 1821 -- Polyplacophores (900 espèces connues vivant entre 0 et - 3 000 m)
  • classe ScaphopodaBronn, 1862 -- Scaphopodes (400 espèces toutes marines)
  • classe SolenogastresGegenbaur, 1878 -- Solénogastres (350 espèces connues vivant dans toutes les mers)
  • classe RostroconchiaCox, 1960
  • Mollusca incertae sedis (groupes fossiles isolés)

Les Solénogastres et les Caudofovéates étaient anciennement regroupés dans une même classe : les Aplacophores.

Au contraire, les Eumollusques regroupent tous les mollusques à l'exception des Solénogastres et des Caudofovéates.

Les Conchifères sont un sous-embranchement regroupant tous les Eumollusques sauf les Polyplacophores.

Les Amphineures sont le deuxième sous-embranchement des Mollusques et regroupent les Aplacophores et les Polyplacophores.

Les Bivalves et les Scaphopodes peuvent être regroupés sous le terme Diasomes.

Nom vernaculaire[modifier | modifier le code]

Article détaillé : Liste des noms vernaculaires de mollusque.

Comme pour les poissons, les noms vernaculaires sont assez peu homogènes et dépendent beaucoup des régions où ils sont utilisés. Ils peuvent être inconnus d'une région à l'autre ou ne pas désigner la même espèce.

Place des mollusques dans le monde animal[modifier | modifier le code]

  • Les types d'organisation présentés ici sont des grades évolutifs ne correspondant généralement pas à des groupes monophylétiques, mais paraphylétiques (ne comportant pas tous les descendants d'un même ancêtre — exemple : les descendants d'ancêtres vermiformes ne sont pas tous aujourd'hui des vers, etc.).
  • En jaune : les principales explosions radiatives.
> Unicellulaires procaryotes(cellule sans noyau) ♦ Échinodermes : Oursins, Crinoïdes, Concombres de mer, étoiles de mer et ophiures ♦ Bivalves(coquillages)  
> Unicellulaires Eucaryotes(cellules à noyau) ♦ Gastéropodes (escargots, limaces, etc.)
> Éponges(organisme multicellulaire)♦ Mollusques♦ Céphalopodes (pieuvres, seiches)
> Polype : hydres, coraux et méduses 
> Vers(mobilité et tube digestif)bilatériens  ♦ Trilobites(de deux à 24 pattes — éteint)
> Poissonsagnathes(sans mâchoire)♦ Arthropodes primitifs type myriapodes(beaucoup de pattes) ♦ Décapodes : crabes et écrevisses(dix pattes)
> Poissons primitifs (poissons cartilagineux)♦ Arachnides : araignées, scorpions et acariens(huit pattes)♦ Libellules
> Poissons typiques (poissons osseux)♦ Serpents> Hexapodes(à six pattes) : Insectes type Apterygota(primitifs sans ailes) ♦ Blattes, mantes, termites
> Poissons type Sarcopterygii(à nageoires charnues)♦ Dinosaures(éteint)♦ Orthoptères (sauterelles, grillons)
> Tétrapodes primitifs (type Amphibiens)♦ Crocodiles♦ Marsupiaux♦ Hémiptères (punaises, cigales…)
> Reptiles primitifs (Amniotes de type Lézards) ♦ Tortues♦ Insectivores (Taupes, Hérissons…)♦ Coléoptères (hannetons, coccinelles…)
 ♦ Oiseaux♦ Chiroptères(Chauves-Souris)♦ Hyménoptères (abeilles, guêpes, fourmis)
 ♦ Primates♦ Diptères(mouches)
 > Mammifères primitifs type monotrèmes ♦ Rongeurs et Lagomorphes(lapins)♦ Lépidoptères(papillons)
♦ Carnivores
♦ Ongulés

 

Voir aussi[modifier | modifier le code]

Articles connexes[modifier | modifier le code]

Références taxonomiques[modifier | modifier le code]

Bibliographie[modifier | modifier le code]

  • Adam W (1960) Faune de Belgique. Mollusques 1 - Mollusques terrestres et dulcicoles. Bruxelles. (Institut royal des Sciences naturelles de Belgique): 402 pages.
  • Germain L (1931) Mollusques terrestres et aquatiques (première partie). Faune de France, Paris. (Paul Lechevalier): 477 pages.
  • Germain L (1931) Mollusques terrestres et aquatiques (seconde partie). Faune de France, Paris. (Paul Lechevalier): 479-893 pages.
  • Sander Rang, Atlas des mollusques composé de 51 planches représentant la plupart des mollusques nus et des coquilles décrits dans le Manuel d’histoire naturelle, Paris, Éd. Roret, [1843], 16 p., in-16 (lire en ligne[PDF]). — Les 51 pl. sont regroupées à la suite du texte.
  • Sander Rang et Louis François Auguste Souleyet, Histoire naturelle des mollusques ptéropodes : monographie comprenant la description de toutes les espèces de ce groupe de mollusques, Paris, Éd. J.-B. Baillière, , IV-88 p., grd in-4° (lire en ligne[PDF]).
  • Sander Rang, Manuel de l’histoire naturelle des mollusques et de leurs coquilles…, Paris, Éd. Roret, , IV-390 p., in-18 (lire en ligne[PDF]).
  • Sander Rang, « Note sur le Ropan d’Adanson et quelques autres observations sur les mollusques », Annales des sciences naturelles, Paris, Éd. Crochard, vol. 21,‎ , p. 352 (chiffrée par erreur 351) (lire en ligne[PDF]).
  • Sander Rang, « Notice sur la Galathée, genre de mollusque acéphale de la famille des Conchacées », Annales des sciences naturelles, Paris, Éd. Crochard, vol. 25,‎ , p. 152-164 (lire en ligne[PDF]).
  • Sander Rang, « Notice sur le Litiope, nouveau genre de mollusque gastéropode », Annales des sciences naturelles, Paris, Éd. Crochard, vol. 16,‎ , p. 303-307 (lire en ligne[PDF]).
  • Sander Rang, « Notice sur quelques mollusques nouveaux appartenant au genre Cléodore, et établissement et monographie du sous-genre Creseis », Annales des sciences naturelles, Paris, Éd. Crochard, vol. 13,‎ , p. 302-319 (lire en ligne[PDF]).

Liens externes[modifier | modifier le code]

Sur les autres projets Wikimedia :

  • Gargominy, O., Prié, V., Bichain, J.-M., Cucherat, X., Fontaine, B. 2011. Liste de référence annotée des mollusques continentaux de France. MalaCo, 7 : 307-382.
  • Voir un schéma détaillé de l'anatomie interne d'un mollusque
  • Antoine Morin, « Cours-Mollusque », Université d'Ottawa
  • MalaCo : le journal électronique de la malacologie continentale française

Notes et références[modifier | modifier le code]

  1. ↑Romaric Forêt, « Malacologie », Dico de Bio, 2e éd. De Boeck Université, 2006 (ISBN 2804152480).
  2. ↑G. Lecointre & H. Le Guyader, Classification phylogénétique du vivant, 2006, 3e édition, Belin, Paris
  3. ↑Locard A (1890) Les huîtres et les mollusques comestibles, moules, praires, clovisses, escargots, etc: histoire naturelle, culture industrielle, hygiène alimentaire. J.-B. Baillière et fils.
  4. ↑Locard A (1890) Les huitres et les mollusques comestibles. Baillière et Fils.
  5. ↑Brusca, R.C., Brusca, G.J. (2003). Invertebrates (2 ed.). Sinauer Associates. pp. 702. (ISBN 0878930973).
  6. ↑David M. Raup & Steven M. Stanley (1978). Principles of Paleontology (2 ed.). W.H. Freeman and Co., p. 4-5. (ISBN 0716700220).
  7. ↑Ruppert, E.E., Fox, R.S., and Barnes, R. D. (2004). Invertebrate Zoology (7 ed.). Brooks/Cole. p. 367–403. (ISBN 0030259827).
  8. ↑World Register of Marine Species, consulté le 11 avril 2016

Mollusca is a large phylum of invertebrate animals whose members are known as molluscs or mollusks[Note 1] (). Around 85,000 extantspecies of molluscs are recognized.[2] The number of fossil species is estimated between 60,000 and 100,000 additional species.[3]

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrialhabitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomicclasses, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total.

The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, 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. The main body cavity is a hemocoel through which blood circulates; as such, 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 trochophorelarvae, more complex veliger larvae, or miniature adults.

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 541 to 485.4million years ago[dead link]. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.

Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.

Etymology[edit]

The words mollusc and mollusk are both derived from the French mollusque, which originated from the Latinmolluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τα μαλακά (ta malaká), "the soft things", which he applied to cuttlefish. The scientific study of molluscs is accordingly called malacology.[4]

The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods, bryozoans, and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As it is now known these groups have no relation to molluscs, and very little to one another, the name Molluscoida has been abandoned.[5]

Definition[edit]

The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.[6]

Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies (defining characteristics) to apply to all modern groups.[7] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.[8] The following are present in all modern molluscs:[9][10]

Other characteristics that commonly appear in textbooks have significant exceptions:

 Whether characteristic is found in these classes of Molluscs
Supposed universal Molluscan characteristic[9]Aplacophora[11]Polyplacophora[12]Monoplacophora[13]Gastropoda[14]Cephalopoda[15]Bivalvia[16]Scaphopoda[17]
Radula, a rasping "tongue" with chitinous teethAbsent in 20% of NeomeniomorphaYesYesYesYesNoInternal, cannot extend beyond body
Broad, muscular footReduced or absentYesYesYesModified into armsYesSmall, only at "front" end
Dorsal concentration of internal organs (visceral mass)Not obviousYesYesYesYesYesYes
Large digestive cecaNo ceca in some AplacophoraYesYesYesYesYesNo
Large complex metanephridia ("kidneys")NoneYesYesYesYesYesSmall, simple
One or more valves/ shellsPrimitive forms, yes; modern forms, noYesYesSnails, yes; slugs, mostly yes (internal vestigial)Octopuses, no; cuttlefish, nautilus, squid, yesYesYes
OdontophoreYesYesYesYesYesNoYes

Diversity[edit]

Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.[1] In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial. The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs.[19] In 2009, Chapman estimated the number of described living species at 85,000.[1] Haszprunar in 2001 estimated about 93,000 named species,[20] which include 23% of all named marine organisms.[21] Molluscs are second only to arthropods in numbers of living animal species[18]—far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.[22] About 200,000 living species in total are estimated,[1][23] and 70,000 fossil species,[9] although the total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today.[24]

Molluscs have more varied forms than any other animal phylum. They include snails, slugs and other gastropods; clams and other bivalves; squids and other cephalopods; and other lesser-known but similarly distinctive subgroups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes.[7] About 80% of all known mollusc species are gastropods.[18]Cephalopoda such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates.[25] The giant squid, which until recently had not been observed alive in its adult form,[26] is one of the largest invertebrates, but a recently caught specimen of the colossal squid, 10 m (33 ft) long and weighing 500 kg (1,100 lb), may have overtaken it.[27]

Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered nonmarine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species.[28]

Hypothetical ancestral mollusc[edit]

Further information: Mollusc shell

Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modern monoplacophorans, and some suggest it may resemble very early molluscs.[7][10][13][29]

The generalized mollusc is bilaterally symmetrical and has a single, "limpet-like" shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot".[10] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs.[8]

Mantle and mantle cavity[edit]

The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air. The cavity was at the rear in the earliest molluscs, but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity.[10] The whole soft body of bivalves lies within an enlarged mantle cavity.[8]

Shell[edit]

Main article: Mollusc shell

The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs[8]) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate),[10][30] except the outermost layer, which in almost all cases is all conchiolin (see periostracum).[10] Molluscs never use phosphate to construct their hard parts,[31] with the questionable exception of Cobcrephora.[32] While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.[33]

The shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite, often nacreous.[8]

[edit]

The underside consists of a muscular foot, which has adapted to different purposes in different classes.[34]:4 The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.[10] In bivalves, the foot is adapted for burrowing into the sediment;[34]:4 in cephalopods it is used for jet propulsion,[34]:4 and the tentacles and arms are derived from the foot.[35]

Circulatory system[edit]

Molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.[8] The blood contains the respiratory pigmenthemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.[10]

The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.[10]

Respiration[edit]

Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.[10]

Eating, digestion, and excretion[edit]

Members of the mollusc family use intracellular digestion to function. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ.[8] The radula is unique to the molluscs and has no equivalent in any other animal.

Molluscs' mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string".[10]

At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it.[10]

The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect.[10]

Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.[10]

As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus.[8]

Nervous system[edit]

The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar.[10]

The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of the three functioning as the principal center of "thinking". Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow.

Reproduction[edit]

See also: Reproductive system of gastropods and Reproductive system of cephalopods

The simplest molluscan reproductive system relies on external fertilization, but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems.[10]

The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult.[37]

Ecology[edit]

Feeding[edit]

Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as a 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts.[38]Filter feeders are molluscs that feed by straining suspended matter and food particle from water, typically by passing the water over their gills. Most bivalves are filter feeders.

Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophoreStannophyllum.[39]Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,[40] whereas doridnudibranchs and some Vetigastropoda feed on sponges[41][42] and others feed on hydroids.[43] (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). "Molluscan diets". Journal of Molluscan Studies. 31 (3–4): 144. .)

Classification[edit]

See also: List of mollusc orders

Opinions vary about the number of classes of molluscs; for example, the table below shows eight living classes,[20] and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and solenogasters into one class, the Aplacophora.[11][29] Two of the commonly recognized "classes" are known only from fossils.[18]

ClassMajor organismsDescribed living species[20]Distribution
Gastropoda[44]All the snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterfly70,000marine, freshwater, land
Bivalvia[45]clams, oysters, scallops, geoducks, mussels20,000marine, freshwater
Polyplacophora[12]chitons1,000rocky tidal zone and seabed
Cephalopoda[46]squid, octopus, cuttlefish, nautilus, spirula900marine
Scaphopoda[17]tusk shells500marine 6–7,000 metres (20–22,966 ft)
Solenogastres[11]worm-like organisms200seabed 200–3,000 metres (660–9,840 ft)
Caudofoveata[11]worm-like organisms120seabed 200–3,000 metres (660–9,840 ft)
Monoplacophora[13]An ancient lineage of molluscs with cap-like shells31seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
Rostroconchia †[47]fossils; probable ancestors of bivalvesextinctmarine
Helcionelloida †[48]fossils; snail-like organisms such as Latouchellaextinctmarine

Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora.[49] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. Tentaculita may also be in Mollusca (see Tentaculites).

Evolution[edit]

Main article: Evolution of molluscs

See also: Evolution of cephalopods

Fossil record[edit]

Good evidence exists for the appearance of gastropods (e.g. Aldanella), cephalopods (e.g. Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes.[50] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.

Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about 555 million years ago, has been described by some paleontologists as "mollusc-like",[51][52] but others are unwilling to go further than "probable bilaterian",[53][54] if that.[55]

There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms.[53][56] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510million years ago are fragments of a genuinely mollusc-like radula.[57] This appears to contradict the concept that the ancestral molluscan radula was mineralized.[58]

However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,[59][60] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites.[48] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults.[61]

Some analyses of helcionellids concluded these were the earliest gastropods.[62] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head.[14][63][64]

Volborthella, some fossils of which predate 530 million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella's classification is uncertain.[65] The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface.[66] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago.[67] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.[68]

The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.[69][70][71][72] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443million years ago.[73] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event.[74] Even so, bivalves remain abundant and diverse.

The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life.[75]

Phylogeny[edit]

Anatomical diagram of a hypothetical ancestral mollusc

Snail radula at work
  = Food       = Radula
  = Muscles
  = Odontophore "belt"
Simplified diagram of the mollusc nervous system

Apical tuft (cilia)

Prototroch (cilia)

Stomach

Mouth

Metatroch (cilia)

Mesoderm

Anus

/// = cilia