What are Molluscs?
- Molluscs are multi-cellular animals (cells with a nucleus), they live in and create the shell.
- Bilateral plane of symmetry that when observed, divides the animal into symmetrical halves
- Soft-bodied (invertebrates – lacking a backbone)
- Wholly or partially enclosed in a calcium carbonate shell
- They are an extremely diverse group in the animal kingdom ranging from snails to squid) (Table below: 7 Classes of Molluscs)
- Sizes range from 1mm (millimeter) to 1.3 meters)
- Varied means of reproduction – (separate sexes) a) eggs and sperm are released in the water and fertilized; b) water currents may cause internal fertilization; c) eggs are fertilized by using a muscular penis; d) both sexual organs present in one organism.
- Feeding Patterns – a) burrows in muddy sediments consuming microorganisms & detritus filter-feeding; b) scrape algae from rock surfaces, c) catching algae from water currents.
- Defense – a) withdraw into its shell; a snail has the added advantage of having a hardened plate on the foot that blocks the shell opening. once the animal has withdrawn. b) Poisons and fluid excreted to confuse the predator.
- External features – the mantle, the foot, the head, and the mantle cavity.
7 Classes of Molluscs
|Bivalvia (Mainly filter feeders)||Clams, mussels, oysters, and scallops||20 000||Marine and freshwater||Upto 40 years|
|Cephalopoda (Carnivores)||Octopi, squid, cuttlefish, and nautili||900||Marine only||1–15 years|
|Gastropoda (Carnivore or Herbivore)||Snails and slugs||70 000||Marine, freshwater, land||20–50 years|
|Polyplacophora(Carnivore & Grazers)||Chitons||940||Seabed and rocky tidal zone||-|
|Scaphopoda (Deposit feeders)||Tusk shells||500||Marine Only||-|
|Monoplacophora||Cap like shells||31||Seabed from 200 meters to 7000 meters|
How do Molluscs create their shells? and how do they grow?
The shell is made of a lattice of different minerals and organic molecules. Like the Mammalian bones in humans (a matrix of minerals of calcium and protein of collagen). the molluscs shell is an incredibly strong substance able to survive a variety of stresses The molluscs shell though, is different from the mammalian bone by the type and proportions of minerals in their shell, they have a mineral content of 95 to 99 percent per weight to 1-5 percent organic, making the shell a biological rock that is alive. Interwoven between the minerals is the organic protein giving strength and flexibility, and most important the light weight.
The shell begins to develop in the tiny larval stage, the process begins with the shell gland, a small invagination from which the ingredients to make the shell are secreted and structured. The gland is located in the mantle a hard region where all their organs are located. First, an organic molecule layer is secreted, which will become the future periostracum (a thin, organic coating that is the outermost layer of the shell), this layer becomes the base in which the snails can perform biomineralization (the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues).
The minerals are deposited on the organic matrix and slowly built upon, some molluscs deposit aragonite on the inside of the shells, while the outside is calcite, as the snail begins to grow the shells begin to spiral around creating the dome inside, in which they will reside (The spiraling can happen in two directions, right-handed spiraling and left-handed spirling.) The molluscs absorb the minerals, carbon, and hydrogen from the water and food they eat, converting these elements into the required compounds and enabling them to start building their shells.
Shells found in Fiji
|Murex Shells||Common Name||Description|
|Caltrop Murex||The shell of Murex tribulus can reach a length of 65–160 millimetres (2.6–6.3 in). This quite common snail has a shell with a very long siphonal canal and numerous very long, fragile and acute spines, providing protection against predators. It feeds on other mollusks. ("Murex tribulus", 2021)|
|Purple Pacific Drupe||Thick, globose shell, up to 5 cm, with low spire, large body whorl, and flat base. Colour white with dark brown nodules. Dark violet, narrow aperture with conspicuous groups of denticles. Columella with three strong, plicate ridges. ("Drupa morum", 2021)|
|Ramose Murex||A large, solid, very rugged, and heavy shell, of up to 330 mm in length. It has a relatively globose outline, possessing a short spire, a slightly inflated body whorl, and a moderately long siphonal canal. One of its most striking ornamentations is the conspicuous, leaf-like, recurved hollow digitations. It also presents three spinose axial varices per whorl, with two elongated nodes between them. The shell is colored white to light brown externally, with a white aperture, generally pink towards the inner edge, the outer lip, and the columella. ("Chicoreus ramosus", 2021)|
|Sauls Murex||The shell size varies between 60 mm and 142 mm|
|Cone Shells||Cone Shells||Description|
|Admiral Cone||The size of the shell varies between 35 mm and 109 mm. The color is the shell is chestnut with darker revolving lines, and upper, basal and one or two approximate bands, finely reticulated with yellow on a white ground. This pattern is overlaid with large, irregularly disposed triangular white spots ("Conus ammiralis", 2022)|
|Geography Cone||Has a broad, thin shell, cylindrically inflated. Geography cones grow to about 10 to 15 cm (4 to 6 in) in length. The size of an adult shell varies between 43 and 166 mm (1.7 and 6.5 in). The ground color of the shell is pink or violaceous white, occasionally reddish. It has a mottled appearance, clouded and coarsely reticulated with chestnut or chocolate, usually forming two very irregular bands. The geography cone has a wide, violaceous white or pink aperture and numerous shoulder ridges or spines. ("Conus geographus", 2021)|
|Imperial Cone||The size of an adult shell varies between 40 mm and 110 mm. The color of the thick shell is yellowish-white or cream, with numerous interrupted revolving lines and spots of dark brown and two irregular and wider light brown bands. In the synonym Conus fuscatus, the light brown coloring extends in clouds and irregular markings over the surface, so that the bands can scarcely be defined. The shell has a flat but nodular spire and shoulders. ("Conus imperialis", 2021)|
|Leaden Cone||The thin shell is striated throughout. The color of the shell is yellowish or violaceous white, clouded. with chestnut, with distant revolving series of chestnut spots and short lines, most conspicuous on two irregular lighter bands. ("Conus circumcisus", 2021)|
|Cone||The thick shell is obconic, with the whorls enrolled upon themselves. The spire is short, smooth or tuberculated. The narrow aperture is elongated with parallel margins and is truncated at the base. The operculum is very small relative to the size of the shell. It is corneous, narrowly elongated, with an apical nucleus, and the impression of the muscular attachment varies from one-half to two-thirds of the inner surface. The outer lip shows a slight sutural sinus. ("Conus", 2021)|
|Weasel Cone||The size of an adult shell varies between 40 mm and 107 mm. The low spire is striate, flamed with chocolate and white. The body whorl is yellowish, or orange-brown, encircled by rows of chestnut dots, usually stained chocolate at the base. There is a central white band, with chocolate hieroglyphic markings on either side, and a shoulder band, crossed by chocolate smaller longitudinal markings. The border markings of the bands are reduced to spots. The aperture has a chocolate color with a white band. ("Conus mustelinus", 2021)|