Mollusks or Molluscs are a group of soft-bodied invertebrates distributed across marine, freshwater, and terrestrial habitats. They belong to the phylum Mollusca, the second largest after Arthropoda, with around 76,000 extant species worldwide, accounting for around 23% of all marine species. The phylum derives its name from the Latin word mollis, which means ‘soft,’ referring to their body form.
These invertebrates are characterized by a dorsal muscular mantle covered by a hard, calcareous shell. The mantle encloses a large cavity, the mantle cavity, that bears the anus and the genitals. They use a specialized tongue called the radula, bearing multiple rows of chitinous teeth, which help grasp filamentous and microscopic algae.
The enormous shellfish, the giant clam, is the largest living mollusk on Earth, growing up to 1.3 m (4.26 ft) and weighing around 250 kg (551 lbs). In contrast, Angustopila psammion, the smallest snail in the world, measures only 0.6 mm in diameter.
Mollusks have a soft, unsegmented, bilaterally symmetrical body dorsally covered by a hard shell made of chitin and conchiolin, a protein hardened with aragonite (a crystal form of calcium carbonate).
Shell
It has three layers: the outer periostracum, composed of organic matter; the middle layer of calcite; and the inner layer of laminated calcite.
In some snails, like abalones, the shells contain holes for respiration and release of gametes, whereas, in cephalopods, like nautiluses, a strand of tissue called the siphuncle passes longitudinally through the shell and throughout the body chambers. The shells of chitons are composed of eight plates, penetrated by nerves and other sensory structures.
Mantle
Between the shell and the body lies the mantle or pallium that secretes the shell above it. The mantle gets infolded into an inner cavity called the mantle cavity, lined with epidermis. This cavity houses the hindmost pairs of gills, the gonads, the openings of the anus and the genitals, and the exit opening of the nephridia or the ‘Organ of Bojanus.’
Foot
The ventral part of the body has a muscular foot that bears a pair of balancing organs called statocysts. In different groups of mollusks, the foot serves different purposes, such as locomotion, burrowing, or feeding.
All mollusks (except cephalopods) have an open circulatory system. Their heart consists of one or more pairs of auricles that receive oxygenated blood from the gills. This blood is then pumped into the ventricle, which, in turn, shunts it into the aorta for circulation throughout the hemocoel.
Molluscan blood contains hemocyanin, a pigment that binds to oxygen and helps transport it around the body. However, ram’s horn snails contain hemoglobin, which performs the same function.
Mollusks have either singular or paired feather-like vascular gills to oxygenate their blood. These gills have filaments composed of three types of cilia, which drive the water current through the mantle cavity and filter waste out of the gills.
Almost all mollusks have radula, a toothed, chitinous ribbon that serves as the muscular mouth. The radulae are supported by a cartilaginous organ called the odontophore, which, in turn, is connected to mucous-secreting glands.
The stomach is tapering towards the rear end, followed by the prostyle, a backward-pointing cone of feces and mucous. The acid in the stomach makes the mucus of the food less sticky and helps release the particles from it.
The pouch-like cecum digests the remaining food particles and sends the waste for expulsion from the body. Finally, the circular muscles at the entrance of the hindgut help pinch and excrete parts of the prostyle, thus preventing it from growing too long.
All cephalic mollusks have two main pairs of nerve cords surrounding a set of pairs of ganglia. The cerebral, pleural, and visceral ganglia are located above the esophagus, while the pedal ganglia that control the foot are below it. These pairs of corresponding ganglia are interconnected through large bundles of nerves called commissures.
The commissures connecting the pedal ganglia to the cerebral and pleural ganglia form a nerve collar or a circumesophageal ring around the esophagus.
There are only three pairs of ganglia in the bivalves: the cerebral, parietal, and visceral. The last pair is the largest and most significant as the principal ‘center of thinking.’
Many mollusks have a chemoreceptor sense organ called the osphradium that aids in sensing the water currents that flow through the mantle cavity. However, this organ is highly reduced in scaphopods, cephalopods, and gastropods.
While cephalopods have well-developed, vertebrate-like eyes, scallops and related mollusks bear a pair of photoreceptors on the margins of their mantle.
Mollusks are generally considered members of Lophotochozoa, a clade defined by organisms with trochophore larvae. Though the exact phylogenetic relationships within the molluscan family tree remain unclear, they are currently listed under seven extant and three extinct classes.
Mollusks are generally considered members of Lophotochozoa, a clade defined by organisms with trochophore larvae. Though the exact phylogenetic relationships within the molluscan family tree remain unclear, they are currently listed under seven extant and three extinct classes.
Though most mollusks are marine and are found anywhere between seashores and oceanic depths, many species, like land snails, are terrestrial. Freshwater mussels and clams inhabit rivers, canals, springs, lakes, and other freshwater environments.
Almost all mollusks are herbivores, feeding on bacteria and algae, especially filamentous algae and microalgae. However, gastropods are omnivores that often feed on sponges, echinoderms, bivalves, and other gastropods.
Mollusks that graze on algae employ two primary feeding strategies. Those feeding on microscopic algae comb up algal filaments from the floor using their radula, while those feeding on macroscopic plants, such as kelp, grate the plant surface using the radula. In contrast, sea slugs pierce the cell wall of algae and suck sap from them.
In most mollusks, the individuals are either males or females (gonochoric), with their gonads located next to the coelom. They release their gametes, either the eggs (ova) or the sperm, into the coelom, which are then extracted by the nephridia and delivered to the mantle cavity. The gametes then fertilize externally.
However, most gastropods (either hermaphroditic or gonochoric) undergo internal fertilization, in which the ova and the sperm fertilize inside the mollusk’s body. Some populations of Cylindrus obustus undergo self-fertilization or autogamy, where the same individual contributes the two gametes.
The fertilized eggs typically develop into the planktonic trochophore larva, which uses the two bands of cilia encircling the middle of their body to feed on floating food particles. With the gradual growth of the larva, new mesodermal tissue grows in the interior of the body, pushing the apical tuft of cilia and the anus away from each other. Slowly, this larva transforms into the veliger larva, characterized by a velum or veil, a pair of cilia-bearing lobes modified from the equatorial cilia of the trochophore. The veliger then swims to the seafloor, bores into it, and finally metamorphoses into adulthood.
However, in cephalopods, the hatchling directly develops into miniature adults and bypasses all the steps of metamorphosis.
In snails, like limpets, the foot is modified into suckers for attaching to hard surfaces, while in bivalves, they are well-adapted for burrowing into sediments. Similarly, the foot secretes a lubricating mucus in gastropods to facilitate smooth movement.
According to the IUCN Red Data List (2016), there are around 581 critically endangered (CR) and 507 endangered (EN) mollusks. Additionally, 27% of all evaluated mollusks (around 1988 species) are data deficient (DD).
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