Information about Mollusks
Mollusks are animals of great ecological and adaptive success, with representatives in almost all habitats, although their variation is greater in the marine environment. Most mollusks range from 1 to 20 cm in length, with some barely visible and others reaching large sizes, such as giant squids, which can reach lengths of up to 18 m.
Among the most well-known mollusks are clams, oysters, snails, slugs, octopuses, and squids.
The first fossil mollusks appeared in rocks from the Cambrian period, about 570 million years ago.
Living representatives exist in eight classes of the phylum: caudofoveates, marine organisms without shells and worm-like appearance, with 70 species; solenogastres, worm-like organisms lacking shells, with 250 species; chitons, with 600 species; monoplacophorans, with 10 species; bivalves, like clams, with 7,500 species; scaphopods or tusk shells, with 350 species; gastropods, like snails and slugs, with about 40,000 species, and cephalopods, like octopuses and squids, with 600 species.
Several classes and thousands of fossil species are also known.
Did you know? Some mollusks are exquisite delicacies. Mussels, clams, octopuses, or squids are some mollusks that we eat. Have you ever tried them?
What are Mollusks Like?
Although most mollusks are small, some reach large dimensions. Giant clams can exceed one and a half meters in length. Giant squids can reach a length of 18 meters, including tentacles, and weigh up to 2,000 kilograms.
Mollusks are invertebrate animals because they lack a backbone. Their bodies are soft and not divided into rings or segments. Some have an external shell with which they protect themselves and can also hide. They have a layer, called a mantle, that covers the internal organs. The mantle is what secretes the shell or protective covering.
Have you ever noticed the foot of snails? They use it to move; but other mollusks use the foot to dig and bury themselves in the ground or to capture their prey.
Characteristics
Mollusks have three main characteristics that differentiate them from other animals: the presence of a structure known as the mantle, an organ called the radula, and the structure of their nervous system. In some species, the radula is more developed, allowing them to pierce shells of other animals or inject venom into their prey.
It is also worth noting that the foot is an important part of these animals’ bodies, although its function varies depending on the class of mollusk. These animals also stand out due to their powerful and complex musculature.
The most important function of this organ is locomotion. However, in bivalves, it is used for digging, while in cephalopods, it helps provide propulsion.
We must also highlight that mollusks have organs that perform many important functions for them.
For example, their heart or nephridia play an important role in reproduction. In many cases, some of their gills are necessary for breathing, and they are also involved in excretion or reproduction.
Feeding
Their diet varies depending on the species, including filter feeders that feed on microscopic floating organisms such as algae and particles.
There are also grazers like mollusks that feed on microorganisms like filter feeders, but the difference is that they ingest them from rocks.
And the rest of the species feed in various ways: carnivorous, herbivorous, scavengers, and deposit feeders.
Reproduction
Reproduction in mollusks is sexual, with individuals being unisexual or hermaphroditic, depending on the species.
Types of Mollusks
There are three main groups of mollusks: gastropods, bivalves, and cephalopods. Gastropods are the most numerous.
Gastropods
Snails, slugs, and winkles are gastropods. Their bodies are protected by a single shell, except for slugs, which lack shells. Most gastropods live in the sea and breathe through gills; but terrestrial ones, like snails, use lungs.
Generally, gastropods have a wide foot that they use to move. Some are herbivores; others feed on plant and animal remains, and there are also carnivorous ones.
Many snails have a kind of plate on the foot called an operculum, which they use like a door. When they feel in danger, they retract their body into the shell and close it with the operculum. They also retreat into the shell when there is no moisture and remain there until conditions change.
Bivalves
Clams, mussels, and oysters are bivalves. They have a shell or covering formed by two pieces, called valves, which are joined by powerful muscles. They do not have a well-differentiated head. All bivalves live in the sea and breathe through gills.
However, they do not have a radula. Do you know how they feed? Most are filter feeders and obtain their food from the water. They use their gills to capture food. Some bivalves, like clams or razor clams, use their foot to bury themselves in the sand. Others, like mussels, use it to move along the seabed.
Cephalopods
Nautiluses, squids, and octopuses are cephalopods. Nautiluses have an external shell; squids have a small shell inside their body, called a pen, and octopuses do not have a shell. Cephalopods are marine animals and breathe through gills.
They have several long arms or tentacles around their mouths, equipped with suction cups. Some octopus tentacles can reach up to 5 meters in length! Cephalopods use their tentacles to capture prey. Cephalopods have a fairly developed nervous system and two large eyes that provide them with excellent vision.
Some cephalopods have an ink sac, which serves as protection; when they feel threatened, they release ink into the water, making it difficult for their enemies to see them, allowing them to escape quickly without being seen. Squids have ten tentacles and are good swimmers. Octopuses have only eight tentacles.
Habitat
Mollusks have managed to colonize almost all environments; they are found from high altitudes over 3000 m above sea level to depths near 5000 m in polar and tropical waters. They are mostly common in coastal areas worldwide.
Many are found in highly humid environments because they need them to breathe and live properly since their bodies are moist and soft.
Some snails living in slightly dry areas tend to seal their shells to maintain the moisture of their bodies.
Importance of Mollusks in the World
There is a variety of mollusks worldwide, which is one of the main reasons why they are important in the food chains of many habitats. Some animals, like tusks, are mollusks that feed on material found on the seabed, while most bivalves filter suspended material in the water.
Many are carnivores; most of these attack stationary or moving animals, while others are active predators of large animals, such as crabs.
Numerous mollusks are an important food source for humans, but some of them are not as well represented since they damage crops, and others are the main authors of disease-causing parasites.
Therefore, my advice to you is to take care of the areas where these animals live.