Mollusks: Classes, Characteristics, and Importance

Posted on Mar 10, 2025 in Biology

There are three main classes of mollusks: Gastropoda, Bivalvia, and Cephalopoda.

Gastropoda

Gastropoda includes snails, slugs, and abalones. The name means “stomach foot.”

Movement: They use their foot to slowly crawl.

Bivalvia

Bivalvia includes clams, oysters, scallops, and mussels. They have two shells.

Movement: They are often sessile, anchored in place for filter feeding. Some can use their foot to dig through the sand.

Cephalopoda

Cephalopoda includes octopi, squid, and cuttlefish. The name means “head foot.”

Movement: They move through the water with jet propulsion, using muscles to propel water. Some can even “fly” for short periods (up to 5 minutes).

Mollusks are soft-bodied animals that have an internal or external shell. They appeared over 545 million years ago.

Feeding and Respiration

Radula: Herbivores (most gastropods) use a tongue-shaped structure with hundreds of tiny teeth to scrape algae off rocks. Carnivores use it to drill holes in the shells of their prey.

Filter Feeding: Mussels draw in water from one side and pump it out the other. In between, filters act as gills, extracting oxygen from the water and filtering out food particles.

Respiration:

Aquatic mollusks have gills in their mantle cavity, which absorb gases and exchange them between the blood and surrounding water.
Terrestrial mollusks have primitive lungs to absorb oxygen directly from the air.

Clams use a muscular foot to burrow into the sediment for protection. When the tide comes in, they stick their siphons out to inhale fresh seawater for oxygen.

Circulatory Systems:

Open Circulatory System: Blood is pumped from the heart into the body cavity, goes through tissues, and then returns to the heart.
Closed Circulatory System: A continuous system of vessels where blood is not pumped into body cavities.

Octopuses have highly developed brains and organs, while clams have only simple organs and no brain.

Nephridium: This is the unit of the excretory system. It expels waste from the body cavity to the exterior.

External Fertilization: A male organism’s sperm fertilizes a female organism’s egg outside of the female’s body. After fertilization, the zygote undergoes cell division, and in many mollusks, the fertilized eggs develop into larvae.

Protection Without Shells:

Ink Defense: Visual distraction.
Camouflage: Nudibranchs and cuttlefish can blend in with their surroundings.
Toxicity: Sea slugs can release toxic compounds from their prey, such as jellyfish, and store these toxins in their tissues.

Economic and Environmental Importance

Helpful Aspects:

Economic Importance: Oysters, clams, mussels, and scallops are valuable food sources.
Environmental Benefits: Maintaining the balance of ecosystems. For example, filter feeding helps improve water quality, and snails and limpets contribute to nutrient cycling and algae control.

Unhelpful Aspects:

Pests: Certain snails and slugs can be agricultural pests, causing damage to crops by feeding on leaves, stems, and fruits.
Disease Transmission: Freshwater snails can act as hosts for parasitic worms that cause diseases in humans.

Larval Stage: Veliger.

Internal Fertilization: Used by cephalopods (squid, octopus, cuttlefish), some gastropods (land snails and slugs), and some bivalves (freshwater mussels).

Cephalopods, such as squid and octopuses, have more developed nervous systems than bivalves. They possess a complex and centralized nervous system, including a large brain and well-developed sensory organs, allowing for advanced behaviors and intelligence.

Solid waste leaves through the anus.

Liquid waste is eliminated through excretory structures called nephridia. They filter waste products from the blood and release them as liquid waste into the surrounding environment.