Arthropods: Hard Case

From the slow, methodical glide of a garden snail to the brilliant intelligence of an octopus, the phylum Mollusca represents one of the most successful and widespread groups of animals on Earth. With over 100,000 recognised species, they inhabit marine, freshwater, and terrestrial environments across the globe.

Defining Characteristics

While molluscs are not true arthropods, they are remarkable invertebrates that share several key features:

• Soft Bodies: Most have soft, unsegmented bodies.

• Protective Shells: Many species secrete a hard shell made of calcium carbonate for protection, though some (like slugs and octopuses) have reduced or lost them entirely.

• Muscular Foot: Used for movement, whether it is the creeping foot of a snail or the specialised tentacles of a squid.

The Diversity of Molluscs

The Phylum Mollusca contains several distinct groups, each with unique adaptations:

Group Key Features Examples

Gastropods Often have a single, coiled shell; name means "stomach-foot". Snails, slugs, nudibranchs.

Bivalves Two-part hinged shells; specialised filter feeders. Clams, mussels, oysters.

Cephalopods Advanced brains, sophisticated camouflage, and tentacles. Octopuses, squids, cuttlefish.

Ecological Significance: Nature’s Water Filters and Grazers

Molluscs are "ecosystem engineers" that provide critical services:

• Water Quality: Bivalves like oysters filter water, removing excess nutrients and improving clarity.

• Plant Regulation: Gastropods help control plant and algae populations through grazing.

• Food Web Foundation: They serve as a primary food source for fish, birds, and mammals.

• Habitat Providers: Their discarded shells provide homes and refuges for other marine life, such as hermit crabs.

Conservation Challenges

Human activities pose significant threats to these master sculptors:

• Habitat Destruction: Coastal development and bottom trawling destroy critical reefs and seagrass beds.

• Pollution: Industrial runoff can degrade water quality, impacting sensitive filter feeders.

• Climate Change: Rising sea temperatures and ocean acidification make it difficult for molluscs to build and maintain their shells