Clams are invertebrates, bivalves characterized by their two-part, hinged shell. These mollusks live in both marine and freshwater aquatic environments. Clams often live buried within sand or mud substrates.
Master Filter Feeders
Clams obtain food through filter feeding. They use siphons to draw in and expel water. Water enters through an incurrent siphon, passes over their gills, and exits through an excurrent siphon.
The gills of a clam are covered in cilia. These cilia create a current that pulls water, along with microscopic food particles like plankton, algae, and detritus, into the clam’s body. As water moves across the gills, food particles are trapped in strands of mucus. Food-laden mucus is then transported to the clam’s mouth by palps, where it is ingested.
A single small clam can filter a significant volume of water, potentially clearing nearly ten gallons per hour. This feeding behavior is important for their survival and ecological role.
Life in the Sediment: Burrowing and Beyond
Clams use a muscular foot for movement and anchoring in sand or mud. The foot extends into the substrate, swells with blood to create an anchor. Once anchored, the clam contracts its muscles, pulling its shell downward into the sediment.
This burrowing action often involves a “double-anchored wriggle” or seesaw motion, where the foot and shell valves alternate in providing leverage. The shell valves can contract inward, narrowing the body and pushing blood into the foot, which helps reduce friction during movement. While many clams are active burrowers, some species become largely sedentary once they have settled into a suitable location. The strong shell also provides protection while buried.
Clams’ Role in the Ecosystem
Clams play an important role in aquatic ecosystems through filter-feeding. They contribute to maintaining water quality by removing suspended particles, including phytoplankton, algae, and detritus, which improves water clarity. This filtration process can reduce turbidity and prevent excessive algal growth, benefiting other aquatic organisms and allowing more sunlight to penetrate the water for seagrass growth.
Clams also participate in nutrient cycling. As they filter feed, they accumulate organic matter and minerals, which are then released as biodeposits, consisting of feces and pseudofeces. This processing of organic matter and release of waste products, such as ammonia, influences nutrient levels like nitrogen and phosphorus, supporting the productivity of the food web. Beyond filtering, clams serve as a food source for aquatic animals, including fish, birds, crabs, and mammals like walruses and seals. Their burrowing activities further influence sediment structure, promoting aeration and nutrient exchange, which benefits other organisms living in the sediment.
The Clam Life Cycle
Most clam species reproduce through external fertilization, a process known as spawning. During spawning, male clams release sperm and female clams release eggs into the water, where fertilization occurs. This process typically happens when water temperatures are suitable, often between April and October.
After fertilization, the eggs develop into free-swimming larval stages, beginning with trochophore larvae. These microscopic larvae have cilia that aid in movement and feeding, allowing them to drift in the water column. They then transform into veliger larvae, which possess a rudimentary shell and develop a foot, enabling more controlled swimming.
Eventually, the veliger larvae undergo metamorphosis and settle onto a suitable substrate, such as sand or mud. Once settled, they develop into juvenile clams, growing their shells and internal organs, and eventually mature into adult clams capable of reproduction and filter feeding. The lifespan of clams can vary significantly by species, ranging from a few years to over 500 years for some, like the ocean quahog.