Mussels are bivalve mollusks that primarily consume microscopic algae, known as phytoplankton, through filter feeding. This feeding strategy is essential for their survival and profoundly impacts water quality and the entire aquatic food web.
The Mechanics of Filter Feeding
The feeding process begins with the mussel drawing water into its shell cavity using an inhalant siphon. The water is moved by the coordinated beating of tiny hair-like structures called cilia, which cover the gills. The gills function as a sieve, trapping suspended particles, including microscopic algae, from the water column. Particles retained on the gills are then encased in mucus and transported toward the mouth.
At the mouth, labial palps sort the collected material. These palps separate nutritious, edible particles from indigestible matter, such as silt or sand. Edible particles are directed into the digestive tract for ingestion. Rejected material is bound with mucus into masses called pseudofeces.
Pseudofeces are expelled from the mantle cavity, often through the inhalant siphon or by a sudden contraction of the shell, without passing through the digestive system. This mechanism allows mussels to filter large volumes of water efficiently, even in turbid conditions.
Defining the Primary Diet
The bulk of a mussel’s diet consists of phytoplankton, which are single-celled photosynthetic organisms suspended in the water. Mussels are highly size-selective, generally capturing particles smaller than 25 micrometers (µm). Some smaller particles are retained with nearly 100% efficiency.
While phytoplankton is the main food, mussels also consume detritus, which is decaying organic matter providing a secondary source of nutrition. Mussels reject large, macroscopic algae, like seaweed, because their filtering apparatus processes only microscopic particles.
Ecological Impact of Mussel Feeding
The sheer volume of water mussels filter gives them enormous influence on the aquatic environment, establishing them as ecosystem engineers. By continuously removing suspended particles, they dramatically increase water clarity, allowing more sunlight to penetrate the water column. This increased light availability is essential for the growth of submerged aquatic vegetation, which provides habitat and food for other organisms. Filtration activity also helps to cycle nutrients within the ecosystem.
Mussels manage excess nutrients, such as phosphorus and nitrogen, which can lead to the overgrowth of algae. They sequester these nutrients into their bodies and shells as they grow, effectively removing them from the water. Additionally, they deposit filtered material as feces and pseudofeces onto the sediment, transferring nutrients from the water column to the bottom.
Mussels are also capable of filtering out harmful algal blooms (HABs), which are dense concentrations of toxic algae. By consuming or sequestering these toxic organisms, mussels can reduce the overall concentration of the bloom in the water. However, the toxins can accumulate in the mussel tissue, making them dangerous for human consumption.