Oyster larvae are the microscopic, free-swimming, early life stage of an oyster. Following egg fertilization, they drift in the water column for a limited time before transforming into stationary adults. Their successful development and settlement are significant for maintaining wild oyster populations and supporting global aquaculture practices.
The Larval Life Cycle
The larval life cycle begins with a fertilized egg, which undergoes rapid cell division. Within about six hours, this embryo develops into the trochophore larva, a shell-less stage characterized by cilia that enable movement. These cilia allow the larva to swim by spinning, marking its entry into open water.
The trochophore quickly transitions into the veliger stage within 12 to 24 hours of fertilization. During this stage, the larva develops its first shell, which takes on a distinct D-hinge shape. A specialized organ called the velum also forms, equipped with cilia that allow for both swimming and filtering algae for food. As the veliger grows, its shell continues to develop, eventually forming an umbonate shape where a hinge between the two shells becomes more prominent.
The final larval stage is the pediveliger, appearing after about two and a half weeks of development. This stage is marked by the development of a muscular foot, signaling the larva’s readiness to transition from a free-swimming existence to a stationary life on the seafloor. A black spot may also become visible on the shell, though it is not used for vision.
Settlement and Metamorphosis
The pediveliger stage represents an important point, as the larva actively seeks a permanent attachment site. It uses its newly developed foot to explore and “taste” various underwater surfaces. This exploration can involve descending to the seafloor, moving horizontally across potential surfaces, or rejoining the water column if a suitable location is not found.
A significant factor guiding this search is the presence of specific chemical cues in the water, particularly those released from adult oyster shells and associated biofilms. These chemical signals indicate a safe and suitable environment, often where other oysters have already established themselves. Once a satisfactory location is identified, the pediveliger cements itself to the chosen surface, such as an old oyster shell, by secreting a glue-like substance.
Following attachment, the larva undergoes metamorphosis into a “spat,” or juvenile oyster. During this process, the larva loses its swimming organs, like the velum, and begins to develop the recognizable form of a miniature oyster. The spat then starts to grow, filtering food from the water and remaining permanently affixed to its chosen substrate.
Environmental Influences and Threats
Oyster larval survival is dependent on environmental conditions in the water column. Water temperature plays a significant role, with adult oysters reproducing when temperatures rise above 68°F (20°C). Salinity levels also influence development, and larvae require a consistent supply of phytoplankton, or algae, as their primary food source.
Despite these needs, oyster larvae face numerous threats during their planktonic existence. They are small and vulnerable to predation by various zooplankton and filter-feeding organisms. Environmental stressors, such as pollution, can impair their development and reduce survival rates.
Ocean acidification, caused by increased absorption of carbon dioxide by seawater, poses another threat. It can hinder the proper formation of their calcium carbonate shells, making them more fragile and susceptible to damage. These combined environmental pressures make the larval stage a delicate period in the oyster’s life cycle.