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Hard Clams

Aquaculture Production of Hard Clams, Mercenaria mercenaria

Hard Clam broodstock are brought into the Castagna Shellfish Research Hatchery each winter to condition, or encourage gonad development for Spring spawning. These animals are held in a temperature-controlled static tank with filtered seawater and fed calculated doses of cultured microalgae on an hourly basis. Clams are held in the conditioning system for 4-8 weeks, depending on the extent of conditioning necessary.

Once ripened, the clams are exposed to mimicked environmental cues that will trigger them to spawn in a closed hatchery system. Once spawning has been initiated, egg fertilization is monitored, and soon after the fertilized embryos can be removed from the spawning tank and placed into a larval culturing tank. In scenarios where selective breeding is necessary, clams are isolated in individual dishes and cued to spawn. The gametes are collected then fertilization is initiated by strategic mixing of gametes according to project goals.  

The larval stage of a hard clam can last for 8-15 days depending on culturing conditions. During this time, the larvae are kept at a constant temperature and fed calculated rations of microalgae each day. The cultures are drained every other day and the larvae are collected on a sieve sized appropriately for the developing veligers. This allows for close observation of the larvae, but also allows any accumulated waste products, contaminants, or unconsumed food or excess nutrients to be removed from a culture as the larvae are retained on the sieve, and anything smaller will pass through the sieve as waste. After quantifications and observations are recorded, the larvae are returned to a clean, pre-heated larval tank according to specific larval stocking densities.

When hard clams are ready to set, they do not develop an eyespot as bay scallop and oyster larvae do. Upon reaching a shell height of ~170-180 um, the larvae develop a foot in addition to their swimming organ, the velum, at which point they are classified as pediveligers. Oftentimes, clam pediveligers will begin to sink to the bottom of larval tanks as they become ready to set. When any of these signs are observed, the larvae are passed over a grading sieve to determine readiness for transfer to a closed downweller where they will set, lose their swimming organ, or velum, and metamorphose into their juvenile stage.

During the downweller phase, clams are rinsed and moved to a clean, heated downweller system loaded with calculated rations of microalgae on a daily basis. These early post-set clams are kept in a downweller system until they have grown to a size capable of being retained on nursery sieves. Once this size has been reached, these animals are transferred to a flowing upweller system in ESL’s land-based nursery.

For hard clams, flow-through upweller troughs are maintained in ESL’s land-based nursery using raw, unfiltered seawater coming from the Wachapreague Channel. The upwellers are drained, and the sieves and trough are rinsed with freshwater daily. Daily rinsing promotes mixing and aeration of clams inside each sieve to stimulate uniform growth, prevent juveniles from ingesting sediment, prevent mortality from smothering, and prevent biofouling inside the system. Daily maintenance also allows for basic daily observations relating to size and health.

When ready to be deployed (>9 mm), the juveniles are placed in a 5 mm mesh bag which is then placed into a 3’x3’ bottomless 1” wire mesh cage. This allows the bag to settle down into the sandy bottom at which point the clams can bury in the sediment while still being contained within the bag. Once the clams reach a size of more than 1”, they can be removed from the bags and planted directly within the bottom cage. Animals and gear are maintained regularly and the clams’ survival and growth is monitored throughout development.