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Manila Clam (Ruditapes philippinarum)

Photo: manila clam. Photo: Graham Gillespie.
Manila Clam (Ruditapes philippinarum). Photo credit: Graham Gillespie.

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Species overview 

Physical description

Manila Clam, also known as Japanese Littleneck Clam, is a medium-sized intertidal clam. It has 2 thick shells that are oval in shape. The outside of Manila Clam shells has rings that run both parallel and perpendicular to the edge. These rings can be used to determine a clam’s age. The outside edge of Manila Clam shells is smooth. Flat or depressed lunules are present on the shells.

The inside of Manila Clam shells is glossy white or yellow with a purple stain towards the outside. The outside of shells can be:

Young Manila Clam shells can have strips of:

Older clams are less colourful. Manila Clam grows up to 75 millimeters (mm) long. It can live to 14 to 16 years. 

Distribution

The native range of Manila Clam is:

Manila Clam was accidentally introduced to Canada in the 1930s with shipments of Pacific Oyster seed. It was first found at Ladysmith Harbour, British Columbia (BC) in 1936. In less than 10 years, it spread throughout the Strait of Georgia. By the late 1950s, Manila Clam had spread across the west coast of Vancouver Island. It was most likely introduced by accident to Barkley Sound with Pacific Oyster seed. Manila Clams around Bella Bella were likely introduced as larvae from Quatsino Sound. Today, Manila Clam is found from central BC to California.

Manila Clam lives in shallow, temporary burrows down to 20 centimeters (cm) deep. It stays shallow because its siphon is short. It prefers to burrow in:

Manila Clam lives in the upper half of the intertidal zone. There are no significant subtidal populations of Manila Clam in BC.

Because Manila Clam does not burrow deeply, it can die during the winter from frost and cold winds. This is known as “winter kills”.  It can also die from heat stress. The northern distribution of Manila Clam is limited by the minimum temperatures they need to survive.

Life cycle

Manila Clam maturity depends on its size, not its age. Manila Clam matures between 20 to 25 mm long. It reaches this length when it is between 1 and 2 years old. Environmental conditions affect Manila Clam growth rates. It grows quickly for the first few years of life and then its growth slows.

In BC, it spawns from June to September. Manila Clam is a broadcast spawner, meaning males and females release eggs and sperm into the water column for fertilization. A minimum temperature of 14 to 15°C is needed for spawning. Female Manila Clams that are 40 mm long can produce 2,350,000 eggs.

Larvae are pelagic for 3 to 4 weeks before settling on the ocean floor. Larvae can be transported to new beaches by ocean currents. Once settled, adults cannot move to another beach.

Recruitment of Manila Clam in BC depends on environmental conditions and is irregular. This causes fluctuations in adult populations over time.

Fishery history  

The intertidal clam fishery plays an important role in coastal communities in BC:

Commercial clam fisheries have been carried out for over 100 years. Butter Clam was the target species of the commercial clam fishery in BC until a market shift to Littleneck and Manila in the 1980s. Manila Clam is valuable because its meat separates easily from the shell after cooking. Manila Clam is commonly sold as a live product to restaurants in the U.S.

While Manila and Littleneck clams have been reported in commercial landings since 1951, the directed fishery did not develop until the late 1970s. Manila Clam landings averaged 189 tonnes (t) from 1951 to 1979. Peak landings of Manila Clam were 3909 t in 1988. Landings began to decrease after 1989 when more restrictive management measures were introduced due to concerns of overharvesting. Licence limitations were introduced in 1998.

In recent years, commercial fishery openings have been reduced in most areas to only a few days per year. Clam harvesters fish during low tides. They use rakes or scrapers to turn Manila Clams out and collect them by hand.

A commercial Manila Clam fishery was developed along the central coast near Bella Bella in the 1990s in partnership with the Heiltsuk Nation.

Ecosystem context 

Manila Clam is a filter feeder. It uses its siphon to remove food particles from the water column. Feeding only happens when Manila Clam is underwater. When the tide is low, Manila Clam closes its shells tightly and waits to feed until the tide covers it again.

Bivalve shellfish are very sensitive to water quality because they feed by filtering microscopic organisms from the water. While feeding, harmful bacteria, viruses, and marine biotoxins can build up in their tissues. If eaten, this can make humans sick.

Be careful when harvesting shellfish. Check for contamination in the subarea where you plan to harvest. Check our real-time bivalve shellfish safety harvesting map. If there are no contamination closures, check species regulations and additional harvest restrictions.

In addition to human consumption, Manila Clam are prey for:

Manila Clam overlaps with native Littleneck Clam in the intertidal zone. It does not overlap with Butter Clam. Manila Clam does not hybridize with native clams in BC.

Science advice and research  

Science advice and research for Manila Clams

Integrated Fisheries Management Plan  

Integrated Fisheries Management Plan for Intertidal Clams

Sources 

Related links

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