Hard clam (Mercenaria mercenaria)

Description

The hard clam or northern quahog (Mercenaria mercenaria) has two valves of equal size with an ovate-trigonal shape (Eversole 1987). An external ligament joins the two valves, and obvious growth lines radiate concentrically from the umbo. These concentric ridges are sharp in young clams, but become smooth and irregular with age (Weiss 1995). The shell is off-white to fawn colored with a thin periostracum, and the shell interior is white or pale yellow with purple markings. Northern quahogs have two short siphons, the tips of which range in color from white to black, and a ring of small tentacles surrounds the incurrent siphon. The quahog's foot is proportionately large and hatchet shaped.

Background

Distribution. Northern quahogs are distributed along the eastern and Gulf coasts of the U.S., ranging from the Gulf of St. Lawrence to Florida and into Texas (Grosslein and Azarovitz 1982, Eversole 1987). It is most abundant from Virginia to Massachusetts (Eversole 1987) and has also been introduced to California and Europe.

Hard clams inhabit the subtidal regions of bays and estuaries to approximately 15 meters in depth (Eversole 1987). They are generally found in firm bottom areas consisting of sand or shell fragments (Chesapeake Bay Program 1987), optimal settling substrates for juveniles (Eversole 1987). Franz and Harris (1988) identified northern quahogs as fine-sand species in Jamaica Bay.

Feeding. Hard clam larvae, juveniles, and adults consume phytoplankton, zooplankton, and detrital material by trapping particles in the mucus lining of their gills (Grosslein and Azarovitz 1982, Chesapeake Bay Program 1987). They are also able to absorb dissolved organics directly from the water (Eversole 1987). Water is brought through the inhalant siphon and passed over the gills where the food particles are captured and transported to the mouth via cilia. The particles pass through the digestive system, and waste is released through the exhalant siphon in the shape of rod-shaped fecal pellets (Eversole 1987).

Fishery. Northern quahogs are a demanded fisheries species. They have a large population, estimated in the millions of bushels, with the leading production area in the mid-Atlantic region (Eversole 1987, MacKenzie 1990). The clams can be harvested in many ways and have one of the oldest fisheries in the United States. Both small clams and large clams are harvested and classified into one of four commercial size grades. These include 1) seed clams; less than 50 millimeters in length, 2) littlenecks; 50 to 65 millimeters, 3) cherrystones; 66 to 79 millimeters, and 4) chowders; greater than 80 millimeters (Eversole 1987).

Hard clams have a long-standing history within the New York/New Jersey Harbor area as one of the staple foods of the native Americans and were a special food item consumed during celebrations. They were also a popular food item with the colonists (MacKenzie 1990). According to data collected by Grosslein and Azarovitz (1982), peak harvest of northern quahogs occurred during the late 1940s and early 1950s, averaging 9,500 metric tons (MT). By the 1960s it had dropped to 6,000 MT, where it remained until the mid-1970s. Data from the National Marine Fisheries Service (NMFS 1999) indicate that the fishery only brought in around 1,000+ MT per year in the 1980s and early 1990s. Grosslein and Azarovitz (1982) reported that production in the New York Bight increased from 2,700 MT in the 1950s to 5,000 MT in 1971, after which time the harvest began to decline. Possible explanations for this decline are poor monitoring of catches, harvesting of undersized clams, and the by-catch of seed clams. Recreational harvesting of northern quahogs does not appear to affect the commercial take (Eversole 1987).

Life Cycle

Spawning occurs when males and females release gametes into the water column during the summer months, as water temperatures reach approximately 22 to 24 degrees Celsius (Chesapeake Bay Program 1987). Eggs are buoyant and have a diameter of 0.07 millimeters plus an outer envelope 0.03 to 0.1 millimeters thick. Fertilization occurs in the water column followed by development into planktonic larvae. The larvae pass through various developmental stages, marked by the formation of shell valves, umbo, and ciliated foot. After approximately four weeks of development, settlement occurs with the larvae attaching to sand grains and taking up a benthic lifestyle. During this stage, the siphons develop, the mantle fuses, and the shell develops ridges. As the juveniles grow, they burrow into the sediment, maintaining contact with the surface using only the siphon (Eversole 1987). Prior to sexual maturity, hard clams go through a hermaphroditic stage (occurring at 6 to 7 millimeters in length) having both male and female gonadal cells while functioning mostly as males (Eversole 1987). At the end of this stage they become either male or female and reach maturity by age two and at lengths of 3.2 to 3.8 centimeters. Hard clams in the south reach maturity in about one year, while their northern relatives mature in two years, thus sexual maturity in northern quahogs is dependent on size rather than age (Eversole 1987). In their first 5 to 6 years, quahogs can reach sizes of 5 to 6 centimeters (littleneck) and reach their maximum length of 15 centimeters at an estimated 20 years.

Environmental Influences

Salinity. Northern quahogs are an osmoconforming, euryhaline species generally occurring at salinities ranging 15 to 32 practical salinity units (psu; Grosslein and Azarovitz 1982, Chesapeake Bay Program 1987). They have been found in salinities as low as 4 psu, but optimal growth occurs at 24 to 28 psu (Eversole 1987). At extreme salinities, clams can tightly close their shell valves and respire anaerobically, allowing them to survive up to a few weeks in harmful conditions. Adults are more tolerant of salinity extremes than are larvae and eggs (Eversole 1987), and normal egg development requires salinities of 20 to 35 psu. Below 17.5 psu, larvae fail to metamorphose and juvenile growth ceases (Chesapeake Bay Program 1987).

Predation and Competition. Predation on juvenile hard clams is very high, especially below 15 to 20 millimeters in length (Grosslein and Azarovitz 1982). Larval clams are consumed by planktivores, and Eversole (1987) indicated that open areas often result in few adult clams, even if the set quantity is high, due to predation of young clams. Areas with vegetation and shell matter tend to be more conducive to survival of the young. Adult northern quahogs are preyed upon by gastropods, crabs, starfish, and some fish species (Eversole 1987). A predator exclusion experiment detailed by Grosslein and Azarovitz (1982) resulted in a 7- to 8-fold increase in hard clam abundance.

The distribution of hard clams may be affected by the existence of the competitors, such as the amethyst gem clam (Gemma gemma). One study showed that in muddy areas with high densities of G. gemma, newly settled hard clams were unable to effectively compete for food resources, resulting in high mortality (Ahn et al. 1993).

Pollution. Northern quahogs are often found in very polluted habitats. Adults can tolerate wide ranges in water quality and can survive in changing concentrations of ammonia, nitrites, nitrates, phosphates, and sulfur compounds (Eversole 1987). Although they tend to survive in such habitats, these beds are unsafe for human consumption. One cause for concern is their ability to filter and concentrate harmful bacteria from domestic sewage leading to shellfish contamination. Contaminated shellfish can transfer typhoid, hepatitis, cholera, and other diseases. Oil pollution is also a concern, and can lead to closure of fisheries. Adult clams are also susceptible to inhibited growth, reduced feeding rates, and delayed sexual maturity from exposure to pollutants.

Planktonic larvae are more susceptible to harm from pollution than are adults, and improved conditions following clean-up of the Raritan River and Bay led to dramatic increases in hard clam recruitment (Menzel 1979). Larvae have been experimentally shown to suffer from inhibition of growth and increased mortality as a result of exposure to various pesticides, herbicides, detergents, and heavy metals. Effluent waste from pulp mills has been determined to cause mortality during all northern quahog life stages. In areas with typically cold water, industrial plants are known to release warm water, which can affect the recruitment of non-native species (e.g., hard clams in the British Isles; Menzel 1979). Pollution also indirectly affects hard clams by inhibiting algal growth, thereby reducing the food supply (Grosslein and Azarovitz 1982).

Additionally, human activities, such as dredging and drainage of marshes and wetlands, have contributed to the reduction of active clam beds (Grosslein and Azarovitz 1982). Dredging of navigation channels destroys bottom habitat in the dredged areas, spoil placement alters the habitat at the disposal site, and resuspension of silts can smother shellfish. Besides navigational dredging, dredging for the purposes of shellfish harvesting has similar effects (Menzel 1979).

Return to Top