The primary species of bay clams found in Oregon are butter clams (Saxidomus gigantea), cockles (Clinocardium nuttallii), gaper clams (Tresus capax, T. nuttallii), littleneck clams (Leukoma staminea, Venerupis philippinarum ), purple varnish clams (Nuttallia obscurata), and softshell clams (Mya arenaria). The first four clams listed are the most common targets for sport clammers. Each of these clams has a unique life history and biology (Table 1) that is used to inform management.

Bay Clam Life History

The basic life history is similar among the species of bay clams. Bay clams are broadcast spawners, meaning that eggs and sperm are released by clams into the water column where eggs are fertilized. The baby clams spend several weeks to months in their early development as free-swimming larvae in the water column during this pelagic stage, passing through several larval stages (trochophore, veliger, and pediveliger). It is during this pelagic stage when clam larvae can be transported within a given estuary or even to neighboring estuaries.

During the pediveliger larval stage, shell starts to develop and the larvae begin settling down in the sediment. The benthic stage of living beneath the sediments starts as the larvae goes through metamorphosis and slowly matures. Juvenile clams grow for 1-3 years before they become mature adults.

Bay clam species differ based on the time of year when they spawn, the amount of time spent in the water column during their larval stages, and when the juvenile clams settle to the estuarine seafloor. These differences in timing can account for why some species have a better recruitment year and survival to maturity than others.

Some life history attributes that are important when considering clam harvest regulations are: age at maturity, patterns of reproduction, mobility, larval and recruitment dynamics, and lifespan. Table 1 provides ranges for these attributes for the recreationally and commercially important bay clams in Oregon estuaries. All these life history attributes may vary depending on the substratum, food supply, temperature, competition, and many other environmental factors. Also, drifting planktonic larvae likely provide connectivity between populations that inhabit subtidal and intertidal habitats, between populations in the nearshore ocean and bays, and possibly between populations in neighboring bays.

Table 1. Life history and habitat attributes for four Oregon bay clams

	Cockle Clam Clinocardium nuttallii	Gaper Clam Tresus capax	Butter Clam Saxidomus gigantea	Native Littleneck Clam   Leukoma staminea
Maximum Age	10 – 14 years	16 years	20+ years	6 – 10 years
Age at Maturity	2 years	3 – 4 years	3 – 4 years	2 – 3 years
Maximum Size	5"	10"	4 – 5"	3 – 4"
Burial Depth	Surface to 6"   Can "walk" on surface with foot.	1 – 3 feet	6" – 14"	Surface to 10"
Spawning Season in Oregon	Hermaphroditic, June - October	January – April	April - July	April - October
Duration of Larval Stages	No published larval duration	24 – 34 days	28 – 36 days	3 weeks
Geographical Range	Japan, AK – San Diego	Kodiak, AK – San Francisco, CA	Aleutian Islands to Monterey, CA	Aleutian Islands to Central Mexico
Sediment Preferences	Sand, muddy sand	Sand, muddy sand	Sand, gravel	Sand, muddy sand

Cockles are mobile and typically bury themselves just below the sediment surface. This clam typically matures in its second year, grows to a shell length of approximately 1½ - 2" after 2 years, and reaches a maximum size of 5" after 10-12 yrs. Cockles are hermaphroditic and spawn mostly during the summer. Their lifespan is approximately 10-14 yrs.

Oregon's native littleneck clam can also be found on or close to the surface in sandy or muddy sediments. Littleneck clams mostly spawn through the late spring and summer. They mature between 2-3 years, reach a maximum size of 3-4", and live up to 10 years. Littleneck clams have declined along the Pacific Northwest over past decades and biologists have not yet identified the specific broad-scale causes for this decline. 

In contrast, gaper and butter clams mature later, occur deeper in the sediments and are chiefly stationary, especially as they grow larger. These larger clams are also less capable of re-burying after disturbance.

Gaper clams generally occur at lower tidal elevations where they are submerged and able to feed longer than if they were higher in the intertidal. Unlike the other bay clams, spawning typically occurs during the winter to early spring. Recruitment can be sporadic and punctuated over longer time scales; this is often recognizable by finding cohorts of gaper clams that are all the same size at a particular location. They grow to a shell length of 2" after 2 years, mature at 3-4 years, and reach a maximum size of approximately 7" after 10-12 years. The estimated lifespan for gapers is up to 16 years. 

Butter clams can be found in gravel and cobble more often than other species, but they also occur in higher sandy "butter bars".  Spawning occurs during the early spring and summer. They begin to reproduce after 3 or 4 years and can live over 20 years.

Purple varnish and softshell clams are the other two bay clams that are often targeted by sport clammers. They differ from the other bay clams in that they are both non-native clams that have been introduced to Oregon. Purple varnish clams were introduced to Oregon estuaries in the late 1990s and have rapidly expanded. They are most commonly found in the sandy substrates of the upper intertidal, often co-occurring with burrowing shrimp. They are the major recreational clam in Alsea and Siletz bays. Softshell clams are thought to have been unintentionally introduced to the Pacific Northwest in the 1800s and populations have been established in Oregon estuaries for over a century. They are more common in the muddier, brackish portion of estuaries where they are buried 4-8" deep in mud or sandy mud habitats.