(DRAFT) - Taxonomy
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
TAXONOMY
NAME - horseneck gaper
OTHER COMMON NAMES - Alaskan gaper, fat gaper, blue clam, empire clam, gaper, gaper clam, greyneck clam, horseneck clam, horse clam, bigneck clam, giant rockdweller, butter clam, money shell and giant saxidome
ELEMENT CODE -
CATEGORY - Aquatic Molluscs
PHYLUM AND SUBPHYLUM - Mollusca,
CLASS AND SUBCLASS - Bivalvia,
ORDER AND SUBORDER - Veneroida,
FAMILY AND SUBFAMILY - Mactridae,
GENUS AND SUBGENUS - Tresus,
SPECIES AND SSP - capax,
SCIENTIFIC NAME - Tresus capax
AUTHORITY -
TAXONOMY REFERENCES - 01
Taxonomy - 1 (DRAFT) - Status
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
STATUS
Coded Status
Commercial
Commercial/consumption
Commercial/industrial
See Comments
REFERENCES FOR STATUS - 01
COMMENTS ON STATUS -
COMMERCIAL
Harvested commercially from N. California to British Columbia. Recent
harvests have averaged about 225 tons annually, placing them 5th in volume
for the entire U.S. and Canada Pacific Coast clam harvest. Species is taken
year round, but most are harvested from July to December in B.C. and Oregon.
Although the species is a large clam that provides excellent meat for
chowder or clam steaks, it is not often sold fresh. Instead, it is usually
canned because it has a fragile shell that breaks easily and its valves
gape, reducing shell life and allowing water loss. Also, a tough outer
covering on its neck increases processing/packaging time and meat yield
during processing is low (25-30% of total body weight). *01*
RECREATIONAL
The horseneck gaper is harvested recreationally from Humboldt Bay, CA to
Puget Sound WA. No more than 10/day can be taken in California, 12/day in
Oregon, and 7/day in Washington. It is harvested primarily by hand (using
shovels, rakes, etc.,) during low tides. *01*
Status - 1 (DRAFT) - Distribution
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
DISTRIBUTION
Distribution - 1 HABITAT ASSOCIATIONS
LAND USE -
Bays and Estuaries
REFERENCES FOR LAND USE - 01
NATIONAL WETLAND INVENTORY CODES
NWI NWICLS NWIMOD NWISPEC
Estuarine 2
Estuarine 4
Estuarine 2
Estuarine 4
REFERENCES FOR NWI - 01
COMMENTS ON HABITAT ASSOCIATIONS -
Predation can cause very high mortalities on some clambeds. High mortality
of small juveniles is probably due to low salinities, temperature stress
and predation. As they grow, horseneck gapers burrow deeper, escaping many
physical and biological stresses. Recruitment may be highly variable on
some clam beds, resulting in beds dominated by only 1 or 2 age classes. In
general, intertidal populations of this species are affected by numerous
alterations and disturbances, including siltation, storms, freshwater
runoff, floods, erosion, dredging and marine development. Diseases may also
affect horseneck gaper populations; it is often infected with a
haplosporidan parasite, (43% in Yaquina Bay, Oregon). Two species of
pinnotherid crabs are known to inhabit the mantle cavity of horseneck
gapers, but apparently cause little harm to the clam.*01*
ANIMAL/PLANT SPECIES ASSOCIATIONS -
PREDATION
Eggs and larvae are probably preyed on by many planktivorous organisms.
Predators of juveniles include: worms, snails, crustaceans, and copper
rockfish (Sebastes caurinus). Common predators of juveniles and adults
include moon snails (Polinices spp.), Dungeness crab (Cancer magister), bat
ray (Myliobatis californica) and sea stars (Pisaster spp). *01*
REFERENCES FOR SPECIES ASSOCIATIONS - 01
Habitat Associations - 1 (DRAFT) - Food Habits
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
FOOD HABITS
TROPHIC LEVEL -
FILTERER
REFERENCES FOR TROPHIC LEVEL - 01
LIFESTAGE FOOD FOOD PART
Adult Detritus Not Specified
Adult Phytoplankton Not Specified
Juvenile Detritus Not Specified
Juvenile Phytoplankton Not Specified
REFERENCES FOR ADULT FOOD - 01
REFERENCES FOR JUVENILE FOOD - 01
COMMENTS ON FOOD -
Juveniles and adults are suspension/filterfeeders. Food particles travel in
water through the inhalent siphon and are collected on the gills, sorted by
the palps and passed to the mouth. Energy reserves are stored as glycogen in
the gonads and as fat.*01*
Juveniles and adults feed on suspended diatoms, flagellates,
dinoflagellates, and fine detritus, including small eelgrass.*01*
Food Habits - 1 (DRAFT) - Environment Associations
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
ENVIRONMENTAL ASSOCIATIONS
REFERENCES FOR ENVIRONMENTAL ASSOC_ - 01
COMMENTS ON ENVIRONMENTAL ASSOCIATIONS -
Juveniles and adults are found primarily in bays and estuaries, occurring
from mid-tide levels (+2 m) down to 30 m below mean lower low water (MLLW).
In Puget Sound and Humboldt Bay, they are most abundant at depths 1-5 m
below MLLW.*01* The horseneck gaper is found primarily in substrates
consisting of shell fragments and dense sand, as well as silty-sand and
gravel. In Humboldt Bay, clam densities are greatest in silty-sand
substrates covered with eelgrass. Sediment structure affects burrowing
depth; clams burrow deeper in mud and sand substrates than in clay
substrates. *01* Juveniles and adults are found in polyhaline-euhaline
waters, at temperatures of 2-20 degrees C. Larvae do not survive at 20
degrees C. Optimum conditions for somatic growth are 13 degrees C water
temperatures (range 11-18 degrees C), 28% salinities (range 26-31%), and
food suspension density of 95 mg/l (range 15-200 mg/l). *01*
Eggs and larvae are pelagic. Juveniles and adults are benthic infauna,
burrowing into sediments to depths <= 1 m, but usually 25-50 cm.*01*
Environment Associations - 1 (DRAFT) - Life History
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
LIFE HISTORY
Migrations and Movements: Eggs and larvae are dispersed by currents.
Juveniles and adults do not move laterally once they become established.
Clams older than 2 years (77 mm shell length) lose the ability to reburrow.
Reproduction Mode: The horseneck gaper is gonochoristic, oviparous, and
iteroparous. It is a broadcast spawner, hence eggs are fertilized
externally.*01*
Mating and Spawning: Spawning begins when waters warm after the seasonal
minimum, usually late winter to early spring. In British Columbia and Puget
Sound, spawning occurs from February-May, peaking primarily in March. In
California and Oregon, spawning occurs from January-March, peaking in
February. The horseneck gaper may spawn more than once during the spawning
season. Fecundity: Unknown.*01*
Age and Size of Larvae: Larvae range from 0.06-0.7 mm to 0.26-0.27 mm in
diameter. Metamorphosis to spat takes 24 days at 15 degrees C., 26 days at
10 degrees C, and 34 days at 5 degrees C. Larval settlement occurs primarily
between early spring and summer.*01*
Juvenile Size Range: Juveniles range in size from 026-0.28 mm to about 70 mm
shell length. They may grow to 2.54 cm after 1 winter. Most growth occurs
during the spring and summer when phytoplankton is abundant.
Age and Size of Adults: Size appears to determine maturity; most horseneck
gapers mature at about 70 mm shell length (SL). In British Columbia, this
takes 4 years, but only 3 years in California and Oregon. In Oregon,
subtidal clams between the ages of 4 and 7 years grow faster than intertidal
clams of similar ages. The horseneck gaper can live to 16 years and can
reach 254 mm SL. The oldest clams found in Oregon were 10-12 years old.*01*
LIFE HISTORY CODES -
Foraging Strategy: Filtering
Breeding/Spawning Season: February
Breeding/Spawning Season: March
Breeding Spawning Season: April
Breeding/Spawning Season: May
Breeding/Spawning Season: January
Spawning Site: Flowing Water
Dispersion: Random
REFERENCES FOR LIFE HISTORY- 01
Life History - 1 (DRAFT) - Management Practices
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
MANAGEMENT PRACTICES
RESULT MANAGEMENT PRACTICE
Beneficial Maintaining undisturbed/undeveloped areas
Beneficial
Beneficial Developing/maintaining suitable salinity
Adverse Dredging
Beneficial Controlling sedimentation
Adverse Salinity alteration
Adverse Constructing/maintaining piers
Adverse Constructing/maintaining moring piles, dolphins, and bouys
Adverse Constructing/maintaining bulkheads, seawalls, and dikes
Adverse Constructing/maintaining jetties, groins, and breakwaters
Adverse Deposition of fill
Beneficial Predator control
REFERENCES FOR BENEFICIAL MANAGEMENT PRACTICES - 01
REFERENCES FOR ADVERSE MANAGEMENT PRACTICES - 01
COMMENTS ON MANAGEMENT PRACTICES -
Predation can cause very high mortalities on some clambeds. High mortality
of small juveniles is probably due to low salinities, temperature stress
and predation. As they grow, horseneck gapers burrow deeper, escaping many
physical and biological stresses. Recruitment may be highly variable on
some clam beds, resulting in beds dominated by only 1 or 2 age classes. In
general, intertidal populations of this species are affected by numerous
alterations and disturbances, including siltation, storms, freshwater
runoff, floods, erosion, dredging and marine development. Diseases may also
affect horseneck gaper populations; it is often infected with a
haplosporidan parasite, (43% in Yaquina Bay, Oregon). Two species of
pinnotherid crabs are known to inhabit the mantle cavity of horseneck
gapers, but apparently cause little harm to the clam.*01*
Management Practices - 1 (DRAFT) - References
Species horseneck gaper
Species Id M060010
Date 26 AUG 96
References
01 Emmett, R. L., S. L. Stone, S. A. Hinton, and M. E. Monaco. 1991
Distribution and abundance of fishes and invertebrates in west coast
estuaries, Volume II: species life history summaries. ELMR Rep. No. 8.
NOAA/NOS Strategic Environmental Asessments Division, Rockville, MD,
329p.
References - 1