Roger Steeb, USA
Info
Tresus capax (A. Gould, 1850)
Tresus capax is a marine mussel in the family Mactridae. Among other things, it also has the colloquial name horse mussel like the Pacific horse mussel Tresus nuttallii, a species that is similar to it in morphology and lifestyle. Both species are somewhat similar to the geoduck (Panopea generosa, which belongs to the family Hiatellidae), but are smaller, with shells up to 20 cm long and weighing 1.4 to 1.8 kg.
The two species live in the intertidal zones of the Pacific coast: the Pacific clam, Tresus nuttallii, which is more common south of California; and the fat clam Tresus capax, which is more common north of Alaska. Both have oval and chalky white or yellow shells with brown spots of periostracum (leathery skin) on the shell. These clams are also popularly known as gaper clams because their shells are flared around the siphon and do not close completely, similar to geoduck clams. Like geoducks, they are unable to fully retract the siphon into the shell, although less conspicuously since the siphon is not as large in Tresus species.
Although it is possible to distinguish between the two Tresus clam species, it is very difficult for laypeople.
Their habitat is the lower intertidal zone to depths of 30m. They prefer sand, mud, and gravel substrates and typically bury 12-16 inches into the ground, making them much easier to excavate than geoducks. Their preferred substrates are also preferred by butter clams and neck clams, so horse clams are often included in the commercial harvest.
Sea clams often have a relationship with the small commensal pea crabs (Pinnixa faba), which often enter as a pair through the large siphon and live in the mantle cavity of the sea clam. Pea crabs have no negative impact on the mussels.
Tresus nuttalii spawns in summer and Tresus capax in winter.
The mussels are also eaten in their distribution area. Valued by local pre-contact Native Americans for their size, abundance, and relative ease of capture, they are now less sought after than geoducks, which have achieved marketing cachet.
Some clam collectors find that these gaping clams are not as tasty as others, so it is not uncommon to find specimens left on the beach during clam season. The shells are more fragile than they appear at first glance. Therefore, it is important that they are not damaged when they are first dug up. The mussels will die soon. Adult clams are unable to burrow back in because they need the pressure of their environment to stay intact and move. They cannot hold their two large valves together to protect their soft tissues. Responsible collectors carefully bury them again to approximately the depth at which the shells were found.
Prehistoric use of gaping clams is known among Native Americans on California's Pacific Coast. For example, archaeological finds at Chumash sites in San Luis Obispo County have revealed the use of open clam shells as shoveling implements. An unusually well-decorated example was found during archaeological excavations in what is now the city of Morro Bay.
Synonymised names:
Lutraria capax A. Gould, 1850 · unaccepted (original combination)
Lutraria maxima Middendorff, 1849 · unaccepted (invalid: junior homonym of...)
Schizothaerus capax (Conrad, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii capax (A. Gould, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii var. capax (A. Gould, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii var. maxima (Middendorff, 1849) · unaccepted > junior homonym (non Lutraria maxima Jonas, 1840)
Tresus capax is a marine mussel in the family Mactridae. Among other things, it also has the colloquial name horse mussel like the Pacific horse mussel Tresus nuttallii, a species that is similar to it in morphology and lifestyle. Both species are somewhat similar to the geoduck (Panopea generosa, which belongs to the family Hiatellidae), but are smaller, with shells up to 20 cm long and weighing 1.4 to 1.8 kg.
The two species live in the intertidal zones of the Pacific coast: the Pacific clam, Tresus nuttallii, which is more common south of California; and the fat clam Tresus capax, which is more common north of Alaska. Both have oval and chalky white or yellow shells with brown spots of periostracum (leathery skin) on the shell. These clams are also popularly known as gaper clams because their shells are flared around the siphon and do not close completely, similar to geoduck clams. Like geoducks, they are unable to fully retract the siphon into the shell, although less conspicuously since the siphon is not as large in Tresus species.
Although it is possible to distinguish between the two Tresus clam species, it is very difficult for laypeople.
Their habitat is the lower intertidal zone to depths of 30m. They prefer sand, mud, and gravel substrates and typically bury 12-16 inches into the ground, making them much easier to excavate than geoducks. Their preferred substrates are also preferred by butter clams and neck clams, so horse clams are often included in the commercial harvest.
Sea clams often have a relationship with the small commensal pea crabs (Pinnixa faba), which often enter as a pair through the large siphon and live in the mantle cavity of the sea clam. Pea crabs have no negative impact on the mussels.
Tresus nuttalii spawns in summer and Tresus capax in winter.
The mussels are also eaten in their distribution area. Valued by local pre-contact Native Americans for their size, abundance, and relative ease of capture, they are now less sought after than geoducks, which have achieved marketing cachet.
Some clam collectors find that these gaping clams are not as tasty as others, so it is not uncommon to find specimens left on the beach during clam season. The shells are more fragile than they appear at first glance. Therefore, it is important that they are not damaged when they are first dug up. The mussels will die soon. Adult clams are unable to burrow back in because they need the pressure of their environment to stay intact and move. They cannot hold their two large valves together to protect their soft tissues. Responsible collectors carefully bury them again to approximately the depth at which the shells were found.
Prehistoric use of gaping clams is known among Native Americans on California's Pacific Coast. For example, archaeological finds at Chumash sites in San Luis Obispo County have revealed the use of open clam shells as shoveling implements. An unusually well-decorated example was found during archaeological excavations in what is now the city of Morro Bay.
Synonymised names:
Lutraria capax A. Gould, 1850 · unaccepted (original combination)
Lutraria maxima Middendorff, 1849 · unaccepted (invalid: junior homonym of...)
Schizothaerus capax (Conrad, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii capax (A. Gould, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii var. capax (A. Gould, 1850) · unaccepted > superseded combination
Schizothaerus nuttallii var. maxima (Middendorff, 1849) · unaccepted > junior homonym (non Lutraria maxima Jonas, 1840)
