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| Habitats & ecology | |||||||||
Studies on burrowing are considered here, while COMPETITION is presented in another section. Butter clam Saxidomus gigantea exposed in its burrow.
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| Burrowing | |||||||||
| This section on burrowing is divided into subsections on soft substrata and hard substrata. | |||||||||
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Soft substrata |
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Research study 1 |
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Research study 2 |
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In comparison, Siliqua patula lives in impermanent burrows on wave-exposed sandy beaches, and its burrowing ability is more enhanced. The pattern is similar to that described for Solen, starting with foot extension in an anterior-ventral direction. The thrust is rapid and drives the foot into the sand. The tip of the foot is fringed and highly expansible (see schematic on Left). The tip expands to form an anchor and contraction of foot-retractor muscles pulls the shell towards the anchored foot. This first rapid thrust places the body well into the substratum, and only 1-2 more cycles are required for complete burial. The author’s data show that a 3cm shell-length individual can bury completely in 7sec, while an 8cm individual can bury in 27sec.
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Research study 3 |
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Cockle siphons are well endowed with sensory tentacles, and other |
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Research study 4 |
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Research study 5 |
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NOTE the authors sample 11,154 geoducks from 6 major zones in Puget Sound
Siphons of a geoduck Panopea abrupta positioned in front |
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Research study 6 |
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These siphon tips originate from 3 geoducks Panopea abrupta whose bodies may be as deep as 1m. Based on yield from commercial harvesting in British Columbia between 2000-2003, some 30-40 million dollars is added yearly to the provincial economy. Harvest of geoducks is by hydraulic blasting that creates massive “peripheral” damage to the local habitat.
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Research study 7 |
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Littleneck clams Protothaca staminea in San Juan Island, Washington inhabit sand/mud shores in the usual clam-like way, in burrows, but may also be found in in other shore regions lying openly on pebbles and cobbles on one or other of the shell valves. This circumstance is of interest NOTE the study area is at Argyle Creek, where the 2 habitats are located within 20m of one another
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Hard substrata |
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Research study 1 |
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We commonly think of clams as sand- or mud-burrowiing organisms, but shipworms and piddocks bore into wood and rock, respectively. Bankia setacea is a common wood-borer on this coast and causes much damage to unprotected submerged wood. Maximum settlement of veliger larvae in Monterey Note that at about 5mo age two things happen. One, the clam digs a short side-branch, possibly |
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Research study 2 |
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Bivalves such as Penitella conradi parasitically drill into the shells of other molluscs, such as red abalone Haliotis rufescens. The bivalve settles as a veliger larva onto the shell of the abalone, metamorphoses, and begins drilling. The anterior edges of the shell valves bear prominent teeth, suggestive of predominantly mechanical action, but their role in drilling appears to be secondary. Most of the “drilling” is reported to be via secretion of chemicals, likely shell-dissolving carbonic-anhydrase enzymes. Rotational movements of shell likely loosen the shell crystals and fragments of the organic matrix of the shell, which are then gotten rid of via the exhalent siphon of the clam. Measurements in California show boring rates of Penitella of about 20um per day. When the burrow breaks through the abalone shell, the “host” immediately plasters over the opening, first with regular soft crystals of calcium carbonate, then with harder nacre. These create the blister pearls so characteristic of the inner shell surface of abalones, especially those of H. NOTE Fr. word for “mother-of-pearl”. While the main portion of an abalone’s shell is made up of larger, soft crystals of calcium carbonate (much like in a clam shell), the nacre is comprised of multiple flat layers of harder crystals. The multi-hued and variable colouration of nacre owes to differential penetration and re-emission of light rays. The light is broken up, or diffracted, and some wavelengths are absorbed while others are emitted at different wavelengths. This creates the colours of an abalone shell, which change with angle of viewing, and also the colour and luster of pearls
Inside and outside views of an infested |
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Most clams burrow into sand or sand-mud and live buried at a depth dictated by the length of their siphons. Competition between clams is not well studied for west-coast species, but would be expected to be 
exploitative, not interference.
Observations on razor clams Siliqua patula in Pacific Grove, California show that the foot is disproportionately longer than in other species, commensurate with a good ability to burrow. Associated with this is a pronounced pedal gape that allows the foot considerable ventral motion. Shell growth is primarily in a posterior direction. This, combined with a smooth proteinaceous covering or periostracum on the shell, greatly increases streamling and burrowing ability. 
A similar pattern to that described above for general bivalves, also applies to burrowing of razor clams on beaches near San Diego, California. Solen rosaceus lives in permanent burrows in mud flats. Burrowing starts with foot extension into the substratum, expansion of the tip to form a bulbous-shaped anchor, and contraction of the foot-retractor muscles (see schematic upper Right). This raises the shell and draws the 
body into the sand towards the anchor. The cycle is repeated. 
Interestingly, there is a change in shell shape during growth, as shown here for 2.5mm and 8cm individuals (see drawings lower Right). Although scaling relationships suggest that it would be more functionally advantageous for the small, younger stage also to have a sleeker body design, perhaps absolute burying speed is more important. The author notes that young clams are able to bury themselves much more quickly than the older, despite costs being relatively higher based on their less streamlined shell shape. 
The small, thin-shelled clam Cryptomya californica commonly lives with ghost shrimps Neotrypaea californiensis embedded in the walls of the shrimp’s burrow (see drawings). Adaptations for this life include extremely short siphons, which may enable it to live within the burrow system without risk of having its siphons nipped off by the shrimp, and a relatively large foot and associated pedal musculature, perhaps enabling fast burrowing. Cryptomya may be found as deep as 50cm within the shrimps’ burrows. Studies on animals collected at Yaquina Bay and Coos Bay, Oregon show that their food is suspended detrital material, including bacteria and 
diatoms. There is a prominent crystalline style and, additionally, an abundant flora of spirochete bacteria Cristispira sp. in the stomach and matrix of the crystalline style. The author suggests that the bacteria may help with food digestion and, after death, be digested by the clam. 
Throughout 
to a researcher from the University of Buenos Aires, Argentina who studies the condition of the shells from a taphonomic point of view, specifically, how the shells are modified differently in the 2 habitats and how these changes may be interpreted after their later fossilisation. Results, in short, show that the infaunal burrowers suffer much less shell damage than the ones lying open on the gravel, and indicate that damage on fossilised shells, even of the same species, may not necessarily be incurred post-mortem, but may represent effects incurred during an animal’s lifetime in a certain habitat. 
Bay is during winter, and burrowing rates can exceed 10cm per month at temperatures >10°C. The composite X-ray photos show burrow development in a 1month-old Bankia in a piece of Douglas-fir panel floating in Monterey Harbor just below the sea surface. 
rufescens.