Habitats & ecology

photograph of a horse clam Tresus sp. spurtingMost clams burrow into sand or sand-mud and live buried at a depth dictated by the length of their siphons.  Topics in this varied section deal with burrowing, competition both intra- and interspecific, and habitat effects on distribution and abundance. Competition between clams is not well studied for west-coast species, but would be expected to be photograph of butter clam Saxidomus gigantea in its burrowexploitative, not interference, and to be mostly for space, not food.

Studies on burrowing are considered here, while COMPETITION and HABITAT EFFECTS ON ABUNDANCE are presented in their own sections.

Butter clam Saxidomus gigantea exposed in its burrow.
This small individual is at a depth about 1.5X its shell length,
but larger individuals burrow much deeper than this 0.33X

Horse or gaper clam Tresus sp. spurting at low tide.
The clue to the genus of the spurter is in the horny
lappets at the tip of the siphon pair 0.5X

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  This section on burrowing is divided into subsections on soft substrata and hard substrata.
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Soft substrata

The burial process in soft substrata varies with species, but generally involves cyclical repetition of the stages shown below.  Drawings modified from Ansell & Nair 1969 Am Zool 9: 857.
first of a series of drawings showing process of burrowing in a clam

1. adductor muscles relax, springy hinge opens the valves, creating a penetration anchor

2. contraction of body musculature forces fluid into the foot, which extends it downwards

3. contraction of muscles at the foot’s tip creates a penetration wedge into sand

NOTE  lit. “to/toward/lead” L., referring the fact that contraction of these large muscles pulls the shell valves together.  This force antagonises the natural springiness of the hinge ligament which continuously acts to force open the valve

second of a series of drawings showing process of burrowing in a clam

4. adductor muscles contract and relax several times, forcing water from the mantle cavity and “puddling” the sand around the valves

5. muscles at the tip of the foot relax, hemolymph is pumped in, and a terminal anchor is created

NOTE the foot is extended hydraulically by the fluid being forcibly pumped into large hemocoelic spaces within it. Studies in Victoria, British Columbia  of cardiovascular pressures in several bivalve species, including Tresus spp., reveal that the heart is too weak to contribute to siphon (or foot) extensions.  Smith & Davis 1965 J Exp Biol 43: 171.

third of a series of drawings showing process of burrowing in a clam

6. foot retractor muscles now contract, pulling the body downwards towards the now anchored foot

7. the cycle repeats until suitable depth is reached

8. Once buried, most clams have no ability for quick movement of the body.  Water spurts seen in a clam-bed represent water ejected from the siphons when they are quickly pulled down, either for defense or to eject sediments and pseudofeces. 

NOTE  the retractor muscles function normally to pull the foot back into the shell. However, with its tip anchored and the sand "puddled", contraction of these muscles now pulls the body of the clam towards the foot

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Research study 1

drawing of razor clam Siliqua patula to show main body featuresphotograph of razor clam Siliqua patula on sand, courtesy Dave Cowles, Walla Walla University, WashingtonObservations 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. Yonge 1952 Univ Calif Publ Zool 55: 421. Photograph courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

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Research study 2

drawings to show burrowing in razor clams Solen rosaceusA 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 schematic showing burial into sand by razor clam Siliqua patulabody into the sand towards the anchor.  The cycle is repeated. 

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. 

comparison of shell shapes in juvenile and adult razor clams Siliqua patulaInterestingly, 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.  Pohlo 1963 Veliger 6: 98.

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Research study 3

photograph showing close view of siphons of a cockle Clinocardium nuttallii
Cockles Clinocardium nuttallii have short siphons and for that reason live close to the sediment/water interface.  It is reported that they have a green-algal symbiont, possibly Chlorella sp. in their mantle tissues.  Although the symbiont is similar in morphology to Chlorella found in sea anemones, it is unclear whether it makes a similar photosynthetic contribution to its host.  Cooke 1975 Phycologia 14: 35.

Cockle siphons are well endowed with sensory tentacles, and other
tentacles are abundant along the mantle edges. Clinocardium nuttallii
has a rapid and dramatic escape behaviour, cued no doubt by stimulation
of these tentacles. The ventral, or inhalent, siphon is on the Left 3X

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Research study 4

drawing of clam Cryptomya californicaThe 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 drawing of a mud-shrimp burrow Neotrypaea californiensis showing locations of clams Cryptomya californica withindiatoms.  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.  Lawry 1987 Veliger 30: 46; drawing of clam from Yonge 1952 U Calif Pub Zool 55: 395.

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Research study 5

photograph showing geoduck Panopea abrupta siphonsThroughout Puget Sound, Washington geoducks Panopea generosa prefer to live in sand/mud habitats at around 20m depth.  Densities of geoducks at this depth in Case Inlet, Washington are 5-10 times greater than at shallower or deeper depths.  Largest sizes are realised in habitats of sand and mud/sand (139 and 137mm shell length, respectively, as compared with 130 and 129mm in habitats of mud and pea-gravel, respectively).  Goodwin & Pearse 1991 J Shellf Res 10: 65.

NOTE  the authors sample 11,154 geoducks from 6 major zones in Puget Sound



Siphons of a geoduck Panopea generosa positioned in front
of a coralline-alga encrusted beer bottle. Exhalent and
inhalent siphons are difficult to determine in geoducks

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Research study 6

photograph of siphons of 3 geoduc clams Panope abrupta at the sediment surface

These siphon tips originate from 3 geoducks Panopea generosa 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.  Campbell et al. 2004 J Shellf Res 23: 661.



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Research study 7

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 photographs of littleneck clams with varying types of shell damage/growthsto 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.  Lazo 2004 Palaios 19 (5): 451.

NOTE  the study area is at Argyle Creek, where the 2 habitats are located within 20m of one another

Live littleneck clams Protothaca staminea showing
different degrees of shell blemishing 0.5X


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Research study 8

drawings of juvenile geoduck Panopea generosa burying itselfGeoducks Panopea spp. are the world’s largest burrowing bivalves, and as adults in sandy/muddy sediments usually dwell at depths between 60-100cm. A group of Baja California and Washington researchers describe burrowing of juvenile P. generosa from sizes of 10mm shell length through to 21mm shell length over about 6mo in the laboratory. The digging process in mud is quite interesting, involving an initial vertical positioning of the shell using the foot and a rocking motion of the shell (see drawings). The siphon is then cyclically inflated and emptied, with the water being forcibly expelled through the gap in the shell and mantle where the foot emerges. Note in the drawing on the Left that the authors have indicated that filling of the siphon is via both inhalent and exhalent siphons (blue arrows), but the latter is questionable because one guesses it would involve cilia-reversal, something not discussed by the authors. The expulsions puddle the mud and soften it, allowing the foot to dig deeper into the sediment. The authors do not report specifically on the use of pedal anchors by these burrowing juvenile geoducks, just that they do anchor.

histogram showing effects of size on burrowing rates in juvenile geoduck clams Panope generosaOlder, 2-times larger animals appear to burrow at a rate a little greater than twice that of younger ones (see histogram). Such linearity would not be predicted by scaling relationships, however, and the authors could have made a more interesting contribution had they incorporated exponential scaling aspects of length/surface area/metabolism into their research protocol. Tapia-Morales et al. 2015 J Shellf Res 34 (1): 63.

NOTE the authors actually compare burrowing behaviour of this species, which occurs from southeast Alaska to about halfway down the Baja Peninsula on the Pacific side, with that of P. globosa, which occurs throughout the Gulf of California around the southern end of the Peninsula. However, as the burrowing methods are similar in the 2 species, the researchers provide just a generalised description for Panopea spp. (see drawings)

NOTE a graph of burrowing rate over shell length that could have been helpful in this regard is actually provided by the authors, but they have used arithmetic axes instead of logarithmic, perhaps unaware that that a rate function would not scale linearly with a body-dimension function. In the authors' defense they do suggest that the apparent linearity seen may only apply over the range of sizes studied. Two other comments can be made regarding these data. The first is that the Y/X plot just referred to is just a different iteration of the same data used in the bar graph shown above; in other words, the authors have plotted the same data twice. Also, although the 2 presentations should be the same, they are not, as though some data have been added or dropped. Another comment relates to the statistical analysis used, a one-way ANOVA. This would be fine except for the fact that it assumes independence of the data, which is not the case because the data are from the same individuals tracked over 5 sizes (ages). A more appropriate test would have been a repeated-measures ANOVA or perhaps even a series of dependent t-tests (much less robust than ANOVA, however)

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Hard substrata

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Research study 1

We commonly think of clams as sand- or mud-burrowing 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 photograph of several shipworms within a piece of wood taken with X-ray imageryryBay 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.  Haderlie & Mellor 1972 Veliger 15: 265.

Note that at about 5mo age two things happen. One, the clam digs a short side-branch, possibly
encounters the wood surface, and abandons it. Second, when it encounters the opposite wood
surface it veers away, and the burrow doubles back on itself. The clam lines its burrow with a
layer of calcium carbonate, but this process lags up to 20cm behind the front of the burrow,
so the clam can readily change direction. The dark objects in the photos are the clam's shell valves

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Research study 2

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. photographs of inside and outside of an abalone shell Haliotis rufescens to show drilling activity of parasitesrufescens. Smith 1969 Am Zool 9: 869.

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 lustre of pearls


Inside and outside views of an infested
Haliotis rufescens shell 1X

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