Habitats & ecology
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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.  Aspects of competition are considered here, while BURROWING and HABITAT EFFECTS ON ABUNDANCE are presented in another section.

The following studies are arranged by major taxa, starting with shipworms, and in reverse alphabetical order (for no reason).

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Research study 1

photograph of a piece of wood riddled with shipworms Teredo navalisCompetitive interactions among clams mostly takes the form of competition for space, whether it be in sand, mud, rocks, or wood.  Competition for food, if it occurs at all, is unlikely to be measurable in those species that filter-feed, but may be assessed in ones that deposit-feed. Shipworms burrow into wood and, as seen in the accompanying photograph, competition for space, both intra- and interspecific, may be fierce.

A log filled with wood-burrowing shipworms, judging
by their size, likelyTeredo navalis. Bankia setacea is
much larger in size than Teredo navalis and has
greater calcification of its burrows

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Research study 2

drawing showing conditions inside a block of wood infested with shipwormsAn early study on wood-borers in San Francisco Harbor, California suggests that competition between shipworm species in the same piece of wood may be minimised by differences in life cycle.  In comparison with Teredo spp., Bankia setacea is apparently more specialised and less adaptable. Teredo navalis, one of the more common shipworms on the west coast, was introduced from the Atlantic coast to California in 1914.  It is a smaller species than its potential competitor, but is robust and fast growing. Additionally, because it incubates its eggs it ensures a larger proportion of successful larvae and has the potential to “swamp out” Bankia. The latter releases eggs into the water for fertilisation.  Bankia is thought to survive, in part, because of its earlier breeding season (Feb-May) than Teredo (Jul-Dec), which gives its newly colonising larvae a head-start to grow to a considerable size and depth in the timber before the larvae of Teredo arrive in late summer and autumn.  Nonetheless, only the larger and stronger  Bankia are able to survive the honey-combing onslaught of Teredo once it arrives. On the Right are drawings of the wood-boring Bankia setacea, singly and in a space-constrained block of wood. Miller 1926 Ecology 7: 247.

NOTE  Bankia setacea is essentially a cold-water species, occurring from northern California to Alaska. Optimal conditions for breeding in San Francisco Bay occur during the colder months of the year.  In comparison, Teredo navalis is more adaptable, but prefers the warmer months for breeding.  Teredo is also much more tolerant of low salinity than is Bankia, and can even withstand temporary exposure to fresh water

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Research study 3

Shipworms Bankia setacea burrow into wood and their burrows increasingly fill the available space.  When one burrower encounters another, it veers X-ray photograph of a piece of wood infested with shipworms Bankia setaceaaway, and a labyrinth is created within the wood.  Eventually, as illustrated by this X-ray view of 145 well-established individuals at about 9mo of age, available wood habitat is exhausted and further growth is impossible.  The substrate is now weak and friable.  Under such crowded conditions, size of the clams is about half what it is under uncrowded conditions.  Haderlie & Mellor 1972 Veliger 15: 265.

NOTE  it is not known what sensory input is used to detect neighbouring burrowers.  An individual could be sensitive to the calcareous lining of its neighbour’s tube, to diffusing chemicals, or even to rasping vibrations coming from next door

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Research study 1

photograph of a rock piddock Penitella penitaAnother bivalve that bores by mechanical means, in this case into rock, is the piddock Penitella penita.  It is also sensitive to crowding, which often occurs because more larvae settle on the rock surface than can be accommodated at depth.  During normal boring, accomplished by a back-and-forth rasping action of the shell valves, an individual senses the presence of neighbouring individuals and turns its burrow.  Often, only a millimeter of wall thickness remains between adjacent burrows.  Studies at Fossil Point, Oregon show that when competition for space is intense, a clam with insufficient room to burrow will change to pictorial representation of a rock with different ages of rock piddocks Penitella penita showing stultified growth in some individualsa stunted, reproductively useless adult.  The diagram shows several burrows 34mo after larval settlement, and next to this is a projection showing expected conditions after 47mo.  Some individuals are stunted (indicated in brown colour in the diagram), while others have shifted direction of their burrowing to avoid contact. After 47mo from settlement, 2 individuals have become reprodutively mature adults. Evans 1968 p.1 In, Proceedings of the International Congress on Marine Corrosion and Fouling, Athens.


A rock containing 34mo-old clams has about 50% of
the available surface area riddled at a burrow length of 7cm

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Research study 2

graph showing % occupation of space by rock piddocks Penitella penita over time

Shown in the graph is the maximum carrying capacity of a rock for piddocks Penitella penita at Fossil Point, Oregon.  If the data are extrapolated from the curves measured at 12 to 34mo from larval settlement, then at a projected time of 47mo the burrows would occupy almost 100% of the available space at a depth of about 8-9cm.  This so weakens the fabric of the rock that whole chunks may be washed away, leaving fresh surfaces for recolonisation. The author notes that such rock-boring clams may be a major erosive influence of sedimentary rocks.  The photograph of a piece of sandstone containing old burrows of rock piddocks Penitella penitalong-term effects of competition for space, namely stultification and possibly extinction of the species, are avoided by this erosion, which creates new surfaces for larval settlement.  Evans 1968 p.1 In, Proceedings of the International Congress on Marine Corrosion and Fouling, Athens.

Rocks riddled with burrows can be found
on any beach where Penitella is common

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Research study 1

histogram showing seasonal growth of several clam species with focus on potential competitive interactions between them
Do sand- or mud-dwelling clams compete for space?  Researchers at the Marine Science Institute, UC Santa Barbara, California do some field experiments to find out.  In Mugu Lagoon, California at least 3 species of deep-burrowing bivalves Sanguinolaria nuttallii (20-50cm depth), Tresus nuttallii (30-50cm depth), and Saxidomus nuttalli (25-50cm depth) are present, but with the first species not residing in close proximity with the other 2.  Other clams are also present, including the shallow-burrowing littleneck clam Protothaca staminea.  First, the researchers construct and install several wire-mesh enclosures in the clam habitat.  Into one set of enclosures they transplant some deep-burrowing Sanguinolaria nuttallii, hereafter called the “test species”.  Into other sets of enclosures they transplant equal numbers of the test species and shallow-burrowing Protothaca staminea.  Into other enclosures they transplant equal numbers of the 3 deep-burrowing species.  Into still others are placed individuals of the test species along with equal numbers of dead shells of the other 2 deep-burrowing species at their natural depths.  The enclosures are maintained over 2 growing seasons.  After each of 2 seasons all clams are dug up and measured for growth (see results in histogram). The researchers find no significant difference between growth of the test species Sanguinolaria nuttallii alone or with the shallow-burrowing P. staminea. There are, however, significant reductions in growth of Sanguinolaria nuttallii when it grows with the other 2 species or with their shells. Peterson & Andre 1980 Ecology 61:129.

NOTE Sanguinolaria nuttallii (the "test species", Tresus nuttallii, and Saxidomus nuttalli

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Our conclusion is that some sort of interaction between the deep-burrowing species is affecting the growth of Sanguinolaria nuttallii, but what sort? Consider these answers, then CLICK HERE to see explanations.

Food competition. 

Space competition. 

Feces/urine contamination. 

Oxygen depletion. 

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Research study 2

graph showing effects of crowding on growth and reproduction in clams Protothaca stamineaThe most abundant bivalve species in Mugu Lagoon, California are Protothaca staminea and Chione undatella.  Both are shallow-burrowing and suspension-feeding, and both inhabit sand and mud habitats.  The question arises as to the importance of competition for food and space on the population ecology of the 2 species.  A researcher at the Marine Science Institute, UC Santa Barbara, California uses a complex matrix design of shallow, subtidal field enclosures to assess both intra- and interspecific effects of density on growth, recruitment, reproductive effort, and survivorship in both species.  Results show that growth rates are significantly lower in both species as intraspecific densities increase (data shown only for P. staminea). Note also in the graph that higher densities also result in smaller gonadal sizes ( shown for P. staminea but also true for C. undatella).  In contrast to these results suggesting strong intraspecific competition, results for interspecific effects are mostly non-significant and are consistently smaller in magnitude.  The results suggest that both species are resource-limited, but only intraspecifically.  The 2 species seem to be ecologically segregated.  Because both species live at approximately the same depth, the author reasons that food is more likely than space to be the limiting resource.  If space were also limiting, then both intra- and interspecific effects should be evident.

Interestingly, in sand habitats the recruitment rate of Protothaca is not related to adult density, although some significant effects are evident in mud habitats.  Chione’s photograph of clam Protothaca staminearecruitment is much more variable than Protothaca’s, and is about an order of magnitude lower.  Survivorship of Chione is surprisingly high over the course of the 2yr study: 88-96%.  In comparison, survivorship of Protothaca is substantially lower than that of Chione (by 16-61%), in many instances as a response to higher intraspecific densities.  The study is broad in scale and presents much useful data on a subject that has been notably neglected by researchers, both on the west-coast and elsewhere.  Peterson 1982 Ecol Monogr 52: 437; for more on this subject see Peterson 1983 J Exp Mar Biol Ecol 68: 145.

NOTE  this species apparently is tropical/subtropical in distribution, and at Mugu Lagoon is at the northern limit of its range

NOTE  an ancillary experiment on predation using exclusion cages is also done, and is briefly considered elsewhere in the ODYSSEY: LEARNABOUT CLAMS: PREDATORS & DEFENSES: CRABS

Pacific littleneck clam Protothaca staminea 1X

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Research study 3

photograph of Manila clam Venerupis philippinarumEven though buried in mud, bivalves may not be completely protected from environmental vicissitudes such as extremely cold weather.  This is shown in observations of massive mortality of Japanese littleneck clams Venerupis philippinarum in Departure Bay, British Columbia following several days of -6 to -10oC temperatures coupled with low-tide cycles.  Dead clams show necrosis of gill tissues and abnormal clumping of the gill filaments.  Littleneck clams live close to the substratum surface (4-6cm) and would be more susceptible to freezing weather than, say, deeper-burrowing butter or gaper clams. Bower 1992 J Shellf Res 11: 255.

Shells of a Japanese littleneck
clam Venerupis philippinarum 0.25X

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Research study 4

photograph of varnish clams Nuttallia obscurata dug up from sand habitatAn investigation at the Bamfield Marine Science Centre, British Columbia of invasion dynamics of varnish clams Nuttallia obscurata (first appearance in local B.C. waters in 1991) uses a matrix-modeling approach to analyse growth, survival, and fecundity over a 2-yr period at 2 sites.  The sites are a quiet-water one in Saanich Inlet, B.C., first invaded in 1994, and a more wave-exposed one in Barkley Sound, B.C., invaded in about 2000.  Of particular interest is the life-history stage that has contributed most to differences in population-growth rates.  The data show that adult survival has the greatest impact on population-growth rates at the 2 sites.  Other site-specific differences relate to recruitment dynamics.  Clams at one site experience annual recruitment with high post-settlement mortality, while clams at the other site have episodic recruitment and lower post-settlement mortality.  The authors conclude, as found for other other marine populations, that removal of the largest breeding individuals will have the greatest impact on population growth of N. obscurata Dudas et al. 2007 Ecology 88: 2084.

NOTE  this technique identifies the life-history characteristics most crucial for population growth

NOTE  data analysis in this study is complex, and the reader is directed to the original paper for the fine details


Ten varnish clams Nuttallia obscurata removed
from a sand beach at Cortez Island, British Columbia

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Research study 5

photograph of quahogs Mercenaria mercenaria, taken on the east coastAn interesting study of quahogs Mercenaria mercenaria at Colorado Lagoon, Los Angeles County, California concerns a population that was thriving in the 1970s to its relatively recent demise.  Quahogs are a commercially important east-coast species. and were introduced intentionally into several west-coast areas for commercial purposes from as early as 1935.  Their appearance in the Lagoon dates from the 1950s.  From these origins the Lagoon population thrived, then died out en masse.  Interestingly, the native community of bivalves initially competitively displaced by the quahogs never recovered.  Six of 14 species identified by the authors in their study collections are non-indigenous, with one of these, the Manila clam Venerupis philippinarum, representing 88% of the 2,490 bivalves collected in 77 study plots.  Thus, instead of native species taking up the slack after the quahog die-out, the population remains dominated by another non-indigenous species.  As for M. mercenaria, the researchers think it is possibly now extinct on the west coast.  The authors discuss several possible reasons for the species’ demise in the Lagoon, including harvesting by humans, predation by fishes, and chemical changes to the water in the lagoon, but dismiss all for lack of convincing evidence.  Burnaford et al. 2011 Mar Biol 158: 1915.

NOTE  the authors cite distributional records from up and down the west coast indicating that the population at Colorado Lagoon may, in fact, have been the only population ever to have become established

NOTE  the authors cite several references to populations living in southern British Columbia, but comment that none is substantiated

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Research study 6

photograph of siphon tips of gaper clam Tresus capaxAlthough not relating to competition among clams as such, an observation by researchers at the Bamfield Marine Sciences Centre, British Columbia that invasive non-native seaweed Sargassum muticum can gain inroads into eelgrass beds by colonising siphons of native gaper clams Tresus capax is of interest.  The seaweed itself is thought to be a competitor of native eelgrass Zostera marina, and under some circumstances will come to dominate and displace resident eelgrass and other associated native seaweed species.  Interestingly, the seaweed’s morphology differs depending upon whether it grows on its normal rock substratum or on a clam’s siphon plates.  On the former it expresses a typical wave-exposed morphology, while on the latter it shows a sheltered-habitat morphology.  Imported with the Tresus  morph is an epibiont community of isopods, amphipods, worms, snails, and bay mussels The presence of S. muticum, moreover, facilitates later settlement of 2 other non-native tunicate species, Styela clava and Botrylloides violaceous onto its own foliage.  These and other longer-term effects may lead to eventual loss of eelgrass in the habitat.  White & Orr 2011 Mar Ecol Progr Ser 424: 87. Photograph of photograph of red alga Polysiphonia growing on siphon tips of gaper clam Tresus capax, courtesy Dave Cowles, Walla Walla Universityalgal-infested siphon tips courtesy Dave Cowles, Walla Walla University wallawalla.edu.

NOTE  the seaweed is thought to have been introduced to British Columbia in the 1940s along with Japanese oysters Crassostrea gigas, the latter for aquaculture use

NOTE  significant differences between the 2 morphs relate to overall length, holdfast size, basal-axis size, and extent of branching from holdfast

Top: protruding siphon tips of gaper clam Tresus
in an eelgrass bed 1X; Right: red alga
Polysiphonia sp. on the siphons of T. capax 1X

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Research study 7

Two introduced clam species, Manila Venerupis philippinarum and varnish Nuttallia obscurata, are highly abundant on beaches in the Strait of Georgia, British Columbia, yet never co-occur at high densities.  A detailed study on the reasons for this by a University of British Columbia graduate student show simply that the 2 species do better at different intertidal levels, varnish clams in the high intertidal levels and Manila clams in low intertidal levels.  However, if either species is translocated to the other species’ habitat at varying densities, no significant reduction in growth occurs in either species.  These results suggest an absence of both intra- and interspecific competition for food or space, at least, not in the adult stage.  Selective predation on either species also appears not to be an issue, at least, not over the 27wk duration of study.  Lum 2011 MSc Thesis, University of British Columbia 71pp.

NOTE  varnish clams are native to Japan and Korea, and were introduced to the west coast in the early 1990s likely via ballast waters.  Manila clams in B.C. likely originated from Japan in the 1930s along with shipments of Pacific oysters Crassostrea gigas

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