Habitats & ecology
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Habitat effects on abundance


Aspects of habitat effects on abundance are considered here, while BURROWING and COMPETITION are presented in other sections

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Research study 1

A study on gaper clams Tresus capax in Humboldt Bay, California provides information on recruitment, distribution, and common predators.  Recruitment is in early spring and there appears to be no correlation between density in a clam bed and density of recruits.  Distribution of Tresus is random at low densities and aggregated at high densities.  Predators are mainly moon snails Polinices lewisii, Dungeness crabs Metacarcinus magister, and ochre stars Pisaster ochraceusWendell et al. 1976 Cal Fish Game 62: 41.

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Research study 2

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 3

graph comparing densities of horse clams Tresus capax and T. nuttallii at different depthsFisheries scientists from the Pacific Biological Station, British Columbia investigate habitat preferences of gaper or horse clams Tresus capax and T. nuttallii in southern British Columbia.  Preferred habitats for both species are sand/mud/gravel, but depth preferences appear to be quite different (see graph).  Densities of T. nuttallii are greatest at about 3-5m depths (relative to Chart Datum, equivalent to MLLW level), while those of T. capax are greatest at about 0-1m depth.  Although these data are obtained from different regions and combined into a single graph here for convenience, they suggest that in areas with mixed populations (if these exist) the different depth preferences might tend to separate the 2 species, thus minimising the potential for interspecific competition. Campbell & Bourne 2000 J Shellf Res 19 (2): 933.

NOTE  mean lower low water, representing the average height of the lowest tide recorded at a site each day over a 19y period (updated roughly every 20y)

NOTE  study plots in the areas sampled for this graph are characterised by sand/broken shell substrata at all depths.  The data line for T. nuttallii represents a population in Ritchie Bay, British Columbia where T. capax  is virtually absent (<0.8% numerical representation), while the line for T. capax represents a population in Seal Island, B.C. where T. nuttallii is virtually absent (<0.25%).  The actual extent of intermixing at other sites is not mentioned by the authors, nor is the possible significance of absence of one species or the other at certain sites discussed 

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Research study 4

histogram comparing mortality of clams Nuttallia obscurata in relation to intertidal height and burrowing substratum over 24h periodTwo decades after its successful invasion of the west-coast the varnish clam Nuttallia obscurata has spread to, and is flourishing on, beaches from southern British Columbia to mid-Oregon.  It lives somewhat higher intertidally than other bivalve species and prefers sandy substrata to muddy ones.  The extent to which habitat plays a role in Nuttallia’s success is investigated by a researcher at Friday Harbor Laboratories, Washington who first tethers individuals to the sand surface at both high (+1.6m above MLLW) and low (+0.5m) intertidal levels, and establishes that crab predators Cancer productus regularly forage in both areas and readily eat the unprotected clams.  Comparably placed but unrestrained crabs that are able to bury normally suffer heavy predation only in the low intertidal area.  The researcher then transplants sediment within and between the intertidal sites and places unrestrained clams on these substrata for 1d.  Clams placed on low-intertidal mud substrata at both intertidal heights suffer >60% mortality, while those on high-intertidal loose-sand substrata suffer only 10% mortality.  The reason is that they are able to bury more deeply in sand than in tightly packed mud (by about 3cm).  Caged photograph of accumulation of shells of Nuttallia obscurata at Savary Island, British Columbiaclams at both intertidal levels suffer no mortality.  Results from these simple experiments show that physical features of a habitat can mediate the susceptibility of Nuttallia to a native predator and thus possibly control its invasive successByers 2002 Oecologia 130: 146.

NOTE  15cm-long garden stake, up to 40cm waxed dental floss, and superglue

NOTE  in a later paper the author compares abundances of 3 invasive bivalve species, including N. obscurata, Mya arenaria, and Venerupis philippinarum, in 3 marine-reserve sites versus 8 non-reserve sites, and finds no significant differences in overall abundances.  Byers 2005 Ecology 86 (2): 487.

Accumulation of dead shells of Nuttallia obscurata on
a sand beach at Savary Island, British Columbia 0.25X

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Research study 5

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 2yr 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


Varnish clams Nuttallia obscurata dug up from
a sand beach at Cortez Island, British Columbia

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Research study 6

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 7

map of study sites in Puget Sound used in clam recruitment/survival studyAn axiom of population dynamics of benthic-marine invertebrates is that density and distribution of adults is linked, or coupled, to post-recruitment survival.  That makes sense.  When it doesn't work that way, one looks to processes such as predation or competition, or to temperature or other non-biological stresses to explain the decoupling.  This is what a group of researchers at the University of Washington and Washington Department of Natural Resources has done for clam populations Puget Sound, Washington, at 2 inner, more estuarine, sites and one outer, more marine, site (see map; note that there are 3 beaches per site).  On finding that juveniles and adults are more rare at the inner estuarine sites than at the outer site and suspecting predator involvement, the researchers implant plastic 15cm-diameter mesh-covered plastic cylinders at all sites that allow normal settlement of clam larvae, but exclude all but the smallest predators (>1cm).  Open pipes serve as controls.  Results after 4mo in summer show significantly greater abundance of clam recruits in the predator-exclusion treatments than in the controls.  Later analyses of the results, however, reveal that, rather than predators, it is position along the estuarine gradient that has the greater negative influence on clam assemblages in the plastic tubes.  The operative factor is conjectured by the authors not to be salinity (other experiments show that growth of the clams is actually greater in the inner part of the estuary than in the outer) but, rather, physical stress of higher summer temperatures on the inner beaches. Dethier et al. 2012 J Exp Mar Biol Ecol 422/423: 48; see also Dethier 2010 Mar Ecol Prog Ser 410: 43 and Dethier & Schoch 2005 Estuarine, Coastal & Shelf Sci 62: 253.

NOTE  the authors remind us of the importance of predators in the survival of juvenile clams by noting the widespread use of netting on grow-out beaches by culturists

NOTE  12 species are involved, of which the following are most abundant: Rochfortia tumida, Macoma inquinata, Clinocardium nuttallii, Leukoma staminea, and Tellina modesta

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Research study 8

With all the recent attention on molecular methodologies, such as genomics and transcriptomics, being used in marine biological studies it is refreshing to find a “boots-on-the-ground” approach being used to answer a fundamentally important question relating to recruitment. The question addressed by a group of Washington and Oregon scientists is whether present distributions of Manila clams Ruditapes philippinarum in Willapa Bay, Washington owe to settlement preferences or differential post-settlement mortality. The basic experiment uses a randomised-block design with 4 different starting substrate types (natural sand, sand + shell, crushed clam-shells 40% vol, natural rock (1cm pebbles), and rock + shell, contained in small mesh baskets buried flush in sand/mud substratum. Variations on this design create 4 experiments that span all habitat types in the Bay through one complete reproductive period. Settlement and survival of clam recruits are monitored at beginning and end, or sometimes throughout the durations, of each experiment. As expected, results vary greatly among the plots, but generally show that the main influence on post-settlement distributions is habitat-specific mortality and not settlement preferences of the larvae. Settlement occurs evenly throughout June-Sept and is generally insensitive to substratum characteristics. In contrast, post-settlement survival is significantly enhanced in plots containing crushed rock (= gravelling). The authors speculate that this material may exclude motile predators such as crabs and birds, and therefore may be beneficial for aquaculturists to use. In addition to the practical aspect just mentioned, the study adds useful insight into the age-old debate over importance of settlement preference vs. post-settlement mortality as determinants of distributions. Ruesink et al. 2014 J Exp Mar Biol Ecol 459: 23.

NOTE another non-native species, the soft-shell clam Mya arenaria, is also included in the study but, as similar results are obtained for both species, only Manila clams are considered here. Note also the name change of Manila clams from that used in Research Study 2 above

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Research study 9

map showing distribution and density of geoducks Panopea generosa in Hood Canal, WashingtonAn interesting and useful study by a group of 8 researchers associated with the University of Washington determines habitat preferences for geoducs Panopea generosa in Hood Canal, Washington (see Research Study 6 above for its location). Tethered cameras towed behind boats (used for sites deeper than safe SCUBA depth: 20-50m) or visual observations by transect-tracking SCUBA divers (sites at <20m depth) provide distributional data for 87 locations over a 3yr period that later histogram showing substratum preferences of geoducks Panopea generosa in Hood Canal, Washingtonare correlated with known seasonal patterns of dissolved oxygen and with substratum type. Results for densities along the canal are shown in the illustration on the Left and sediment preferences in the histogram on Right. Preferred depths are 5-20m; at depths deeper than 30m, densities fall off to essentially zero (<99.5% of maximum values). Densities are generally higher in the northern section of the Canal and lower in the southern, a pattern thought by the authors to correlate with generally lower seasonal levels of dissolved oxygen in the southern areas. For geoducks in Hood Canal, habitats with loosely consolidated sand seem to be most preferred of the ones available. The authors suggest that their novel sampling methods combined with special Bayesian-habitat analytical models, may provide other researchers or commercial harvesters a new, less expensive, less labour-intensive, and less habitat-destructive methodology for assessing geoduck densities. McDonald et al. 2015 J Shellf Res 34 (1): 137.

NOTE the SCUBA divers count all visible siphons along vertical transects originating at the deepest depth of about 20m. These records are then corrected for proportion of “no-shows”, that is, individuals present in their burrows but with siphons withdrawn. This correction factor, known as the “show-factor multiplier”, is determined weekly from data from similar SCUBA-transects done at nearby government-maintained sites containing known densities of geoducks

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