title for learn-about section of A SNAIL'S ODYSSEY
  Feeding & growth
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  Growth
 

The topic of growth is presented in this section, while topics of FEEDING, and EFFECT OF PARASITES ON GROWTH & SURVIVAL are considered in other sections.

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Research study 1
 

graph showing growth of sea mussels Mytilus californianus over the first 4 years of life in La Jolla, California
Some statistics on growth of Mytilus californianus at Scripps Institution of Oceanography, La Jolla, California are shown here.  Approximate mean lengths during the first 4yr of life are 70mm for the 1st year, 105mm for the 2nd year, 125mm for the 3rd year, and 130mm for the 4th year. Growth during subsequent years is relatively small.  Other data on shell sizes, including volumes and ratios of various shell dimensions, are provided by the authors but not presented here.  Coe & Fox 1942 J Exp Zool 90: 1.

NOTE the means are based on measurements of more than 1000 individuals

 
Research study 2
 

graph showing growth of sea mussels Mytilus californianus at 2 locations around La Jolla, Californiaphotograph of pier at the Scripps Institution of Oceanography, La Jolla, CaliforniaGrowth is highly dependent upon location, both height in the intertidal zone, and degree of water movement.  As an example of the latter, note the much greater shell growth of Mytilus californianus living on rock as compared with pier habitat at the Scripps Institution of Oceanography, La Jolla, California.  The authors address the question of calcium source for shell growth.  Their calculations show that a 100-mm mussel of 1.5yr of age could obtain no more than 60mg calcium from a year’s feeding on dinoflagellates, yet its shell increases by 17g calcium from 1-2yr of age.  The annual volume of seawater filtered by a 100-mm length mussel is estimated by the authors t be 22,000 liters, which would contain about 9kg calcium. Thus, if seawater were the source, the mussel would require to extract only 0.2% of the calcium available to support a year’s worth of shell growth.  Fox & Coe 1943 J Exp Zool 93: 205.

 
Research study 3
 

histograms showing growth of mussels Mytilus californianus in relation to various environmental factors over the year 1941In a later paper the same authors investigate in more detail temperature effects and food supply on growth rates in Mytilus californianus.  A number of wire-screen boxes containing between 100-400 individuals is kept continually immersed on the pier at the Scripps Institution of Oceanography at La Jolla for a 44mo period.  Seawater temperatures are recorded monthly and water samples taken for phytoplankton- and bacterial-content analyses.  The authors provide data for each of the 4 years but, as the clearest set is that for 1941, it will be presented here.   The authors note that best growth occurs in the springtime months when water temperatures are warming and dinoflagellate concentrations high, but also reiterate their belief that it is not the live organisms being eaten but, rather, the organic debris left by their passage.  Neither diatoms nor bacteria seem much consumed.  When all 4 years of data are considered, maximum feeding and growth occur at temperatures of about 20 and less so at higher temperatures of 24-28oC.  Ingestion and fecal discharge can still be observed at temperatures down to 7-8oC.  Gut dissections confirm the authors’ previous assertions that M. californianus is primarily a scavenger, utilising as food not only small unicellular organisms and dissociated cells, but also the particulate disintegration products of plants and animals that die in the vicinity or are carried to the mussels from a distance by currents.  Coe & Fox 1944 Biol Bull 87 (1): 59.

NOTE  substances commonly present in the guts include finely divided organic detritus, dinoflagellates and other flagellates, diatoms, bacteria, ciliates and other protists, algal cells and algal fragments, spores, sperm and ova (including from mussels), and inorganic particles (sand and shell bits)

 
Research study 4
 

graph comparing pumping rates in mussels Mytilus californianus at different latitudesIf growth rates in mussels Mytilus californianus show latitudinal compensation to temperature (see Research Study below), then it is likely that pumping rates would show similar latitudinal compensation, and this is indeed so.  Note in the accompanying graph that pumping rates in mussels collected from 3 locations separated by 14o latitude are comparable when measured at the minimum annual temperature found at each location, indicating latitudinal compensation. Other data show that mussels from Friday Harbor, Washington have the same rate of pumping at 6.5oC as ones from Fort Ross at 10oC and Los Angeles at 12oC. Rao 1953 Biol Bull 104: 171.

NOTE  the locations are Los Angeles, Fort Ross, and Friday Harbor, with minimal temperatures of  10oC, 9oC, and 5oC, respectively

 
Research study 5
 

graph showing growth rates of mussels Mytilus californianus at sites in southern California and Alaska
A comparison of summer growth rates in populations of Mytilus californianus both latitudinally between southern California and southern Alaska, and vertically between 0.3 and 1m tidal height at Los Angeles, shows that differences in growth are almost completely accounted for by differences in duration of submergence. For example, the data point in the graph labelled "La Jolla, CA" represents mussels that are totally submerged.  The data point at 0.3m tidal height represents mussels submerged about 90% of the time or 21h per day, while the one at 1m represents mussels submerged about 60% of the time or 14h.  Finally, the point at 1.7m for Alaska represents mussels submerged about 40% of the time or 10h. 

Estimated filtration rates of the California mussels are almost double that of Alaska mussels (2 liter . h-1 vs. 1 liter . h-1, respectively).  This means that California mussels at the 1-m level are filtering about 2.8 times more food than those at 1.7-m tidal level in Alaska per day.  This compares quite well with the 2.1 growth ratio between the 2 populations and suggests that if the mussels had been sampled from tidal levels with comparable submergence times, then growth rates would be comparable despite the lower temperatures in Alaska.  The author concludes that growth in the northern population has acclimated to the lower temperatures found there.  Dehnel 1956 Biol Bull 110: 43.

NOTE growth is measured in situ by scratching lines on shells and measuring them again 30d later

NOTE the mean difference in surface-seawater temperatures between California and Alaska in summer is about 7oC but, as noted, this appears to account for much less of the variability in the growth data than submergence time.  The author does not explain why a 1.7m height is selected for the Alaska mussels, rather than a 0.3 or 1.0m height for direct comparison with the California populations

 
Research study 5.1
 

drawings showing extent of back-and-forth movements of a date mussel Lithophaga plumula in its burrowphotograph of date mussels Lithophaga sp. courtesy uBioPortal websiteThe date mussel Lithophaga plumula  is unusual in that it burrows into soft rocks such as calcareous shale and limestone.  Within its burrow the mussel is attached by byssus threads, and this attachment provides the purchase for back-and-forth, and rotational movements.  The burrow is lined by calcium carbonate that is shiny, hard, and mechanically strong.  The question addressed by a researcher at Kerckhoff Marine Laboratory, California is whether the burrowing is done by mechanical or chemical means.  Observation that Lithophaga does not burrow naturally into mudstone or into artificially created burrows in mudstone in the laboratory suggests that boring is by chemical means.  In these laboratory tests, individuals are seen to rotate in the artificial burrows, but no abrasion occurs and the burrows are not  lengthened.  Conversely, an individual stuck in limestone and unable to rotate its shell is still able to burrow.  The mode of dissolution of the substratum is not known, whether an acid-mucus is secreted by the mantle or whether a more complex ion-exchange between mantle and rock is involved.  A series of mark-recapture experiments shows that growth in limestone is extremely slow, less than 20% per year in one set of data (but sample sizes are small: see NOTE below).  Hodgkin 1962 The Veliger 4 (3): 123. Photograph courtesy uBioPortal.

NOTE  individuals are grooved with a dental drill and the grooves stained with India ink.  Recovery of marked animals is poor.  Of 508 animals marked and released over a 2yr period, only 22 are recovered in several collections of over 3800 individuals in the subsequent 6-9mo from release, representing a recovery rate of only 0.6%

 
Research study 6
 

photograph of 3 species of west-coast mytilids courtesy Linda Schroeder, Pacific Northwest Shell ClubMost investigations on mussel growth are on intertidal populations whose feeding is interrupted during low-tide periods.  In comparison, a study on effects of water temperature and food availability on growth of mussels Mytilus galloprovincialis in Santa Barbara Channel, California uses specimens submerged continuously in baskets suspended from an offshore oil platform.  Best growth is obtained at 9m depth, and is correlated most closely with phytoplankton biomass and not with annual seawater temperature, which at 9m depth ranges from 12-17oC.  Time to achieve a shell length of 50mm from recruitment is estimated at 6-8mo.  Page & Hubbard 1987 J Exp Mar Biol Ecol 111: 159. Photograph of Mytilus spp. courtesy Linda Schroeder and Pacific Northwest Shell Club, Washington PNWSC.

NOTE  the authors report their data for M. edulis but, based on recent genetic evidence, the species used is most likely to have been M. galloprovincialis

 
Research study 7
 

graph of growth of mussels Mytilus trossulus in relation to wave exposureA study in British Columbia compares growth of Mytilus trossulus at 2 sites on either side of Vancouver Island differing in degree of wave exposure.  Departure Bay is a wave-protected site (blue site on map) map of study sites for experiments on growth of mussels Mytilus trossulus on Vancouver Island, British Columbiaand MacKenzie Anchorage is a wave-exposed site (pink site on map).  

Both experimental populations grow to 50mm shell length within 14-16mo of settlement (see graph upper Right), but high mortalities during the second summer kill 90% of both populations (see graph lower Right).

 “Summer mortality” of mussels has later become the subject of much research by British Columbian fisheries scientists and aquaculturists. At the time of this study the graph showing summer mortality of mussels Mytilus trossulus at sites on Vancouver Island, British Columbia authors suggest that its main cause may relate to reproductive stress, perhaps combined with high summer seawater temperatures.  Emmett et al. 1987 J Shellf Res 6: 29.

 
Research study 8
 

photograph of Mytilus californianus "sculpture"The most popular species for aquaculturists the world over are Mytilus edulis and M. galloprovincialis.  Most mussels for sale in west-coast markets are Mytilus edulis cultured on the east coast of North America and in Puget Sound, Washington, along with some M. trossulus and M. galloprovincialis  What about the larger west-coast species M. californianus? A direct comparison of growth of M. trossulus and M. californianus cultured on long-lines in Winchester Bay, Oregon over a 5mo period in summer shows that growth rate of the former species is about half that of the latter.  This translates to about twice the production of flesh mass in M. californianus over the same period in comparison with that in the smaller species M. trossulus.  Although the authors suggest that the aquaculture potential of M. californianus merits further investigation, market trials have shown that the features of large size, orange colour, and rich texture of M. californianus are generally off-putting to potential buyers.  Behrens-Yamada & Dunham 1989 Aquaculture 81: 275.

 

 

 

 

 

Several Mytlus californianus collected from the field
and arranged "artistically". What's not to like?! 0.6X

 
Research study 9
 

graph of growth of mussels Mytilus trossulus in Port Valdez, Alaska
In Port Valdez, Alaska a large Mytilus trossulus of 4cm shell length is about 8yr old, and individuals may live to 10yr or more.  Blanchard & Feder 2000 Veliger 43: 34.

 
Research study 10
 

photographs showing methodology of annulus/acetate-peel method of estimating growth of mussels Mytilus trossulus  in AlaskaA study on growth of Mytilus trossulus in Prince William Sound, Alaska provides data with which to test the suitability of different methodologies and growth models.  The researchers tag mussels at 13 sites and collect them 1yr later.  Shell lengths are measured and ages determined from surface growth rings on the shells, made visible by radial sections and acetate peels.  Annual deposition of growth rings can then be determined and estimates of growth made. By this method the researchers identify up to 12 annuli in some individuals, indicating 11+ years of age. The photographs on the Left indicated how the annulus/acetate-peel method is used to determine the number of annuli on the shell of a 6yr-old 3cm-long individual.

graph of growth of mussels Mytilus trossulus in Alaska, comparing 2 methods of estimating ageAs shown in the graph on the Right, individuals with 8 annuli have shell lengths of about 35mm, results that agree closely with estimates made from traditional tagging or mark-recapture methods.The authors discuss the results in relation to their applicability to various growth models and conclude that the von Bertalanffy submodel best describes growth of M. trossulus at the Alaska sites.  More importantly, use of the annulus growth-increment method as done here requires only a single visit to the field, rather than at least 2 visits over a minimum of 1yr if more traditional mark-recapture methods are used.  Millstein & O’Clair 2001 J Exp Mar Biol Ecol 262: 155.

 
Research study 11
 

histograms comparing effect of current flow-rate on clearance rates in mussels Mytilus trossulus and M. californianus from the Bamfield, British Columbia areahistogram showing growth of mussels Mytilus californianus & M. trossulus in different current velocitiesMussels rely on water currents to bring phytoplankton to them for filter feeding, but can current flows become too strong for effective filtration and, thus, for good growth?  This is tested at the Bamfield Marine Sciences Centre, British Columbia on the sympatric species Mytilus californianus and M. trossulus using flow-through chambers both in the laboratory and mounted dockside.  Clearance rates for 2cm-sized individuals of M. californianus are not significantly affected by current velocity between 1-18 cm . sec-1 (see lower histogram on Left) while those for the same-sized M. trossulus are significantly higher at the highest velocity tested (see upper histogram on Left). 

The authors report that growth of both species tends to decrease with current velocity (see histogram on Right) and that rates are consistently higher in M. californianus than in M. trossulus. However, no statistical tests appear to have been done on these data and the differences noted may not be statistically significant. Even so, the authors suggest that the presence of such a “fluid dynamically mediated growth advantage” in M. californianus may contribute to its dominance in wave-exposed habitats.  Ackerman & Nishizaki 2004 J Mar Syst 49: 195.

NOTE  the small letters on the bars indicate significant differences

NOTE  the authors provide several sets of data for different dates, different starting sizes, laboratory chamber vs. dockside tests, but just one set representing the best data is displayed here

 
Research study 12
 

map showing study sites for study on upwelling effects on growth in sea mussels Mytilus californianushistograms showing growth and condition index of sea mussels Mytilus californianus in various conditions of upwelling along the southern California coastUpwelling increases nutrient concentrations and thus enhances planktonic productivity in nearshore areas.  Thus, mussels Mytilus californianus living in areas where upwelling is typically weak as, for example, in the Santa Barbara Channel south of Point Conception, California (see map on Left) would be expected to grow more slowly than ones living in areas where upwelling is typically strong as, for example, at and to the north of Point Conception.  However, studies on growth of sea mussels Mytilus californianus at several sites to the north and south of Point Conception show just the opposite, that springtime growth is significantly greater in the southern region than in the northern region (see upper histogram). Overall, growth and body condition are up to 3-6 times higher at sites furthest south of the Point in the area of weakest upwelling. Neither food availability nor tidal height explain the pattern of growth.  Water temperatures (warmer in the south) do, however, vary in a pattern consistent with growth rates.  The author suggests that factors other than, but perhaps in conjunction with, plankton productivity are responsible for the large-scale differences in growth rates exhibited by these mussels.  Phillips 2005 Mar Ecol Progr Ser 295: 79.

NOTE spring and summer data are presented for mussels and 2 species of barnacles, but only the mussel data are considered here.  Growth is measured by collecting mussels in February, notching their shells, returning the marked individuals to their shore locations, and collecting and measuring them 4mo later(in June. The small letters on each bar of the histograms indicate significant differences between the means. The author neglects to provide Y-axis units for the growth histogram, but the trend is obvious.

NOTE condition index measured as in this study, as a proportion of dry mass of body to shell length, effectively indicates the extent to which a mussel fills its shell

 
Research study 13
 

A method for marking bivalve larvae for release and capture-type growth studies involves calcein.  This dye has been used for marking of fish otoliths and elasmobranch skeletons, and for conducting growth studies on mussels and other shelled invertebrates. As to the question of possible deleterious effects of the dye on survival and growth, tests with mussel larvae Mytilus trossulus marked at 4d of age show that survival is actually better than in unmarked control veligers (38 vs. 21%, respectively), with no significant deleterious effect on growth.  The authors caution that release and subsequent recovery of marked larvae in the field is a “daunting task”, but the ease of the technique, especially for large numbers of larvae, makes it feasible and may aid in the study of growth and dispersal.  Moran & Marko 2005 J Shellf Res 24: 567.

NOTE  also known as fluorexon (a fluoroscein complex), calcein is a yellow-green fluorescent dye with excitation at 495nm that binds to calcium. The method uses 6.25g . liter-1 calcein in DW buffered to pH 6 with sodium bicarbonate, made up in seawater to a concentration of 100ppm.  Treatment involves immersing larvae in the marking solution for 72h or, in the words of the authors, "whatever works best in the least time possible"

 
Research study 14
 

map showing seawater temperature differences on either side of Point Conception, Californiagrowth of mussels Mytilus californianus on either side of Point Conception, CaliforniaPoint Conception, California is known from many studies to be a major oceanographic and biogeographic boundary to the distribution of marine-invertebrate species.  Conditions north of the Point are characterised by strong upwelling, cooler water temperatures, and high wave exposure, while south of the Point there is weak upwelling, warmer temperatures, and calmer waters (see map on Left).  How these differences might affect growth of mussels Mytilus californianus is assessed in a large, comprehensive study at the Marine Science Institute, Santa Barbara using intertidal and offshore-moored stations at several sites north of the Point and several south of the Point, spanning 300km (see map).  Results show significantly faster growth rates at the southern sites with an increasing gradient in rates from north to south (see graph on Right, data from 1998 only).  The gradient is not correlated with inshore concentrations of phytoplankton but, rather, with gradients of decreasing wave exposure and increasing intertidal temperature.  Growth of mussels on offshore moorings is significantly greater than that of intertidal mussels, but no difference is found in growth rates at moored locations on either side of the Point.  In explanation of this, the researchers propose that the southward gradient in increasing temperature may be offset by a decreasing gradient in phytoplankton productivity, although why this does not likewise affect the intertidal mussels is not clear.  The study further emphasises the importance of  comprehensive “bottom-up” productivity assessments in comparative geographic studies of mussel communities. Blanchette et al. 2007 J Exp Mar Biol Ecol 340: 126.


NOTE  the researchers monitor seawater temperatures, wave exposure, chlorophyll-a concentrations, and mussel body temperatures (using mussel-mimics) over a total 4-5yr study period.  Eleven intertidal sites are monitored (11), but only 5 of these are associated with offshore moored sites.  Not all sites yield all types of data for all years

 
Research study 15
 

graph showing effects of temperature and food on growth of mussels Mytilus californianusWays to test effects of climate change on marine shallow-water ecosystems are understandably limited, but researchers at Oregon State University, Corvallis have come up with the idea to use short-term oscillations in coast current flows as a proxy for assessing effects of climate change.  Specificially, they investigate effects of  the El Niño-Southern Oscillation and Pacific Decadal Oscillation on growth of mussels Mytilus californianus.  The first has a 3-7yr cycle; the second, a multidecadal one.  Both Oscillations involve changes in current intensity/direction and  seawater temperature, with associative effects on upwelling, and on intensity and duration of phytoplankton blooms.  The study focuses on 2 sites on the Oregon coast, but includes other sites, monitored over a span of 8-15yr.  The 2 main sites, Boiler Bay (characterised by higher phytoplankton productivity) and Strawberry Hill (lower productivity) have featured in several previous studies by the same research group.  Mussels are notched at the beginning of an experiment on the posterior edge, enabling annual shell growth to be assessed in specific individuals.  In situ data loggers provide long-term air and seawater temperatures, and daily/monthly water samples provide chlorophyll concentrations from which to estimate productivity.  Notched mussels are translocated from a common site to standardised mid-intertidal levels at the test sites and monitored for growth.  Results and analyses are understandably complex and are not presented here.  Overall, almost half of the variance in growth is explained by seawater temperature and phytoplankton availability (see graph), with contributions from other factors (e.g., sea-level changes)  being non-significant. Both main factors respond to climate-driven changes in upwelling.  The data show that mussel growth tracks annual changes in both Oscillation patterns, with warming trends being associated with enhanced growth.  The authors conclude that as long as temperatures reached are not stressful, then warming trends along the west coast can have positive effects on M. californianus populations.  The results may seem obvious after-the-fact, but the study is a marvel in concept, design, and execution.  It is a valuable contribution to the growing field of climate-change research by west-coast scientists.  Menge et al. 2008 Ecol Lett 11: 151.

NOTE  both Oscillations have warm-water (e.g., El Niño) and cool-water (El Niña) components

NOTE  for information on stress-effects of temperature on mussels see LIFE IN THE INTERTIDAL ZONE: HEAT-SHOCK PROTEINS

 
Research study 16
 

Factors known to influence shell shape in a growing mussel include wave action, predation, parasite load, and intertidal height, to name just a few.  Research on sea mussels Mytilus californianus at the University of California Santa Barbara Marine Laboratory has recently added 2 other features that may be related to shell shape.  Thus, individuals with wider shells relative to length have significantly greater discoloration of labial palps (see photograph) and a lower ratio of Sr/Ca in their shell nacre.  The 2 features are not known to be related.  The authors photograph of discoloration in labial palp of a sea mussel Mytilus californianusspeculate on the significance of their results, most notably on possible relationship of shell shape (greater tissue content) to basal metabolism, with differences perhaps being associated with physiological conditions of labial-palp discoloration the Sr/Ca ratio.  Blythe & Lea 2008 J Shellf Res 27 (2): 385.

NOTE the authors describe this as being red, but this does not show as such in the accompanying photograph


Discoloration in labial palp of Mytilus
californianus,
viewed from ventral side 25X

 
Research study 17
 

map showing study sites in mussel growth/recruitment investigationSea mussels Mytilus californianus in California are generally more abundant in the north than in the south, and numbers are also more variable in the south.  Possible reasons for this are investigated by researchers from the University of California, Los Angeles by comparing recruitment and growth rates over a 2-yr period at 9 sites separated by 900km (see map).  Of particular interest are several sites in southern California that differ markedly in mussel abundances.  Results are quite variable, but show generally that despite high abundances in northern California, recruitment and growth rates are lower than in southern California. Also, paradoxically, sites in southern California with higher abundances are generally characterised by higher rates of recruitment and growth.  The authors suggest that the large differences seen between north and south likely owe to large-scale oceanographic patterns, such as upwelling affecting larval transport and temperature affecting growth.  Similar events, but on a smaller scale, are thought to explain the differences between neighbouring sites in the southern region.  Smith et al. 2009 J Sea Res 61: 165.

 
Research study 18
 

A study by researchers from the University of North Carolina investigates whether geographical distributions of mussel species can be correlated with availability of energy for growth at high and low ends of geographical seawater temperature ranges.  The authors determine energy budgets for 3 mussel species Mytilus trossulus, M. galloprovincialis, and M. edulis at 6 temperatures in the laboratory over a range from 5-30oC.  Mussels are acclimated to the experimental temperatures over a 2wk period.  Results show, as expected, reasonable correlation with the temperature regimes of habitats presently occupied, with M. trossulus exhibiting positive scope for growth up to 17oC, M. edulis up to 23oC, and M. galloprovincialis, a warm-water species, up to 30oC.  The high end of each species’ spectrum correlates positively with the seawater temperature at which that species scope for growth tends to become negative, that is, when metabolic expenditure exceeds energy acquisition, in summer/autumn. Things are less clear, however, at the cold end of the ranges for all species.  Based on their data the authors consider M. edulis to be the most energetically “cosmopolitan” of the 3 species, yet this does not translate into good invasive ability.  In graph showing effect of temperature on scoope for growth in 3 species of mussels Mytilus trossulus, M. edulis, and M. galloprovincialiscontrast, M. galloprovincialis has greater tolerance to warm temperatures and, as history has shown, is more likely to be successfully introduced to novel, warm habitats.  Fly & Hilbish 2013 Oecologia 172: 35.

NOTE  this is known as scope for growth and represents the energy remaining after all maintenance requirements have been accounted for.  This residual energy can be used for somatic or reproductive growth, or both.  The authors calculate scope for growth as the product of filtration rate x absorption efficiency – respiration costs, expressed in joules

NOTE  M. trossulus is a boreal species on both Pacific and Atlantic coasts; M. galloprovincialis is a temperate warm-water species native to the Mediterranean area but also invasive to California; and M. edulis is a temperate cold-water species found on both sides of the Atlantic Ocean


Sample graph showing scope for growth for the 3 species in
springtime. Note the considerable metabolic advantage shown by
the warm-water species M. galloprovincialis at higher temperatures

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