Filtration rate of the blue mussel, Mytilus edulis, during exposure to plankton and fucoid zygote diet treatments

Photo by: Marissa McNally

Marissa McNally

Corning School of Ocean Studies, Maine Maritime Academy

Advisor: Dr. Jessica Muhlin

Fall 2014

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Abstract

The blue mussel, Mytilus edulis, is a widely distributed mollusc that is known for its role

in the ecosystem as a filter feeder. M. edulis individuals from Penobscot Bay, Maine were

analyzed for differences in filtration rate during exposure to two different diets for three different

time periods. The expectation was that the filtration rate of mussels in the zygote diet treatment

would be lower than the filtration rate of mussels in the plankton diet treatment, and that

filtration rate in both treatments would decrease over time. It was determined that the filtration

rate of M. edulis was higher in a diet consisting of fucoid zygotes than in a diet consisting of

plankton, which was opposite of the expectation. In addition to differences in filtration rate

between diets, the filtration rate in both diets decreased over time. A difference in filtration rate

between two diets may indicate a higher nutritional value of one diet over another, in particular

that the zygote diet was higher in nutrients than the plankton diet. A decreased filtration rate over

time may suggest that mussels sense particle depletion within the water column, or that the need

to feed decreases after feeding for a given time.

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Introduction

Mytilus edulis is a commercially and ecologically-important marine bivalve commonly

known as the edible blue mussel (FAO Fisheries Statistics 2006; ITIS Report 2014). M. edulis

suspension feeds to consume plankton and detritus in bays, estuaries, and the brackish waters of

the intertidal zone across the Northern Atlantic and Pacific Oceans (FAO Fisheries Statistics

2006; Bracken et al. 2012; Schulz 2013). Individual mussels utilize strong byssal threads that are

secreted from a gland in their foot in order to attach to substrate such as rocks and shells in

environments with constantly moving water (Pechenik 2010). Once attached, groups of mussels

create patches called beds (Schulz 2013). Mussel beds create commensal homes for organisms

such as: small crabs, sea stars, and various algal species (Figure 1; Schulz 2013) and have been

found to regulate the populations of surrounding fauna; larger beds promote higher faunal

diversity and abundance within the environment (Koivisto et al. 2011). A decline in mussel

population could therefore lead to a decline in their predators such as birds and whelks. A

removal of mussels could also cause a decrease in seston (suspended particles such as plankton,

detritus, and salts) depletion and therefore a decrease in water clarity (Grant et al. 2007) which

may be detrimental to the photosynthetic algae and plankton that rely on light to survive. For

these reasons, M. edulis is known as an ecosystem engineer (Koivisto et al. 2011).

M. edulis plays a critical role in the ecosystem by filtering particles out of the water

(Grant et al. 2007). As an organism that regulates the surrounding ecosystem and provides a

habitat and food source for other organisms, the mussel is considered to be important within its

environment (Koivisto et al. 2011). Mussels have been shown to exhibit a physiological response

to changes in composition and abundance of seston throughout seasons and over time by

increasing or decreasing filtration rate in response to these changes (Lesser et al. 2010).

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Changes in M. edulis populations are often used as an indicator for presence of pollution

within its environment (Pechenik 2010). Dimethyl Sulfide (DMS) is an odorous substance often

found in fish, shrimp, and shellfish due to ingestion of algae (including plankton) that produce

the chemical during osmoregulation (Schulz 2013). The presence of DMS in shellfish such as

mussels can make them unpalatable for human consumption which is disadvantageous to

fisheries. Although DMS is not harmful to the organism, there are diseases such as parasitic

infections, Vibriosis, and Rickettsiosiis that are potentially life threatening to shellfish such as

mussels. In cases where pollution or additional environmental factors could cause declining

mussel populations, the diet of the mussel is of concern because the decline could have an impact

on the ecosystem.

M. edulis responds to environmental cues, such as changes in concentration of plankton

or detritus within the water column by altering its filtration rate to suit its nutritional needs

(Clausen and Riisgard 1996). The diet of M. edulis consists primarily of plankton but mussels

also ingest lower nutrient foods such as detritus suspended in the water column (Bracken et al.

2012). Hing Wong and Levinton (2004) observed that mussels exhibit the highest filtration rate

and the highest growth rate in a treatment containing both phyto-and zoo-plankton compared to

treatments containing either only phytoplankton (which promoted the second highest growth

rate) or only zooplankton. Mussels feed on phytoplankton and zooplankton within the water

column, but plankton high in nutrients or abundance are not always available (Bracken et al.

2012). As such, a more nutrient-rich food source could be beneficial to the mussel population in

Maine and in commercial mussel farms.

Alternative sources of nutrients for M. edulis may be present in Maine and intertidal

zones worldwide. In addition to ingesting plankton within the water column, M. edulis also

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ingests microscopic algae (plankton) such as Rhodomonas salina (Clausen and Riisgard 1996),

so the potential for ingestion of other microscopic species of algae (and their reproductive

zygotes) exists. In autumn in the Gulf of Maine, the fucoid macroalga, Fucus vesiculosus is

reproductive (Berndt et al. 2002; Muhlin et al. 2011). Fucus vesiculosus males and females

broadcast their gametes synchronously on calm and sunny days (Serrao et al. 1996; Berndt et al.

2002; Muhlin et al. 2011); the result is a high reproductive success rate (typically very close to

100%) meaning that all of the released eggs are fertilized by the released sperm (Berndt et al.

2002). Although all of the eggs are fertilized, not all of the algal zygotes settle and subsequently

attach to substrate (Muhlin et al. 2011). Zygotes that do not attach may be ingested by various

filter feeders, such as mussels and snails. Lehane and Davenport (2004) demonstrated that

mussels ingest the larvae of other bivalves and potentially even their own larvae; therefore the

zygotes of the alga Fucus vesiculosus are a potential high nutrient food source (Kraufvelin et al.

2012) and the mussels could utilize them as an alternative where plankton are present in low

densities or not at all.

In addition to an ecological role, M. edulis also plays an economic role in ecosystems

throughout the world. In the year 2010 alone, nearly 100,000 tonnes of M. edulis were cultivated

worldwide (FAO Fisheries Statistics 2006). Denmark and Canada were the top commercial

producers of M. edulis in 1999 but cultivation is also common in Maine (USA; FAO Fisheries

Statistics 2006). Higher filtration rates and growth rates due to the presence of a high nutrient

diet could be beneficial to mussel farms worldwide.

M. edulis is an important organism in the ecosystem of Penobscot Bay, Maine (not only

for the ecosystem, but also for the economy) and a decline there could be detrimental to

populations of sea stars, crabs, and alga that rely on mussel beds as a habitat. In Penobscot Bay,

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mussels prey on plankton; seagulls, whelks, and other gastropods prey on the mussels (FAO

Fisheries Statistics 2006; Schulz 2013), so a decline in mussel population could in turn effect

populations of those species as well. Fucus vesiculosus thrives in the intertidal zones of

Penobscot Bay, so there is potential for fucoid zygotes as a diet source there. A better

understanding of the filtration habits of M. edulis in Maine could lead to a better understanding

of the filtration of M. edulis worldwide.

The expectation of this study was that mussels would alter their filtration rate depending

on the availability and quality of food within the ecosystem. Studies have shown that the

mussel’s normal diet consists mostly of plankton (Bracken et al. 2012), but fucoid zygotes are

potentially a more nutrient rich food that may be available to mussels. The potential for a food

source higher in nutrients than plankton may allow mussel populations to reach a maximum

growth rate faster in both the wild and in aquaculture farms. The goal of this study was to

determine whether the filtration rate of a M. edulis individual would exhibit a quantitative

change when exposed to two different diet treatments. The first objective of this study was to

determine whether mussel filtration rate changed in response to diet treatments of differing

qualities, particularly whether the filtration rate would be higher in a diet treatment consisting of

only plankton than in a diet consisting of fucoid zygotes. The second objective was to determine

whether filtration rate changed over time (specifically whether the filtration rate decreased over

time which may be indicative of a physiological response to change in diet concentration).

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Materials and Methods

The expectation of this study was that blue mussel (Mytilus edulis) individuals would

alter their filtration rate in response to exposure to different diets. An experiment was designed

and implemented to test for these differences; mussels were collected and exposed to a diet

treatment of plankton or fucoid zygotes for 1, 2, or 4 hours. Differences in filtration rates

between the three time periods in each treatment and between the two treatments at each time

period were analyzed for three days of data collection.

Organism Collection

One hundred adult (larger than 40 mm in length as defined by FAO Fisheries Statistics

2006) blue mussel, Mytilus edulis, individuals were collected in the fall of 2014 from the floating

docks at the Maine Maritime Academy waterfront in Castine, Maine. Sixty mussels, of an

average length of 54.5+3.43 mm, width of 27.7+1.78 mm, and weight of 14.3+2.7 g with no

shell damage (e.g. cracked or broken areas, epiphyte growth), were chosen for the experiment.

Mussels were maintained in the Andrews Hall running seawater laboratory prior to data

collection. Twenty mussels were used each day for a three day experiment. All mussels were

exposed to artificial seawater containing no food for 24 hours prior to the start of the experiment.

Mussels were randomly assigned to a beaker numbered one through twenty. Subsequently, the

diet treatment (fucoid zygotes or plankton in artificial seawater) and a time period (1, 2, or 4

hours) were randomly assigned to each beaker.

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Diet Treatments

All treatment beakers were aerated and circulated via an air pump throughout data

collection to ensure that the seawater remained homogeneous. Four air pumps and splitters with

five hoses each were used to aerate the twenty beakers equally throughout the experiment. The

twenty individual beakers were contained within the laboratory table and water was maintained

at the same level as the water within the beakers to ensure constant water temperature was

maintained (Figure 2). Each treatment group of mussels was exposed to an Instant Ocean

Aquarium Salt seawater (salinity of 33 psu as measured on an RHS-10ATC Portable

Refractometer) treatment containing both phytoplankton and zooplankton or zygotes of the alga

Fucus vesiculosus.

Plankton were collected from natural seawater with a 50μm SeaBird plankton net from a

seawater hose in the wet laboratory. After collection in a plankton net, plankton were removed

from natural seawater by centrifugation (on a VWR Galaxy 10 Centrifuge at 1000 rpm) and

removal of the supernatant. The pellet of plankton was added to artificial seawater. Zygotes were

removed from the filtered, autoclaved seawater and added to artificial seawater by centrifugation

and removal of the supernatant. The centrifugation process was repeated until a concentration of

250 cells mL-1 was reached. The concentration (cells mL-1) of the seawater was measured via

Sedgewick Rafter counting cell on a Nikon Eclipse E200 compound light microscope.

Fucus vesiculosus male and female gametes were released following the methods of

Brawley (1991). Female individuals were stirred in an ice bath for 5 minutes, dried for 5

minutes, stirred again for 5 minutes, and dried again for 5 minutes, then placed in sunlight for

two hours in autoclaved, filtered seawater. Male individuals were stirred in an ice bath for 5

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minutes, dried for 5 minutes, stirred again for 2 minutes, and dried again for 5 minutes, then

placed in sunlight for two hours in autoclaved, filtered seawater to release. Upon gamete release,

the eggs were fertilized with sperm at a 5000:1 sperm:egg ratio (Brawley 1991). The zygotes

were pelleted and added to the artificial seawater following the same centrifugation method used

for plankton.

The diet treatments were both prepared to approximately 250 cells mL-1 (one-fourth of

the concentration measured at the Maine Maritime Academy Waterfront). The seawater for each

treatment was then diluted by one half with artificial seawater to a final concentration of

approximately 125 cells mL-1.

Data Collection

Beakers were labeled one through twenty and arranged in the laboratory table in two

rows of ten. Two hundred mL of the treatment water was placed in each beaker and the water

was given ten minutes to homogenize and to adjust to temperature within the beakers. The

mussels were added to the beakers and timing for data collection began when all mussels were

added to their respective beakers. Either three or four mussels were removed at each time period

for each treatment (Table 1).

Upon mussel removal and homogenization of treatment water, 50 mL water samples

were collected for all treatments on all three days and stored in 50mL centrifuge tubes in the

refrigerator. After the three day data collection was complete, the samples were thoroughly

shaken to mix and analyzed for final concentration (cells mL-1). For each of the sixty samples, 1

mL of water was transferred to a Sedgewick rafter cell and the cells were counted at 200x

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magnification on a Nikon compound light microscope. This process was repeated three times for

each sample.

The final cell concentration was subtracted from the initial cell concentration of the

seawater and the filtration rate was calculated using Equation 1.

Initial Cell Concentration (cells mL -1 ) - Final Cell Concentration (cells mL -` ) Equation 1 Mussel Weight (g) * Exposure Time (h)

Data Analyses

To test for differences in filtration rate over time and between treatments, the data were

analyzed in IBM SPSS Statistics 20. A Kruskal-Wallis Analysis of Variance (ANOVA) for

nonparametric statistics was used to test for significant differences in the filtration rate at each of

the three time periods for each of the two treatments. Individual differences in filtration rate were

analyzed for each diet treatment using a Tukey’s post-hoc multiple comparison test. The data

were graphed to represent the filtration rate (cells mL-1 g-1 hr-1) of the mussels exposed for 1, 2, or

4 hours to a diet of plankton and a diet of fucoid zygotes. A Mann-Whitney U-test for

nonparametric statistics was used to test for a significant difference in filtration rate between the

two diet treatments for each of the three time periods. The data were graphed to represent the

filtration rate (cells mL-1 g-1 hr-1) of the mussels in two different diet treatments after exposure for

1, 2, and 4 hours.

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Results

In both the plankton diet treatment and the fucoid zygote diet treatment, there was a

significant decrease in filtration rate of Mytilus edulis over time. A significant difference was

found in filtration rate between the two diet treatments after one hour of exposure; the mussels in

the zygote diet treatment exhibited higher filtration rates. For the remaining two exposure times,

there were no significant differences in filtration rate between the two diet treatments, however

the mussels in the zygote diet treatment continued to exhibit higher filtration rates than those in

the plankton diet treatment.

The filtration rate of M. edulis individuals exposed to the plankton diet differed

significantly between the three exposure times (Kruskal-Wallis ANOVA, p<0.001, H=25.812,

df=2, n1hour=n2hours=n4hours=10, Figure 3). The filtration rate was significantly higher (2.88+0.38

cells mL-1g-1hr-1) after one hour than after two hours (Tukey Post-Hoc, p<0.001) and after four

hours (Tukey Post-Hoc, p<0.001). The lowest filtration rate (0.87+0.07 cells mL-1g-1hr-1) was

observed after four hours, and the filtration rate after two hours fell in the middle (1.59+0.10).

The filtration rate of M. edulis individuals exposed to the fucoid zygote diet also differed

significantly between the three exposure times (Kruskal-Wallis ANOVA, p<0.001, H=21.572,

df=2, n1hour=n2hours=n4hours=10, Figure 4). The filtration rate was significantly higher (4.32+1.12

cells mL-1g-1hr-1) after one hour than after two hours (Tukey Post-Hoc, p<0.001) and four hours

(Tukey Post-Hoc, p<0.001). There was no significant difference (Tukey Post-Hoc, p=0.107)

between the filtration rates at two hours and four hours, however the filtration rate was lower at

four hours (1.02+0.36 cells mL-1g-1hr-1) than at two hours (1.73+0.54 cells mL-1g-1hr-1).

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There was a significant difference (Mann-Whitney U-test, z=2.570, p=0.010, df=18,

n=20, Figure 5) in filtration rate between M. edulis individuals in the plankton treatment and

mussels in the fucoid zygote treatment after one hour. The filtration rate (4.32+1.12 cells mL-1g-

1hr-1) of individuals in the fucoid zygote treatment was nearly twice that of the filtration rate

(2.88+0.38 cells mL-1g-1hr-1) of individuals in the plankton treatment. There were, however, no

significant differences in filtration rate between individuals in the plankton treatment and

mussels in the fucoid zygote treatment after two hours (Mann-Whitney U-test, z=0.605, p=0.545,

df=18, n=20, Figure 6) or after four hours (Mann-Whitney U-test, z=1.588, p=0.112, df=18,

n=20, Figure 7). Although the difference was not significant, the filtration rate for mussels in the

fucoid zygote treatment was higher (1.73+0.54 cells mL-1g-1hr-1 and 1.02+0.36 cells mL-1g-1hr-1

after two hours and four hours, respectively) than the filtration rate for mussels in the plankton

treatment (1.59+0.10 cells mL-1g-1hr-1 and 0.87+0.07 cells mL-1g-1hr-1 after two hours and four

hours, respectively).

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Discussion

An altered filtration rate in response to a change in diet quantity and quality has been

observed for Mytilus edulis individuals in the past in studies (e.g. Hing Wong and Levinton

(2004); Strohmeier et al. (2009); Bracken et al. (2012)) and was expected to be found in this

study. M. edulis did exhibit a difference in filtration rate between a plankton diet treatment and a

fucoid zygote diet treatment over time during experimental exposure; a higher rate of filtration

was observed in the fucoid zygote treatment and after one hour of exposure. These results may

indicate a physiological response by M. edulis to different diet qualities and quantities similar to

the results of Hing Wong and Levinton (2004).

When hunger was satiated, the filtration rate of M. edulis individuals was expected to

decrease or stop altogether. The expectation was that mussels would feed immediately when

given a food source, results supported by a study by Maire et al. (2007). Mussels are known to

physiologically regulate their filtration rate over time in response to changes in diet quality or

quantity (Hing Wong and Levinton 2004); there is also a possibility that mussels select for algae

over detritus and silt in the water column (Clausen and Riisgard 1996). The rate of seston

depletion by a mussel increases and decreases accordingly with an increase or decrease in food

availability in order to conserve the maximum amount of energy (Clausen and Riisgard 1996,

Strohmeier et al. 2009). Mussels and other bivalves have the ability to significantly decrease the

seston concentration in a given volume of water (Grant et al. 2007). In both the plankton diet

treatment and the fucoid zygote diet treatment, the filtration rate decreased over time as hunger

was satiated and the seston concentration decreased. This result supported the hypothesis that

filtration rate would decrease over time, and was consistent with the results of Grant et al. (2007)

which may indicate that mussels may sense seston depletion with time. A decrease in filtration

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rate in both diets not only indicated that both of the diets could satiate the hunger of the mussel,

but also that both potentially did so quickly in order to trigger a response to decrease the

filtration rate in the mussel. Contrastingly, a decrease in filtration rate over time could also be in

response to the decrease in cells mL-1 over time.

A mussel’s filtration rate was expected to be the highest in a diet consisting of only

phytoplankton (as opposed to phytoplankton, zooplankton, and detritus; Clausen and Riisgard

1996). On the contrary, the filtration rate of M. edulis individuals was shown to be the highest in

a diet consisting of both phytoplankton and zooplankton, as opposed to a diet consisting of only

one or the other (Hing Wong and Levinton 2004). Studies have shown that M. edulis individuals

alter their filtration rate depending on food availability in order to increase their metabolism and

achieve the maximum growth rate (Clausen and Riisgard 1996; Hing Wong and Levinton 2004;

Strohmeier et al. 2009). Environments containing an abundance of phytoplankton promote the

maximum growth rate for M. edulis (Bracken et al. 2012). In environments with a low natural

seston concentration, the mussel may reduce filtration rate in order to conserve energy and

reduce consumption (Strohmeier et al. 2009). In this study, the results showed that the filtration

rates were the opposite of expected: the filtration rate in the fucoid zygote diet treatment was

significantly higher after one hour than the filtration rate in the phytoplankton and zooplankton

diet treatment. If a higher filtration rate is indicative of an optimal seston concentration with a

high nutrient concentration, the results of the study demonstrated that zygotes may be a higher

nutrient diet for mussels than plankton which was expected.

Plankton is not always available to mussels in high quantities in the wild (Bracken et al.

2012). In environment where the plankton concentration is too low to promote a maximum

growth rate for mussels, fucoid zygotes may be a viable alternative or supplement to the plankton

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diet. The high reproductive success of Fucus vesiculosus (Berndt et al. 2002) can prevent

settlement of all of the zygotes which may in turn be ingested by bivalves including M. edulis in

a similar manner to the ingestion of bivalve larvae found in a study by Lehane and Davenport

(2004). The higher filtration rate in the zygote treatment likely suggests that mussels respond to

the presence of a high nutrient diet by increasing filtration to maximize growth; a study exposing

mussels to diets containing both plankton and fucoid zygotes could confirm this. Although

zygotes are a supplementary food source, F. vesiculosus is reproductive in the fall which limits

this option to the colder seasons. Due to the photosynthetic properties of phytoplankton, there is

a likelihood that the plankton concentration and primary production naturally decrease during the

fall and winter months (Grant et al. 2007), potentially creating a need for mussels to feed on the

high nutrient zygote diet that is available in those seasons.

Measurements in this study were taken in similar units to comparative literature. A study

by Clausen and Riisgard in 1996 used the units of cells mL-1 for cell concentration which were

also used in this study. Clausen and Riisgard (1996) also measured filtration rate in the units of

mL min-1 ind-1. The units for filtration rate in this study were cells mL-1 g-1 hr-1. Although the

units were not the same, the units in this study accounted for cells ingested and the comparative

study did not.

A potential limitation of this study is that the cell concentration were unnaturally low

(approximately one-eighth of the natural concentration found in Penobscot Bay). A study by

Clausen and Riisgard (1996) observed a low filtration rate in a lab environment containing an

unnaturally high concentration of cells. Mussels may decrease their filtration rates in cell

concentrations that are not optimal, so a concentration that is either unnaturally high or

unnaturally low could cause a decreased filtration rate, especially in a lab setting.

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An experiment with a third diet consisting of larvae from another bivalve could be an

addition to the results of this study. Bivalves exhibited larviphagy (a form of cannibalism in

which a bivalve ingests the larvae of another bivalve) in a study by Lehane and Davenport

(2004); the bivalve larvae were completely digested a majority of the time, so there is a potential

that bivalve larvae are a high nutrient food source as well. The use of bivalve larvae could be an

alternative diet similar to fucoid zygotes.

In addition to observing the filtration rate of M. edulis in diet treatments other than the

ones used in this study, it may be beneficial to observe changes in filtration rate throughout

seasons. As previously mentioned, phytoplankton are less productive in the fall (Grant et al.

2007) and their abundance may decrease in the colder months. Due to the reproductive nature of

F. vesiculosus (Berndt et al. 2002), the population increases during those months. A

determination of the abundance of both plankton and fucoid zygotes throughout the seasons

could be beneficial in determining when mussels may need another food source available to

them. M. edulis was shown to physiologically adapt to changes in the environment throughout

the seasons in a study by Lesser et al. (2010), so there is potential that changes in diet abundance

would have an effect on mussels throughout the year. A seasonal study would likely be better

suited in the wild than in the laboratory environment such as the one used for this study; there

seasonal fluctuations in temperature, salinity, and currents that would be difficult to simulate.

Bivalve molluscs, such as M. edulis, are responsible for approximately two thirds of

global fisheries production (Aquaculture 2014). M. edulis is commonly farmed throughout North

America and Europe (FAO Fisheries Statistics 2006). Maintaining plankton at an optimal

concentration could maximize the growth rate in a farming environment (Clausen and Riisgard

1996; Grant et al. 2007). An increasing issue with farms is that suspension-feeding bivalves

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cause a high level of seston depletion (a decline in the plankton population as a result), especially

in a small area (Grant et al. 2007), that has the capability of altering the surrounding ecosystem.

A change in the ecosystem could have a negative impact on the organisms that live there. If an

additional food source were added in the vicinity of mussel farms, there may be a decrease in

seston depletion that could prevent such changes. In addition to preventing seston depletion, the

introduction of another food source could promote a higher growth rate which would be ideal for

production in a farming environment. The growth of F. vesiculosus in M. edulis farms has the

potential to increase filtration rate, and in turn growth rate, during the reproductive season of F.

vesiculosus due to the higher filtration rate observed in a fucoid zygote diet treatment compared

to the filtration rate in the plankton diet treatment.

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Acknowledgements

I would like to thank my project advisor, Dr. Muhlin, for her assistance in project

development, proposal composition, experimental work, and paper writing throughout OS400

and OS401. In addition to Dr. Muhlin’s assistance throughout the two courses, instruction from

Dr. Alan Verde and Dr. Lauren Sahl was helpful in the development and execution of the project

from start to finish. This project would not have been possible without the help of Pamela

Grindle in material planning and collection. Lastly, I would like to thank Alison Mitchell,

Kristina Kelley, Kevin Kelly, Nathan Rooney, Heather Ponto, Patrick Graham, Dina Bahloul,

Austin Hubbard, and Dillon Davidson for the hours spent in the lab with me as I prepared the

experiment and counted cell concentrations as well as for countless peer revisions on my

developing paper.

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Strohmeier T, Strand, O, Cranford P (2009) Clearance rates of the great scallop (Pecten

maximus) and blue mussel (Mytilus edulis) at low natural seston concentrations. Mar Biol

156: 1781-1795

21

Tables

Table 1. Mussel removal method for each day of data collection in a three day diet experiment.

1 Hour 2 Hours 4 Hours

Mussels to Remove from each Treatment (Day One)

3 3 4

Mussels to Remove from each Treatment (Day Two)

4 3 3

Mussels to Remove from each Treatment (Day Three)

3 4 3

22

Figures

Figure 1. A blue mussel, Mytilus edulis, individual in Penobscot Bay, Maine providing a

commensal home to a sea star, algae, and skeleton shrimp (Caprellidae sp.).

23

Figure 2. Experimental set up in the Andrews Hall running seawater laboratory. Mussels were

arranged in two rows of ten and air pumps aerated five beakers each. Water level was maintained

at same level as water within the beakers.

24

1 2 40.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Exposure Time (Hours)

Filtr

atio

n R

ate

(cel

lsm

L-1

g-1h

r-1)

p<0.001H=25.812

A

B

C

Figure 3. Mean filtration rate (cells mL-1g-1hr-1) of Mytilus edulis individuals exposed to a

plankton diet for 1, 2, or 4 hours (Kruskal-Wallis ANOVA, p<0.001, H=25.812, df=2,

n1hour=n2hours=n4hours=10). Histogram bars that do not share letters are significantly different from

one another (Tukey Post-Hoc, p<0.001). Data are mean+SD.

25

1 2 40.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Exposure Time

Filtr

atio

n R

ate

(cel

lsm

L-1

g-1h

r-1)

Figure 4. Mean filtration rate (cells mL-1g-1hr-1) of Mytilus edulis individuals exposed to a fucoid

zygote diet for 1, 2, or 4 hours (Kruskal-Wallis ANOVA, p<0.001, H=21.572, df=2,

n1hour=n2hours=n4hours=10). Histogram bars that share letters are not significantly different from one

another (Tukey Post-Hoc, p=0.107). Data are mean+SD.

p<0.001H=25.812

A

B A

B

26

Plankton Zygote0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Diet Treatment

Filtr

atio

n R

ate

(cel

lsm

L-1

g-1h

r-1)

Figure 5. Mean filtration rate of Mytilus edulis individuals exposed to either a plankton diet

treatment or a fucoid zygote diet treatment for one hour. Mussels in the fucoid zygote diet

treatment had a significantly (Mann-Whitney U-test, z=2.570, p=0.010, df=18, n=20) higher

filtration rate (cells mL-1g-1hr-1) than mussels in the plankton diet treatment. Data are mean+SD.

P=0.010z=2.570

27

Plankton Zygote1.50

1.55

1.60

1.65

1.70

1.75

Diet Treatment

Filtr

atio

n R

ate

(cel

lsm

L-1

g-1h

r-1)

Figure 6. Mean filtration rate of Mytilus edulis individuals exposed to either a plankton diet

treatment or a fucoid zygote diet treatment for two hours. There was no significant difference

(Mann-Whitney U-test, z=0.605, p=0.545, df=18, n=20) in filtration rate (cells mL-1g-1hr-1)

between mussels in the plankton diet treatment and mussels in the fucoid zygote diet treatment

after two hours. Data are mean+SD.

P=0.545z=0.605

28

Plankton Zygote0.75

0.80

0.85

0.90

0.95

1.00

1.05

Diet Treatment

Filtr

atio

n R

ate

(cel

lsm

L-1

g-1h

r-1)

Figure 7. Mean filtration rate of Mytilus edulis individuals exposed to either a plankton diet

treatment or a fucoid zygote diet treatment for four hours. There was no significant difference

(Mann-Whitney U-test, z=1.588, p=0.112, df=18, n=20) in filtration rate (cells mL-1g-1hr-1)

between mussels in the plankton diet treatment and mussels in the fucoid zygote diet treatment

after four hours. Data are mean+SD.

P=0.0.112z=1.588