Factors influencing the attachment strength ofDreissena polymorpha (Bivalvia)

JAROSŁAW KOBAK

Department of Invertebrate Zoology, Institute of General and Molecular Biology, Nicolaus Copernicus University,

Torun, Poland

(Received 20 October 2005; accepted 10 January 2006)

AbstractThe effects of several factors (shell length, exposure time, substratum orientation in space, illumination, temperature,conspecifics) upon the attachment strength (measured with a digital dynamometer) of the freshwater, gregarious bivalveDreissena polymorpha were studied under laboratory conditions. A rapid increase in attachment strength was observed onresocart (a thermosetting polymer based on phenol-formaldehyde resin, with paper as filler) substrata during the first 4-dexposure, after which it stabilised at ca 1 N. The attachment strength increased also with mussel size. Mussel adhesion onvariously oriented surfaces (vertical, upper horizontal and lower horizontal) was similar. Illumination inhibited attachmentstrength, as expected for a photophobic species, but only after a 2-d exposure. After 6 d, no effects of light were detected.Thus, illumination seemed to influence the attachment rate, rather than the final strength. The optimum temperature formussel attachment was 20 – 258C. At lower and higher temperatures (5 – 158C and 308C), their adhesion strength decreased.The presence of conspecifics stimulated mussel attachment strength.

Keywords: Zebra mussel, adhesion, light, substratum orientation, temperature, conspecifics

Introduction

The zebra mussel (Dreissena polymorpha, Pallas, 1771)

is a freshwater, gregarious bivalve with high invasive

potential (Orlova, 2002). It retains the ability to

attach to hard substrata throughout its entire adult life

and often fouls various underwater structures, dete-

riorating structural function. Therefore, the zebra

mussel is regarded as a nuisance (O’Neill, 1997). Its

effects upon the environment are also considerable

(Karatayev et al. 2002): it is an efficient filtrator of

particulate matter and strongly influences zoobenthos

community structure (Botts et al. 1996; Lewandowski,

2001), element cycling (Fahnenstiel et al. 1995) and

food webs (Molloy et al. 1997).

Attachment to a hard substratum is essential for

the survival of a byssate bivalve. Therefore, zebra

mussels try to attach even in unsuitable environ-

mental conditions, e.g. during starvation (Clarke,

1999) or in the presence of toxic compounds

(Rajagopal et al. 2002). Strongly attached marine

bivalves of various species are less vulnerable to

predation (Reimer & Tedengren, 1997), dislodge-

ment (Bell & Gosline, 1997; Hunt & Scheibling,

2001; Schneider et al. 2005) and toxins (Rajagopal

et al. 2005). The latter phenomenon, observed also

in zebra mussels, emphasises that the attachment

status of bivalves used in bioindication or research

should be taken into account, as it may affect their

responses (Rajagopal et al. 2002). Reattachment

strength, in itself, also may be an easy (although not

very rapid) bioindication tool (Mersch et al. 1996).

Mussels would produce fewer byssal threads and

their adhesion would be weaker in worse conditions.

Thus, environmental factors that could potentially

affect mussel attachment should be examined in

detail.

Previous studies have shown that zebra mussels

in the field are influenced by substratum quality,

viz. its type (e.g. Walz, 1973; Lewandowski, 1982;

Marsden & Lansky, 2000; Kobak, 2004), shape

(Czarnołeski et al. 2004; Kobak, 2005) and orienta-

tion (Marsden & Lansky, 2000; De Lafontaine et al.

2002). These factors determine mussel recruitment

density and post-settlement migrations. Furthermore,

mussel distribution may be controlled by water

movement (Wacker & von Elert, 2003), illumination

(Marsden & Lansky, 2000; Kobak, 2001), salinity

Correspondence: Jarosław Kobak, Department of Invertebrate Zoology, Institute of General and Molecular Biology, Nicolaus Copernicus University, Gagarina

9, 87 – 100 Torun, Poland. Fax: þ48 56 6114772. E-mail: jkob@biol.uni.torun.pl

Biofouling, 2006; 22(3): 141 – 150

ISSN 0892-7014 print/ISSN 1029-2454 online � 2006 Taylor & Francis

DOI: 10.1080/08927010600691895

(Rajagopal et al. 1996), and food conditions

(Vanderploeg et al. 1996). Another factor influencing

zebra mussels is the presence of conspecifics, which

stimulate larval settlement (Wainman et al. 1996) but,

on the other hand, adversely affect life conditions

in dense aggregations (Burks et al. 2002). The above-

mentioned factors also may affect zebra mussel attach-

ment strength (Ackerman et al. 1995; Dormon et al.

1997), byssal thread production (Clarke & McMahon,

1996; Rajagopal et al. 1996; Clarke, 1999) or per-

centage of reattaching individuals (Kobak, 2001;

Kavouras & Maki, 2003). Although all these measures

are somewhat related to one another, as has been

demonstrated for marine bivalves (Carrington, 2002),

attachment strength seems to be the most direct

estimate of the relationships between a mussel and its

environment, as it determines the actual level of

protection provided by byssal threads.

The aim of the present study was to investigate the

impact of several factors upon zebra mussel attach-

ment strength. Mussel attachment was hypothesised

to be stronger on vertical surfaces and on the under-

side of horizontal surfaces due to the higher risk of

losing contact with hard material after dislodgement

from such substrata. Moreover, mussel adhesion was

assumed to be weaker in the light than in darkness, as

bivalves are photophobic organisms (Kobak, 2001;

Toomey et al. 2002). Furthermore, it was predicted

that high and low temperatures would inhibit mussel

attachment. Finally, the presence of conspecifics was

expected to stimulate attachment of this gregarious

animal (Wainman et al. 1996; Mortl & Rothhaupt,

2003).

Materials and methods

Experimental design

The mussels were collected by a diver from the dam

of the Włocławek Dam Reservoir (the Vistula River,

central Poland) and kept in a 500-l aquarium filled

with settled, aerated tap water (ca 16 – 198C). The

mussels could attach themselves to glass Petri dishes,

which were used to facilitate handling. Each mussel

was tested only once, not earlier than 2 weeks after

collecting. The average shell length of the tested

mussels was 14.5 mm (range: 13.0 – 18.0 mm, SD:

1.11 mm), except for the experiment dealing with

the relationship between mussel size and attachment

strength (Experiment 3).

The experiments were run in 10-l tanks (bottom:

2406 240 mm, water level: 170 mm) filled with

settled, aerated tap water, at room temperature

(mean: 18.58C, range: 16 – 218C, SD: 1.408C),

except for Experiment 5 (temperature as a factor).

If light was not an examined factor, the experiments

were conducted in darkness, obtained by covering

the tanks with cardboard boxes. The illumination

under such a box was below the detection limits

of the luxometer Sonopan L-20A (i.e. 50.1 l6).

Mussels were allowed to attach to square plates

(1006 1006 5 mm) made of resocart (a thermo-

setting polymer based on phenol-formaldehyde

resin, with paper as filler). This material was found

to be a suitable substratum for zebra mussels

(Kobak, 2004; 2005). The resocart plates were

assembled into boxes consisting of the bottom and

four walls, joined by rubber bands and covered with

1-mm nylon mesh (Figure 1). This design prevented

mussels from leaving the plates and allowed for easy

disassembling and making measurements. The plates

were roughened with sandpaper and kept in aqua-

rium water for 1 week before use. This period seems

to be sufficient for the development of a biofilm that

makes a substratum more suitable for mussels

(Wainman et al. 1996; Kavouras & Maki, 2003).

At the beginning of each trial, 12 (unless stated

otherwise) mussels were put onto the bottom plate

lying on the tank floor and then surrounded by the

wall plates and mesh. Only those mussels that were

found attached to the substratum in the rearing tank

were used in the experiments. At the end of a trial

Figure 1. Schematic of box made from resocart plates used to study mussel attachment strength. A¼disassembled; B¼ assembled.

142 J. Kobak

(after 2 or 6 d), the boxes were removed from

the water, disassembled and mussel attachment

strength was measured with a digital dynamometer

(FG-5000A, Lutron Electronic Enterprise Company

Ltd) connected with forceps holding a mussel.

The device was pulled gently in the direction per-

pendicular to the plate, until the mussel was

detached. Individuals that were too crowded to

access with the forceps, or that had attached to con-

specifics, rather than to the plates, were dropped

from further analysis. The mussels that attached to

the vertical walls of the boxes were included in the

data, as the attachment strengths on vertical and

horizontal surfaces were similar (see the results of

Experiment 2, below). The numbers of mussels

examined in each experiment are given in the Figures

presenting the results.

The results were analysed with ANOVA (details

given below). If necessary, data transformations were

applied to meet the homoscedasticity and normality

assumptions (tested by Levene and Kolmogorov-

Smirnov tests, respectively). If ANOVA results

were statistically significant, Tukey test was used

for post hoc comparisons.

The details of all the experiments carried out in

this study are summarised in Table I.

Experiment 1. Exposure time

The main purpose of this experiment was to find the

appropriate exposure time for testing mussel attach-

ment in the rest of this study. Two hundred mussels

were placed in a tank with the bottom and walls lined

with resocart plates. Each day, the attachment

strengths of 12 randomly chosen individuals were

measured, until there were no mussels left in the tank

(i.e. after 10 d, as some individuals were too

crowded to measure their adhesion, or had attached

to conspecific shells).

The experiment was carried out in triplicate. Its

results were analysed with the two-factor ANOVA

(replicate: random factor, day of measurement: fixed

factor) performed on the square root transformed

data.

Experiment 2. Substratum orientation in space

To check whether substratum orientation affected

mussel adhesion, mussels were put into resocart

boxes (25 individuals in each). On the following day,

the boxes were disassembled and plates with

initially attached individuals were suspended in

the tanks in the following positions: i) vertically,

ii) horizontally with mussels on the upper surface,

and iii) horizontally with mussels on the lower surface.

Mussel attachment strength was measured after an

additional 6 d. Unlike in Experiments 4 and 6,

Tab

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Attachment strength of D. polymorpha 143

measurements after only 2 d exposure at the new

orientation were abandoned, because they would

be largely affected by the preliminary attachment,

during which the conditions in all treatments were the

same.

This experiment was replicated eight times. The

results were analysed with the two-factor ANOVA

(replicate: random factor, surface orientation: fixed

factor).

Experiment 3. Mussel size

To check the effect of mussel size on attachment

strength, mussels of various shell lengths (range:

5.8 – 22.1 mm, mean: 12.73 mm, SD: 3.71 mm)

were tested in 20 trials (i.e. separate boxes), with

12 individuals in each. Their attachment strength

was measured after 6 d exposure.

Spearman rank correlation coefficient was used to

assess the relationship between shell length and

attachment strength. Then, differences among 13 size

classes of mussels (57.0, 7.0 – 7.9, 8.0 – 8.9, 9.0 – 9.9,

10.0 – 10.9, 11.0 – 11.9, 12.0 – 12.9, 13.0 – 13.9,

14.0 – 14.9, 15.0 – 15.9, 16.0 – 16.9, 17.0 – 17.9 and

�18.0 mm) were tested with the single-factor

ANOVA. The data were square root transformed.

The results of this analysis allowed selection of the

size range (13 – 18 mm) of mussels used in the other

experiments in this study. The attachment strength

of mussels within this size range was the most uni-

form (see the Results section).

Experiment 4. Illumination

To study the effect of light on mussel attachment,

the boxes with mussels were put into tanks that

were either illuminated (constant electrical incan-

descent light, ca 700 lux at the surface) or darkened.

Attachment strength was measured after either 2

or 6 d.

This experiment was replicated 10 times for each

exposure time. The two-factor ANOVA (replicate:

random factor, illumination: fixed factor) was used

to analyse the results. The trials with various

exposure times were not carried out simultaneously,

so they were analysed separately.

Experiment 5. Temperature

To examine the effect of temperature on attachment

strength, mussels were tested at 5, 10, 15, 20, 25 or

308C. Temperatures were kept constant (ca+ 18C)

by means of a refrigerator, an aquarium heater with

thermostat, or a combination of both these appli-

ances. Although such a set-up is not a very precise

method of controlling temperature, it was sufficient

to detect roughly the optimum temperature range for

mussel attachment. Mussels taken from the breeding

aquarium (without removing from the Petri dishes

to which they were attached) were acclimated for

1 week at the test temperature. The preliminary trials

showed that a longer, 2-week acclimation did not

change mussel responses. After the acclimation, the

mussels were detached from the dishes and placed in

the resocart boxes, in which they were exposed to the

test temperature for the next 6 d. Only the longer

time measurements of adhesion strength were made

to prolong the period during which mussels stayed

in the experimental conditions and to dampen the

effect of the differences between the ambient and test

temperatures on the results.

This experiment was replicated 10 times. The

mussels kept at 58C did not attach to the substratum

at all, so there was no variance within this group and

no transformation could eliminate the data hetero-

scedasticity and ensure their normal distribution.

Therefore, the results were analysed by means of the

non-parametric Kruskal-Wallis test, in which the

average attachment strengths for each box were used

as data points. The Bonferroni-corrected pairwise

Mann-Whitney U tests were applied to make the

post hoc comparisons.

Experiment 6. Conspecifics

To check whether the presence of conspecifics

influences mussel attachment, mussels were tested

in the following treatments: (i) nine mussels in a

single box placed in a tank, (ii) one mussel in a box,

nine such boxes placed in a tank, and (iii) one mussel

in a single box placed in a tank. In the first treatment,

mussels could touch one another and detect con-

specific chemical signals (if such signals exist). In the

second treatment, only detection of chemical cues

was possible, and in the third treatment mussels

were completely separated from conspecifics. Unlike

in the other experiments, the boxes consisted of

756 756 5 mm plates. Using smaller plates was

necessary to fit nine boxes into a 2406 240 mm tank

bottom. Attachment strength was measured after

either 2 or 6 d.

This experiment was replicated 10 times for each

exposure time. The square root transformed

data were analysed with the two-factor ANOVA

(replicate: random factor, treatment: fixed factor).

Results

Experiment 1. Exposure time

Mussel attachment strength increased with exposure

time (ANOVA: F9,18¼ 13.99, p5 0.0001). The

rapid increase in attachment strength was observed

during the first 4 d of exposure, after which it

144 J. Kobak

stabilised at a comparatively constant level (slightly

more than 1 N) (Figure 2).

According to these results, two exposure times

were chosen for the other experiments, viz. 2 d, to

assess initial, rapid mussel responses, and 6 d, to

measure attachment strength after its stabilisation.

Experiment 2. Substratum orientation in space

Substratum orientation did not significantly affect

mussel adhesion after 6 d (ANOVA: F2,14¼ 1.70,

p¼ 0.2143), although slightly greater attachment

strength was observed on the vertical surfaces

(Figure 3).

Experiment 3. Mussel size

Mussel attachment strength after 6 d positively cor-

related with shell length (Spearman r¼ 0.52,

p5 0.0001). Mussels could be divided into two

groups differing in attachment strength (ANOVA:

F12,188¼ 10.02, p5 0.0001): smaller (511 mm)

were comparatively weakly attached and larger

(413 mm) were strongly attached. The attachment

strengths of mussels with shell lengths from

11 – 13 mm were intermediate (Figure 4).

To make sure that differences in mussel size would

not influence the results of the other experiments,

the size class 13 – 18 mm was chosen for the rest of

this study, as the attachment strengths within this

group were the most uniform (Figure 4).

Experiment 4. Illumination

After 2 d exposure, the attachment strengths of

mussels kept in darkness were stronger than those of

illuminated mussels (Figure 5). This difference

was statistically significant (ANOVA: F1,9¼ 25.81,

p5 0.0001). After 6 d exposure, the difference

between the two treatments decreased (Figure 5)

and became insignificant (ANOVA: F1,9¼ 2.84,

p¼ 0.1255).

Experiment 5. Temperature

Temperature strongly influenced mussel attachment

(Kruskal-Wallis test: H¼ 50.32, df¼ 5, p5 0.0001).

Mussel attachment strength increased between

5 and 208C (Figure 6). The greatest attachment

strengths, found at 20 and 258C, differed significantly

from mussel adhesion at all other temperatures.

Figure 2. Mussel attachment strengths after various exposure times. Horizontal black bars at the same level¼ groups that did not differ from

each other in their attachment strength. Error bars¼SEs of the mean. Values above the abscissa (n)¼ the numbers of the mussels examined

in each group.

Figure 3. Mussel attachment strengths on variously oriented

surfaces. (See Figure 2 for key to marks and descriptions).

Attachment strength of D. polymorpha 145

Individuals tested at 58C did not attach at all, while

mussels kept at 308C were only slightly attached. The

latter group was the only one in the entire study in

which mussel mortality (34%) occurred.

Experiment 6. Conspecifics

Mussel attachment strength was greater in the

presence of conspecifics (ANOVA: F2,18¼ 7.39,

p¼ 0.0045 and F2,18¼ 5.09, p¼ 0.0173, for the 2 d

and 6 d exposures, respectively). After 2 d exposure,

the possibility of physical contacts among mussels

seemed to be unimportant. The attachment strengths

in both treatments, in which several mussels were

kept in the same tank, were similar to each other and

differed significantly from that in the single mussel

treatment (Figure 7). After longer exposure, the

results were less clear, because the average attach-

ment strength of the mussels placed in a single tank

in separate boxes did not differ from those found in

the two other treatments.

It should be noted that in Experiment 6, mussel

attachment strength measured after 2 d exposure

was much greater than in Experiments 1 and 4, des-

cribed above (compare Figure 7 with Figures 2

and 5). These results are difficult to compare,

however, due to different dates and conditions (e.g.

mussel densities and box sizes) of the experiments.

Discussion

Experiment 1. Exposure time

After the first 4 d exposure, mussel attachment

strength was stabilised at ca 1 N, which corresponds

Figure 4. Attachment strengths of mussels of various shell lengths.

(See Figure 2 for key to marks and descriptions).

Figure 5. Attachment strengths of mussels kept in darkness and in

illuminated boxes. (See Figure 2 for key to marks and

descriptions).

Figure 6. Mussel attachment strengths at different temperatures.

(See Figure 2 for key to marks and descriptions).

Figure 7. Mussel attachment strengths in the presence of

conspecifics. [9 in 1]: 9 mussels per tank in a single box; [9 in

9]: 9 mussels per tank, each in a separate box; [1 in 1]: 1 mussel

per tank. (See Figure 2 for key to other marks and descriptions).

146 J. Kobak

to the values noted earlier for zebra mussels (e.g.

Hubertz, 1994; Ackerman et al. 1995). Dormon

et al. (1997) found higher mean attachment strength

of dreissenids (ca 2 N), but with substrata that had

been exposed in the field for as much as 2 years.

On the other hand, the attachment strength of the

blue mussel, Mytilus spp., is greater by an order of

magnitude (Bonner & Rockhill, 1994; Schneider

et al. 2005), probably due to the greater hydro-

dynamic forces experienced by marine bivalves

(Hunt & Scheibling, 2001; Schneider et al. 2005).

Clarke and McMahon (1996) observed a longer

initial phase of accelerated byssal thread production

by zebra mussels, lasting for 7 d. This difference may

follow from the different materials used (resocart

in the present study vs. Plexiglas in the Clarke &

McMahon study) and/or a different method of

assessing attachment (a dynamometer was used in

the present study vs counting byssal threads in the

Clarke & McMahon study).

It should be noted that it is difficult to estimate the

real field attachment rate in a laboratory experiment.

However, laboratory experiments may be used to

detect the impact of various factors upon attachment

rate. For instance, the main emphasis in Experiment

1 was to determine the appropriate exposure times

for the other experiments carried out in this study.

The longer phase of initial attachment extends the

period during which a mussel is more vulnerable to

various environmental dangers, such as predation or

dislodgement. This is why the final attachment

strength, as well as the attachment speed, may in-

fluence mussel survival and resistance to unsuitable

environmental conditions. Therefore, in the other

experiments, attachment strength was measured

before and after the stabilisation of attachment (i.e.

after 2 and 6 d, respectively).

Experiment 2. Substratum orientation in space

Measuring the attachment strength of mussels that

moved actively onto the vertical surfaces or onto the

underside of the horizontal surfaces could cause a

bias, as the physiological condition of migrating

individuals could be better (resulting in a stronger

attachment) than that of mussels staying at the

bottom. On the other hand, the latter would have

more time for producing the byssal threads than the

relocating individuals, which would lead to their

greater attachment strength. Therefore, in order to

study the direct effect of substratum orientation

upon mussels, it was more appropriate to reorient

the position of the plates on which mussels had

preliminarily settled, as was done in this experiment.

Mussel attachment strength on variously oriented

surfaces did not differ from one another in this study.

Zebra mussels inhabit vertical substrata, such as

macrophytes (Lewandowski, 1982) or the walls of

hydrotechnical structures (Bivens et al. 1992) and

horizontal ones, e.g. boulders on the lake bottom

(Thorp et al. 1994). Some evidence exists that they are

well adapted to life on vertical substrata, and most of

the population may occupy such sites in the presence

of a competing species, the quagga mussel (Dreissena

bugensis Andrusov) (Diggins et al. 2004). The prob-

ability of losing contact with a vertical surface is

higher, as dislodgement from such a place would

always result in falling down, possibly into unsuitable

soft sediments. Thus, it could be expected that mussel

attachment on vertical surfaces would be stronger.

However, substratum orientation does not elicit such

a reaction from zebra mussels. Probably other factors

influencing attachment in the field prevail, so that the

effect of substratum position does not further increase

the adhesion strength of this species.

Recruitment of zebra mussel larvae in the field

also does not differ between vertical and horizontal

substrata (Kobak, 2005). On the contrary, some

studies reported greater abundance of mussels settling

on horizontal substrata rather than vertical substrata

(e.g. Marsden & Lansky, 2000; De Lafontaine et al.

2002). Furthermore, they were observed to settle

in greater numbers on upper horizontal surfaces

(Marsden & Lansky, 2000) or on lower hori-

zontal surfaces (Walz, 1973; Lewandowski, 2001;

De Lafontaine et al. 2002). Thus, there seems to

be no clear tendency in zebra mussel preferences for

substrata of various orientations. It suggests that

some other factors, differing with surface orientation,

rather than orientation per se, determine mussel dis-

tribution. These factors could be, for instance,

exposure to predators (Djuricich & Janssen, 2001),

light (De Lafontaine et al. 2002; Kobak, 2004),

or hydrodynamics (Kobak, 2004; 2005). Thus, re-

cruitment studies seem to support the results of

Experiment 2, showing that mussels do not respond

directly to substratum orientation.

The results of Experiment 2 justified including

mussels attached to the vertical walls of the boxes

into analyses of data from the other experiments.

Experiment 3. Mussel size

The border between strongly attached larger mussels

and weakly attached small mussels (ca 11 – 13 mm)

roughly corresponds to the division into more and

less mobile individuals (5 and 410 mm, respec-

tively) found by Toomey et al. (2002). In some

bivalves, e.g. Mytilus sp., weaker attachment strength

increases the probability of dislodgement (Schneider

et al. 2005). Higher motility may counteract its

negative consequences and help find a new attach-

ment site. On the other hand, young zebra mussel

recruits often settle onto temporary substrata, such

Attachment strength of D. polymorpha 147

as macrophytes decaying in autumn (Lewandowski,

1982) or in dense adult colonies (Wainman et al.

1996; Mortl & Rothhaupt, 2003), in which chemical

and food conditions may suddenly deteriorate

(Burks et al. 2002; Tuchman et al. 2004). Such

mussels often relocate to find a new, more suitable

site (Burks et al. 2002). Perhaps that is why young

zebra mussels do not partition much energy into

initial attachment.

Contrary to the results of this study, Rajagopal

et al. (1996) found no differences in byssal thread

production among zebra mussels of various sizes. In

another study, Rajagopal et al. (1997) demonstrated

a significant increase of byssal thread production

with mussel size, but the relationship was rather weak

and concerned only the smallest individuals (5 mm

shell length). On the other hand, a larger number of

threads reduces the stress on each individual

thread, which should result in increased attachment

strength, as found in blue mussels (Bell & Gosline,

1996; Carrington, 2002). Explaining this disagree-

ment will require further research, but it may be

speculated that the difference involves a change in

byssus quality with mussel age.

Experiment 4. Illumination

Short-time attachment strength was greater in dark-

ness but, after a longer exposure, differences in

mussel adhesion were similar for both treatments.

The earlier study demonstrated that the percentage

of individuals reattaching after exposure for 1 d in

darkness was also greater than in the light (Kobak,

2001). Zebra mussels always prefer shaded sites

rather than illuminated sites (Kobak, 2001; Toomey

et al. 2002), as well as dark substrata rather than light

substrata (Kobak, 2001). In the field, light usually

indicates a site exposed to open water and/or close to

the water surface, where threats of desiccation and

predation are more probable. That is why mussels

usually are located closer to the surface at shaded

sites, compared with areas exposed to full sun

(Hanson & Mocco, 1994). Thus, the mussel

response to light, observed in this study, could be

accounted for by a delayed attachment of the

illuminated individuals, which in the first place tried

to find a more suitable, shaded site. On the other

hand, Toomey et al. (2002) have shown that the

distances travelled by zebra mussels are similar in

darkness and in light, which seems to contradict the

above hypothesis, as an individual searching more

intensively for a better site should move a longer

distance. However, the cited authors illuminated one

end of the experimental tank to detect a directional

phototactic mussel movement. In the present study,

mussels were illuminated by uniform light, so there

was no preferable direction in the tank. They could

only move at random. This difference could modify

mussel behaviour.

The results of Experiment 4 show that, in various

conditions, mussels can reach the same final attach-

ment strength, but at a different rate. It may be

crucial for their survival, as the longer times needed

for creating firm bonds with the substratum prolongs

the period during which they are more susceptible to

environmental dangers.

Experiment 5. Temperature

Temperature clearly influenced mussel attachment

in this study. The results of other studies concerning

mussel responses to high temperature are rather

ambiguous. Rajagopal et al. (1996) found that byssal

thread production by zebra mussels was the highest

at 15 – 208C and decreased above and below these

values. Other aspects of zebra mussel biology, e.g.

lower filtration rates and higher oxygen consump-

tion, also indicate the deteriorating condition of

mussels kept at high (4288C) temperature (Aldridge

et al. 1995). On the contrary, Clarke and McMahon

(1996) demonstrated that byssal thread production

by zebra mussels increased with temperature up to

the value of 308C, which is sublethal for this species

(according to Karatayev [1995], the lethal tempera-

ture is ca 328C). The present experiment supported

the results of the study by Rajagopal et al. (1996),

although a shift toward higher temperature was

observed. Differences in acclimation temperatures

before the experiments may determine mussel re-

sponses at different exposure temperatures (Payne &

Aldridge, 1993).

A variety of results also have been obtained in

studies of the effects of temperature upon the marine

mussel Mytilus edulis. Some authors reported a

decrease in byssal thread production at high tem-

perature (Van Winkle, 1970), while others observed

no such inhibition just below the lethal temperature

(Young, 1985).

The observation reported here, that low tempera-

ture inhibited zebra mussel attachment, confirmed

the results of other studies (Clarke & McMahon,

1996; Rajagopal et al. 1996). Inhibited attachment

probably was caused by a lower metabolic rate,

resulting in the production of fewer byssal threads.

Temperatures of 10 and 158C, at which the reduced

attachment strength was observed, are regarded as

the lower limits for growth and spawning of zebra

mussels, respectively (Karatayev, 1995).

Experiment 6. Conspecifics

Mussels responded positively to the presence of

conspecifics, regardless of the possibility of physical

contacts among them. The results of a short-time

148 J. Kobak

(1 d) reattachment study of zebra mussels (Kobak,

2001) were similar, suggesting that chemical sub-

stances released by conspecifics might be involved in

this behaviour. Aquatic invertebrates commonly use

chemical cues to find a settlement site, mating partner,

or food (Zimmer & Butman, 2000). Zebra mussels are

gregarious animals, living in high densities and

preferring settlement in the presence of conspecifics

or their shells (Wainman et al. 1996; Mortl &

Rothhaupt, 2003). For blue mussels, this mode of life

provides protection against predators (Reimer &

Tedengren, 1997; Reimer & Harmsringdahl, 2001)

and enhances the vicinity of mating partners

(particularly important for a sessile animal).

In Mytilus sp., the attachment strength of indivi-

duals living in dense colonies is lower than that of

solitary mussels, because the former are subjected

to relatively smaller hydrodynamic forces (Bell &

Gosline, 1997). In the present study, however, zebra

mussels seemed to respond to the presence of

conspecifics by increasing their attachment strength,

even if they could touch each other during the test.

This may be a result of comparatively low experi-

mental density (but was sufficient to elicit a clear

response of mussels). Otherwise, chemical stimuli

released by conspecifics might enhance byssal thread

production, as demonstrated for another freshwater

bivalve, Limnoperna fortunei (Uryu et al. 1996).

Conclusions

The results presented show that the attachment

strength of zebra mussels is modified by a number of

environmental factors, such as light, temperature and

conspecifics, as well as mussel size and exposure

time. Surface orientation in space does not influence

mussel adhesion strength.

Sometimes the effect of a given factor (e.g. light)

can be observed only during the initial phase of

mussel attachment and later disappears, but still may

be essential for assessing their relationship with the

environment.

As the same substratum type was used in all the tests,

modification of byssal thread production seems to be

the most probable reason for the observed differences.

Because the attachment status influences zebra

mussel responses to various stimuli, e.g. resistance to

toxins (Rajagopal et al. 2002), the above-mentioned

factors must be taken into account in studies of

bioindication techniques involving mussels.

Acknowledgements

I am grateful to Mr Andrzej Denis, Szymon Denis

and Jozef Liczkowski for collecting the mussels for

the experiments.

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