byssogenesis of invasive marine mussels perna viridis and perna perna: implications for their...

MARINE

Marine Environmental Research 62 (2006) 98–115

www.elsevier.com/locate/marenvrev

ENVIRONMENTAL

RESEARCH

Mussel colonization of a high flow artificialbenthic habitat: Byssogenesis holds the key

S. Rajagopal a,*, V.P. Venugopalan b, G. van der Velde a,H.A. Jenner c

a Department of Animal Ecology and Ecophysiology, Institute for Wetland and Water Research,

Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlandsb Biofouling and Biofilm Processes Section, BARC Facilities, Kalpakkam 603 102, India

c KEMA Environmental Research and Sustainables, P.O. Box 9035, 6800 ET Arnhem, The Netherlands

Received 16 September 2005; received in revised form 27 February 2006; accepted 8 March 2006

Abstract

Water flow is an important characteristic determining the settlement and growth of macro-invertebrates in the marine environment. Intake systems of coastal power stations offer a uniqueopportunity to study the effect of water flow on benthic organisms under field conditions. Thecooling water intake system of a tropical coastal power station is used as an experimental facilityto study the effect of flow on the recruitment and growth of three mussel species, viz, Brachidontes

variabilis, B. striatulus, and Modiolus philippinarum. The study was prompted by earlier observa-tion that these mussels were numerically abundant in the biofouling community present insidethe seawater intake tunnel of the power station, even though their occurrence in the benthic com-munity in the coastal waters outside was only nominal. Recruitment data showed that the threemussel species very successfully colonised surfaces exposed to the intake mouth (characterisedby relatively high flow) of the power station. Significant difference was observed in the recruitmentrecorded at the intake point and the ambient environment outside. Under high flow condition, thegrowth rates of all the three mussel species were uniformly enhanced. It is argued that recruitmentof the different species is related to the number of byssus threads produced by each mussel and thestrength of the byssus threads. The results indicate that byssus number and byssus strength of themussels are important criteria that decide successful colonization and establishment in high flowenvironments.� 2006 Elsevier Ltd. All rights reserved.

0141-1136/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marenvres.2006.03.004

* Corresponding author. Tel.: +31 24 3652417; fax: +31 24 3553450.E-mail address: [email protected] (S. Rajagopal).

mailto:[email protected]

S. Rajagopal et al. / Marine Environmental Research 62 (2006) 98–115 99

Keywords: Mussels; Biofouling; Benthic community; Seawater intake; Water flow; Recruitment; Growth rate;Physiology

1. Introduction

In coastal benthic environments where attachment substratum is a limiting factor, ses-sile organisms employ a wide range of competitive strategies (Smedes and Hurd, 1981;Nandakumar, 1996). However, local factors may play a very important role, modifyingthe competitive capabilities of species. Competitively dominant groups, such as mussels,are often successful benthic colonisers due to their large body size, fast growth rate,longevity and their ability to overgrow other colonisers (Richmond and Seed, 1991;Rajagopal et al., 2005). Since they lead an attached mode of life, water flow plays animportant role in their settlement, growth and physiological responses. Mussels are avery successful group that colonise the cooling water systems of coastal electric powerstations. Such artificial habitats are characterized by high flow rates (Perkins, 1974; Neit-zel et al., 1984; Rajagopal, 1997; Rajagopal et al., 1998), which ensure increased larvalflux rate, increased availability of suspended food, faster oxygen replenishment and effi-cient waste product removal (Nixon et al., 1971; Perkins, 1974; Rajagopal, 1991; Wildishand Kristmanson, 1997). Such habitats provide unique opportunities to test hypothesesregarding the influence of flow on marine benthic invertebrates (Venugopalan et al.,1991).

We examined the relative distribution of three mussel species, Brachidontes variabilis

(Krauss), Brachidontes striatulus (Hanley) and Modiolus philippinarum (Hanley), growingon the intake screens (located at the mouth of the intake tunnel) of a coastal power plantand compared the data with data from the adjoining coastal waters. The objective was totest the hypothesis that increased water flow at the intake gates would modify the recruit-ment of the three mussel species in such a way that colonisation pattern on the intakescreens would be significantly different from that in the adjacent coastal waters. Since bys-sus threads play a significant role in the colonisation of mussels, it was also hypothesisedthat byssogenic activity of the mussel species would play a major role in the colonisation ina high flow environment.

2. Materials and methods

2.1. Site description

The study was carried out using the seawater intake system of Madras Atomic PowerStation (MAPS) as the artificial (specialised) intake habitat. The power station is locatedat Kalpakkam (12�32 0N and 80�11 0E), 65 km south of Chennai (Madras) on the east-coastof India (Fig. 1a). MAPS is a seawater cooled station and uses a 468 m long sub-seabed(located 50 m below the mean sea level) tunnel to draw cooling water (design flow rate35 m3 s�1) from the Bay of Bengal for its twin reactors (for details refer Rajagopalet al., 1991). The submerged intake mouth is located at a depth of about 7 m. The seawaterenters the intake shaft of the tunnel through 16 windows, each 3.2 m high and 2 m wide,

Fig. 1. (a) Map showing Kalpakkam, (b) schematic representation of the Madras Atomic Power Station seawaterintake tunnel showing two sampling stations (not drawn to scale).

100 S. Rajagopal et al. / Marine Environmental Research 62 (2006) 98–115


located radially around the circular intake structure. From the intake shaft, seawater flowsby gravity via the tunnel to a forebay pump house, from where it is pumped to the steamcondensers. Each one of the submerged seawater intake windows is guarded by a steelweld mesh screen (intake screen) to prevent the entry of debris into the cooling circuit.The maximum seawater velocity in the tunnel when all pumps are running is about3 m s�1 (Madras Atomic Power Station Design Manual, 1975). Chlorine (antifouling bio-cide) is added in the intake tunnel, a short distance downstream of the intake windows.

2.2. Field studies

2.2.1. Species and sampling stations

Three mussel species namely B. variabilis, B. striatulus and M. philippinarum were cho-sen for the study. The mussels were selected as they were found to be numerically abun-dant fouling species during a diving inspection of the tunnel. Distribution of themussels was studied at two stations. Station 1 (Sta 1), representative of the coastal waters,was 8 m deep and experienced coastal currents of velocity of the order of 0.1–0.3 m s�1.Station 2 (Sta 2) was located at the seawater intake point of the power station, which ischaracterised by high water velocity (up to 3 m s�1, depending on the number of seawaterpumps in operation). The samples were collected from steel mesh screens placed radiallyaround the circular intake structure. Sta 1 represented the natural habitat, while Sta 2 rep-resented the intake habitat (high flow rates, but no chemical stress). The distance betweenSta 1 and Sta 2 was about 25 m. The physicochemical characteristics of the water were,therefore, identical at both the stations, except for water velocity. Since Sta 2 (intakescreens) was located upstream of the chlorine dosing point, it did not encounter any resid-ual chlorine.

2.2.2. Hydrographical features

Hydrographical features of the study site were studied by collecting surface water sam-ples at fortnightly intervals during January 1989 to December 1989. Parameters like tem-perature, salinity, dissolved oxygen (DO) and chlorophyll-a were monitored (Stricklandand Parsons, 1972) to understand their variation and their possible influence on the settle-ment pattern and growth rate of the mussels.

2.2.3. Recruitment

Concrete blocks (20 · 20 · 20 cm) were used to sample spatfall in coastal waters (Sta 1),as described by Nair et al. (1988) and Rajagopal et al. (1997). Three test blocks were sus-pended at 1, 4 and 7 m using nylon ropes and retrieved after 30 d to estimate spatfall. AtSta 2, spat samples were collected from the steel intake screens at three depths, viz., 2, 4and 6 m. The samples were collected randomly from three (out of the total 16) differentintake screens (facing three different directions) and treated separately as triplicates. Ear-lier trials had shown that mussel settlement on the steel surfaces was comparable to that onconcrete surface (Rajagopal et al., 1998). The samples (in triplicate) were collected at eachdepth, and the data were averaged and represented as numbers dm�2 (dm2 = 100 cm2).

2.2.4. Growth rateGrowth measurements were initiated by suspending concrete test blocks at 1 m (Sta 1)

at the beginning of spat settlement (April 1989). Every month about 30 mussels of each


species were randomly collected from the concrete blocks (1 m depth at Sta 1) and fromthe intake screens (2 m depth at Sta 2) for monitoring growth. For more details, referRajagopal et al. (1998).

Fig. 2. Seasonal variations in the hydrographic parameters (a) temperature, (b) salinity, (c) dissolved oxygen and(d) chlorophyll-a) in Kalpakkam coastal waters from January 1989 to December 1989. Data are presented asmean ± SD.

Table 1Numerical abundance (number dm2) and relative percentage contribution of various mussel species (Sta 1:Coastal waters at 4 m depth; Sta 2: Intake screens at 4 m depth; data from long term panels)

Station P. viridis P. perna B. variabilis B. striatulus M.

philippinarum

Numericalabundance

% Numericalabundance




%

Coastal waters 529 87.1 3 0.4 18 3.0 46 7.7 10 1.7Intake screen 775 65.2 12 1.0 48 4.0 243 20.5 111 9.3

P, Perna; B, Brachidontes; M, Modiolus.


2.3. Laboratory studies

2.3.1. Test animals

The byssus thread production of different mussel species was studied in the laboratory,using mussels collected from coastal waters near the MAPS intake area. In the laboratory,the mussels were acclimated for two weeks in seawater under the following conditions:salinity 34.2 ± 0.5&, temperature 29.2 ± 0.7 �C temperature and dissolved oxygen5.7 ± 0.6 mg l�1 dissolved oxygen.

2.3.2. Byssus thread production

After the acclimation period, the mussels were carefully cleaned of epizoic organismsand their byssus threads were nipped off with scissors, disturbing the animal as little aspossible. The mussels were placed on a rectangular plexiglass slate (refer Rajagopalet al., 1996 for schematic representation of experimental set-up) in each of the twenty11 capacity experimental chambers (one mussel in each chamber). The experiments wereconducted under static conditions. The cumulative number of threads produced by mus-sels (1 mussel in each experiment · 20 replicates · 3 size groups · 3 species = 180 mussels)was recorded daily over a 7 day period following procedures outlined by Clark andMcMahon (1996) and Rajagopal et al. (1996).

Byssus thread production and byssus strength were also studied under dynamic condi-tions using laboratory shaker (Rajagopal et al., 1996). The experimental mussels weretaken in a 1 l glass beaker and placed on the shaker, set at 0 (control), 5, 10 and20 cycles min�1. The number of byssus threads produced by each mussel was countedafter 24 h. Similarly, detachment strength of the mussel byssus was measured using aspring balance (Holmes, 1970; Lee et al., 1990; Rajagopal, 1991). The mean strength ofa single byssus thread (byssus thread strength) was calculated from the total number ofthreads and weight required to break the byssus attachment (Lee et al., 1990; Rajagopal,1991).

2.4. Statistical analysis

A three-way analysis of variance (ANOVA) was used to examine variability in spatrecruitment and growth of mussels, taking into account season (sampling time), samplingstation and different species as independent variables (Zar, 1984). Spat recruitment of mus-sels at different depths was tested by one-factor ANOVA (Sokal and Rohlf, 1981). for


post-hoc comparison of monthly means, we used student t-tests for comparison of twomeans and Student–Neuman–Keuls (SNK) tests for comparison of multiple means(Zar, 1984). Prior to the analysis the data were tested for normality and homogeneity of

Fig. 3. Monthly variations of recruitment of Brachidontes variabilis, Brachidontes striatulus and Modiolus

philippinarum along with Perna viridis (Rajagopal et al., 1998) at different depths in coastal waters from January1989 to December 1989. Recruitment data of P. viridis are plotted on the right Y-axis. Data are presented asmean ± SD (n = 3).


variance. All analyses were performed using a Statistical Analysis Systems package (SAS,1989).

3. Results

3.1. Environmental data

Seawater temperature showed a bimodal pattern of distribution with higher tempera-tures recorded during May–June and October (Fig. 2). Salinity was uniformly highthroughout most of the study period (between 33& and 34&), except during November,when there was significant reduction in salinity caused by rainfall. Chlorophyll-a showedhigher values during May–June and August. A definite pattern of dissolved oxygen con-centrations was not discernible (Fig. 2).

3.2. Relative distribution of mussels at the two sites

Data on the relative composition of various mussel species collected from the long-termblocks exposed in the coastal waters and intake screens are presented in Table 1. The threemussel species were present in the coastal waters (Sta 1), in terms of number of individuals,in the order B. striatulus > B. variabilis > M. philippinarum. However, on the intake screens

Table 2Data on spat settlement of Brachidontes variabilis, Brachidontes striatulus and Modiolus philippinarum at differentdepths in coastal waters and intake screens from January to December 1989

Coastal waters Intake screens

Depth Depth

1 m 4 m 7 m 2 m 4 m 6 m

Brachidontes variabilis

Mean 11 28 14 18 21 21SD 15 34 22 25 29 31SE 4 10 6 7 8 9Lower 95% Cl 1 7 1 2 3 1Upper 95% Cl 20 49 28 33 40 39ANOVA F = 4.020; P < 0.05 F = 0.6853; P > 0.05

Brachidontes striatulus


Modiolus philippinarum


One-way ANOVA followed by multiple comparison tests (SNK tests) were used to determine whether depthwisesettlement of mussels differed significantly in coastal waters and intake screens.

Fig. 4. Monthly variations of recruitment of Brachidontes variabilis, Brachidontes striatulus and Modiolus

philippinarum along with Perna viridis (Rajagopal et al., 1998) at different depths on intake screens of Madrasatomic power station from January 1989 to December 1989. Recruitment data of P. viridis are plotted on the rightY-axis. Data are presented as mean ± SD (n = 3).


(Sta 2) the corresponding composition was B. striatulus > M. philippinarum > B. variabilis.The relative contribution of B. striatulus increased from 7.7% in coastal waters to 20.4% onthe intake screens. The corresponding increases in the case of B. variabilis (from 3.0% to


4.0%) and M. philippinarum (from 1.7% to 9.3%) were relatively small. In contrast, the per-centage contribution of Perna viridis, another prominent mussel in the coastal benthic hab-itat, decreased from 87.1% in the coastal waters to 65.1% on the intake screen (Table 1).

3.3. Recruitment

The data on spat recruitment of B. variabilis, B. striatulus and M. philippinarum in thecoastal waters (Sta 1) are given in Fig. 3. B. striatulus was recorded in relatively large num-bers when compared to the other two mussels; M. philippinarum showed the lowest recruit-ment rates among the three (SNK tests, P < 0.001). Recruitment density in the coastalwaters was maximum at 4 m depth (Table 2), when compared to either 1 or 7 m depth(ANOVA, F(2,33) = 3.72, P < 0.05). However, the recruitment pattern (Fig. 4) was differenton the intake screens (Sta 2). Here, the overall recruitment density was significantly highwhen compared to that at Sta 1 (ANOVA, F(1,22) = 194.07, P < 0.0001). Moreover, depth-wise differences in recruitment were not significant at Sta 2 (ANOVA, F(2,33) = 0.53,

Fig. 5. Relationship between recruitment of Perna viridis (Rajagopal et al., 1998) and other mussel species inKalpakkam coastal waters and on the intake screens of Madras atomic power station.

Fig. 6. Relationship between percentage recruitment of Brachidontes variabilis, Brachidontes striatulus, Modiolus

philippinarum, Perna perna (Rajagopal, 1991) and Perna viridis (Rajagopal et al., 1998) in Kalpakkam coastalwaters and intake screens of Madras atomic power station.


P > 0.05). P. viridis settled at high frequencies at both the stations but their density wasrelatively greater at Sta 2 (Figs. 3 and 4). The relative distribution of the mussel recruit-ment at Sta 1 and Sta 2 are given in Figs. 5 and 6, in which the differences in the settlementpattern of the mussels in the coastal waters and also in the artificial habitat can be clearlyseen. B. striatulus formed about 50% of the total spat settled at both the stations, irrespec-tive of P. viridis recruitment. While B. striatulus was the dominant mussel species at Sta 2,M. philippinarum consistently colonised in excess of B. variabilis (t-tests, P < 0.001), indi-cating the selection of M. philippinarum as compared to the latter (Fig. 6).

3.4. Growth rate

All the three mussel species registered significantly higher growth rates (increase in shelllength) on the intake screens (Fig. 7) as compared to the coastal waters (ANOVA,F(1,22) = 4.86, P < 0.005). At Sta 1, B. variabilis grew at the rate of 6.2 mm in 30 days,and reached 23.4 mm in 370 days (Fig. 7). At the same time, B. variabilis growing atSta 2 showed a significantly higher growth rate of 9.7 mm in 49 days and had a shell lengthof 29.6 mm after 375 days (t-tests, P < 0.0001). About 27% increase in the growth rate ofB. variabilis was observed at Sta 2, when compared to Sta 1. The difference in percentage


increase of growth rate among the three species was not significant, indicating that higherflow benefited the three species to the same extent. P. viridis also showed a significantincrease (24% after one year) in growth rate at Sta 2, as compared to that at Sta 1 (Rajag-opal et al., 1998).

3.5. Byssus thread production

The differences in byssus thread production between the species were significant (specieseffect; F(2,177) = 231.59, P < 0.0001, Fig. 8). For example, 7 mm long B. striatulus

Fig. 7. Seasonal variations in the growth rate of Brachidontes variabilis, Brachidontes striatulus and Modiolus

philippinarum in Kalpakkam coastal waters and on the intake screens of Madras atomic power station. Data arepresented as mean ± SD (n = 27–30).

Fig. 8. Cumulative byssus thread production in different size groups of Brachidontes variabilis, Brachidontes

striatulus and Modiolus philippinarum over a period of 7 days. Data are presented as mean ± SD (n = 20).


(184 threads mussel�1 after 7 days) produced more threads per animal than comparablysized M. philippinarum and B. variabilis (SNK tests, P < 0.001). Smaller individuals ofall the three species invariably produced more number of threads than their larger coun-terparts (size effects; F(2,177) = 38.53, P < 0.0001). For comparison, byssus thread produc-tion data of P. viridis is given in Fig. 9. Experimental methods (including mussel size andage) used for P. viridis were similar to those used for the other three mussel species. Thebyssus thread production of P. viridis (95 ± 6 threads in 7 days) was relatively lower thanthat of B. striatulus, M. philippinarum and B. variabilis.

Data on byssus thread production under dynamic conditions are presented in Table 3.Byssus thread production and strength of B. striatulus, B. variabilis and M. philippinarum

significantly increased with increasing frequency of agitation (see Table 3).

Table 3

Effect of mechanical shaking on byssus thread production (threads mussel�1 day�1; mean ± SD; n = 20) and byssus strength

(g thread�1 mussel�1; mean ± SD; n = 20) of Brachidontes variabilis (shell length in mm ± SD: 16.3 ± 1.4), Brachidontes

striatulus (18.9 ± 1.5) and Modiolus philippinarum (22.7 ± 1.9)

Musselspecies

Mechanical shaking rate (cycles min�1)

0 5 10 20

Byssusproduction

Byssusstrength

Byssusproduction

Byssusstrength

Byssusproduction

Byssusstrength

Byssusproduction

Byssusstrength

B. variabilis 49 ± 6 4.2 ± 1.1 56 ± 7 4.9 ± 1.0 64 ± 7 5.3 ± 1.4 69 ± 8 5.4 ± 1.3B. striatulus 71 ± 9 6.9 ± 1.6 86 ± 8 8.1 ± 1.5 91 ± 8 8.7 ± 1.6 98 ± 9 9.2 ± 1.7M. philippinarum 59 ± 7 5.3 ± 1.3 63 ± 8 6.0 ± 1.2 72 ± 9 6.9 ± 1.1 80 ± 8 7.7 ± 1.5

Fig. 9. Relationship between cumulative byssus thread production of Brachidontes variabilis, Brachidontes

striatulus, Modiolus philippinarum, Perna perna (Rajagopal, 1991) and Perna viridis (Rajagopal, 1997) over aperiod of 7 days. Data are presented as mean ± SD (n = 20).


4. Discussion

Bivalve mussels constitute an important constituent of hard bottom communities incoastal waters all over the world. Among them, mussels are a very successful group, oftenforming extensive biomass accumulations on rocks, jetty piers, navigation buoys etc. Inthe warm coastal waters of the Indo-Pacific regions, P. viridis and related species (P. perna

and P. canaliculus) have been reported to be important species and therefore, goodamount of work has already been done on them (Lee, 1988; Cheung, 1993; Rajagopalet al., 2005). Nevertheless, B. striatulus, B. variabilis and M. philippinarum, all of compa-rable size, occasionally form an abundant group of macro-invertebrates in the hard bot-tom benthic communities (Morton, 1977; Jenner et al., 1998). In addition, they havealso been reported to constitute important pest organisms fouling seawater intake systems


of power stations (Rajagopal, 1997). We have earlier reported significant occurrence of thethree mussel species (B. variabilis, B. striatulus and M. philippinarum) in the cooling waterconduits of Madras Atomic Power Station (Rajagopal et al., 1991). However, not muchdata exist on the effect of flow on the settlement and growth of these mussel species.

Analysis of the environmental data showed that temperature distribution in the coastalwaters of Kalpakkam was characterized by two well-defined maxima – one in May–Juneand the other in October (Fig. 2). The seasonal distribution of salinity and chlorophyll-aalso showed relatively high values in summer (May–June). Recruitment patterns of mostsessile benthic macro-invertebrates, especially barnacles and mussels, also followed abimodal pattern, with a peak during May–June and another in September–October (Sas-ikumar, 1991; Rajagopal, 1997). Recruitment of the three mussel species in the coastalwaters also showed certain similarities. There were two peaks during May–June andAugust–September. For comparison, recruitment of P. viridis is given in Figs. 3 and 4.It shows two clear peaks, one in April–June and another in October. We have earlierreported that the spawning period, gonadal index and larval abundance of P. viridis matchwith the temporal distribution of temperature and that the intensity of spatfall was relatedto chlorophyll-a concentration in the coastal waters (Rajagopal et al., 1997, 1998). In theother three mussel species, the spatfall intensity is comparatively weak, when compared tothat of P. viridis (Figs. 3 and 4). The second peak in spatfall is relatively subdued inP. viridis, while in the other three mussel species, the second peak is much more pro-nounced than the first one. Apparently, the environmental factors which influence theintensity of spatfall in three mussels are not the same as in the case of P. viridis. The singlemost important limiting resource for sessile organisms in the intertidal zone is substratumspace required for settlement (Seed, 1976). According to Rajagopal (1997), P. viridis is acompetitive and dominant species in the intertidal community and monopolises the spaceresources quite efficiently. One may argue that recruitment of the three minor mussel spe-cies during April–June was suppressed by the heavy recruitment of P. viridis (Fig. 5). How-ever, this kind of domination by P. viridis over the three minor mussel species observed inthe coastal waters was not observed in the altered flow regime existing at the intake point(Fig. 5).

In the coastal waters, recruitment of B. striatulus was consistently high (59%), followedby that of B. variabilis (29%). M. philippinarum (12%) showed the least amount of recruit-ment (Fig. 6). However, at Sta 2, which is characterized by high flow rate, we observed achange in the relative distribution of the three mussel species. While B. striatulus (61%)was numerically still the most abundant, M. philippinarum (28%) consistently settled inexcess of B. variabilis (11%) (ANOVA, P < 0.005). This observation points to the impor-tance of flow regimes in species selection among the three bivalves. It appears that M. phil-ippinarum, which is byssogenically more active (Fig. 9), has a selective advantage over B.

variabilis, which produces comparatively less number of byssus threads. The higher num-ber of byssus threads produced by B. striatulus and M. philippinarum may have permittedthem to retain their attachment under high flow conditions and possibly has been respon-sible for their higher abundance at Sta 2 (Fig. 4). It may be noted that increased watervelocity at Sta 2 did not specifically favour any of the three mussel species in increasingits byssus thread production or byssus strength. Flow enhanced both the parameters moreor less uniformly in all the three species (Table 3). Nevertheless, the inherent ability of B.striatulus and M. philippinarum to produce greater number of stronger threads held themin good stead under altered flow conditions at Sta 2.


Mussel larvae, in general, are capable of settling at high water velocities (Neitzel et al.,1984; Rajagopal et al., 2005). Neitzel et al. (1984) reported that at velocities as high as3.5 m s�1, mussels could settle and colonise new surfaces. In the present study, the two sta-tions, though located close to each other, were different in terms of their flow characteristics.Recruitment data show that depth-related variation, which is obvious at Sta 1, is lacking atSta 2 (Table 2). At Sta 1, the mussels clearly prefer the intermediate depth (4 m), when com-pared to the near-surface (1 m) or near-bottom (7 m) (Fig. 3). This behaviour is probablydue to the subtidal habitat of these mussels (Rajagopal, 1991). Depth-wise differences inspatfall are likely if the larvae are non-uniformly distributed in the water column or alter-natively, the settling larvae prefer discrete light regimes (Rajagopal et al., 2005). However,this depth-related feature was not apparent at high flow conditions (as was observed at theSta 2, Fig. 4), owing probably to higher water flow, which would have disturbed any verticaldistribution of larvae or prevented the larvae from exercising their light preferences.

Growth rate data of the three mussel species in the coastal waters as well as at theintake screens clearly showed that growth rates were enhanced at the latter station. Thisobservation emphasizes the importance of flow in influencing growth rate of filter-feedingmussels (Nixon et al., 1971; Seed, 1976; Rajagopal et al., 1998). In an earlier paper, Rajag-opal et al. (1998) demonstrated the importance of flow conditions in the recruitment andgrowth rate of P. viridis. The present data showed that growth rates of all the three musselspecies were uniformly enhanced at higher flow conditions. Differences in flow enhancedgrowth rates cannot, therefore, account for the differences in the abundance of musselsbetween Sta 1 and Sta 2.

The three mussel species, though not major bivalve species in the hard bottom commu-nity in the coastal waters of Kalpakkam, are the major group in the specialized habitatcreated by the power station. If we examine the relative composition of the three mussels(B. variabilis, B. striatulus and M. philippinarum), it is obvious that the ability to producemore byssus threads provided a selective advantage in the high flow conditions that existedat Sta 2. Consequently, B. striatulus and M. philippinarum scored over B. variabilis (interms of numerical abundance) under higher flow conditions. This observation supportsour hypothesis that byssus production is an important factor that determines the coloni-sation of the three species of mussels in a high flow environment as that exists at the intakepoint of a coastal power station.

5. Conclusions

1. Higher flow rate significantly enhanced the growth rate and recruitment of B. striatulus,

B. variabilis and M. philippinarum.2. Mussel species with relatively high byssus thread production (as well as greater byssus

strength) were selected inside the cooling water circuit, where high flow conditionsexisted.

3. Due to their higher byssus production rate and greater byssal strength, B. striatulus andM. philippinarum became numerically dominant mussel species inside the intake tunnelin comparison to B. variabilis; they appear to be more adapted for survival in suchconditions.

4. Altered benthic habitats, such as those provided by power station intake systems offerexcellent settings to study the effect of environmental factors on communitydevelopment.


Acknowledgements

We express our thanks to The Chief Superintendent of Madras Atomic Power Station forproviding facilities. Financial support from Council of Scientific and Industrial Research,New Delhi, India and Department of Animal Ecology and Ecophysiology, RadboudUniversity Nijmegen, The Netherlands is gratefully acknowledged. This is publication num-ber 392 of the Centre for Wetland Ecology.

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byssogenesis of invasive marine mussels perna viridis and perna perna: implications for their...

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