INTRODUCTION

Mytilus is widely distributed throughout thecooler waters of the northern and southern hemi-spheres and is commonly found dominating thecommunities of littoral and shallow sub-littoral

shores (Seed and Suchanek, 1992). Recent morpho-logical and genetic evidence strongly suggests thatMytilus edulis chilensis from the Falkland Islands issynonymous with northern Mytilus edulis(McDonald et al., 1991). M. edulis is the most wide-ly distributed species within the genus and has theability to withstand a wide range of fluctuating envi-ronmental conditions (Seed and Richardson, 1990and references therein).

ECOLOGY OF MYTILUS EDULIS CHILENSIS IN THE FALKLAND ISLANDS 39

SCI. MAR., 61 (Supl. 2): 39-48 SCIENTIA MARINA 1997

ECOLOGY OF MARINE MOLLUSCS. J.D. ROS and A. GUERRA (eds.)

Reproduction and growth of Mytilus edulis chilensisfrom the Falkland Islands*

A.P. GRAY, R. SEED and C.A. RICHARDSON

School of Ocean Sciences, University of Wales-Bangor, Menai Bridge, Gwynedd LL59 5EY, UK.

SUMMARY: Although widely distributed and locally abundant throughout much of the Falkland Islands, surprisingly littleinformation is available for M. edulis in this particular part of its geographic range. In this paper we therefore document sev-eral ecologically important aspects of mussel populations from three sites. Annual reproductive cycles were determinedstereologically from stained histological preparations. Reproductive development proceeded through early to late spring(September-December), whilst spawning occurred mainly during the southern summer (January-February). The age andgrowth rate of individual mussels were determined from microgrowth bands present in acetate peel replicas of shell sections.Growth curves fitted using the von Bertalanffy growth model predicted L∞ and k values ranging between 52.9 and 73.6 mmand 0.419 and 0.579, respectively. Population structure varied significantly between study sites, and there was no congru-ence between modal distributions and the size at age estimated from shell sections. The incidence (ranging from 2-46%) ofa hitherto unidentified isopod living within the mantle cavity, and its effect on host condition, are described.

Key words: Mytilus edulis, growth, reproduction, association, isopod, Falkland Islands.

RESUMEN: REPRODUCCIÓN Y CRECIMIENTO DE MYTILUS EDULIS CHILENSIS EN LAS ISLAS MALVINAS. – Aunque es una especiede amplia distribución y es localmente abundante en gran parte de las islas Malvinas, es sorprendente que se disponga depoca información sobre M. edulis en esta zona concreta de su área de distribución. En este artículo se documentan variosaspectos importantes desde el punto de vista ecológico de poblaciones de mejillones de tres localidades. Los ciclos de repro-ducción anuales se determinaron estereológicamente a partir de preparaciones histológicas teñidas. El desarrollo reproduc-tor se extiende desde principios a finales de primavera (septiembre-diciembre), mientras que la freza tuvo lugar principal-mente durante el verano austral (enero-febrero). La edad y la tasa de crecimiento de individuos concretos se determinarona partir de las bandas de microcrecimiento presentes en réplicas de acetato de secciones de la concha. Las curvas de creci-miento ajustadas utilizando el modelo de crecimiento de von Bertalanffy predijeron valores de L∞ y de k que oscilan entre52.9 y 73.6 mm y 0,419 y 0.579, respectivamente. La estructura de la población varió de forma significativa entre las dis-tintas localidades de estudio, y no se advirtió congruencia entre las distribuciones modales y el tamaño a la edad estimadaa partir de las secciones de la concha. Se describe la incidencia (del 2 al 46%) de un isópodo hasta ahora no identificado quevive en el interior del manto, así como su efecto sobre la condición del patrón.

Palabras clave: Mytilus edulis, crecimiento, reproducción, asociación, isópodo, islas Malvinas.

*Received October 1995. Accepted August 1996.

The reproductive cycle in M. edulis varies, bothspatially and temporally and generally also exhibitsa marked seasonal pattern. The timing and durationof this cycle probably results from complex interac-tions between several exogenous and endogenousfactors. Although temperature and food supply areknown to be principal regulators (Bayne, 1975;Newell et al.,1982), mussels will develop gameteseven when temperatures are close to zero (Kautsky,1982a), suggesting that the quantity and quality offood supply are the principal limiting factors (Bayneand Worrall, 1980; Newell et al.,1982).

Growth in Mytilus has been extensively docu-mented. The growth history of each mussel is per-manently recorded in the shell either as a series ofannual checks in the periostracum (Seed, 1969a;Davenport et al., 1984) and/or inner nacreous layers(Lutz, 1976; Kautsky, 1982a), or as microgrowthbands present in the middle prismatic layer(Richardson 1989; Richardson et al.,1990). Size-fre-quency distributions have also been used successful-ly by some authors to assess population growth (e.g.Bayne and Worrall, 1982; Rodhouse et al.,1984). Inthe case of M. edulis however, this method often haslimited application due to the generally extendedperiod of recruitment and/or variable individualgrowth rates which lead to the merging or overlap ofsuccessive year classes (Seed, 1976; Kautsky,1982b).

Although widely distributed and locally abun-dant throughout much of the Falkland Islands, thereis surprisingly little information on Mytilus in this

part of its geographic range. This paper investigatesthe growth rates and annual reproductive cycles inmussels from three sites in the Falkland Islands andbriefly reports the presence of a hitherto unidentifiedisopod living within the mantle cavity of the hostmussel.

MATERIALS AND METHODS

The selected study sites lie on the relatively shel-tered isthmus that joins the northern and southernparts of East Falkland (Fig. 1). The three sites:Darwin, Goose Green and Camilla Creek are shel-tered from strong wave action and the substratumranges from exposed bed-rock and shingle to shingleand mud. Camilla Creek is situated in an estuarywith a salinity of approximately 28 ppt, whereasGoose Green and Darwin are coastal beaches withsalinities of 34-36 ppt. Tides in the Falkland Islandsare mixed semi-diurnal with a maximum tidal rangeof approximately 1.90 m. Average monthly seawatertemperatures were estimated from daily records col-lected between September 1993 and March 1995 byTinyTalk temperature loggers (Orion ComponentsChichester Ltd).

In order to determine the timing and duration ofthe reproductive cycle 25 mussels, 40 - 50 mm inshell length, were collected monthly from the mid-dle of the intertidal mussel zone (~ +0.7 m c.d.) ateach site. A section of mantle tissue was excisedfrom each mussel and fixed in 5% saline buffered

40 A.P. GRAY et al.

FIG. 1. – Locations of the study sites in the Falkland Islands.

ECOLOGY OF MYTILUS EDULIS CHILENSIS IN THE FALKLAND ISLANDS 41

FIG. 2. – Photomicrographs of gonads at different stages in the annual cycle. A, Resting or spent. B, Developing stage III, male (left) andfemale (right). C, Stage V, fully ripe male. D, Stage V, fully ripe female. E, Partially spawned stage III male. F, Partially spawned stage I

female.

formalin for at least 24 hours then stored in 70%alcohol. Tissues were embedded in paraplast wax,sectioned at 7 µm and stained with Ehrlich’s haema-toxylin. From these histological preparations a meangonad index (G.I.) was determined using the schemepreviously described by Seed (1969b, 1976), where-by each individual mussel can be assigned to one ofseveral stages depending on whether the gonad is inthe developing, ripe, spawning or spent condition(Fig. 2). The average G.I. for each sample is thendetermined by multiplying the number in each stageby the numerical ranking of the stage and dividingthe sum of these products by the total number ofindividuals in the sample. The index can vary fromzero, if the entire population is spent or resting, tofive when fully ripe.

Reproductive condition was also determinedstereologically and expressed as the “gamete vol-ume fraction” (GVF), which is the proportion of themantle tissue that is composed of follicles contain-ing developing or ripe gametes. The GVF can varybetween zero for a reproductively quiescent mussel,and one for a mussel in peak reproductive conditionand is calculated from point counts (using a statisti-cally derived grid) applied to random thin sectionsof mantle tissue (see Lowe et al.,1982). In this studyan average GVF was determined for a sub-sample often animals, with four replicates for each individual.This method is somewhat less subjective than thatinvolving the calculation of a G.I. and providesinterval rather than nominal measurements.

To investigate the periodicity of microgrowthbands present in the prismatic layer of the shell,small mussels, each approximately 20 mm in shelllength were collected from the low shore, marked by24 hours emersion and returned to the shore in meshcages situated in the low, middle and upper levels ofthe natural mussel bed. Ten mussels were collectedfrom each cage at 2, 4 and 8 week intervals. Cleanshells were embedded in resin, sectioned along themajor growth axis using a diamond saw, and the cutsurface ground on increasingly finer grades of wetand dry paper fitted to a rotating table. Ground sec-tions were then polished using household Brassoand etched for 30 minutes in 1% Decal. Acetate peelreplicas were prepared using the methodologydescribed by Richardson et al. (1979) andRichardson (1989) and viewed under the lightmicroscope.

To determine the periodicity of the bands thenumber present between the point of marking andthe growing edge were counted and compared to the

predicted number of tidal emersions that the musselsexperienced over the equivalent period of time.Richardson et al. (1990) have previously demon-strated that tidal microgrowth bands in the shell ofM. edulis become progressively narrower through-out the autumn and winter as growth slows downwith declining seawater temperatures thus givingrise to the annual growth check. In order to estimatethe age and annual growth rate of M. edulis chilen-sis, five of the largest mussels were collected fromthe middle of the mussel bed at each site and acetatepeels prepared as outlined above. The position ofeach annual check within each shell was identifiedfrom narrowing of the growth bands, and markeddirectly on the peel. The age of each mussel wasnoted and the distance between each successivecheck and the umbo measured to the nearest 0.1 mmusing vernier calipers. Von Bertalanffy growthcurves were then fitted to these measurements ofsize at age from the three populations using a com-puter program, Fishparm.

To assess the population structure at each siteapproximately 300 animals were collected random-ly by quadrat (0.4 m2) from the middle of the mainmussel zone and the shell length (maximum anterio-posterior dimension) of each individual measured to0.1 mm using vernier calipers. These lengths weresubsequently grouped into 2.5 mm size classes andthe size-frequency distributions analysed using themethod of Bhattacharya (1967).

In order to assess the infection rate and possibleeffect of a hitherto unidentified isopod on the hostmussels, samples were collected from the low, midand upper levels of the mussel bed and sorted into10 mm size classes. Each mussel was measured to0.1 mm and the number and size of isopods presentwithin the mantle cavity noted. Thirty infected and30 uninfected mussels from each size class wereselected and the dry weights of the flesh and shelldetermined after 3 days and 24 hours respectively, at65 oC. The data were log-transformed and infectedand uninfected dry weights compared using the gen-eral linear model with a covariate.

RESULTS

The mean monthly sea temperature at GooseGreen is illustrated in Fig. 3A, whilst the G.I. andGVF of mussels from this site are presented in Figs.3B and C respectively. Temperature ranged from amaximum of 14oC in summer to a minimum of 2oC

42 A.P. GRAY et al.

during the winter months. Gametogenesis starts inearly spring (August-September) and mussels peakin reproductive condition during the early summermonths (November-December), when the G.I. andGVF reach their highest values. A single spawningperiod occurs between December and February, andthe population then becomes reproductively quies-cent by early autumn (February-March). Virtuallyno further reproductive activity occurs until the fol-lowing spring with most individuals remaining inthe resting or spent condition during this period. TheGVF and G.I. are closely correlated (see alsoRodhouse et al., 1984). The peak reproductive con-dition broadly coincides with the time at which themean monthly temperature reaches approximately12oC. Although the timing of gametogenesis wassimilar at the three study sites, the actual gonadindices differed, with maxima of approximately 4.0,

3.0 and 4.7, for Darwin, Camilla Creek and GooseGreen mussels respectively (Fig. 4).

There was close agreement between the numberof clearly defined microgrowth bands present in theprismatic layer and the predicted number of tidalemersions (Table 1), strongly suggesting that thesebands have a tidal periodicity. The spring/neap cycleof bands is clearly evident in Fig. 5A. Bands arestronger in definition during spring tides due to theincreased period of emersion, and weaker duringneap tides when the mussels were not always uncov-

ECOLOGY OF MYTILUS EDULIS CHILENSIS IN THE FALKLAND ISLANDS 43

FIG. 3. – A, Mean monthly seawater temperature; B, gonad index(solid circles), percentage of ripe (solid columns) or spent (opencolumns) mussels; C, gamete volume fraction (GVF). All data

relate to the Goose Green site.

FIG. 4. – Gonad index of M. edulis chilensis at the three study sites.

ered. Fig. 5B illustrates the slowing of growth dur-ing winter when the microgrowth bands almost con-verge and a check is produced on the outer shell sur-face. Population growth curves including L∞ and kvalues predicted by the von Bertalanffy growthmodel are presented in Fig. 6. Growth rates differbetween the populations with mussels from CamillaCreek reaching approximately 50 mm after 4 yearsgrowth and those from Darwin and Goose Greenreaching only 40 mm over the same time interval.

Fig. 7 illustrates the size-frequency distributionsof the three populations together with the modalcurves, predicted using the Bhattacharya method,and the estimated age, determined from shell sec-tions. The population structure differs markedlybetween the three populations. At Darwin the popu-

lation is polymodal with three obvious cohorts at19.1, 35.7 and 43.9 mm shell length, representing68, 17 and 15% of the sample, respectively; atCamilla Creek the population is unimodal with asingle cohort at 27.0 mm in shell length, whilst atGoose Green it is bimodal with a relatively small

44 A.P. GRAY et al.

TABLE 1. – Expected and observed numbers of microgrowth bands present in the prismatic layer of Mytilus edulis chilensis from the lowerregion of the intertidal mussel bed at Camilla Creek.

Experiment Number of shells Duration (days) Expected no. of Average number ofNumber examined emersions clearly defined

(0.5 m above chart datum) bands (mean ±1 S.D.)

1 6 14 12 9.17 ± 3.76

2 9 28 29 24.33±10.22

FIG. 5. – Photomicrographs of acetate peel replicas of M. edulischilensis shell sections. P, periostracum; PL, prismatic layer, scalebars = 200 µm. A, Peel from a mussel grown in the low shore atCamilla Creek and marked by 24 hour emersion (arrow). Groups ofprominent growth bands deposited during the spring tides (S) alter-nate with groups of weaker bands deposited during neap tides (N).B, Peel from a mussel collected from the middle of the mussel zoneat Camilla Creek showing a narrowing of the tidal bands duringautumn (open arrow) and narrow increments in the winter (largefilled arrow), wider growth increments are subsequently deposited

in summer (small filled arrow).

FIG. 6. – Growth rates of M. edulis chilensis at the three study sites.Curves fitted using the von Bertalanffy growth model.

peak at 17.7 mm (12%), and a second larger peak at41.2 mm (88%). No congruence appears to existbetween the modal distributions and the size at ageestimated from shell sections.

The infection rates of low shore mussels by anunidentified isopod at Darwin, Camilla Creek andGoose Green were 2, 46 and 18 %, respectively. Noisopods were recorded in the middle or upper mus-sel zones at Darwin or Goose Green, whilst atCamilla Creek infection rates were similar at alltidal levels. Fig. 8 provides little evidence of anincrease in infection rate with increasing size of hostmussel though the average number of isopods doesappear to increase with host size. When regressionsof the log-transformed dry weights of infected and

uninfected mussels were compared using the gener-al linear model, no significant differences betweenthe slopes could be detected (p > 0.05 in all cases).

(1) Darwin beach infected : log flesh wt = 3.01 log length - 5.63

F = 0.89uninfected : log flesh wt = 3.17 log length - 5.86

(2) Camilla Creekinfected : log flesh wt = 2.65 log length - 5.29

F = 3.24 uninfected : log flesh wt = 2.75 log length - 5.41

(3) Goose Greeninfected : log flesh wt = 2.95 log length - 5.54

F = 2.90uninfected : log flesh wt = 3.13 log length - 5.82

Thus, although often present at high abundance,this isopod seems to have little or no marked effecton mussel condition.

DISCUSSION

The annual reproductive cycle in M. edulis isextensively documented (see Seed and Suchanek,1992 for a review). Typically, gametogenesis is ini-

ECOLOGY OF MYTILUS EDULIS CHILENSIS IN THE FALKLAND ISLANDS 45

FIG. 7. – Size-frequency distributions of the three study populationstogether with individual cohorts (stippled) as predicted by the

Bhattacharya method and estimated age in years (arrowed).

FIG. 8. – Infection rates (left) and average numbers of isopods pre-sent within the mantle cavity (right) of M. edulis chilensis at the

three study sites. Vertical bars indicate + 1SD.

tiated in late autumn/early winter and proceedsthroughout the winter months. Spawning usuallyoccurs in early spring and if environmental condi-tions are favourable, periods of redevelopment andspawning may occur throughout the summermonths until the mussels enter a reproductively qui-escent phase in early autumn (e.g. Wilson andHodgkin, 1967; Kennedy, 1977; Dix and Ferguson,1984). In the present study gametogenesis com-menced during the southern spring (August-September), and mussels were in peak reproductivecondition during early summer (November-December). A single major spawning periodoccurred during the summer (December-March),after which mussels entered a reproductively quies-cent phase with little or no evidence of any furthergametogenesis until the following spring. Dix andFerguson (1984) and Kennedy (1977) bothobserved a major spawning period in latewinter/early spring and subsequent minor spawn-ings during summer and autumn in M. edulis plan-ulatus from Tasmania and M. edulis aoteanus fromNew Zealand respectively. However, M. edulisplanulatus from mainland Australia is predomi-nantly a winter spawner (Wilson and Hodgkin,1967), though the gonad does continue to redevelopthroughout the spring and summer months.

In the Northern Hemisphere populations of M.edulis from warmer more southerly waters general-ly spawn earlier than those from cooler northernwaters (Seed and Suchanek, 1992 and referencestherein) and in Britain M. edulis on the west coastspawns earlier than on the colder North Sea coast atthe same latitude (Seed, 1975). Food supply and seatemperature are probably the two principal environ-mental factors controlling the timing and duration ofgametogenesis. In the present study the timing ofgametogenesis coincides with increasing springwater temperatures. Both the G.I. and GVF increasewith increasing temperature and achieve their maxi-mal values as summer temperatures approach 12oC.This strongly suggests that temperature is an impor-tant controlling factor. However, Kautsky (1982a)found that mussels from the Baltic, when placed infavourable feeding conditions, ripened in Januarywhen temperatures were close to zero and when theG.I. of the natural population was still close to itslowest value. Lubet and Aloui (1987) have suggest-ed that a ‘temperature window’ probably exists, out-side which gametogenesis is negligible, but insidewhich the reproductive strategy depends principallyon food availability.

Although the timing of gametogenesis wassimilar at the three study sites (Fig. 3), the actualgonad index exhibited marked intersite variability,which in turn is probably reflected in marked dif-ferences in reproductive output. The energy allo-cated to reproduction in individual mussels variesaccording to body size and age (Bayne andWorrall, 1980; Kautsky, 1982b). In the presentstudy, however, mussels were all of a similar size(40-50 mm), suggesting that these intersite varia-tions are due to environmental factors, in particu-lar the quantity and quality of the available foodsupply.

Individual and population growth are the resultof complex interactions between several endoge-nous and exogenous factors, for example genotype(Koehn and Gaffney, 1984; Gardner et al., 1993),temperature (Bayne and Worrall, 1980; Kautsky,1982a; Richardson et al., 1990; Sukhotin andKulakowski, 1992), food supply (Incze et al., 1980;Page and Ricard, 1990) and aerial exposure(Rodhouse et al., 1984; Sukhotin and Maximovich,1994). Sites in the present study are relatively closegeographically, they have almost identical tempera-ture ranges and all mussels were collected from asimilar tidal height. Thus differences in growth ratesrecorded between these sites must be a result ofsome other factor(s), e.g. differences in food avail-ability and/or the partitioning of resources withinindividual animals.

Microgrowth bands present in the prismaticlayer of M. edulis have a tidal periodicity (e.g.Richardson, 1989; Richardson et al., 1990). Thesetidal microgrowth bands reveal clearly markedannual patterns of deposition which result fromthe narrowing of the tidal bands during the colderwinter months. Similar microgrowth patterns havebeen observed in M. edulis chilensis shell sectionscollected in this study, enabling the growth histo-ry of individual mussels to be determined. VonBertalanffy growth constants L∞ and k rangedfrom 52.9-73.6 mm and 0.419-0.579, respectively,for our intertidal populations; these values aresomewhat lower than the L∞(78 mm) and k (0.88)values obtained by Davenport et al. (1984) for anatural subtidal population from the hull of the“SS Great Britain” in the Falkland Islands.Davenport et al. (1984), however, determined theage and growth rates of individual mussels fromthe external growth checks present in the perios-tracum, a method which generally provides some-what less reliable estimates of age for two reasons.

46 A.P. GRAY et al.

Firstly, because early stages in the growth historyof the shell may be lost as a result of abrasion inthe umbo region, and secondly because suddenchanges in environmental conditions may producedisturbance checks which are not annual in origin.Analysis of internal shell banding patterns, on theother hand, allows disturbance checks to be distin-guished from bands which are produced annually(Richardson, 1989).

Although size-frequency distributions have beenused by some authors to determine growth rates inM. edulis (e.g. Bayne and Worrall, 1980; Rodhouseet al., 1984), this method is generally of little use inmost natural mussel populations where extendedrecruitment periods and/or variable individualgrowth rates cause individual year classes to over-lap. This appears to be the case in this study, and inone case (Camilla Creek) the population had a uni-modal distribution even though mussels of up to 6years of age were identified from acetate peel repli-cas of shell sections. Similar conclusions weredrawn by Richardson et al. (1990) in the study ofMytilus populations collected from off-shore pro-duction platforms.

Many diseases and parasites are known to beassociated with M. edulis (Bower, 1992).However, we are unaware of any records of a par-asitic/commensal association between an isopodcrustacean and Mytilus. During this study a hither-to unidentified valviferan isopod was found livingwithin the mantle cavity of Falkland mussels. Asurvey (October 1994) showed that this isopod hadno obvious effect on the condition of the mussel. Itis possible therefore, that the isopod is simplyusing the mussel as a refuge from predation, asshown by Stebbins (1988) who investigated therelationship between the commensal isopod,Colidotea rostrata, and the sea urchin,Strongylocentrotus spp. Infection rates differedmarkedly at the three study sites; the highest lev-els were reported at Camilla Creek which is anestuarine environment (approximately 28 ppt),whilst the fully marine sites, Goose Green andDarwin (34-36 ppt), displayed considerably lowerinfection rates (Fig. 7). Some species within thesuborder Valvifera, e.g. Idotea chelipes, areknown to inhabit brackish water and estuarineenvironments, although the majority are found onfully marine coasts (Naylor, 1972). Infection ratedid not vary significantly with host size, althoughlarger mussels generally contained significantlymore individuals than smaller mussels.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Conor Nolan(Falkland Islands Fisheries Dept.) for assisstance incollecting monthly samples; thanks also to Mr. GusClausen, Mr. Bill Kidd and Mr & Mrs. A.McLoed.We are also grateful to the British Antarctic Surveyfor shipping samples to the UK and the FalklandIslands Dept. of Agriculture for use of laboratoryfacilities. Financial support to A. Gray was provid-ed by the Commonwealth Association and theFalkland Islands government.

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