the population structure and ecology of the antarctic ... · rings can be picked out and measured...

ADAMUSSIUM COLBECKI: POPULATION STRUCTURE AND ECOLOGY 15

SCI. MAR., 61 (Supl. 2): 15-24 SCIENTIA MARINA 1997

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

The population structure and ecology of the Antarcticscallop Adamussium colbecki (Smith, 1902) at Terra

Nova Bay (Ross Sea, Antarctica)*

RICCARDO CATTANEO-VIETTI1, MARIACHIARA CHIANTORE2, †

and GIANCARLO ALBERTELLI2

1Istituto di Zoologia, Università di Genova, Via Balbi 5, 16128 Genova, Italy.2Istituto di Scienze Ambientali Marine, Università di Genova., C.P. 79, 16038

S. Margherita Ligure (Ge), Italy.

SUMMARY: One of the main purposes of the core project “Ecology and Biogeochemistry of the Southern Ocean” (ItalianAntarctic Programme-PNRA) is to understand the utilization and ultimate fate of the organic matter sedimenting throughthe water column and its influence in the structure of the macrobenthic assemblages. At Terra Nova Bay (Ross Sea), thescallop Adamussium colbecki (Smith, 1902) constitutes large beds up to 70-80 m depth. The importance of this populationin the local community structure requires a closer examination of its structure and dynamics, in order to assess its role in thecoastal organic matter flux, and for this reason it has been studied during several years (1987-92) in areas close to the ItalianStation and particularly in the Summer 1993/94. Its high density (up to 60 ind m -2) and biomass (up to 120 g m -2 dry weightof soft tissues) values are probably linked to slow growth rate and reduced reproductive capacities. X-ray studies on the shellconfirmed the slow growth rate of this species, which averages about 0.8 cm yr -1. The analysis of the ratio between lengthand height of the shell (generally ~ 1) shows a significative inversion at the age of maturity, when the byssally attached juve-niles become free from the adult valves. A comparison of the gonadosomatic index in the population between Decemberand January suggests that sexual maturity is reached late in this season and is strongly related to the water column food sup-ply consequent to the phytoplankton bloom. Comparing the size-frequency distribution of this population in different years,it is possible to observe a cohort gap, shifting through the study period, and probably caused by unsuccessful recruitmentsfrom 6 to 9 years before 1994. Slow growth rate and intermittent recruitment suggest that an eventual commercial exploita-tion of this species, abundant but patchly distributed in a narrow bathymetric range, would quickly result in overfishing andcommercial failure.

Key words: Adamussium colbecki, Antarctica, ecology, biometrics

RESUMEN: ESTRUCTURA DE LA POBLACIÓN Y ECOLOGÍA DE LA VIEIRA ANTÁRTICA ADAMUSSIUM COLBECKI EN LA BAHÍA TERRANOVA (MAR DE ROSS, ANTÁRTIDA). – Uno de los objetivos principales del proyecto “Ecología y Biogeoquímica del OcéanoAustral” (Programa Italiano de Investigación en Antártida-PNRA) es el de comprender el uso y destino final de la materiaorgánica particulada que sedimenta y su influencia en la estructura de las comunidades macrobentónicas. En la Bahía deTerra Nova (Mar de Ross) la vieira Adamussium colbecki (Smith, 1902) constituye grandes bancos hasta los 70-80 m deprofundidad. La importancia de esta población en la estructura de la comunidad local requiere un examen más atento de suestructura y dinámica, con el fin de evaluar su papel en el flujo de la materia orgánica en el ecosistema costero; por ello hasido estudiada durante diversas expediciones antárticas en áreas vecinas a la Base Italiana (1987-92), particularmente en elverano austral 1993/94. Esta población muestra altos valores de densidad (aproximadamente 60 ind m -2) y biomasa (hasta120 g m -2 peso seco de tejidos blandos) probablemente relacionados con una tasa de crecimiento lenta y a una capacidadreproductiva reducida. El estudio de la concha mediante rayos X confirmó el crecimiento lento de esta especie, cuyo valor

†To whom all correspondence should be sent

*Received November 1995. Accepted September 1996.

INTRODUCTION

In the framework of the Project “Ecology andBiogeochemistry of the Southern Ocean” (PNRA),during several summer Antarctic expeditions(1987/88; 1989/90; 1991/92; 1993/94) data weregathered about macrobenthic communities of TerraNova Bay (Ross Sea), in order to get a better under-standing of their structures (Cormaci et al., 1992; DiGeronimo et al., 1992; Gambi and Mazzella, 1992).

Benthic assemblages play an important role inthe flux of organic matter in the Antarctic littoralecosystem, but this role is often underestimated,and little is known about the trophic couplingbetween the water column and the sea-bottomcommunities. In the austral summer 1993/94, dur-ing the IX Italian Expedition at Terra Nova Bay,the study of the ecological and biogeochemicalprocesses between the water column and the sedi-ments was included in the core-project “Ecologyand Biogeochemistry of the Southern Ocean”. Thegoal was to evaluate the composition and transfor-mation of the organic matter through the water col-umn, as well as the influence of physical andchemical parameters.

Particular attention was focused on theAdamussium colbecki community, a common mac-robenthic assemblage in the Ross Sea fromMcMurdo Sound to Terra Nova Bay from 20 to 80metres depth (Amato, 1990; Berkman, 1990;Berkman and Nigro, 1992; Di Geronimo et al.,1992; Nigro, 1993). The scallop Adamussium col-becki is the dominant species of this community,reaching high values of density (60 ind m-2) andbiomass (120 g m-2, dry weight of soft tissues), andplays an important role in the flux of energy fromthe water column to the benthic compartment.

Preliminary data collected at Terra Nova Bayduring the austral summers 1987/88 and 1989/90were presented by Amato (1990) and Albertelli et

al. (1994) respectively, while Berkman and Nigro(1992) proposed this circumantarctic species (Dell,1990) as a bioindicator in a Mussel Watch AntarcticProgramme. At McMurdo Sound (Stockton, 1984;Berkman, 1990) this species was studied from dif-ferent points of view, underlining its high densityand biomass, slow growth rate and intermittentrecruitment.

Adamussium colbecki is the largest species in theAntarctic bivalve fauna, with a recorded maximumlength of 120 mm (Berkman, 1990). Adult individu-als generally lie free on the bottom, while juvenilesare commonly byssally attached to the adults, bene-fitting from detrital slurry resuspended by the adults.Young individuals can be also found byssallyattached to other substrates, such as macroalgae(Gambi, pers. comm.) or artificial substrates(Cattaneo-Vietti and Chiantore, pers. observ.).

The aims of this study are to analyse the distrib-ution of this species in Terra Nova Bay and its pop-ulation structure in the last years, evaluating itsabundance and size structure, growth rate and otherbiometrical features, in relation to food requirement.These studies are important also to understand thepossible impact of the installation of the ItalianStation on this unexploited resource, both on itspopulation distribution and structure.

MATERIAL AND METHODS

Study area

The sampling area has been restricted insideTerra Nova Bay, at a maximum distance of 1 nauti-cal mile from the Italian Station (Fig. 1). After pre-liminary data (Amato, 1990), major observations onthe population distribution were carried out in1989/90 at Road Cove (40-80 m), Faraglione (40-80m), and Adelie Cove (60-70 m).

16 R. CATTANEO-VIETTI et al.

promedio es aproximadamente 0,8 cm año-1). El análisis de la relación entre la longitud y la altura de la concha (general-mente ~ 1), muestra una significativa inversión a la edad de la madurez, cuando los individuos juveniles fijados por su bisose desprenden de las valvas de los adultos. Una comparación del índice gonadosomático de la población desde finales dediciembre a finales de enero, sugiere que la madurez sexual se alcanza a fines de esta estación y está fuertemente relaciona-da con la disponibilidad alimentaria presente en la columna de agua después de la floración del fitoplancton. Comparando ladistribución de frecuencias de talla de esta población en diferentes años, es posible observar una discontinuidad en las cohor-tes, que se desplazó a lo largo del periodo de estudio, y que probablemente esté causada por el reclutamiento frustrado entre6 y 9 años antes de 1994. La tasa de crecimiento lenta y el reclutamiento intermitente sugieren que una eventual explotacióncomercial de esta especie, abundante pero con una distribución restringida a un limitado rango batimétrico y agregada, daríacomo resultado una sobrepesca y un consecuente fracaso comercial.

Palabras clave: Adamussium colbecki, Antártida, ecología, biometría.

In 1993/94 the study was conducted inside TerraNova Bay in front of Road Cove, on a station placedat 40 m depth (74°41.9’ S; 164°07.5’ E) close to theItalian Station, between the 27th December 1993and the 11th February 1994.

Nearby Terra Nova Bay Station, between 20-80 m depth, the substrate is fairly fine (fraction >212µm constitutes 94.2%) and is completely covered bythe bivalve Adamussium colbecki. Here, large preda-tors such as the gastropod Neobuccinum eatoniSmith, and the nemertean Parborlasia corrugatus(McIntosh), are frequent. The echinoid Sterechinusneumayeri (Meissner) and the starfish Odontastervalidus (Koehler) are very frequent at all depths onboth hard and mobile substrates (Di Geronimo et al.,1992).

Sampling and laboratory techniques

Samples were collected using Van Veen grabs(surface: 60 x 35 cm), for quantitative data, andCharcot-Picard or naturalist dredges for qualitativesamples.

The collected material was frozen and partiallyfixed in 4 % buffered formol and preserved in ethyl-ic alcohol. Specimens were weighed and measured(height and length) with a vernier caliper. Shelllength is the longest dimension of the shell parallelto the hinge line; shell height is measured along thedorso-ventral axis from the umbo region to the ven-tral margin. The soft parts (gonad, adductor muscle,

remains) were dissected. Gonad volume was esti-mated as water volume equivalent (ml).Successively the soft parts were separately weighed,as wet and dry weight (drying oven at 60°C for 24hr). Finally, they were burnt at 550°C in an oven for4 hr to determine the ash weight and deduce theorganic matter value. In 1993/94 specimens werecollected at the beginning and at the end of the studyperiod (end of December, end of January), in orderto observe the increase of the weight of total soft tis-sues and of the gonad in the same size class throughthe summer period. The ratio between dry weight ofthe gonad and of the muscle (G/M) and the gonado-somatic index (GSI: gonad dry weight × 100/totaldry weight) were calculated in order to assess thereproductive state of the population.

The identification of sexes and of the stage ofgonad maturation were made by histological tech-niques. The gonads, preserved at -20°C, were put ina phosphate-saccharose buffer solution for 48 hr at9°C. Then they were cut with a microtome into thinlayers (10 µm), which were put on slides and stainedwith toluidine blue.

The estimate of the growth rate of A. colbeckiwas performed counting the number of externalgrowth rings on the shell. In Antarctic bivalves first-order growth rings are generally assumed to beannual, in association with periods of feeding andmetabolic activity (Adamussium: Ralph andMaxwell, 1977; Berkman, 1990; Laternula ellipti-ca: Ralph and Maxwell, 1977; Lissarca notocarden-sis: Brey and Hain, 1992; Yoldia eightsi: Nolan andClarke, 1993).

As the growth rings are not conspicuous, majorrings can be picked out and measured through X-rayphotographs of the shell (Ralph and Maxwell,1977). X-ray photographs were taken of dry shellswith a General Electric mammography machinewith fixed anode, using a 3M HM monoemulsionfilm coupled with a support screen of rare earths T2.The parameters used were: focal film distance 38cm, voltage 15 kv, current 15 mA and exposure timefrom 7 sec to 15 sec. The outlines of the most con-spicuous rings were transferred to a desktop com-puter using a KD 4300 digitizer (GraphtecCorporation, Tokyo, Japan) in order to calculate thevalues of the perimeter and the area of each ring.

To collect hydrological data (temperature, salini-ty and fluorescence), a multiparametric probe wasused up to 50 metres depth at the same station,where a study on the role of the organic matter fluxinside the Adamussium community was carried on


FIG. 1. – Adamussium colbecki distribution area (dotted) nearbyTerra Nova Bay Station.

(Albertelli et al., 1996, in press). The profiling sys-tem used was a Meerestechnik multi-parametricCTD probe and a back-scat fluorimeter. Data fromthe CTD package were collected only on descent(downcast), at about 1 m/sec, in according with thestandards CTD operations (UNESCO, 1988).

RESULTS

Distribution, abundance and size structure

At Terra Nova Bay dense assemblages ofAdamussium colbecki are limited to a narrow beltfrom Tethys Bay to Adelie Cove between 40 and 80m depth (Fig. 1). A. colbecki specimens can be alsofound from 15 m depth, but its density shows adecreasing trend with decreasing depth (2.3 ind m -

2 at 15 m; 58.8 ind m-2 at 30 m; Nigro, 1993).Around 70-80 m depth, the bottom is completelycovered by empty shells. Data on the presence ofthis species along Victoria Land, south Adelie Cove,

are not available, but its presence is probable,because at McMurdo Sound dense assemblagesoccur.

Its population density may be appreciated bycover values higher than 100 %, as evidenced byROV images (Fig. 2), showing overlapping individ-uals. Quantitative sampling by grab allowed to eval-uate average density and biomass of this bivalve inthe area nearby Terra Nova Bay Station, as well asto assess its sex-ratio (1:1) and population structure(Table 1). The population is dominated mainly bylarge-sized adult individuals, which reach an aver-age density of 60 ind m-2 and an average total bio-mass around 120 g m-2 (dry weight of soft tissues).Young individuals have a scattered distribution andare mainly attached through byssus threads to adultvalves, where a micro-community mainly composedof benthic diatoms on the upper valve and forams,bryozoans and the spirorbid Paralaeospira levinseniPixell on the inferior valve, can be found (Rosso,1992; Berkman, 1994).

Size-frequency distributions in summer1989/90 (Fig. 3) and 1993/94 (Fig. 4) evidencehigh abundance values of large-sized individuals(modal length class: 70-75 mm), but show differenttrends for younger size classes. In 1989/90 the


FIG. 2. – A. colbecki beds from ROV observations, showing covervalues higher than 100%.

FIG. 3. – Size frequency distribution of A. colbecki population in1989/90.

TABLE 1. – Main features of A. colbecki population structure atTerra Nova Bay.

Modal length class 70-75 mmYoung individuals mainly attached to adults’ valvesMaximum length 88 mmAverage length 51.8 ± 25.9Average weight 25.4 ± 22.2 g WW (with shell)Maximum density 93 ind m-2

Average density 60 ind m-2

Average biomass 1.5 kg m-2 WW (with shell)120 g m-2 DW (soft tissues)

Sex-ratio 1:1Sexual maturity 6-7 years old (50-60 mm in length)

FIG. 4. – Size frequency distribution of A. colbecki population in1993/94.

almost complete absence of the smallest size class-es was observed, while four years later (1993/94)youngest individuals were well represented and theprevious gap shifted to size classes between 40 and65 mm length. The relevant presence of adults,belonging to different overlapping size classes, ischaracteristic of stable and K-selected populations(MacArthur, 1960; White, 1984), and is linked tothe coupling of stability of the physical environ-ment and the slow growth rate, which lead to theevolution of efficient, stable community structureswith a low turn-over rate (White, 1984) and largestanding stocks, maximizing share of food input

under the low temperature regime of the Antarctic(Brey and Clarke, 1993).

Growth

X-ray photographs have allowed to measure thesurface of each growth ring (Fig. 5), to calculatetheir average diameter, and, consequently, to assessthe growth increment from each growth ring to thefollowing one, that is the year growth increment, forindividuals of different size (Table 2). The averagegrowth increment is 8.1 ± 1.0 mm yr -1 till the fifthring, while this value falls to 5.1 ± 1.5 mm yr -1 inlarger individuals.

Biometrics

Highly significant linear regressions were foundbetween the main biometrical features (Table 3)measured on the complete size range of collectedindividuals. The relationships between shell length


FIG. 5. –X-ray picture of A. colbecki shell (60 mm height), sho-wing growth rings from the 3rd (white dot) to the 7th. The firsttwo ones are not visible in the picture. Several spirorbids are

visible on the shell.

TABLE 2. – Growth parameters in A. colbecki at Terra Nova Bay.Shell areas (mm2) and diameter (mm) for each annual ring and con-sequent annual growth increment (mm yr-1), estimated on 13 speci-mens. The anomalous value of 11.3 mm yr-1 was not taken intoaccount, as it certainly corresponds to two following growth areas

in which it was not possible to evidence the boundary ring.

ring area (mm2) diameter (mm) growt incrementnumber avg sd avg sd mm–1 yr avg sd

1 251 96 17.5 3.72 587 234 26.7 5.8 9.23 947 307 34.2 5.9 7.54 1480 451 42.9 6.9 8.75 1966 433 49.7 5.7 6.8 8.1 1.06 2667 547 53.3 6.6 3.67 2748 475 58.9 5.7 5.68 3885 535 70.2 4.3 11.39 4729 77.6 7.4

10 5198 81.4 3.8 5.1 1.5

TABLE 3. – Regression parameters of main biometrical features. Slope and constant are calculated according to the equation y= b x + a.Significance level >0.001.

X Y slope constant df r

Length Height 0.967 0 494 0.987Ln Length Ln Wet Weight 2.816 –8.37 241 0.995Total Wet Weight Total Dry Weight 0.292 0 24 0.971Total Wet Weight Soft tissues Dry Weight 0.090 0 36 0.869Soft issues Wet Weight Soft tissues Dry Weight 0.130 0 36 0.888Soft tissues Dry Weight Soft tissues Ash Free Dry Weight 0.852 0 36 0.998Muscle Wet Weight Muscle Dry Weight 0.151 0 24 0.969Muscle Dry Weight Muscle Ash Free Dry Weight 0.910 0 24 0.999Gonad Volume Gonad Wet Weight 0.805 0.181 36 0.886Gonad Wet Weight Gonad Dry Weight 0.139 0 17 0.913Gonad Dry Weight Gonad Ash Free Dry Weight 0.860 0 22 0.999Remains Wet Weight Remains Dry Weight 0.082 0 24 0.883Remains Dry Weight Remains Ash Free Dry Weight 0.828 0 24 0.995

and dry weight of the adductor muscle, and betweenshell length and dry weight of gonad, are shown inFigs. 6 and 7, respectively.

Ratio between shell length and height (aspectratio) turned out to be particularly interesting, show-ing an inversion during the growth (Fig. 8). Until 45mm length this ratio is approximately below 1, andorganisms are fundamentally oval-shaped, whileover 50 mm length the shape is subcircular and

length/height ratio is >1. This change in shapeseems to occur when byssally attached juvenilesrelease from the adults. Other important events seemto take place reaching this critical length, as evi-denced by relevant changes in body component per


TABLE 4. – Biometrical features in male and female 70 mm long individuals (13 females and 13 males). G/M: ratio between the gonad andthe muscle dry weight. GSI: gonadosomatic index (gonad dry weightx100/total soft tissues dry weight).

Females MalesDecember January %increment December January %increment

Size (mm) avg 69.29 73.33 - 71.14 76.00 -sd 9.22 5.93 6.73 6.61

Gonad volume (ml) avg 0.78 1.03 32.05 0.83 2.28 174.70sd 0.35 0.20 0.22 0.39

Muscle g DW avg 0.91 1.41 54.95 0.79 1.46 84.81sd 0.25 0.34 0.19 1.22

Gonad g DW avg 0.07 0.20 185.71 0.20 0.40 100.00sd 2.37 2.24 2.03 2.35

Total g DW avg 7.48 9.84 31.55 7.42 10.33 39.22sd 2.37 2.24 2.03 2.35

G/M avg 0.08 0.15 87.50 0.23 0.28 21.74sd 0.02 0.02 0.13 0.06

GSI avg 0.98 2.08 112.24 2.93 4.07 38.91sd 0.30 0.48 2.19 1.21

FIG. 6. – Relationship between shell length (mm) and muscle dryweight (g).

FIG. 7. – Relationship between shell length (mm) and gonad dryweight (g).

FIG. 8. – Length/height ratio for individuals of different size.

FIG. 9. – Average per cent contribution of different body compo-nents to total dry weight in different sized individuals.

cent contribution to total dry weight (Fig. 9), such asincrease of gonad and muscle weight and decreaseof shell weight relative to total dry weight, suggest-ing the start of the adult phase.

The examination of average gonad volume, of dryweight of the different organs, of the gonadosomaticindex (GSI), of ratio between gonad and muscle(G/M) and of wet and dry weight of each soft parthave been performed on males and females separate-ly, comparing individuals of similar size, collected atthe beginning and at the end of summer (Table 4).

In females a slow increase in gonad volume hasbeen observed from the end of December to the end ofJanuary, but its dry weight increment is much higherthan in muscle tissues (185 % vs 55 %). ConsequentlyG/M ratio and GSI show a drastic increase at the endof January (Fig. 10). Histological slides clearlyshowed a marked increase in oocyte diameter fromDecember (20 µm) to January (45 µm), although greatdifferences were found in the maturation stage in dif-ferent portions of the same gonad.

In males a higher increase in muscle dry weighthas been observed from the end of December to theend of January as well as in gonad volume. Suchincrease in volume was not linked to a comparableincrease in its dry weight and consequently thechanges of G/M ratio and GSI in males were lessremarkable than in females.

Summer food availability

Water column food availabilty showed a strongseasonal trend, as evidenced both by fluorescencedepth profiles (Fig. 11) and by the amount of sink-

ing particulate matter (Albertelli et al., 1996, inpress). The highest values of fluorescence weremeasured in the deeper layers of the water column(below 25 m) until January 5th. Afterwards, theymoved towards shallower levels (between January8th and 11th). On January 20th there was a drop fol-lowed by a sudden increase between the surface and20 m depth (22/01), reaching the highest valuesmeasured in the season. After this date, the peakreturned to lower layers.

DISCUSSION AND CONCLUSIONS

The assessment of the population structure ofAdamussium colbecki in Terra Nova Bay has to beconsidered particularly important, as this scallop,constituting large assemblages and reaching veryhigh values of density and biomass, plays an impor-tant role in the organic matter cycle in the Antarcticlittoral ecosystem (Chiantore et al., in press). Thisspecies is remarkably persistent in the study areathroughout the years, in terms of abundance,although interannual differences have been found inthe size-frequency distribution.

The population structure at Terra Nova Bay andthe growth rate there are in agreement with those


FIG. 10. – Gonadosomatic index (GSI) in female individualssampled in December and January.

FIG. 11. – Water column fluorescence values (volts), measured inTerra Nova Bay between December 30th 1993 and February 11th1994. Dates on X axis are expressed as days from the beginning of

the study period.

mainly recorded at McMurdo Sound. Little differ-ences in growth rate, ranging between 0.73 cm yr -1

(Stockton, 1984), 0.84 cm yr -1 (Berkman, 1990) and1 cm yr -1 (Ralph and Maxwell, 1977), are probablyrelated to different levels in food availability, asobserved in other scallops from temperate waters(Broom and Mason, 1978; Bayne and Newell, 1983;McDonald and Thompson, 1985).

Relevant changes in body shape and in ponderalratios between different morphological parameterstake place in coincidence with the detachment ofyoung individuals from the shell of the adults whenthey reach 30-50 mm in length. Detachment takingplace at this size is confirmed, indirectly, also fromevidences in epizoic assemblages on Adamussiumshell, with an abrupt transition in epizoic biomasson shells larger than 65 mm in shell height(Berkman, 1994).

The detachment from adult shells could representan important behavioural and developmentalmoment. Just detached individuals are probablymore exposed to predation pressure: when attached,they can escape predators, as adults show a strongescape reaction through swimming and jumpingbehaviour (Stockton, 1984).

Metabolic changes, due to sexual maturation,take place in just detached individuals, which, more-over, cannot take advantage of organic matter resus-pended by adult shell clapping (Davis and Marshall,1961), as attached juveniles do.

Little is known about the influence of environ-mental parameters on spawning, which at McMurdooccurs during the austral spring (Berkman, 1990;Berkman et al., 1991). In Terra Nova Bay, the GSIincrease in females at the end of January suggeststhat gonad maturation takes place in summer, at theend of the primary production period (Albertelli etal., in press). This fact could be also supported byevidences of different levels of gonadal Cd infemales compared to males observed in austral sum-mer (Mauri et al., 1990), which is probably relatedto gametogenesis. Consequently there seems to be ashift in reproductive cycle from McMurdo Sound toTerra Nova Bay, probably linked to differences infood availability. In bivalves, the summer food sup-ply influences the species physiology in terms ofingestion rate, digestion and assimilation efficien-cies and differential utilization of biochemical com-pounds (Ansell, 1974; Newell and Bayne, 1980;Kreeger, 1993): a high quality food supply couldinduce rapid growth (Thompson and Nichols, 1988)and support energy storage and gonad maturation

(Barber and Blake, 1981). While metabolicdemands of suspension-feeding bivalves may be sat-isfied also by resuspended detritic material, enrichedin benthic diatoms (Davis and Marshall, 1961;Vernet, 1977; Shumway et al., 1987), a detriticaldiet alone seems to be unfit to support growth ofsomatic and reproductive tissues of adults, as inPlacopecten magellanicus (Grant and Cranford,1989).

This supports the hypothesis that the reproduc-tive cycle is controlled by both endogenous andexogenous factors such as temperature and foodavailability, as in other scallops (Bricelj et al.,1987), mussels (Newell et al., 1982) and cockles(Newell and Bayne, 1980).

The small eggs (20-45 µm) and the low ratio(0.36) between first and second larval shells (prodis-soconchs I and II) suggest a planktotrophic larva(Berkman et al., 1991), although there are no dataabout the duration of the larval stage. As amongother scallops (Sastry, 1979), inside a planktotroph-ic strategy, the recruitment success could be influ-enced strongly by the water column food supply(Mileikovsky, 1971) and this could explain the inter-annual variability in spats occurrence (Stockton,1984; Berkman, 1990; Berkman et al., 1991; Nigroet al., 1994). However, evidences in Antarctic inver-tebrates (Pearse et al., 1991) suggest alternativefood supplies to planktotrophic larvae (Olson et al.,1987), such as dissolved organic matter (DOM:Shilling and Bosch, 1994) and bacterioplankton(Rivkin et al., 1986).

Therefore, the study of environmental conditions,especially food availability, is critical for an under-standing of the population dynamics of Adamussium.It is now clear that the prolonged winter season caninfluence both spawning and larval survival, prevent-ing adults from reaching sexual maturation and larvaefrom finding enough nourishment.

The concomitant occurrence of endogenous andexogenous factors could explain the differences inpopulation structure observed in 1993/94 relative to1989/90, mainly the lack of size classes between 40and 65 mm, coinciding with the absence of youngestindividuals in 1989/90. This fact could be related tounsuccessful recruitments from 6 to 9 years earlier,but also to a high mortality rate when individualsreach the critical size at which they detach from theadults and are more selectively preyed upon.

The studies carried out during the last years onthe population of Adamussium colbecki at TerraNova Bay have resulted in a better knowledge of the


structure of this population in an area close to theItalian Station and, until now, no changes have beenrecorded in the population structure after the con-struction of the Italian Station. This suggests ascarce impact on this peculiar community, whichshould, anyway, be monitored every year. Finally,the small extension of the area in which this scallopoccurs and its intermittent recruitment suggest thatan eventual commercial exploitation would quicklyresult in overfishing and commercial failure.

ACKNOWLEDGEMENTS

This research has been carried out thanks to thefinancial support by PNRA (Italian AntarcticResearch Programme). We are grateful to AlanAnsell (Scottish Association for Marine Sciences,Oban) for his pretious suggestions, Carlo Barbante(University of Venice) and Gianmarco Veruggio(CNR Genoa) for putting at our disposal the multi-parametric probe and the video observations by theROV “Roby2”, respectively. We are grateful to theequipe of the R/V Malippo for facilities during sam-pling activities.

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the population structure and ecology of the antarctic ... · rings can be picked out and measured...

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