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 Antarctic Adamussium colbecki (Smith, 1902) at (, )*

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 ” (Italian Antarctic Programme-PNRA) is to understand the utilization and ultimate fate of the organic matter sedimenting through the water column and its influence in the structure of the macrobenthic assemblages. At Terra Nova Bay (Ross Sea), the scallop Adamussium colbecki (Smith, 1902) constitutes large beds up to 70-80 m depth. The importance of this population in the local community structure requires a closer examination of its structure and dynamics, in order to assess its role in the coastal organic matter flux, and for this reason it has been studied during several years (1987-92) in areas close to the Italian Station 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 weight of soft tissues) values are probably linked to slow growth rate and reduced reproductive capacities. X-ray studies on the shell confirmed the slow growth rate of this , which averages about 0.8 cm yr -1. The analysis of the ratio between length and 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 December and 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 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 recruitments from 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 and commercial failure.

Key words: Adamussium colbecki, Antarctica, ecology, biometrics

RESUMEN: ESTRUCTURA DE LA POBLACIÓN Y ECOLOGÍA DE LA VIEIRA ANTÁRTICA ADAMUSSIUM COLBECKI EN LA BAHÍA TERRA NOVA (MAR DE ROSS, ANTÁRTIDA). – Uno de los objetivos principales del proyecto “Ecología y Biogeoquímica del Océano Austral” (Programa Italiano de Investigación en Antártida-PNRA) es el de comprender el uso y destino final de la materia orgánica particulada que sedimenta y su influencia en la estructura de las comunidades macrobentónicas. En la Bahía de Terra Nova (Mar de Ross) la vieira Adamussium colbecki (Smith, 1902) constituye grandes bancos hasta los 70-80 m de profundidad. La importancia de esta población en la estructura de la comunidad local requiere un examen más atento de su estructura y dinámica, con el fin de evaluar su papel en el flujo de la materia orgánica en el ecosistema costero; por ello ha sido estudiada durante diversas expediciones antárticas en áreas vecinas a la Base Italiana (1987-92), particularmente en el verano austral 1993/94. Esta población muestra altos valores de densidad (aproximadamente 60 ind m -2) y biomasa (hasta 120 g m -2 peso seco de tejidos blandos) probablemente relacionados con una tasa de crecimiento lenta y a una capacidad reproductiva 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.

ADAMUSSIUM COLBECKI: POPULATION STRUCTURE AND ECOLOGY 15 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 biso se desprenden de las valvas de los adultos. Una comparación del índice gonadosomático de la población desde finales de diciembre 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 la distribución 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 entre 6 y 9 años antes de 1994. La tasa de crecimiento lenta y el reclutamiento intermitente sugieren que una eventual explotación comercial de esta especie, abundante pero con una distribución restringida a un limitado rango batimétrico y agregada, daría como resultado una sobrepesca y un consecuente fracaso comercial.

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

INTRODUCTION al. (1994) respectively, while Berkman and Nigro (1992) proposed this circumantarctic species (Dell, In the framework of the Project “Ecology and 1990) as a bioindicator in a Mussel Watch Antarctic Biogeochemistry of the Southern Ocean” (PNRA), Programme. At McMurdo Sound (Stockton, 1984; during several summer Antarctic expeditions Berkman, 1990) this species was studied from dif- (1987/88; 1989/90; 1991/92; 1993/94) data were ferent points of view, underlining its high density gathered about macrobenthic communities of Terra and biomass, slow growth rate and intermittent Nova Bay (Ross Sea), in order to get a better under- recruitment. standing of their structures (Cormaci et al., 1992; Di Adamussium colbecki is the largest species in the Geronimo et al., 1992; Gambi and Mazzella, 1992). Antarctic bivalve fauna, with a recorded maximum Benthic assemblages play an important role in length of 120 mm (Berkman, 1990). Adult individu- the flux of organic matter in the Antarctic littoral als generally lie free on the bottom, while juveniles ecosystem, but this role is often underestimated, are commonly byssally attached to the adults, bene- and little is known about the trophic coupling fitting from detrital slurry resuspended by the adults. between the water column and the sea-bottom Young individuals can be also found byssally communities. In the austral summer 1993/94, dur- attached to other substrates, such as macroalgae ing the IX Italian Expedition at Terra Nova Bay, (Gambi, pers. comm.) or artificial substrates the study of the ecological and biogeochemical (Cattaneo-Vietti and Chiantore, pers. observ.). processes between the water column and the sedi- The aims of this study are to analyse the distrib- ments was included in the core-project “Ecology ution of this species in Terra Nova Bay and its pop- and Biogeochemistry of the Southern Ocean”. The ulation structure in the last years, evaluating its goal was to evaluate the composition and transfor- abundance and size structure, growth rate and other mation of the organic matter through the water col- biometrical features, in relation to food requirement. umn, as well as the influence of physical and These studies are important also to understand the chemical parameters. possible impact of the installation of the Italian Particular attention was focused on the Station on this unexploited resource, both on its Adamussium colbecki community, a common mac- population distribution and structure. robenthic assemblage in the Ross Sea from McMurdo Sound to Terra Nova Bay from 20 to 80 metres depth (Amato, 1990; Berkman, 1990; MATERIAL AND METHODS Berkman and Nigro, 1992; Di Geronimo et al., 1992; Nigro, 1993). The scallop Adamussium col- Study area becki is the dominant species of this community, reaching high values of density (60 ind m-2) and The sampling area has been restricted inside biomass (120 g m-2, dry weight of soft tissues), and Terra Nova Bay, at a maximum distance of 1 nauti- plays an important role in the flux of energy from cal mile from the Italian Station (Fig. 1). After pre- the water column to the benthic compartment. liminary data (Amato, 1990), major observations on Preliminary data collected at Terra Nova Bay the population distribution were carried out in during the austral summers 1987/88 and 1989/90 1989/90 at Road Cove (40-80 m), Faraglione (40-80 were presented by Amato (1990) and Albertelli et m), and Adelie Cove (60-70 m).

16 R. CATTANEO-VIETTI et al. 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 24 hr). Finally, they were burnt at 550°C in an oven for 4 hr to determine the ash weight and deduce the organic matter value. In 1993/94 specimens were collected at the beginning and at the end of the study period (end of December, end of January), in order to observe the increase of the weight of total soft tis- sues and of the gonad in the same size class through the summer period. The ratio between dry weight of the gonad and of the muscle (G/M) and the gonado- somatic index (GSI: gonad dry weight × 100/total dry weight) were calculated in order to assess the reproductive state of the population. The identification of sexes and of the stage of

FIG. 1. – Adamussium colbecki distribution area (dotted) nearby gonad maturation were made by histological tech- Terra Nova Bay Station. niques. The gonads, preserved at -20°C, were put in a phosphate-saccharose buffer solution for 48 hr at 9°C. Then they were cut with a microtome into thin In 1993/94 the study was conducted inside Terra layers (10 µm), which were put on slides and stained Nova Bay in front of Road Cove, on a station placed with toluidine blue. at 40 m depth (74°41.9’ S; 164°07.5’ E) close to the The estimate of the growth rate of A. colbecki Italian Station, between the 27th December 1993 was performed counting the number of external and the 11th February 1994. growth rings on the shell. In Antarctic bivalves first- Nearby Terra Nova Bay Station, between 20- order growth rings are generally assumed to be 80 m depth, the substrate is fairly fine (fraction >212 annual, in association with periods of feeding and µm constitutes 94.2%) and is completely covered by metabolic activity (Adamussium: Ralph and the bivalve Adamussium colbecki. Here, large preda- Maxwell, 1977; Berkman, 1990; Laternula ellipti- tors such as the gastropod Neobuccinum eatoni ca: Ralph and Maxwell, 1977; Lissarca notocarden- Smith, and the nemertean Parborlasia corrugatus sis: Brey and Hain, 1992; Yoldia eightsi: Nolan and (McIntosh), are frequent. The echinoid Sterechinus Clarke, 1993). neumayeri (Meissner) and the starfish Odontaster As the growth rings are not conspicuous, major validus (Koehler) are very frequent at all depths on rings can be picked out and measured through X-ray both hard and mobile substrates (Di Geronimo et al., photographs of the shell (Ralph and Maxwell, 1992). 1977). X-ray photographs were taken of dry shells with a General Electric mammography machine Sampling and laboratory techniques with fixed anode, using a 3M HM monoemulsion film coupled with a support screen of rare earths T2. Samples were collected using Van Veen grabs The parameters used were: focal film distance 38 (surface: 60 x 35 cm), for quantitative data, and cm, voltage 15 kv, current 15 mA and exposure time Charcot-Picard or naturalist dredges for qualitative from 7 sec to 15 sec. The outlines of the most con- samples. spicuous rings were transferred to a desktop com- The collected material was frozen and partially puter using a KD 4300 digitizer (Graphtec fixed in 4 % buffered formol and preserved in ethyl- Corporation, Tokyo, Japan) in order to calculate the ic alcohol. Specimens were weighed and measured values of the perimeter and the area of each ring. (height and length) with a vernier caliper. Shell To collect hydrological data (temperature, salini- length is the longest dimension of the shell parallel ty and fluorescence), a multiparametric probe was to the hinge line; shell height is measured along the used up to 50 metres depth at the same station, dorso-ventral axis from the umbo region to the ven- where a study on the role of the organic matter flux tral margin. The soft parts (gonad, adductor muscle, inside the Adamussium community was carried on

ADAMUSSIUM COLBECKI: POPULATION STRUCTURE AND ECOLOGY 17 (Albertelli et al., 1996, in press). The profiling sys- TABLE 1. – Main features of A. colbecki population structure at Terra Nova Bay. tem used was a Meerestechnik multi-parametric CTD probe and a back-scat fluorimeter. Data from Modal length class 70-75 mm the CTD package were collected only on descent Young individuals mainly attached to adults’ valves (downcast), at about 1 m/sec, in according with the Maximum length 88 mm Average length 51.8 ± 25.9 standards CTD operations (UNESCO, 1988). Average 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 1:1 Sexual maturity 6-7 years old (50-60 mm in length)

are not available, but its presence is probable, because at McMurdo Sound dense assemblages occur. Its population density may be appreciated by cover values higher than 100 %, as evidenced by ROV images (Fig. 2), showing overlapping individ- uals. Quantitative sampling by grab allowed to eval- uate average density and biomass of this bivalve in the area nearby Terra Nova Bay Station, as well as FIG. 2. – A. colbecki beds from ROV observations, showing cover to assess its sex-ratio (1:1) and population structure values higher than 100%. (Table 1). The population is dominated mainly by large-sized adult individuals, which reach an aver- age density of 60 ind m-2 and an average total bio- RESULTS mass around 120 g m-2 (dry weight of soft tissues). Young individuals have a scattered distribution and Distribution, abundance and size structure are mainly attached through threads to adult valves, where a micro-community mainly composed At Terra Nova Bay dense assemblages of of benthic on the upper valve and forams, Adamussium colbecki are limited to a narrow belt bryozoans and the spirorbid Paralaeospira levinseni from Tethys Bay to Adelie Cove between 40 and 80 Pixell on the inferior valve, can be found (Rosso, m depth (Fig. 1). A. colbecki specimens can be also 1992; Berkman, 1994). found from 15 m depth, but its density shows a Size-frequency distributions in summer decreasing trend with decreasing depth (2.3 ind m - 1989/90 (Fig. 3) and 1993/94 (Fig. 4) evidence 2 at 15 m; 58.8 ind m-2 at 30 m; Nigro, 1993). high abundance values of large-sized individuals Around 70-80 m depth, the bottom is completely (modal length class: 70-75 mm), but show different covered by empty shells. Data on the presence of trends for younger size classes. In 1989/90 the this species along Victoria Land, south Adelie Cove,

FIG. 3. – Size frequency distribution of A. colbecki population in FIG. 4. – Size frequency distribution of A. colbecki population in 1989/90. 1993/94.

18 R. CATTANEO-VIETTI et al. 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 into account, 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 increment number avg sd avg sd mm–1 yr avg sd

1 251 96 17.5 3.7 2 587 234 26.7 5.8 9.2 3 947 307 34.2 5.9 7.5 4 1480 451 42.9 6.9 8.7 5 1966 433 49.7 5.7 6.8 8.1 1.0 6 2667 547 53.3 6.6 3.6 7 2748 475 58.9 5.7 5.6 8 3885 535 70.2 4.3 11.3 9 4729 77.6 7.4 10 5198 81.4 3.8 5.1 1.5

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

Growth 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 first X-ray photographs have allowed to measure the two ones are not visible in the picture. Several spirorbids are visible on the shell. surface of each growth ring (Fig. 5), to calculate their average diameter, and, consequently, to assess almost complete absence of the smallest size class- the growth increment from each growth ring to the es was observed, while four years later (1993/94) following one, that is the year growth increment, for youngest individuals were well represented and the individuals of different size (Table 2). The average previous gap shifted to size classes between 40 and growth increment is 8.1 ± 1.0 mm yr -1 till the fifth 65 mm length. The relevant presence of adults, ring, while this value falls to 5.1 ± 1.5 mm yr -1 in belonging to different overlapping size classes, is larger individuals. characteristic of stable and K-selected populations (MacArthur, 1960; White, 1984), and is linked to Biometrics the coupling of stability of the physical environ- ment and the slow growth rate, which lead to the Highly significant linear regressions were found evolution of efficient, stable community structures between the main biometrical features (Table 3) with a low turn-over rate (White, 1984) and large measured on the complete size range of collected standing stocks, maximizing share of food input individuals. The relationships between shell length

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

ADAMUSSIUM COLBECKI: POPULATION STRUCTURE AND ECOLOGY 19 FIG. 6. – Relationship between shell length (mm) and muscle dry FIG. 8. – Length/height ratio for individuals of different size. weight (g).

FIG. 7. – Relationship between shell length (mm) and gonad dry weight (g). and dry weight of the adductor muscle, and between FIG. 9. – Average per cent contribution of different body compo- shell length and dry weight of gonad, are shown in nents to total dry weight in different sized individuals. Figs. 6 and 7, respectively. Ratio between shell length and height (aspect ratio) turned out to be particularly interesting, show- length/height ratio is >1. This change in shape ing an inversion during the growth (Fig. 8). Until 45 seems to occur when byssally attached juveniles mm length this ratio is approximately below 1, and release from the adults. Other important events seem organisms are fundamentally oval-shaped, while to take place reaching this critical length, as evi- over 50 mm length the shape is subcircular and 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 and the muscle dry weight. GSI: gonadosomatic index (gonad dry weightx100/total soft tissues dry weight).

Females Males December 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 2.28 174.70 sd 0.35 0.20 0.22 0.39 Muscle g DW avg 0.91 1.41 54.95 0.79 1.46 84.81 sd 0.25 0.34 0.19 1.22 Gonad g DW avg 0.07 0.20 185.71 0.20 0.40 100.00 sd 2.37 2.24 2.03 2.35 Total g DW avg 7.48 9.84 31.55 7.42 10.33 39.22 sd 2.37 2.24 2.03 2.35 G/M avg 0.08 0.15 87.50 0.23 0.28 21.74 sd 0.02 0.02 0.13 0.06 GSI avg 0.98 2.08 112.24 2.93 4.07 38.91 sd 0.30 0.48 2.19 1.21

20 R. CATTANEO-VIETTI et al. FIG. 10. – Gonadosomatic index (GSI) in female individuals sampled in December and January. cent contribution to total dry weight (Fig. 9), such as increase of gonad and muscle weight and decrease of shell weight relative to total dry weight, suggest- ing the start of the adult phase. FIG. 11. – Water column fluorescence values (volts), measured in Terra Nova Bay between December 30th 1993 and February 11th The examination of average gonad volume, of dry 1994. Dates on X axis are expressed as days from the beginning of weight of the different organs, of the gonadosomatic the study period. index (GSI), of ratio between gonad and muscle (G/M) and of wet and dry weight of each soft part ing particulate matter (Albertelli et al., 1996, in have been performed on males and females separate- press). The highest values of fluorescence were ly, comparing individuals of similar size, collected at measured in the deeper layers of the water column the beginning and at the end of summer (Table 4). (below 25 m) until January 5th. Afterwards, they In females a slow increase in gonad volume has moved towards shallower levels (between January been observed from the end of December to the end of 8th and 11th). On January 20th there was a drop fol- January, but its dry weight increment is much higher lowed by a sudden increase between the surface and than in muscle tissues (185 % vs 55 %). Consequently 20 m depth (22/01), reaching the highest values G/M ratio and GSI show a drastic increase at the end measured in the season. After this date, the peak of January (Fig. 10). Histological slides clearly returned to lower layers. showed a marked increase in oocyte diameter from December (20 µm) to January (45 µm), although great differences were found in the maturation stage in dif- DISCUSSION AND CONCLUSIONS ferent portions of the same gonad. In males a higher increase in muscle dry weight The assessment of the population structure of has been observed from the end of December to the Adamussium colbecki in Terra Nova Bay has to be end of January as well as in gonad volume. Such considered particularly important, as this scallop, increase in volume was not linked to a comparable constituting large assemblages and reaching very increase in its dry weight and consequently the high values of density and biomass, plays an impor- changes of G/M ratio and GSI in males were less tant role in the organic matter cycle in the Antarctic remarkable than in females. littoral ecosystem (Chiantore et al., in press). This species is remarkably persistent in the study area Summer food availability throughout the years, in terms of abundance, although interannual differences have been found in Water column food availabilty showed a strong the size-frequency distribution. seasonal trend, as evidenced both by fluorescence The population structure at Terra Nova Bay and depth profiles (Fig. 11) and by the amount of sink- the growth rate there are in agreement with those

ADAMUSSIUM COLBECKI: POPULATION STRUCTURE AND ECOLOGY 21 mainly recorded at McMurdo Sound. Little differ- (Barber and Blake, 1981). While metabolic ences in growth rate, ranging between 0.73 cm yr -1 demands of suspension-feeding bivalves may be sat- (Stockton, 1984), 0.84 cm yr -1 (Berkman, 1990) and isfied also by resuspended detritic material, enriched 1 cm yr -1 (Ralph and Maxwell, 1977), are probably in benthic diatoms (Davis and Marshall, 1961; related to different levels in food availability, as Vernet, 1977; Shumway et al., 1987), a detritical observed in other from temperate waters diet alone seems to be unfit to support growth of (Broom and Mason, 1978; Bayne and Newell, 1983; somatic and reproductive tissues of adults, as in McDonald and Thompson, 1985). Placopecten magellanicus (Grant and Cranford, Relevant changes in body shape and in ponderal 1989). ratios between different morphological parameters This supports the hypothesis that the reproduc- take place in coincidence with the detachment of tive cycle is controlled by both endogenous and young individuals from the shell of the adults when exogenous factors such as temperature and food they reach 30-50 mm in length. Detachment taking availability, as in other scallops (Bricelj et al., place at this size is confirmed, indirectly, also from 1987), mussels (Newell et al., 1982) and cockles evidences in epizoic assemblages on Adamussium (Newell and Bayne, 1980). shell, with an abrupt transition in epizoic biomass The small eggs (20-45 µm) and the low ratio on shells larger than 65 mm in shell height (0.36) between first and second larval shells (prodis- (Berkman, 1994). soconchs I and II) suggest a planktotrophic The detachment from adult shells could represent (Berkman et al., 1991), although there are no data an important behavioural and developmental about the duration of the larval stage. As among moment. Just detached individuals are probably other scallops (Sastry, 1979), inside a planktotroph- more exposed to predation pressure: when attached, ic strategy, the recruitment success could be influ- they can escape predators, as adults show a strong enced strongly by the water column food supply escape reaction through swimming and jumping (Mileikovsky, 1971) and this could explain the inter- behaviour (Stockton, 1984). annual variability in spats occurrence (Stockton, Metabolic changes, due to sexual maturation, 1984; Berkman, 1990; Berkman et al., 1991; Nigro take place in just detached individuals, which, more- et al., 1994). However, evidences in Antarctic inver- over, cannot take advantage of organic matter resus- tebrates (Pearse et al., 1991) suggest alternative pended by adult shell clapping (Davis and Marshall, food supplies to planktotrophic larvae (Olson et al., 1961), as attached juveniles do. 1987), such as dissolved organic matter (DOM: Little is known about the influence of environ- Shilling and Bosch, 1994) and mental parameters on spawning, which at McMurdo (Rivkin et al., 1986). occurs during the austral spring (Berkman, 1990; Therefore, the study of environmental conditions, Berkman et al., 1991). In Terra Nova Bay, the GSI especially food availability, is critical for an under- increase in females at the end of January suggests standing of the population dynamics of Adamussium. that gonad maturation takes place in summer, at the It is now clear that the prolonged winter season can end of the primary production period (Albertelli et influence both spawning and larval survival, prevent- al., in press). This fact could be also supported by ing adults from reaching sexual maturation and larvae evidences of different levels of gonadal Cd in from finding enough nourishment. females compared to males observed in austral sum- The concomitant occurrence of endogenous and mer (Mauri et al., 1990), which is probably related exogenous factors could explain the differences in to gametogenesis. Consequently there seems to be a population structure observed in 1993/94 relative to shift in reproductive cycle from McMurdo Sound to 1989/90, mainly the lack of size classes between 40 Terra Nova Bay, probably linked to differences in and 65 mm, coinciding with the absence of youngest food availability. In bivalves, the summer food sup- individuals in 1989/90. This fact could be related to ply influences the species physiology in terms of unsuccessful recruitments from 6 to 9 years earlier, ingestion rate, digestion and assimilation efficien- but also to a high mortality rate when individuals cies and differential utilization of biochemical com- reach the critical size at which they detach from the pounds (Ansell, 1974; Newell and Bayne, 1980; adults and are more selectively preyed upon. Kreeger, 1993): a high quality food supply could The studies carried out during the last years on induce rapid growth (Thompson and Nichols, 1988) the population of Adamussium colbecki at Terra and support energy storage and gonad maturation Nova Bay have resulted in a better knowledge of the

22 R. CATTANEO-VIETTI et al. structure of this population in an area close to the gramme to the Southern Ocean. Mar. Poll. Bull., 24: 322-323. Berkman, P.A., T.R. Waller and P. Alexander. – 1991. Unprotected Italian Station and, until now, no changes have been larval development in the Antarctic scallop Adamussium col- recorded in the population structure after the con- becki (: : Pectinidae). Antarctic Sci., 3: 151- 157. struction of the Italian Station. This suggests a Brey, T. and S. Hain. – 1992. Growth, reproduction and production scarce impact on this peculiar community, which of Lissarca notocardensis (Bivalvia: Philobryidae) in the , Antarctica. Mar. Ecol. Prog. Ser., 82: 219-226. should, anyway, be monitored every year. Finally, Brey, T. and A. Clarke. – 1993. Population dynamics of marine the small extension of the area in which this scallop benthic invertebrates in Antarctic and subantarctic environ- ments: are there unique adaptations? Antarctic Sci., 5 (3):253- occurs and its intermittent recruitment suggest that 266. an eventual commercial exploitation would quickly Bricelj, V.M., J. Epp and R.E. Malouf. – 1987. Intraspecific varia- tion in reproductive and somatic growth cycles of bay scallops result in overfishing and commercial failure. Argopecten irradians. Mar. Ecol. Prog. Ser., 36: 123-137. Broom, M.J. and J. Mason. – 1978. Growth and spawning in the pectinid Chlamys opercularis in relation to temperature and phytoplankton concentration. Mar. Biol., 47: 277-285. ACKNOWLEDGEMENTS Chiantore, M, R. Cattaneo-Vietti, G. Albertelli, C. Misic and M. Fabiano. – (in press). Role of filtering and biodeposition by Adamussium colbecki in the circulation of organic matter at This research has been carried out thanks to the Terra Nova Bay (Ross Sea, Antartica). J. Mar. Syst. Cormaci, M., G. Furnari, B. Scammacca and G. Casazza. – 1992. Il financial support by PNRA (Italian Antarctic fitobenthos di Baia Terra Nova (Mare di Ross, Antartide): Research Programme). We are grateful to Alan osservazioni sulla flora e sulla zonazione dei popolamenti. In: V.A. Gallardo, O. Ferretti and H.I. Moyano, (eds.): Ansell (Scottish Association for Marine Sciences, Oceanografia in Antartide. Atti Seminario Internazionale, 7-9 Oban) for his pretious suggestions, Carlo Barbante Marzo 1991, Concepcion, Chile pp. 395-408. ENEA Progetto Antartide. (University of Venice) and Gianmarco Veruggio Davis, R.L. and N. Marshall. – 1961. The feeding of the bay scal- (CNR Genoa) for putting at our disposal the multi- lop, Aequipecten irradians. Proc. Nat. Shellfish. Assoc., 52: 25- 29. parametric probe and the video observations by the Dell, R.K. – 1990. Antarctic Mollusca, with special reference to the ROV “Roby2”, respectively. We are grateful to the fauna of the Ross Sea. Bull. Roy. Soc. N.Z., 27: 1-311. Di Geronimo, I., R. Cattaneo-Vietti, C. 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