Seasonal changes in dry weight and chemical composition of the soft parts of the tellinid bivalve Macoma balthica in the Dutch Wadden Sea

•Vetherlands Journal of Sea Research 11 (1) : 42-55 (1977)

SEASONAL CHANGES IN DRY WEIGHT AND CHEMICAL COMPOSITION OF THE SOFT PARTS OF THE TELLINID BIVALVE MACOMA BALTHICA IN THE DUTCH WADDEN SEA by J . J . B E U K E M A and W. DE B R U I N (Netherlands Instztutefor Sea Research, Texel, The Netherlands)

CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . II. Methods . . . . . . . . . . . . III. Results . . . . . . . . . . . . . . . . a. Seasonal growth b. Ash content . . . . . . . . . . c. Condition factor . . . . . d. Biochemical composition, relative amounts . . . . . e. Biochemical composition : absolute amounts . . . IV. Discussion . . . . . . . . . . . . V. Summary VI. References . . . . . . . .

42 43 44 44 46 47 48 50 52 54 54

I. I N T R O D U C T I O N M a n y studies on the seasonal changes in the composition of the soft parts of bivalves h a v e b e e n published, b u t r a r e l y has the g r o w t h of the a n i m a l s b e e n t a k e n into account. I n the g r e a t m a j o r i t y o f cases the seasonal variations h a v e b e e n estimated for hypothetical, non-existent a n i m a l s o f a constant s t a n d a r d size. T h e feasibility of a c c u r a t e age d e t e r m i n a t i o n in Macoma p e r m i t t e d a s t u d y of the real changes with the seasons of total d r y weight a n d its c o m p o n e n t s in a p o p u l a t i o n of this species. Macoma balthica L. is living p r e d o m i n a n t l y in cooler boreal waters. I n the l a b o r a t o r y , its g r o w t h is restricted to a t e m p e r a t u r e r a n g e of 0 ° to a b o u t 15 ° C (DE WILDE, 1975). I n the W a d d e n Sea, t e m p e r a t u r e s exceed 1 5 ° C d u r i n g m o s t of s u m m e r . T h e growing season of a d u l t Macoma is limited to 3 or 4 m o n t h s d u r i n g spring (LAMMENS, 1967 ; this p a p e r ) . Such short a growing season m a y cause special p r o b l e m s for the energy b a l a n c e o f the species, w a r r a n t i n g a study on its energy storage a n d a n n u a l cycle of energy contents. Macoma shows a relatively high g r o w t h rate in the W a d d e n Sea (see review b y GILBERT, 1973), n o t w i t h s t a n d i n g the short growing season. Macoma is one of the m o s t c o m m o n a n d n u m e r o u s m a c r o - b e n t h i c

SOFT

PARTS MACOMA

43

species of the Wadden Sea. Its standing stock accounts for almost 10 percent of the total macrofaunal benthic biomass (BEuK~.MA, 1976). The species represents an important food source for flatfish and wading birds. Consequently, it has been chosen as one of the main subjects of a long term programme to study production processes in the Dutch Wadden Sea. II. METHODS

Some 100 adult Macoma were sampled with approximately monthly intervals at an intertidal station of the Balgzand in the westernmost part of the Dutch Wadden Sea during the period from J u n e 1973 through J u l y 1975. The sampling station is within 100 m distance from station B described in BEUK~.MA(1974). At this station growth rate and condition of Macoma are close to the average for the whole Balgzand area (BEuK~.MA, CAD#.~. & JANSEN, 1977). The monthly sampled Macoraa were used for determinations of condition, contents of ash, lipid, glycogen, and protein. Additional data on growth rate, condition and ash content have been used from Macoma samples at station B during 1968 through 1975. After sampling by digging and sieving the m u d d y sand in the field, the animals were stored alive in running sea water of 5 ° to 10 ° C in the laboratory till the next day, when they were killed by short immersion in boiling water. The flesh was removed from the measured shells, put into crucibles, and dried during 3 to 5 days at 60 ° C in a well ventilated stove. The dry material was stored in closed vessels. Ash-free dry weight (ADW) was obtained by placing the weighed dried material of about 10 animals separately in a furnace at 500 ° to 600 ° C for 2 hours, and weighing again. The difference between dry weight and ash weight is considered to represent the ADW of the animal. Condition factor (CF) was calculated as the quotient of the ADW and the third power of the shell length along the longest axis (L). CF is therefore the A D W of a 1 cm individual expressed in mg. CF appears to be nearly independent of L in the range of 12 to 20 m m ; therefore, only individuals within this size range have been used. Both very small and very large Macoraa show consistently lower values of CF than m e d i u m sized animals. The dry material of the remaining animals was pooled for each sampling date, ground to a fine powder in a mortar, and stored in closed vessels. Before 1973, the dry material was stored at room tern-

44

j.J.

BEUKEMA

~; W . D E B R U I N

perature. As we observed that such samples lost significant parts of their lipids within one year and their calorific values had decreased by 0.1 to 0.2 cal.mg -1, samples collected from mid-1973 onwards were stored at --20 ° C and only during short periods (generally less than one month). We present results of chemical analyses only from the latter samples. Longer series (from 1968 onwards) of dependable observations are available only for ADW, CF, and ash. Lipid content was measured by Soxhlet refluxing extractions of weighed aliquots, using either pentane or chloroform as a solvent. The latter solvent yielded consistently higher (by 30% in summer to 80% in winter and early spring) amounts of lipids, which were even slightly higher than lipid yields obtained by extraction into a chloroformmethanol mixture according to BLIGH & DYER (1959). We used data obtained by Soxhlet extraction into chloroform as well as figures calculated by adding 30 to 80% to data obtained by pentane extraction. The lipid-free fraction was dried again at 60 ° C, and weighed aliquots were used for glycogen (5 to 20 mg) and nitrogen (100 mg) determinations. Glycogen was estimated as glucose following the colorometric anthrone method according to VAN HAND~.L (1965), but slightly modified, viz. by cooling at --20 ° C after precipitation of the glycogen. Dried glycogen (Merck "glycogen for biochemistry") was used as a standard. Protein was estimated as total N, using a standard mmro-Kjeldahl procedure, with 6.25 as a multiplication factor to obtain protein figures from the observed values for total nitrogen. III. a.

RESULTS

SEASONAL

GROWTH

In the Dutch Wadden Sea, Macoma only grows during a short period of the year. In 1963, LAMMENS (1967) observed an increase in shell size during April through June. The data included in Fig. l a extend this observation of a restricted growing season to an 8 years sampling period. The mean lengths shown concern Macoma of an age between 1½ and almost 3 years. At such low ages, densities are still high and age determination is still relatively easy and accurate (LAMMENS, 1967; and own observations). The observed increase of shell length of 2 to 3 m m during the third

SOFT

PARTS

45

MACOMA

growing season in the life of Macoma in the Wadden Sea agrees with the estimate by LAMMENS(1967). It is high relative to the growth rates of Macoma observed in most other areas (GILBERT, 1973). The first growing season in the life of Macoma ranges over summer and late summer. The second one starts already during March and ends already in J u n e or July, as in older Macoma. Thus the second and following annual periods with a growth stop last 8 or 9 months. During these periods mean shell length remains constant (Fig. la).

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Fig. 1. Changes in mean shell length, in mm (a), and weight of soft parts, in mg ashfree dry weight (b), before, during, and after the third growing season, in Macoma balthica at station B for the year classes 1966 (observations during 1968 and early 1969) (O), 1967 (.), 1968 (&), 1969 (V), 1970 (V), 1971 (VI), 1972 (©), and 1973 (A). Lines fitted by eye.

Fig. lb shows that the dry weight of the soft parts decreases during the long annual period of constant shell length. The annual period of growth in shell length coincides exactly with the period of rapid weight gain. In the two year old Macoma, seasonal weight increment amounts to about 30 rag. During the first growing season it is on average only I or 2 mg. It amounts to about 20 mg in one year old Macoma, and to 40 to 60 mg at ages of 3 or more years. Growth during the third

46

j. j. BEUKEMA & W. DE BRUIN

growing season, shown in detail, thus will be a fair representation of growth during any but the first season. During most of the year the weights decrease at a rate of roughly 5 percent per month. At the start of a next growing season, the net result of a one year cycle is a weight somewhat lower than that observed half-way between the start and end of the foregoing growing period. O n average, almost two-thirds of the gain in weight during the growing season gets lost during the subsequent 8 or 9 months. This weight loss is even a little higher in older Macoma, whereas it is less in younger ones. The annual cycle shown in Fig. lb is close to the average one for the whole population, which generally includes some 7 or 8 yearclasses at a time. b. ASH CONTENT

Fig. 2 shows the annual cycle of the ash content, expressed as a percentage of the total dry weight of the soft parts. Lowest ash contents of 6 to 8% are observed at the end of the growing season, when the weight of the soft parts is at its maximum. An ash content of 6% of the dry weight is equivalent to 1 to 1½% of the live weight of the soft parts and to about 3% of the extra-cellular fluid (SPAAROARENpersonal communication). Such ash percentages may be expected to represent indispensable salts to approximate osmotic balance with the environ% osh 15-

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Fig. 2. Annual cycle of ash content of adult Macoma, expressed as a pelcentage of total dry weight of the soft parts. Data from samples obtained during 1968 (O), 1969 (m), 1973 (if3), 1974 (O), and 1975 (A). Lines fitted by eye.

ment, where salinities of 25 to 30%0 S prevail. Percentages far above this level, as occur especially in winter, will point to the presence of some inorganic material in the mantle cavity or in the intestine. Apparently, during this part of the year, the activity of Macoma is too low for complete clearance within one night in clean sea water.

SOFT

PARTS

47

MACOMA

C. CONDITION FACTOR

Fig. 3 shows the annual cycle of the condition factor CF = A D W . L -s. The data have been calculated for adults of intermediate size (viz. 12 to 20 m m length), which are roughly 2 to 5 years old. The seasonal course of CF is comparable to those of dry weights of animals of standard length, which have been published for m a n y bivalve species (e.g. TREVALLION, 1971 ; ANSELL, 1972). ADW L-~ 18-

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Fig. 3. Annual cycle of condition factor, equal to the ADW in mg of a standard Macoma of 10 mm length, and calculated as the mean of the quotients ADW/L8 for the 9 (or less) 1 mm length classes of 12 to 20 ram. Data from samples obtained during 1968 (e), 1969 (11), 1970 (A), 1971 (T), 1972 (V), 1973 ([]), 1974 (©), and 1975 (A). Lines fitted by eye. The seasonal cycle of CF closely follows that of the ADW. An essential difference can be seen during the growing season, when the rate of increase is much steeper in A D W (Fig. lb) than it is in CF. Such a difference is to be expected as outside the growing season, changes of CF purely reflect changes of ADW, as L (Fig. lb) remains nearly constant during that period. The decline of condition during the annual cycle appears to start earlier than CHAMBERS & MZrNE (1975) observed in Scotland, where

48

j.j.

BEUKEMA

&~ W . D E B R U I N

they found the growing season to extend from March through August for Macoma of 2 q- age classes. d.

BIOCHEMICAL

COMPOSITION:

RELATIVE

AMOUNTS

The results of the chemical determinations have been summarized in Fig. 4 after conversion of the amounts found in the small atiquots of % of ADW

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Fig. 4. Annual cycles of protein (a), glycogen (b), and lipid (c) fracttons of the ashfree dried soft parts of Macoma, expressed as percentages. Data from samples obtained during 1973 (El), t974 (C,), and 1975 (A). Lines fitted by eye.

SOFT

PARTS

MACOMA

49

pooled and grounded material into percentages of total ADW. During all or most of the growing season (April, May, and June), the proportions of glycogen and lipid rise rapidly, whereas the percentage of protein decreases. After the growing season, during most of summer and autumn, just the reverse changes take place. During winter little change is observed in the relative contribution of any of the three components. These main lines of seasonal changes are observed during both years of observation. Significant differences between the two winters can be observed only in lipid content, viz. higher proportions of lipid during the winter of 1973-1974 than during that of 1974-1975. As to the 3 summers, the one of 1974 shows relatively low proportions of glycogen. Apart from these, the differences between the 2 successive years are only slight. With the annual cycle being essentially the same during the 2 years of observation, lines can be drawn in Fig. 4 to represent the average and general course of the proportions of protein, glycogen and lipid. Such lines run nearly horizontal during January, February, and March. During April lipid and glycogen increase, protein decreases. The percentage of lipid reaches a maximum already early in May. Glycogen continues to rise at about the same rate as in April, during May and most ofJune. Thus, at the start of the growing season all three components change at the same time (end of March). The earlier reversal in the change in lipid percentages in comparison to the reversals in protein and glycogen percentages may be connected with spawning, culminating in May (LAMMENS, 1967; DE WILDE, personal communication). Summation of the observed percentages of the three main components of ADW almost invariably yielded totals of less than 100%. Fig. 5 shows the percentages deficit found. They are higher in winter than during summer. On average, the deficit amounted to 6.2 4- 0.7% (n = 22) of ADW. Part of the material included in these percentages will have been free sugars, which have not been determined during the present analyses. In mussels, GABBOT & BAYNE (1973) found 3 to 5% of total ADW of soft parts to consist of free sugars (calculated from their table 3). Assuming similar percentages for free sugars in the tissues of Macoma, most of our deficit percentages would still be too high. Incomplete drying may be an explanation, as further drying during 3 days of pellets of ground material already dried routinely during 3 days at 60 ° C, resulted in further weight losses. These losses amounted to mean percentages of ADW (60 ° C) of 1.1% at 80 ° C, of 2.2% at 100° C, and of 3.5% at 110 ° C. As such weight losses at high drying temperatures

50

j. j. BEUKEMA

~¢ W . D E B R U I N

might be due also to losses of volatile lipids, we made the same observations with fat-free material, obtained by further drying the material remaining after the lipid extraction and dried during 3 days at 60 ° C. Weight losses in percentages of DW at 60°C were 1.2% at 85 ° C, % of AOW

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Fig. 5. Annual cycle of percentages of ash-free dry weight, not accounted for in analyses for proteins, glycogen and lipids and obtained by difference. Data from samples obtained during 1973 (R), 1974 (©), and 1975 (L). Lines fitted by eye. 2.6% at 100 ° C, and 2.9% at 110 ° C. As these losses hardly differ from those found in the fat-containing material, it is concluded that soft parts of Macoma dried at 60 ° C still contain some 2 or 3% of water. Nevertheless, drying at higher temperatures is not to be recommended, because the amounts of lipids we were able to extract, decreased significantly with increasing drying temperatures in the 60 ° to l l 0 ° C range. At the latter temperature this amount was only slightly more than half the amount found at 60 ° C. Extraction from wet material is not a perfect method either, because fresh weight is poorly defined. Our practice of drying at 6 0 ° C is a compromise, though not a completely satisfactory one. e.

BIOCHEMICAL

COMPOSITION:

ABSOLUTE

AMOUNTS

To study the seasonal changes of body composition, expression in relative amounts m a y be misleading, because if the share of one component increases, another percentage is bound to decrease, although both increase in absolute quantity. This is the case in protein (Fig. 4).

SOFT

PARTS

MACOMA

51

Absolute amounts of protein rise during the growing season, but at a slower rate than those of glycogen (Fig. 6) and, in consequence, protein percentages decrease during this period. The reason w h y absolute amounts are not commonly calculated will IT °

24 l

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d' F'M' A'M' d~ J~A~S~O=N~D= dl F'M' Fig. 6. Biochemical composition of an "average" Macoma (showing weight changes as indicated by the line in Fig. Ib) before, during and after its third growing season. Protein, glycogen, lipid and other substances calculated by using mean percentages (as shown by the lines in Figs 4 and 5).

be the difficulty to obtain reliable growth curves. In most species, year marks are not sufficiently clear to allow easy and dependable age determination. Fig. 6 shows the changes in absolute amounts of the main body components in the soft parts of an "average" Macoma growing up from

52

j. j. B E U K E M A & W. DE B R U I N

an age of 1½ to almost 3 years. The net growth from the second to the third winter in the life of Macoma, being about 10 rag, is almost completely in the form of protein. Net changes in amounts of glycogen, lipid, and "other substances" only account for about 1 mg each. Thus of the total net growth about 3/4 is incorporated in protein. Seasonal changes, at the other hand, are greatest in glycogen showing a loss of 13 mg after the growing season from a summer maximum of 15 rag. Simultaneous losses of lipid and protein are about 3 and 6 rag, respectively, from a summer stock of 5 and 24 mg. The strong decline in the amount of glycogen (85% of its summer maximum) points to its importance as an energy storage. Apparently, the energy stored in the form of glycogen is insufficient to last for the long non-growing season, as both protein and lipid decrease as well. However, the losses observed in the latter substances (60 and 25% of the summer maxima, respectively) are less than the one found in glycogen. IV. DISCUSSION

Adult Macoma show a surprisingly short annual growing season in the Dutch Wadden Sea. The early growth stop will be related to the low value of the maximum temperature (only about 15 ° C) at which Macoma is able to grow (DE WILDE, 1975). Already in J u n e or early July the annual maxima for condition factor (Fig. 3 ; and ESSINK, 1970), dry weight (Fig. lb), and its components (Fig. 6) are reached. This is one or two months before the time of maximum water temperatures in the Wadden Sea, but it is close to the starting-point of the period during which the temperatures of the flood water remain above 15°C for some months. Temperature, however, appears to be not the sole determinant of growth, because there is no sign of a second annual growing period in early autumn (Figs 1 and 3; ESSINK, 1970; D~ WILDE, 1975), when temperatures are again for one or two months between 10 ° and 15°C. In the adults of some other species of bivalves in the Wadden Sea, seasonal peaks of condition, and dry weight or its components occur one or two months later, e.g. in July or August in Mytitus edulis (DE Zw~ & ZANDEE, 1972) and in August in Cardium edule (EsslNK, 1970; own observations). In some other species, like Tellina tenuis and Scrobicularia plana, which are more closely related to Macoma, we observed maxima as early in summer as in Macoma. In other coastal areas around the North Sea, the timing of the annual peaks and minima of dry weight and its constituents in bivalves is more or less similar. Minimum values appear to be uniformly situated about March (e.g. ANSELL, 1972; HANCOCK & FRANKLIN, t972; TREVALLION,

SOFT PARTS

MACOMA

53

1971). The time of the annual maximum is somewhat more variable, and there may be two peaks in species spawning during the second half of the growing season, e.g. 9onax vittatus (ANSELL, 1972), Tellina tenuis (TREVALLION, 1971) and Ostrea edulis (WALNE, 1970). In agreement with our observations, CHAMBERS & MILNE (1975) observed single peaks in the dry weight of standard sized Macoma in June or July in Scotland. Also in agreement with our observations in the Wadden Sea, peak values of condition dated in June were observed by HUGHES (1970) in Scrobiculariaplana in Wales and in Tellina tenuis in Kames Bay, Scotland, by ANSELL & TREVALLION (1967). However, as a result of a more detailed study of 7". tenuis at three places in Scotland during three years, TREVALLION (1971) found peaks at rather irregular times in summer or autumn. The condition of Tellina in Scotland appears to be more variable than that of Macoma in the Wadden Sea. The great regularity of the annual cycle in such species as Macoma balthica and Cardium edule will be caused partly by the coincidence of a prolonged spawning period with that of the most rapid growth (see also HANCOCK & FRANKLIN, 1972). The observed annual cycles of condition or dry weight can be explained for the greater part by the cycle of glycogen (compare Figs 1b and 3 with Figs 4b and 6). About 45% of the spring increase of dry weight and 65% of the weight decrease during the remainder of the year can be attributed to changes in the amount of glycogen. In no other important body constituent the annual amplitude is as large as it is in glycogen, the early-summer maximum being almost ten times as high as the late-winter minimum (Fig. 6). Glycogen clearly serves as a reserve, stored during the growing season and used up almost completely during the remainder of the year. Expressed in calories, glycogen with a calorific value of about 4 cal.mg-1 contributes about 50 % of the energy used up during the long annual period of emaciation. Proteins and lipids with about 5 and 9 caJ.mg -1, respectively, provide comparable shares to the other half of the calories. Due to its low calorific value, the contribution of glycogen is less impressive when expressed in energy (50%) than in weight units (65%), whereas the role of lipids becomes more prominent, viz. from about 15% of the decrease in weight to about 25 % of that in calories. Calorimetry of the soft parts of Macoma will be dealt with in more detail in a separate paper. The almost synchronous annual cycles of dry weight (or condition) observed in the various species of bivalves living in temperate or boreal shallow seas has interesting ecological consequences. Standing stocks (in g.m -2 of soft parts) of macro-zoobenthos may be expected to show well-defined annual cycles in such areas, where bivalves contribute

54

j.j.

BEUKEMA

& W. D E B R U I N

significantly to the total macrobenthic biomass. In the Dutch Wadden Sea, no less than 65 percent of the macrobenthic biomass of the tidal flats was found to be made up of bivalves (BEug.~MA, 1976). Summer biomass observed in this area was indeed consistently about twice the winter blomass (B~.uK~.~A, 1974). The decrease of biomass during autumn and winter could be attributed about equally to declines in numerical densities and to individual weight losses (B~.uKEMA, 1974). The annual cycles of dry weight (or condition) thus seriously affect the stocks of benthic food available to birds and fishes in areas like the Wadden Sea. During winter and early spring, predators feeding mainly on bivalves will have more difficulty in finding sufficient food than they have in summer. Firstly, they will have to collect higher numbers of prey, because each prey specimen contains less meat. Secondly, these higher numbers have to be discovered from lower numerical prey densities, as in nearly all species the main period of recruitment to the bottom fauna following reproduction is in summer. V. SUMMARY Intertidal Macoma balthica L. from the westernmost part of the Dutch Wadden Sea exhibited an annual cycle in their ash-free dry weight and biochemical composition. The growing season of adult Macoma was limited to the months April, M a y and June. During this short period there was a rapid build up of total glycogen, protein and lipids. The percentage of protein declined during this period from about 75 to 50% of the ash-free dry weight, whereas the percentages of glycogen (from 10 to 35%) and lipids (from 10 to 15%) rose. During the subsequent 9 months the animals lost about half of the weight of their soft parts. The share of the various components differed: 85% of the glycogen and 60% of the lipids were used up, whereas only 25% of the proteins disappeared. The resulting annual net growth was less than half of the seasonal growth and was almost restricted to protein. VI. REFERENCES ANSELL, A. D., 1972. Distribution, growth and seasonal changes m biochemical composition for the bivalve Donax vzttatus (da Costa) from Kames Bay, Millport.--J, exp. mar. Biol. Ecol. 10: 137-150. ANSELL,A. D. & A. TREVALUON,1967. Studies on Telhna tenuis Da Costa I. Seasonal growth and biochemical cycle.--J, exp. mar. Biol. Ecol. 1 : 220-235. BEUKEMA,J. J., 1974. Seasonal changes in the biomass of the macro-benthos of a tidal flat area in the Dutch Wadden Sea.--Neth. J. Sea Res. 8: 94-107. , 1976. Biomass and species richness of the macro-benthic animals living on the tidal flats of the Dutch Wadden Sea.~Neth. J. Sea Res. 10: 236-261.

SOFT PARTS MACOMA

55

BEUraZMA,J. J., G. C. CADI~E • J. J. M. J~a~SSN, 1977. Variability of growth rate of Macoma balthica (L.) in the Wadden Sea in relation to availability of food. Proc. 1lth Europ. mar. blol. Syrup., Galway, Ireland (in press). BUGH, E. G. & W . J . DYSR, 1959. A rapid method of total lipid extraction and purification.--Can. J. Biochem. Physiol. 37; 911-917. CH~BERS, M. R. & H. MILNE, 1975. The production ofMacoma balthica (L.) in the Ythan estuary.--Est, coast, mar. Sci. $: 443-455. ESSINK, K., 1970. Onderzoek naar de gevolgen van lozing van ongezuiverd industri~el- en huishoudelijk afvalwater door middel van een persleiding van Hoogkerk naar de Waddenzee. Interim-rapport over 1969. Zool. Lab. R.U. Groningen, Z 70/128: 1-29. GABBOTT, P. A. & B. L. BAYNE, 1973. Biochemical effects of temperature and nutritive stress on Mytilus edulis L . - - J . mar. biol. Ass. U.K. 53" 269-286. GILBERT, M. A., 1973. Growth rate, longevity and maximum size of Macoma balthica (L.).--Biol. Bull. mar. biol. Lab., Woods Hole 145.119-126. HANCOCK, D. A. & A. FRANKLIN, 1972. Seasonal changes in the condition of the edible cockle (Gardium edule L.).--J. appl. Ecol. 9: 567-579. HANDEL, E. VAN, 1965. Estimation of glycogen in small amounts of tissue.--Analyt. Biochem. U : 256-265. HUGHES, R. N., 1970. An energy budget for a tidal-flat population of the bivalve Scrobicularza plana (Da Costa).--J. Anita. Ecol. :$9: 357-381. LAM~mNS,J . J . , 1967. Growth and reproduction in a tidal flat population of Macoma balthica (L.).--Neth. J. Sea Res. 3: 316-382. TREVALLION, A., 1971. Studies on Tellina tenuis Da Costa. I I I . Aspects of general biology and energy flow.--J, exp. mar. Biol. Ecol. 7: 95-122. WALNE, P. R., 1970. The seasonal variation of meat and glycogen content of seven populations of oysters Ostrea edulis L. and a review of the litterature.---Fishery Invest., Lond. (2) 26 (3) : 1-35. WmDE, P. A. W . J . DE, 1975. Influence of temperature on behaviour, energy metabolism, and growth ofMaeoma balthica (L.). In: H. B~tNES. Proc. 9th Europ. mar. biol. Symp., Aberdeen University Press: 239-256. Zw~ma~, A. DE & D. I. ZANDEE, 1972. Body distribution and seasonal changes in the glycogen content of the common sea mussel Mytilus edulis.--Comp. Biochem. Physiol. A,kSA: 53-58.