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Egg size and larval survival of Mercenaria mercenaria (L.) and Argopecten irradians (Lamarck)
J. <‘s-p. mar. Biol. Ecol..
Elsevier Biomedical
1982, Vol. 56, pp. 3-8
Press
EGG SIZE AND LARVAL
(L.) AND ARGOPECTEN
MERCENARZA
JOHN
SURVIVAL
N. KRAEUTER,
OF MERCENARZA ZRRADZANS
MICHAEL
CASTACNA
Vi~pirlirrImtilute
of Mark
(Lamarck)’
and ROSA VAN DESSEL Scirwe
and School of Marine
Science.
College of William and Mary
Wachapreague,
VA 23480,
C’.S.A.
Abstract: Eggs of the hard clam, Mercenaria mercenaria (L.), and the bay scallop, Argopec,ten irradians (LamdrCk). were graded to three size categories, split into replicates and these were replicate-sampled. A chi-square analysis compared the proportion of larvae (48-h survival) to the proportions of eggs in each initial replicate experiment. Statistically significant differences attributed to egg size were found for both species. Large eggs survived better than small eggs, while those of intermediate size showed no difference between expected and observed survival.
INTRODUCTION
Bivalves with planktonic larvae characteristically produce large numbers of eggs that offset the high mortalities of early life history stages. Numbers of eggs produced by the hard clam Mercenariu mercenaria have been estimated to be between lo5 and 10’ eggs per female with an average of IO6 (Davis & Chanley, 1956; Ansell, 1967). The bay scallop, Argopecten irradians, produces slightly fewer eggs. Fordham (1970) reported a range of lo5 to lo6 eggs per female, the average between these two values. These species have planktrophic fecundity and high early mortality.
being intermediate larvae with high
Another factor in the development of an egg is the quantity and quality of the stored food reserves. Worley (1944) and Raven (1958) have shown that pelecypod eggs have a fatty yolk consisting chiefly of neutral fat, and Bayne (1972) and Bayne rf al. (1975) found that the fatty yolk reserve is utilized during early development. The quantity of this reserve was correlated with early growth of the brooded young of Ostrea edulis (Helm et al., 1973). The importance of fatty yolk reserve to the growth of planktonic bivalve larvae has not been determined. The present study was designed to determine the relationship between egg size and early survival for the pelecypods Argopecten irradiuns (Lamarck) and Mercenaria mercenaria (L.).
’ Contribution
No. 986 from Virginia
0022-098 Ij82/0000-0000/$02.75
Institute
of Marine
0 1982 Elsevier Biomedical
Science. Press
JOHN N. KRAEUTER ETAL MATERIALS AND METHODS Spawning
stock of M. mercenaria were from wild populations
or from 3-yr-old
field-raised stock from previous spawnings. All groups were mass-spawned on a flow-through spawning tray using thermal shock as a stimulus. Fertilized eggs were collected on a 25-pm mesh sieve, screened into 44-, 35- and 25-pm size classes, and placed in filtered and incubated (cultured) sea water in separated containers. Eggs of each size group were accumulated until there were sufficient numbers to be counted. Densities were adjusted to yield x 30 eggs per ml for each mesh size group by the addition of cultured water. Replicates were made for each sieve size group by distributing eggs into four or five containers. Each replicate was then thoroughly mixed, and a l-ml sample was taken from each container with an automatic pipette. This step was repeated five times to obtain five replicate samples from each container. These samples were individually preserved in 376 buffered formalin and provided the base count against which survival was compared. All live eggs were then dispersed in 50 1 of cultured sea water at densities of z 6 . ml -‘. The culture was drained in 2 days and veliger larvae were collected on the appropriate (25, 35, or 44 pm) mesh size screens. These larvae were then placed in l-liter containers of cultured water, mixed, and replicate-sampled five times. The I -ml samples were preserved in 3% buffered formalin. All samples were placed in the reservoir of a l-ml Sedgwick-Rafter counting cell and counted using a binocular compound microscope. The larvae thus counted were used to estimate survival for each replicate group within each sieve size (see below). Spawning stocks for Argopecten irradians were field grown adults from the previous year’s cultured animals. Methods similar to those used for the clams were used for scallop experiments except that eggs were sized on 20-, 25- and 35-pm mesh screens. Chi-square tests were used to compare 2-day survival against initial counts. The average number of eggs per ml for each replicate at time 0 (X,,,) was calculated by summing the counts of an individual replicate (X,, X?...X,) and dividing by 5 (the number of samples per replicate). The total number of eggs per ml for all egg sizes and replicates was determined by summing the X,,, values (ZX,,)). The proportion (P) of the total count attributed to each replicate was calculated by dividing X,,, for that replicate by XX,,) P l(Z) =
XT,>replicate
1 (25 pm egg size) =X,0
This proportion was then used to determine the expected number qf’survivors for each replicate experiment based on the assumption that the survival of larvae from all egg sizes would be proportional to the egg count.
EGG SIZE AND LARVAL
SURVIVAL
OF .~ER~~~AR~A
5
The number of larvae surviving per ml per replicate at time 1 (A’,,) and the total number of larvae for all size categories and replicates (CX,,,) were calculated in the same fashion as for the eggs. To determine the expected survival for any particular replicate, the original proportion (P,(zs, for the case above) was multiplied by XX,, , This served as the expected level ~f’survival for that particular replicate. A chi-square test was used to compare this expected against the observed (X,, ,_) survival. Similar calculations were made for each replicate experiment at each date. Because of the heterogeneity in the experiments (“-test of heterogeneity), we co&d not pool the replicates to determine overall effects. Two additional experiments were designed to test survival of young from eggs > 35-pm eggs vs. small (those c 3.5 pm) Mercenariu ~e~cenarja eggs to I month post setting. Two spawnings were made, eggs were split as in the 48-h experiments, and eggs were distributed in the 50-l containers at 30 eggs . ml-‘. These batches were drained every 2 days, screened, and placed in new culture water. Details of the hatchery system employed are given in Castagna & Kraeuter (1977). After 8-10 days setting had begun, and individuals were placed in a tray in ambient running sea water. Flow rates were gradually increased and reached 8 1 . min -’ after 2 wk. In order to control fouling during this period, the water was turned off, and the tray drained and allowed to air dry for 0.5 h daily. Four weeks after setting the entire tray was washed clean of clams. The clams were placed in a container, 1 liter of water added, thoroughly mixed and five replicate samples were taken. These samples were immediately counted and used to estimate survival. RESULTS
Survival of individual replicates (Tables I and II) clearly shows that larger eggs had significantly greater 48-h survival than the smaller eggs. The results were similar for both M. mercenaria and Argopecten h-radians. Eggs of intermediate size generally TABLE I Summary of significance of chi-square data of individual (replicate) experiments of egg size vs. survival for :Wer~ncz~in mercPnuriu: less, fewer survived than expected; no difference, no significant difference between observed and expected survival; greater, greater than expected survival; sieve size is the size mesh on which eggs were retained; 25+m eggs range in size from 25 pm to 34 pm; 35-pm eggs range in size from 35 pm to 43 pm; 44-pm eggs include all eggs 44 pm or greater in size; all results tested at chi-square at the 0.05 level.
Mesh size (pm) 2.5 35 44
Significantly less survival
No difference
10 4
3 8
1
2
Significantly greater survival --_~~
I 11
JOHN
6
N. KRAEUTER
ETAL
survived in proportion to the initial counts. For both species, smaller eggs had significantiy less than expected survival, while larger eggs had significantly greater than expected survival. Again, intermediate size eggs showed no difference between the expected and observed survival. TABLE
11
Summary of significance of chi-square data of individual (replicate) experiments of egg size vs. survival for Argopecm irrudians: less, fewer survived than expected; no difference, no significant difference between observed and expected survival; greater, greater than expected survival; sieve size is the size mesh on which eggs were retained: ZO-pm eggs range in size from 20 pm to 24 pm; 2%pm eggs range in size from 25 pm to 34 pm; 3%pm eggs include all eggs 35 pm or greater in size: all results tested at chi-square at the 0.05 level. Significantly less survival __...__. .____._
Mesh size (pm) 20 25
No difference
.._._
3
2
I
8
35
Significantly greater survival
2
7
If egg size and species considerations are ignored and eggs are simply graded as small (20 pm A. jr~ff~~~~~and 25 pm N. ~zerc~~aria), medium (25 ,um A. ir~a~ians and 35 pm M. ~ercenar~a). and large (35 pm A. ~~~a~ja~~ and 44 pm M. ~ner~enari~~ and these data are combined, the results indicate that 72:‘; of the time small eggs had less than expected survival, while large eggs survived at a greater rate than expected 787; of the time (Table III).
Combined survival significantly fewer
of Avgopec~or and h4etcc~mtiu: size. size of eggs (see text for explanation); fewer. survived than expected; greater. significantly more survived than expected; no difference. survival not significantly different than expected.
Size
Fewer (Yb)
Small Medium Large
13 : 72.2 5 : 22.7 I : 4.3
No difference 5 : 27.8 16172.7 4: 17.4
(“,)
Greater
(I’,)
0: 0 I: 4.5 18:X3.3
No. ofexperiments 18 22 2.3
Results of longer term experiments mirrored those of the 48-h tests. Smaller eggs had significantly poorer survival 1 month post set than did larger eggs. In both longer-term experiments, at least 95% more individuals survived from the larger eggs than from the smaller ones.
EGG SIZE AND LARVAL SURVIVAL OF MERCENARIA
7
DISCUSSION Loosanoff
& Davis
(1963) reported
egg sizes for a large number
of different
pelecypod species, but the ranges they presented suggest that there is very little difference in egg size within a species. Our results indicate that even small changes in egg size may be important in survival. This may reflect a change in the amount of fat or other reserves stored in the egg. The mesh sizes we used in the 48-h studies selected larger eggs with x43”/, larger volume than that of the smaller eggs for both species. The volume change of the eggs, assuming an egg to be a sphere, between the smallest and largest eggs was 810/o. If the germinal vesicle portion of the egg either remained the same or increased in proportion to the screen size, then larger eggs must have substantially more stored reserves available for development (Bayne, 1972 ; Bayne et al., 1975). Bricelj (1979) has shown that laboratory-induced spawning of M. mercenariu caused the release of two sizes of eggs and that these sizes approximate a bimodal distribution. Eggs in nature are assumed to be released in a similar manner. Our data indicate that the overall proportions of eggs in any size category may change through time or with different parents. Initial egg size distribution of one of the batches utilized in current experiments was 25 pm 7.5x, 35 pm 34.5x, and 44 pm 58’?& A second batch of different parental stock spawned three weeks later had the following proportions: 15.1, 49.2, and 35.7x, respectively. This follows the trend reported by Bricelj (1979) in that spawnings later in the year have higher proportions of smaller eggs. Our longer-term experiments also indicate that smaller eggs are spawned later in the year. The first spawning had 37.69; small eggs ( < 35 pm) while 1 wk later the proportion from a different set of adults was 50”/;; small eggs. This abrupt change may reflect the lateness of the spawning period (late July) or the different set of parents, but it follows the trend. Although many factors other than egg size may alter survival between egg and 1 month post-set, the similarity between the longer-term results and the 48-h survival data suggest
that larger egg size does have a significant
and
lasting
influence
on
survival. Dey (1979) has reported that larger larvae of M. mercenaria set sooner and continue to grow at rates greater than their smaller siblings. These data are similar to those for the oyster Crassostrea virginica (Haley & Newkirk, 1977) and support and extend our results. In order to meaningfully compare survival rates from batch to batch or from experiment to experiment, it is necessary to record not only egg size and screen size, but size distribution of eggs. Researchers conducting energetics or fisheries studies should be aware that using the smallest screen size will reduce the percent survival, but the actual reduction will depend on the size-frequency distribution of the eggs above this minimum. This distribution may change with the season. Hatchery workers interested in efficient production may wish to eliminate the smaller size eggs and concentrate on better care of the larger egg sizes that are
8
JOHN
N. KRAEUTER
ETAL
more likely to survive. Lower overall survival of batches spawned later in the year is often experienced by hatcheries, Reduced survival is often related to higher temperatures and bacterial contamination, but a significant portion of these losses may be due to the use of smaller eggs with reduced stored reserves. This condition yields less healthy animals which in turn are more susceptible to disease.
REFERENCES
ANSELL, A. D., 1967. Egg production of ~ercenaria mercenari~. Limnof. Oceanogr., Vol. 12. pp. 172- 176. BAYNE, B. L., 1972. Some effects of stress in the adult on the larval development of ~~~tilu,~ edulis. Nature, Lond., Vol. 237. p. 459 only. BAYNE, B. L., P. A. GABBO~ & J. WIDDOWS, 1975. Some effects of stress in the adult on the eggs and larvae of Mytilus edulis L. J. mar. biol. Ass. U.K.. Vol. 55, pp. 675-689. BRI~ELJ, V. M., 1979. Fecundity and related aspects of hard clam (Mercenaria mercenario) reproduction in Great South Bay, New York. MS. thesis, State University of New York at Stony Brook, N.Y., pp. l-98. CAST?\GhA, M. & J. N. KRAEUTER, 1977. Mercenaria culture using stone aggregate for predator protection. Proc. natln. SheNfish. Ass.. Vol. 67, pp. 1-6. DAVIS, H. C. & P. E. CHANLEY, 1956. Spawning and egg production of oysters and clams. Biol. BuN. mar. hiol. Lab., Woods Hole, Vol. 110. pp. 117-128. DEY, N. D., 1979. Growth of sibling hard clams, Mercerzaria mercenaria (L.) in a controlled environment. M.S. thesis, College of Marine Studies, University of Delaware, 66 pp. FoRlJH.4M, E. C., 1970. Scallop culture, hatchery rearing of larval and juvenile stages of the bay scallop, Aequipecten irradians. Grant RSA-70-24. Corm. Res. Comm., Hartford. Corm. HALEY. L. E. & G. F. NEWKIRK, 1977. Selecting oysters for faster growth. Prof. Eighrh Ann. Meeting World Mariculture Sot.. Vol. 8, pp. X%565. HFLM, M. M., D. L. HOLLAND & R. R. STEPHENSON, 1973. The effect of supplementary algal feeding of a hatchery breeding stock of Ostrea edulis L. on larval vigour. J. mar. hiol. Ass. U.K.. Vol. 53, pp. 673-684. LOOSANOFF, V. L. & H. C. DAVIS, 1963. Rearing of bivalve molluscs. In. Advances in mnrine biology. I’ol. I, edited by F. S. Russell. pp. l-136. RAVEN, C. R., 1958. Morphogenesis: the analysis of molluscun development. Pergamon Press. New York, 311 pp. W~RLEY, L. G., 1944. Studies on the vitally stained Golgi apparatus. II. Yolk formation and pigment concentration in the mussel Myrilus cal$ornianus Conrad. J. Morph., Vol. 75, pp. 77-93.
Elsevier Biomedical
1982, Vol. 56, pp. 3-8
Press
EGG SIZE AND LARVAL
(L.) AND ARGOPECTEN
MERCENARZA
JOHN
SURVIVAL
N. KRAEUTER,
OF MERCENARZA ZRRADZANS
MICHAEL
CASTACNA
Vi~pirlirrImtilute
of Mark
(Lamarck)’
and ROSA VAN DESSEL Scirwe
and School of Marine
Science.
College of William and Mary
Wachapreague,
VA 23480,
C’.S.A.
Abstract: Eggs of the hard clam, Mercenaria mercenaria (L.), and the bay scallop, Argopec,ten irradians (LamdrCk). were graded to three size categories, split into replicates and these were replicate-sampled. A chi-square analysis compared the proportion of larvae (48-h survival) to the proportions of eggs in each initial replicate experiment. Statistically significant differences attributed to egg size were found for both species. Large eggs survived better than small eggs, while those of intermediate size showed no difference between expected and observed survival.
INTRODUCTION
Bivalves with planktonic larvae characteristically produce large numbers of eggs that offset the high mortalities of early life history stages. Numbers of eggs produced by the hard clam Mercenariu mercenaria have been estimated to be between lo5 and 10’ eggs per female with an average of IO6 (Davis & Chanley, 1956; Ansell, 1967). The bay scallop, Argopecten irradians, produces slightly fewer eggs. Fordham (1970) reported a range of lo5 to lo6 eggs per female, the average between these two values. These species have planktrophic fecundity and high early mortality.
being intermediate larvae with high
Another factor in the development of an egg is the quantity and quality of the stored food reserves. Worley (1944) and Raven (1958) have shown that pelecypod eggs have a fatty yolk consisting chiefly of neutral fat, and Bayne (1972) and Bayne rf al. (1975) found that the fatty yolk reserve is utilized during early development. The quantity of this reserve was correlated with early growth of the brooded young of Ostrea edulis (Helm et al., 1973). The importance of fatty yolk reserve to the growth of planktonic bivalve larvae has not been determined. The present study was designed to determine the relationship between egg size and early survival for the pelecypods Argopecten irradiuns (Lamarck) and Mercenaria mercenaria (L.).
’ Contribution
No. 986 from Virginia
0022-098 Ij82/0000-0000/$02.75
Institute
of Marine
0 1982 Elsevier Biomedical
Science. Press
JOHN N. KRAEUTER ETAL MATERIALS AND METHODS Spawning
stock of M. mercenaria were from wild populations
or from 3-yr-old
field-raised stock from previous spawnings. All groups were mass-spawned on a flow-through spawning tray using thermal shock as a stimulus. Fertilized eggs were collected on a 25-pm mesh sieve, screened into 44-, 35- and 25-pm size classes, and placed in filtered and incubated (cultured) sea water in separated containers. Eggs of each size group were accumulated until there were sufficient numbers to be counted. Densities were adjusted to yield x 30 eggs per ml for each mesh size group by the addition of cultured water. Replicates were made for each sieve size group by distributing eggs into four or five containers. Each replicate was then thoroughly mixed, and a l-ml sample was taken from each container with an automatic pipette. This step was repeated five times to obtain five replicate samples from each container. These samples were individually preserved in 376 buffered formalin and provided the base count against which survival was compared. All live eggs were then dispersed in 50 1 of cultured sea water at densities of z 6 . ml -‘. The culture was drained in 2 days and veliger larvae were collected on the appropriate (25, 35, or 44 pm) mesh size screens. These larvae were then placed in l-liter containers of cultured water, mixed, and replicate-sampled five times. The I -ml samples were preserved in 3% buffered formalin. All samples were placed in the reservoir of a l-ml Sedgwick-Rafter counting cell and counted using a binocular compound microscope. The larvae thus counted were used to estimate survival for each replicate group within each sieve size (see below). Spawning stocks for Argopecten irradians were field grown adults from the previous year’s cultured animals. Methods similar to those used for the clams were used for scallop experiments except that eggs were sized on 20-, 25- and 35-pm mesh screens. Chi-square tests were used to compare 2-day survival against initial counts. The average number of eggs per ml for each replicate at time 0 (X,,,) was calculated by summing the counts of an individual replicate (X,, X?...X,) and dividing by 5 (the number of samples per replicate). The total number of eggs per ml for all egg sizes and replicates was determined by summing the X,,, values (ZX,,)). The proportion (P) of the total count attributed to each replicate was calculated by dividing X,,, for that replicate by XX,,) P l(Z) =
XT,>replicate
1 (25 pm egg size) =X,0
This proportion was then used to determine the expected number qf’survivors for each replicate experiment based on the assumption that the survival of larvae from all egg sizes would be proportional to the egg count.
EGG SIZE AND LARVAL
SURVIVAL
OF .~ER~~~AR~A
5
The number of larvae surviving per ml per replicate at time 1 (A’,,) and the total number of larvae for all size categories and replicates (CX,,,) were calculated in the same fashion as for the eggs. To determine the expected survival for any particular replicate, the original proportion (P,(zs, for the case above) was multiplied by XX,, , This served as the expected level ~f’survival for that particular replicate. A chi-square test was used to compare this expected against the observed (X,, ,_) survival. Similar calculations were made for each replicate experiment at each date. Because of the heterogeneity in the experiments (“-test of heterogeneity), we co&d not pool the replicates to determine overall effects. Two additional experiments were designed to test survival of young from eggs > 35-pm eggs vs. small (those c 3.5 pm) Mercenariu ~e~cenarja eggs to I month post setting. Two spawnings were made, eggs were split as in the 48-h experiments, and eggs were distributed in the 50-l containers at 30 eggs . ml-‘. These batches were drained every 2 days, screened, and placed in new culture water. Details of the hatchery system employed are given in Castagna & Kraeuter (1977). After 8-10 days setting had begun, and individuals were placed in a tray in ambient running sea water. Flow rates were gradually increased and reached 8 1 . min -’ after 2 wk. In order to control fouling during this period, the water was turned off, and the tray drained and allowed to air dry for 0.5 h daily. Four weeks after setting the entire tray was washed clean of clams. The clams were placed in a container, 1 liter of water added, thoroughly mixed and five replicate samples were taken. These samples were immediately counted and used to estimate survival. RESULTS
Survival of individual replicates (Tables I and II) clearly shows that larger eggs had significantly greater 48-h survival than the smaller eggs. The results were similar for both M. mercenaria and Argopecten h-radians. Eggs of intermediate size generally TABLE I Summary of significance of chi-square data of individual (replicate) experiments of egg size vs. survival for :Wer~ncz~in mercPnuriu: less, fewer survived than expected; no difference, no significant difference between observed and expected survival; greater, greater than expected survival; sieve size is the size mesh on which eggs were retained; 25+m eggs range in size from 25 pm to 34 pm; 35-pm eggs range in size from 35 pm to 43 pm; 44-pm eggs include all eggs 44 pm or greater in size; all results tested at chi-square at the 0.05 level.
Mesh size (pm) 2.5 35 44
Significantly less survival
No difference
10 4
3 8
1
2
Significantly greater survival --_~~
I 11
JOHN
6
N. KRAEUTER
ETAL
survived in proportion to the initial counts. For both species, smaller eggs had significantiy less than expected survival, while larger eggs had significantly greater than expected survival. Again, intermediate size eggs showed no difference between the expected and observed survival. TABLE
11
Summary of significance of chi-square data of individual (replicate) experiments of egg size vs. survival for Argopecm irrudians: less, fewer survived than expected; no difference, no significant difference between observed and expected survival; greater, greater than expected survival; sieve size is the size mesh on which eggs were retained: ZO-pm eggs range in size from 20 pm to 24 pm; 2%pm eggs range in size from 25 pm to 34 pm; 3%pm eggs include all eggs 35 pm or greater in size: all results tested at chi-square at the 0.05 level. Significantly less survival __...__. .____._
Mesh size (pm) 20 25
No difference
.._._
3
2
I
8
35
Significantly greater survival
2
7
If egg size and species considerations are ignored and eggs are simply graded as small (20 pm A. jr~ff~~~~~and 25 pm N. ~zerc~~aria), medium (25 ,um A. ir~a~ians and 35 pm M. ~ercenar~a). and large (35 pm A. ~~~a~ja~~ and 44 pm M. ~ner~enari~~ and these data are combined, the results indicate that 72:‘; of the time small eggs had less than expected survival, while large eggs survived at a greater rate than expected 787; of the time (Table III).
Combined survival significantly fewer
of Avgopec~or and h4etcc~mtiu: size. size of eggs (see text for explanation); fewer. survived than expected; greater. significantly more survived than expected; no difference. survival not significantly different than expected.
Size
Fewer (Yb)
Small Medium Large
13 : 72.2 5 : 22.7 I : 4.3
No difference 5 : 27.8 16172.7 4: 17.4
(“,)
Greater
(I’,)
0: 0 I: 4.5 18:X3.3
No. ofexperiments 18 22 2.3
Results of longer term experiments mirrored those of the 48-h tests. Smaller eggs had significantly poorer survival 1 month post set than did larger eggs. In both longer-term experiments, at least 95% more individuals survived from the larger eggs than from the smaller ones.
EGG SIZE AND LARVAL SURVIVAL OF MERCENARIA
7
DISCUSSION Loosanoff
& Davis
(1963) reported
egg sizes for a large number
of different
pelecypod species, but the ranges they presented suggest that there is very little difference in egg size within a species. Our results indicate that even small changes in egg size may be important in survival. This may reflect a change in the amount of fat or other reserves stored in the egg. The mesh sizes we used in the 48-h studies selected larger eggs with x43”/, larger volume than that of the smaller eggs for both species. The volume change of the eggs, assuming an egg to be a sphere, between the smallest and largest eggs was 810/o. If the germinal vesicle portion of the egg either remained the same or increased in proportion to the screen size, then larger eggs must have substantially more stored reserves available for development (Bayne, 1972 ; Bayne et al., 1975). Bricelj (1979) has shown that laboratory-induced spawning of M. mercenariu caused the release of two sizes of eggs and that these sizes approximate a bimodal distribution. Eggs in nature are assumed to be released in a similar manner. Our data indicate that the overall proportions of eggs in any size category may change through time or with different parents. Initial egg size distribution of one of the batches utilized in current experiments was 25 pm 7.5x, 35 pm 34.5x, and 44 pm 58’?& A second batch of different parental stock spawned three weeks later had the following proportions: 15.1, 49.2, and 35.7x, respectively. This follows the trend reported by Bricelj (1979) in that spawnings later in the year have higher proportions of smaller eggs. Our longer-term experiments also indicate that smaller eggs are spawned later in the year. The first spawning had 37.69; small eggs ( < 35 pm) while 1 wk later the proportion from a different set of adults was 50”/;; small eggs. This abrupt change may reflect the lateness of the spawning period (late July) or the different set of parents, but it follows the trend. Although many factors other than egg size may alter survival between egg and 1 month post-set, the similarity between the longer-term results and the 48-h survival data suggest
that larger egg size does have a significant
and
lasting
influence
on
survival. Dey (1979) has reported that larger larvae of M. mercenaria set sooner and continue to grow at rates greater than their smaller siblings. These data are similar to those for the oyster Crassostrea virginica (Haley & Newkirk, 1977) and support and extend our results. In order to meaningfully compare survival rates from batch to batch or from experiment to experiment, it is necessary to record not only egg size and screen size, but size distribution of eggs. Researchers conducting energetics or fisheries studies should be aware that using the smallest screen size will reduce the percent survival, but the actual reduction will depend on the size-frequency distribution of the eggs above this minimum. This distribution may change with the season. Hatchery workers interested in efficient production may wish to eliminate the smaller size eggs and concentrate on better care of the larger egg sizes that are
8
JOHN
N. KRAEUTER
ETAL
more likely to survive. Lower overall survival of batches spawned later in the year is often experienced by hatcheries, Reduced survival is often related to higher temperatures and bacterial contamination, but a significant portion of these losses may be due to the use of smaller eggs with reduced stored reserves. This condition yields less healthy animals which in turn are more susceptible to disease.
REFERENCES
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