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. Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
3. Results
3.1. Growth, development and body size Juveniles brought into the laboratory in August developed secondary sexual charac-
teristics during September–November, underwent gonadogenesis experiment I and II, and mated experiment I. In the later, by the middle of December, about 30 of
females were ovigerous. Simultaneously, mortality of males increased and, no males were present in the culture by the mid-February. At this point, all females were fertilized
and bearing embryos. The first offspring hatched in the middle of March and by the end of April, all the females were postspawned.
There were no significant difference between the sampling occasions and between the experiments with regard to the mysid size in different life stages juveniles, immature
and mature males and immature and mature females ANOVA, p . 0.05 in all cases.
Pooled data on size variability within each category from the field and laboratory observations are presented in Table 1.
Embryos all stages ranged in size from 0.35 to 3.00 mm, with an overlap between neighboring stages Table 1. As free-living mysids, larvae were released at a mean size
of 2.9 60.08 mm, corresponding well to the smallest M. mixta observed in the field
samples 3.0 mm in both 1995 and 1997. The embryo undergoes a substantial dry weight reduction 32 during the marsupial development Table 1.
In some female specimens, oostegites can be seen already at a size of 8.5–8.7 mm, but in most mysids, sex is not distinguishable until they have reached 10–12 mm. Fully
mature individuals also varied in size from about 14 to 16 mm and 13 to 17 mm in males and females, respectively. The size dimorphism in M
. mixta does not appear until the brooding: there were no significant differences in length p
. 0.6 or DW p . 0.3 between males and non-gravid females. No significant body length difference was found
between mature and gravid females p . 0.05, while the DW of ovigerous and
larvigerous females was significantly higher due to the presence of eggs or embryos
Table 1 M
. mixta: mean size 6S.D. in body length BL and dry weight DW, and size ranges of different life-cycle
categories n denotes the number of observations Life stage
Mean 6S.D., range
n BL mm
DW mg Eggs
0.54 60.13, 0.35–0.71
0.035 60.002, 0.031–0.037
11 Eyeless embryos
1.58 60.21, 1.26–2.19
0.031 60.003, 0.029–0.034
16 Eyed embryos
2.66 60.28, 2.07–3.00
0.024 60.003, 0.020–0.028
12 Juveniles
6.1 61.83, 2.9–9.7
0.52 60.38, 0.05–2.81
58 Immature males
10.8 62.14, 9.0–15.2
2.43 61.62, 1.63–7.12
37 Immature females
11.6 61.25, 8.7–14.7
3.20 61.14, 1.50–6.96
46 Mature males
15.0 60.94, 13.8–16.3
6.24 60.84, 5.38–8.62
22 Mature females
15.2 61.66, 12.8–16.9
6.52 61.05, 4.83–9.59
27 Gravid females
15.3 61.26, 13.4–17.2
11.15 61.08, 8.53–12.47
39 Postspawned females
16.6 61.22, 14.9–17.4
9.65 61.87, 6.73–11.72
16
E . Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
109
p , 0.001. The length of postspawned females did not differ significantly from that of
females bearing embryos p . 0.05 while their weight was lower p , 0.0003,
reflecting loss of marsupial content during hatching. The allometric relationships between DW and body length BL are shown in Table 2.
No significant differences p . 0.5 were found between log–log linear regressions of
DW against BL for juveniles and subadults, neither between those for males and non-ovigerous females nor for wild and laboratory reared subadults. Consequently, all
these categories were pooled for a common regression line as a power function Table
2
2. The regression of DW to BL for gravid females was poor r 5 0.51 and differed
from that for the other life-cycle categories as indicated by significantly different slopes p
, 0.05. Regression lines for the carapace length to body length were similar both slopes and
intercepts are not significantly different, p . 0.3 for slopes and p . 0.05 for intercepts
between females having egg-like embryos and younger stages juveniles, immature and mature males and females; Table 2. The line for females carrying eyed embryos,
however, had significantly different intercept p , 0.02 indicating elongated abdomen,
i.e. increase in linear dimension during brooding period. Overall, females with eggs were significantly smaller than those with eyed larvae length 15.67
61.08 vs. 16.1260.97 mm, p
, 0.01 yielding the intermoult growth of 2.9 in body length. 3.2. Reproductive life history traits
The brood size in experiment I ranged from 14 to 35 27.2 65.3 eggs or embryos per
female. The number of eggs and embryos per individual female F, tended to increase with female size, though the relationship with either BL or CL was poor Table 2.
However if only the eyed embryos were considered, the regression improved slightly Table 2. Furthermore, the average number of eggs per brood compared to that of
Table 2 M
. mixta: morphometric relationships describing dry weight DW, mg, carapace length CL, mm, uropod length UL, mm and fecundity F, number of young per brood as functions of mysid body length BL, mm
a
for different life-cycle categories
2
Life-cycle stages age and sex Type of regression
a b
r n
b
Juveniles, immature and DW
5 aBL 0.0032
2.850 0.98
190 mature males and females
b
Gravid females DW
5 aBL 0.7140
1.582 0.51
39 Juveniles, immature and
CL 5 aBL 1 b
0.42 0.12
0.98 64
mature males and females Gravid females with eggs
CL 5 aBL 1 b
0.40 0.43
0.94 11
Gravid females with eyed embryos CL
5 aBL 1 b 0.42
20.05 0.82
12 All stages
UL 5 aBL 2 b
0.28 20.39
0.98 67
Gravid females F
5 aBL 1 b 3.76
231.98 0.34
39 Gravid females with eyeless
F 5 aBL 1 b
4.46 243.73
0.44 28
and eyed embryos
a 2
n denotes number of observations and r is the corresponding regression coefficient. Value for all linear regressions p
,0.0001. Size ranges are presented in Table 1.
110 E
. Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
Fig. 1. M . mixta. Number of embryos in all developmental stages, fertilized eggs, and eyed embryos per
brood. The box extends from the 25th percentile to the 75th percentile, with a horizontal line at the median. Whiskers show the range of the data.
embryos was significantly higher 29.2 65.1 vs. 24.364.5; p , 0.04; Fig. 1 indicating
either embryo loss from marsupia or prenatal mortality during marsupial development. Larger females carried larger eggs, with a linear relationship between egg DW and
2
female DW: Eggs DW 5 0.0012 ? Female DW 1 0.022 r 5 0.61, p , 0.005.
3.3. Carbon and nitrogen content The elemental composition did not differ between years and within the different life
stage categories juveniles, immature males and females; p . 0.05 in all cases. Pooled
data on the carbon and nitrogen content within each category from field and laboratory experiments I and II observations are presented in Fig. 2. There was a considerable
variation in carbon content in somatic tissues both within and between different life-cycle stages of M
. mixta Fig. 2a. The proportion of carbon in abdominal muscle decreased gradually from juveniles 42.9
61.12 to mature males and gravid females 40.0
61.61. The carbon content in muscle tissue did not differ between sexes until after the peak of the mating period Table 3, Fig. 2a. In specimens sampled from the
end of December to the mid-January males: 38.55 60.76, n 5 11 and gravid females:
39.27 60.88, n 5 15, a significantly lower percentage of carbon was found in the males
p , 0.04. The nitrogen content was relatively constant 11.4 in average with
significant differences only between juveniles and mature females 11.3 vs. 11.6 respectively; Table 4. The C:N ratio reflected the change in somatic carbon content, and
the ratio decreased 6.2 from juveniles and subadult to mature males and gravid females Fig. 2c. Thus, at the conditions provided, the specific content of carbon C,
2
changed with size of mysids C 5 2 0.32 ? Body DW 1 43.30; r 5 0.31, p , 0.0001,
while size specific nitrogen N, content showed lower variability N 5 0.019 ? Body
E . Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
111
Fig. 2. M . mixta. Pooled data on carbon A and nitrogen B content, and C:N C ratio for different age and
sex categories field samples, experiments 1 and 2. J, juveniles; IM, immature males; IF, immature females; MM, mature males; MF, mature females; GF, gravid females; PF, postspawned females. The box and whiskers
parameters are the same as in Fig. 1.
112 E
. Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123 Table 3
M . mixta: Results of ANOVA and Bonferroni’s test for multiple comparisons on carbon content in abdominal
a
muscle tissue ANOVA results
p value ,0.0001
F 8.131
2
R 0.3040
Bartlett’s test for equal variances Bartlett’s statistics corrected
10.91 p value
.0.09 ANOVA table
SS df
MS Treatment between groups
56.86 6
9.476 Residual within groups
178.3 153
1.166 Total
235.2 159
Bonferroni’s multiply comparison test Mean difference
t p value
J vs. IM 1.389
3.492 ,0.01
J vs. IF 1.227
2.776 ,0.05
IM vs. IF 20.1620
0.4022 .0.05
IM vs. MM 1.464
4.786 ,0.001
IF vs. MF 1.309
3.590 ,0.01
MM vs. MF 20.3164
1.248 .0.05
MF vs. GF 0.651
2.507 .0.05
PF vs. GF 0.9227
2.153 .0.05
a
J, juveniles; IM, immature males; IF, immature females; MM, mature males; MF, mature females; GF, gravid females; PF, postspawned females.
2
DW 1 11.34; r 5 0.03, p , 0.03, resulting in a significant change in C:N ratio
2
C:N 5 2 0.033 ? Body DW 1 3.81; r 5 0.23, p , 0.0001.
In embryos, carbon and nitrogen content were highest in early stages 58.6 and 14.3, respectively, and decreased to 51.4 for carbon p
, 0.0001, Fig. 3a and to 12.6 for nitrogen p
, 0.04, Fig. 3b to the end of the marsupial development. In eggs, there was no correlation between DW versus carbon and nitrogen content Pearson r
5 2 0.13,
2 2
p . 0.7, R 5 0.02 and Pearson r 5 0.09, p . 0.8, R 5 0.01 for C and N, respectively.
Variations in the C:N ratios in embryos were weak Fig. 3c. 3.4. Ash content
In mysids, the ash content was slightly, but not significantly p . 0.4, higher in
juveniles than in older groups Fig. 4, probably reflecting changes in the body surface to volume ratio. The overall mean value of 8.9 was used in calculations of ash-free-
dry-weight AFDW, mg for all age and sex categories of mysids. In developing embryo, ash content increased almost twice Fig. 4 from fertilized egg 3.4 to moulted larvae
6.4.
E . Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
113 Table 4
M . mixta: Results of ANOVA and Bonferroni’s test for multiple comparisons on nitrogen content in abdominal
a
muscle tissue ANOVA results
p value ,0.02
F 2.757
2
R 0.0975
Bartlett’s test for equal variances Bartlett’s statistics corrected
12.12 p value
.0.06 ANOVA table
SS df
MS Treatment between groups
3.106 6
0.5177 Residual within groups
18.05 153
0.1345 Total
20.22 159
Bonferroni’s multiply comparison test Mean difference
t p value
J vs. IM 0.0526
0.5603 .0.05
J vs. IF 20.0868
0.8278 .0.05
J vs. MF 0.2123
1.789 ,0.05
IM vs. IF 20.1394
1.465 .0.05
IM vs. MM 20.1696
2.351 .0.05
IF vs. MF 20.1772
2.056 .0.05
MM vs. MF 20.1467
2.450 .0.05
MF vs. GF 0.1632
2.658 .0.05
PF vs. GF 0.0014
0.01292 .0.05
a
J, juveniles; IM, immature males; IF, immature females; MM, mature males; MF, mature females; GF, gravid females; PF, postspawned females.
3.5. Energy content
21
The weight-specific energy content of the mysid muscle tissue fell from 20.0 KJ g
21 21
DW 20.9 KJ g AFDW in the young mysids in August field samples to 17.6 KJ g
21
DW 18.9 KJ g AFDW in mature animals in the winter laboratory-reared mysids.
21
The lowest caloric value 17.9 KJ g AFDW was obtained for males in the end of
mating period. The total energy content of ovigerous females somatic tissues and
21 21
fertilized eggs was 18.7 KJ g DW or 207.7 J ind
in the beginning of the marsupial development December–January. By this time energy contained in ripe eggs composed
about 10–17 of the total female energy content Fig. 5a. The relationship between female body size and the energy they devote to reproduction Fig. 5b, based on
regressions describing fecundity and egg size as functions of female DW and egg elemental composition, could be established. Larger females invest more to their
offspring both in terms of the absolute values and relative to their DW.
21
The energy density of embryos varied from 29.7 KJ g AFDW in earliest embryos to
21
25.9 KJ g AFDW in eyed larvae. About 40 of individual embryo energy content
were lost during marsupial development. Considering both intermediary metabolic losses
114 E
. Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
Fig. 3. M . mixta. Carbon A and nitrogen B content, and C:N ratio C for different developmental stages of
embryo experiments 1. The box and whiskers parameters are the same as in Fig. 1.
E . Gorokhova, S. Hansson J. Exp. Mar. Biol. Ecol. 246 2000 103 –123
115
Fig. 4. M . mixta. Ash content for all ontogenetic stages field samples, experiments 1 and 2. E, egg; ELE,
eyeless embryo; EE, eyed embryo; J, juveniles; IM, immature males; IF, immature females; MM, mature males; MF, mature females; GF, gravid females; PF, postspawned females. The box and whiskers parameters
are the same as in Fig. 1. Numbers above plots show the number of analyses.
of embryos and their prenatal mortality, the energy content of brood decreased by 50 during the complete embryogenesis.
4. Discussion
