mangrove crab Sesarma hrockii de Man .... III Zoea 15 3.6116* 9.8672* 4.5613* 4.5128*. 20. 5.8689 0.1960 ... Mega- 15 3.3748* 7,7375* 5.3878* 6.8304*. 1°P^.
Indian J. Fish., 47(4) : 343-348, Oct.-Dec, 2000
Influence of salinity on larval development of t h e mangrove crab Sesarma hrockii de Man T. KANNUPANDI, G. VIJAYAKUMAR AND P. SOUNDARAPANDIAN CAS in Marine Biology, Annamalai University, Parangipettai-608 502, Tamil Nadu, India ABSTRACT The influence of salinity on the larval development of the mangrove crab Sesarma brockii was studied in 8 different salinities from 5 to 40 ppt with 5 ppt increment. The larval development was completed in the salinity ranged from 15 to 35 ppt with 25 ppt as optimum. In the lower (5 to 10 ppt) and higher (40 ppt) salinities complete morality was recorded within 24 hours at the maximum. Further the intermoult duration of the development was shortest (18 days 10 hrs and 5 mts) in 25 ppt.
Introduction Although there are considerable studies on the larval development of mangrove crabs, very little is known on the effect of salinity during the larval development. Studies on influence of salinity on larval development of mangrove crabs indicate that most of the species have optimal salinity at 25 ppt (Krishnan and Kannupandi, 1987; Vijayakumar and Kannupandi, 1987a) but each larval stage shows different optimal salinity (Selvakumar et al., 1987). Recent study of Kannupandi et al. (1997) also reports that each zoeal and megalopal stage requires different optimal salinity to complete larval development of Thalamita crenata. Sesarma brockii is one of the most dominant species in the Pitchavaram mangroves and is available throughout the year (Vijayakumar and Kannupandi, 1987a). Hence it was decided to study the influence of salinity on the larval moulting frequency, the duration of
larval life history and survival of the mangrove crab, -S. brockii which has 4 zoeal and 1 megalopal stages to complete larval development.
Materials and methods Ovigerous females of S. brockii were collected from the Pitchavaram mangroves and maintained in the laboratory at a salinity of 25 ± 1 ppt., temperature 28 ± 1°C and photoperiod 12h L: 12h D. After hatching, the larvae were maintained using technique of Vijayakumar and Kannupandi (1987b). The zoeae were maintained in triplicate at 8 different salinities viz. 5, 10, 15, 20, 25, 30, 35 and 40 ppt. Ten zoeae were placed in a glass bowl (100 ml capacity) containing approximately 80 ml of filtered sea water. Durations of zoeal stages (days) at different salinities were analysed using students 't' test, ANOVA and multiple range test. One hundred larvae were subjected to each test salinity in the range of 5 to 40 ppt.
T. Kannupandi et al.
However, freshly hatched larvae were separated into 8 groups of 100 individuals each and all the larvae were gradually acclimated in steps of 5 ppt for 2 hours to their final rearing salinities of 40, 35, 30, 25, 20, 15, 10, 5 ppt. Each treatment was done in 10 bowls with 10 larvae per glass bowl. Experimental salinities were obtained by filtering sea water (35 ppt) and diluting with glass distilled water. Seawater of 40 ppt was obtained by evaporating sea water. Daily counts of exuviae and surviving larvae were made. The larvae were transferred daily to clean bowls containing freshly filtered sea water of the same sahnity and were fed daily with freshly hatched nauplii of the brine shrimp Artemia. Experiments were terminated when all the larvae either died or moulted to the first crab stage.
Results In 5 ppt salinity, the first zoea survived for only one hour. In 10 ppt salinity, when the larvae were first introduced, they remained motionless and all of them settled at the bottom of the finger bowl within 15 minutes. Complete mortality was observed after 24 hrs. In 15 ppt sahnity the highest mortality rate was observed in first zoea. Mortality rate was 66,15, 9, 4 and 3% in the first, second, third and fourth zoeal and megalopal stages respectively. On an average, 97% of the larvae died before they metamorphosed to the I crab stage. In 20 ppt salinity, the mortality rates were 54, 15, 13, 4 and 3% in the first, second, third and fourth zoeal and megalopal stages respectively, 89% of larvae died without moulting to I crab stage. In 25 ppt salinity, the mortality rates were 16, 2, 2, 3 and 2% in the first, second, third and fourth
344
20 25 Soli n i t y
30 (ppt)
35
Fig. 1. Survival percentage during larval development of Sesarma brockii at various salinities.
zoeal and megalopal stages respectively. 25% of larvae died before the I crab stage. In 30 ppt sahnity, 24, 13, 9, 7 and 9% of mortality rates were observed respectively during the first, second, third and fourth zoeal and megalopal stages of development. Thus 62% of larvae died before the I crab stage. In 35 ppt sahnity, the mortality was 39% in the first, 20% in the second, 15% in the third and 10% in the fourth zoeal stage and 3% in the megalopal stage. Totally, 87% of the larvae died before the I crab stage. In 40 ppt salinity, the
.Fig. 2. Intermoult duration during larval development of Sesarma brockii at various salinities (days).
Salinity
and larval development
in a crab species
first zoea survived for only one hour (Fig. 1). The mean intermoult duration of larval development along with statistical analysis and multiple range test are presented in Fig. 2 and Tables 1 to 3. The 't' and 'F' test showed significant values and the multiple range test showed the 'accept', in all the test concentrations in I, II and III zoeal stages but 'reject' in all the test concentrations of IV zoeal stage and almost in all the test concentrations in the megalopal stage. TABLE 1. 't' values for the differences in mean intermoult duration of larval development of Sesarma brockii at various salinities Stages I Zoea
Salinity (ppt)
15 ppt
15
0.6754*
20
20 ppt
25 ppt
0.6963*
5.1962*
5.1467*
0.3154
0.3448
14.5360
1.1969
0.1470
25 30
30 ppt
1.9794
II Zoea 15 4,6885* 9.1841* 4.7490* 4.5420* 20
4.5530
25
7.2170 0.2473
30
0.1477 4.6560 0.1979
III Zoea 15 3.6116* 9.8672* 4.5613* 4.5128* 20
5.8689
25
0.1960
0.1470
1.1439
5.7197
30 IV Zoea 15 4.9744* 20
4.9349 4,4861* 5.3410* 0.5390
25
4,6846*
0.4242
0.2609
0.8455
0.2451
30
0.5991
Mega-
15 3.3748*
7,7375*
5.3878*
6.8304*
1°P^
20
5.8788
7.9874
0,0786
25 30
1,8710
0.2781 0.8710
*Indicates statistically significant differences from the control value (P < 0.05).
345 Discussion The results of the present study indicate that 25 ppt is the optimum salinity level at which the survival of larvae was highest (75%) and the duration of development was also shortest (18 days 10 hrs and 5 mts) in S. brockii. Duration of zoeal development and the mortality rate of larvae increased as the salinity increased or decreased. The larval mortality in low salinity (5 & 10 ppt) is possibly due to imbalance in the osmoregulatory mechanism. Mortality at highest salinity (40 ppt) is likely to be due to the inability of the larvae to osmoregulate and also high salinity caused shrinkage of tissue cells. However, the berried females ofS. brockii are available t h r o u g h o u t t h e y e a r in Pitchavaram mangroves, where the water salinity ranges from 1 ppt in monsoon to 35 ppt in summer (Palaniappan and Baskaran, 1985). S. brockii requires a salinity range from 15 to 35 ppt for the completion of larval development. Previous studies on the influence of salinity on larval development of mangrove crabs show that most of the species have optimal salinity at 25 ppt., Metaplax distincta (Krishnan and Kannupandi, 1987), Sesarma andersoni (Vijayakumar and Kannupandi, 1987a), Neoepisesarma mederi (Selvakumar et al., 1987), Macrophthalmus elegans (Balagurunathan and K a n n u p a n d i , 1993) and Thalamita crenata (Kannupandi et al., 1997). Further, the mangrove crabs, S. brockii (Vijayakumar and Kannupandi, 1987b), Macrophthalamus depressus (Pasupathi and Kannupandi, 1988), M. distincta (Krishnan and Kannupandi, 1989) and Sesarma edwardsi (Kannupandi and Pasupathi, 1994) also successfully completed their larval development in 25
346
T. Kannupandi et al.
TABLE 2. Analysis of variance for the differences in mean intermoult duration of larval development of Sesarma brockii at various salinities Sum of square
Source I Zoea
D.F.
Mean square
252.89
5
50.58
Deviation
51.44
218
0.24
Main effect
83.42
5
16.69
Deviation
33.44
158
0.22
Main effect
52.32
5
10.46
Deviation
64.34
118
0.54
262.80
5
52.56
Deviation
25.22
93
0.27
Main effect
89.81
5
17.96
Deviation
43.27
66
0.66
Main effect
F
214.3515*
II Zoea
78.8245*
III Zoea
19.1894*
IV Zoea
Main effect
193.751*
Megalopa
27.395*
*Indicates statistically significant difference from control value (p < 0.05).
ppt salinity with maximum survial and shortest duration. Thus it is clear that 25 ppt salinity is ideal for larval development of mangrove crabs in the natural environment. Though S. brockii is available throughout the year in Pitchavaram mangroves, maximum ovigerous females are obtained during post monsoon season when salinity is around 25 ppt. During summer the salinity was around 35 pp in Pitchavaram mangroves and during monsoon it was around 1 ppt. The present study clearly extends support to the above observation by showing low survival in both higher 35 ppt salinity and also lower salinities (15 and 20 ppt). Thus, the population density of this species in the Pitchavaram mangroves coincides with salinity range which appears to be a limiting factor. However, in the natural environment, the larvae of S. brockii can avoid
stressful low salinity by sinking to the bottom where the salinity will be higher. This kind of avoidance behaviour has been reported by Anger (1985) and it appears to be a general response of decapod larvae to low salinity stress. The high survival of IV zoea and megalopal stages of the S. brockii than the earlier stages in the salinities lower and higher than the optimum level may be due to well developed gills which act as potential salt observing tissue (Kannupandi et al., 1997). Besides, reports on the effects of salinity on larval development of Neoepisesarma mederi (Selvakumar et al., 1987) and T. cranata (Kannupandi et al., 1997) show that each larval stage requires different optimum salinities. In view of this, further studies on the influence of salinity on the larval development and also larval recruitment pattern of mangrove crabs will throw light on their population density.
347
Salinity and larval development in a crab species
TABLE 3. New Man-Keuls multiple range test for the intermoult duration of larval development of Seasarma brockii at various salinities Stages
Difference
S.E.
I Zoea
0,97 1.05 1.04 1.00
11.9348 11.9348 11.9348 11.9348
0.0813 0.0880 0.0871 0.0879
2 3 4 5
2.919 3.532 3.901 4.039
A A A A
12 13 14 15
II Zoea
0.98 1.90 1.99 0.95
3.5321 3.5321 3.5321 3.5321
0.2775 0.5379 0.2803 0.2689
2 3 4 5
2.919 3.532 3.901 4.039
A A A A
12 13 14 15
III Zoea
0.90 2.08 0.94 0.93
18.7797 18.7797 18.7797 18.7797
0.0480 0.1108 0.0501 0.0495
2 3 4 5
2.919 3.532 3.901 4.039
A A A A
12 13 14 15
IV Zoea
1.02 1.62 0.79 0.64
193.7450 193.7450 193.7450 193.7450
5.2645 8.3615 4.0775 3.3032
2 3 4 5
2.919 3.532 3.901 4.039
R 12 R 13 R 14 R 15
Megalopa
0.88 2.28 1.39 0.74
160.7800 160.7800 160.7800 160.7800
5.4699 0.0142 8.6399 4.5997
2 3 4 5
2.919 3.532 3.901 4.039
R 12 A 13 R 14 R 15
A indicate 'accept', R indicate 'reject' 15 ppt. = 1; 20 ppt = 2; 25 ppt = 3; 30 ppt
q. 0.05 p. 60
Conclusioin
4; 35 ppt. = 5.
Acknowledgments Thanks are due to the Director and authorities of Annamalai University for facihties and ICAR for funding.
Kannupandi, T. and K. Pasupathi 1994. Laboratory reared larval stages of mangrove crab Sesarma edwardsi De Man 1887 (Decapods : Grapsidae). Mahasagar, 27(2) : 105-115.
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