MOLLUSCAN FAUNA OF INTERTIDAL ROCKY LEDGES OF KARACHI A COMPARATIVE ECOLOGICAL STUDY

The distribution, abundance and biomass patterns of rocky intertidal mollusks were studied at four study site i.e. Manora, Buleji, Nathiagali and Cape Monze along the Karachi coast. Visits were made, on quarterly basis, for a period of two years from December 1993 to December 1995. The spatial and temporal variations in the biota were studied with reference to tidal level, frequency, density, wet weight, dry weight, species diversity, evenness, richness, index of distribution i.e. B:A ratio, abundance-biomass (ABC) curves and species assemblages to characterize the spectrum of intertidal ecosystem. The regression analyses were conducted for the abundant gastropod species from the four study sites. Abundance of Molluscan Species: A total of 97 species of mollusks were collected from the four study sites. More species were sampled from Buleji (68) and Manora (63)compared to Nathiagali (55) and Cape Monze rocky shores. Gastropod species dominated on all the four sites. Few apecies of mollusks were commonly abundant on all the four sites such as: Turbo coronatus, Nerita albicilla, Morula granulate and Cerithium rubus. The average number of species decreased with an increase in tidal heights at Manora, Buleji and Nathiagali rocky shores, whereas, the average number of species was higher in the high tidal zone at Cape Monze rocky shore. Biomass: During the present study, the overall mean wet and dry biomass were highest at Nathiagali (616.804; 518.684 gm/m2), followed by Buleji (495.725; 418.938 gm/m2) and Manora (488.225; 408.088 gm/m2). Cape Monze (398.03; 337.365 gm/m2) showed the lowest wet biomass of the areas sampled. The average wet and dry biomass of mollusks was highest in the mid tidal zone followed by high and low tidal zone at Manora, Buleji and Cape Monze rocky shores whereas, the average biomass was high in low tidal zone followed by high and mid tidal zones at Nathiagali rocky shores. On Manora rocky shore only 11 species were found to contribute biomass greater than 1%. Nine species contributed 90% of the total molluscan biomass on Buleji and on Nathiagali rocky shores, whereas 9 species contributed 93% of the molluscan biomass on Cape Monze rocky shore. Species Richness: The average values of species richness was high (6.613) at Cape Monze followed by Manora (6.16), Nathiagali (5.98) and Buleji (5.848). It may also be seen that seasonal changes in species richness values are more noticeable at Cape Monze. The species richness was inver5sly proportional to the tidal height at Manora, Buleji and Nathiagali whereas the species richness was maximum in high tidal zone at Cape Monze rocky shore. Hill Diversity Index N1: The average value of N1 from the total area was high at Nathiagali (9.71) followed by Manora (8.49), Cape Monze (8.365) and Buleji (7.89) rocky shores. Results showed that the Nathiagali posses high N1 values almost throughout the study period. A marked seasonal variation in values may also be noticed at Cape Monze. The values of Hill diversity number N1 decreases with an increase in tidal height at Manora, Buleji and Nathiagali whereas values of index was directly proportional to tidal height at Cape Monze rocky shores. Hill Diversity Index N2: The average N2 values of the total area were high at Nathiagali (6.83) followed by Buleji (5.82) cape Monze (5.26) rocky shores. The figure shows a marked seasonal variation in values at Nathiagali and Cape Monze. Modified Hill Ratio E5: Molluscan species was more evenly distributed at Buleji (0.701) and less evenly at cape Moze (0.587), Nathiagali (0.663) and Manora (0.615) rocky shores. Seasonal variation in E5 values were less evident from the four sites. The average value of evenness E5 were directly proportional to tidal height of Manora, Nathiagali and Cape Monze whereas the average values were high in the mid tidal zone at Buleji Rocky shore. It was seen that values of both type of Hill Diversity indices (N1 & N2) followed almost the same pattern as that of species richness index, however, inverse relationship exists between values of species richness and modified hill ratio. Two way nested analysis of variance: Two way nested analysis of variance showed that trends in species richness, diversity, evenness and biomass are not significantly different among zones and sites. However, the number of molluscan individuals are significantly different among zones and among sites in the most of the samples. Multivariate Analyses: Numerical classification, multidimensional scaling (MDS) ordinations and SIMPER analyses derived from Bray-Curtis measures of similarity based on abundance and biomass data showed that the grouping of sites appeared to be due to habitat structure, substrate instability, human disturbances and degree exposure. The results of the present study using ANOSIM randomization test showed that the molluscan abundance and biomass were more significantly different among sites than tidal zones. Regression Analyses: The analysis of regression coefficient of Turbo coronatus from Manora, Buleji, Nathiagali; Trochus stellatus from Cape Monze and Nerita albicilla from all four study sites were carried out to study the following relationships: shell height-shell width; shell height total dry weight; shell height water content and shell width total dry weight. The results of the present study showed that linear relationship exist between these parameters. The relationship between shell height and total dry weight was exponential to shell length in all the gastropod species. In a majority of samples, the shell height was growing faster than total dry weight of T. coronatus except in one sample of Manora in which both the variables increased with an identical rate. Faster increase in shell height than total dry weight was noticed in most of the samples (7 out of 9) of T. stellatus except two samples, when both the variable increased with identical rate. The shell height and total dry weight increased with identical in most of the samples of Nerita albicilla. The shell height increased with faster than total dry weight in two samples from Buleji and in two samples from Nathiagali. However, total dry weight increased faster than shell height in three samples from Manora, two samples from Buleji, three samples from Nathiagali and four samples from Cape Monze. A seasonal variation in the b value was noticed. Shell height and water content ratio of Turbo coronatus was isometrically related in 6 samples from Manora and 3 samples from Buleji; shell height increased faster than water content in 1 sample from Manora, 5 samples from Buleji and 8 samples from Nathiagali whereas in rest of the samples, reverse is true. In majority of the samples (6out of 9) of Trochus stellatus from Cape Monze, the shell height and water content increased with identical rate; in 1 sample water content increased faster than shell height, whereas in 2 samples reverse was true. The shell height and water content showed positive allometry in 15 out of 36 samples of Nerita albicilla i.e. 3 samples from Manora, 5 samples from Buleji and 7 samples from Cape Monze. The remaining 21 samples indicated isometric relationship between two variables. Shell width increased faster than total dry weight in most of the samples of Turbo coronatus. The shell width and total dry weight showed identical rate of growth in three samples of Manora, four samples of Buleji and two samples of Nathiagali rocky shore. However, the total dry weight showed faster rate of increase than shell width in two samples of Manora, two samples of Buleji and in one sample of Nathiagali. The total dry weight increased faster than shell width in all samples of Trochus stellatus and Nerita albicilla, except in few samples when both the variables increased with identical rate.