Abstract: The study was conducted for one year on population structure and dispersion pattern of the common gastropod species Potamides cingulatus was conducted in a mangrove habitat of Karachi. Population diversity levels were moderate in summer but maximum in late autumn and winter. It was also revealed that the species consisted of two groups of individuals, which differed from each other in their colour, size-class structure, population diversity and dispersion pattern. It is, therefore, suggested, they may be considered for two different varieties or forms of the species.
Introduction
Potamides cingulatus is a very common gastropod species of the polluted mangrove habitats of Pakistan and also of other countries (Macnae, 1968; Saifullah, 1982, 1996; Jones, 1986, Dybdahl, 1995; Jacobsen, 1996). It predominately occurs on soft mud in mangrove habitat at low tide. P. cingulatus consists of a small (up to 30 mm) elongate robust shell sculptured with rows of spiral beads, the anterior canal is prominent and the columella very much twisted. There appear to be two types of individuals in this species, one large and black and the other small and brown in colour.
Inspite of its abundance in the country, there is no information available on the population density and its seasonal variation which is very important for understanding the dynamics of the mangrove ecosystem. Tirmizi and Barkati (1983) studied allometric growth of this species. The present work, therefore, takes into account the population structure and pattern of this species in a mangrove habitat of Sandspit area at Karachi coast.
Materials and Methods
Sampling: Individuals of P. cingulatus were counted in 10 quadrates (50×50 cm) in the intertidal zone of mangrove stands of Sandspit area, δ Karachi on each sampling occasion. Such observations were made in the area fortnightly for a period of 12 months. Simultaneous observation on temperature and salinity of sea water of the channels located within the sampling area were also recorded using a thermometer and refractometer respectively. Soil samples were collected and studied for (observations were made on) soil texture. The shells after being brought to the laboratory were washed thoroughly with detergent to study the structure and colour. Their length was measured to the nearest 0.1 mm from apex to the siphnoid canal tip.
Pattern Detection: An index of pattern detection that is relatively insensitive to changes in density is required to detect spatial pattern (Myers, 1978). Accordingly, Lloyd (1967) index of patchiness© and Morisita (1971) index (1δ) that are unaffected by changes in density caused by random thinning, were used. First mean crowding (m) (Lloyd, 1967) was calculated as:
where Xi is the number of individuals in the ith quadrate, Q is the number of quadrats and
Index of patchiness C is then calculated as c = m/λ where λ equals the mean density per quadrate. Morisita’s index which is close to Lloyd's index of patchiness is:
Estimates of the pattern detection indices in literature are invariably reported without any indication of sampling variance. The Jackknife method (Tukey, 1958) not only allows estimation of variance from one sample but also permits bias reduction. The variance/mean ratio was also computed. Aggregation pattern was also explored by using Taylor's power law (Taylor, 1971; Taylor et al., 1978). This approach involves relating variance to mean by a power equation as follows:
and, therefore,
where S2 and m are the sample variance and mean density per quadrate respectively at successive time intervals. The parameter b is a measure of aggregation that is generally regarded as characteristic and constant for a species (Taylor et al., 1978).
Results and Discussion
Environmental variation: The seasonal variation in temperature and salinity of sea water in the channels occurring at the sampling sites is shown in Fig. 1. It is evident that high temperature as well as high salinity prevailed in summer.
| Fig. 1: | Changes in water temperature and salinity at Sandspit mangrove site during 1992-93 |
The grain size distribution of the soil shows it to be mainly muddy in texture, that is consisting mostly of silt and clay particles (Saifullah and Elahi, 1992). It was black in colour as a result of accumulation of dead organic matter, which was very high as compared to other normal soil and, therefore, may be termed as organic soil (Snedaker and Snedaker, 1984).
Size Class Structure: Observation on the individuals of P. cingulatus revealed that there are two different morphological types of populations co-occurring in the area. One consisted of individual that were relatively larger in size (11 to 32 mm in length, 'X = 22.462 mm) and black in colour whereas the individuals of the other population were smaller (11 to 26 mm in length, 'X = 20.472 mm) and brown in colour (Fig. 2).
Population Density: In general, there was considerable seasonal change in the density levels of both black and brown populations and the density of black population was considerably higher than that of brown population. Density of black population was moderate during May to mid-August (monsoon season} but, thereafter, declined (September to October). Density levels of black population again increased from mid-October 1992 to early February 1993 (northeast monsoon season). Subsequently, the population !, remained at a lower level up to mid-April. Density level of brown population showed a similar trend to that of black (population. From May to early August the brown population density was moderate; it declined from mid-August and remained so up to mid-September. The density of brown population again increased from early October and remained at a low level during January and February, thereafter, it showed fluctuations.
| Fig. 2: | Size-class structure of brown (A) and black (B) populations of P. cingulatus at Sandspit, Karachi. x̄ = mean, SD = standard deviation, SE = standard error, CV% = coefficient of variation, g1 = skewness, g2 = kurtosis. |
Population dispersion pattern: Dispersion pattern of black, brown and total population as measured by Lloyd's index of mean crowding (m), Morisita’s index (Id), patchiness© and variance/mean ratio (S2/x̄) is presented in Table 1. In general, the dispersion pattern of both black and brown populations was aggregated as evidenced by all the four measures of dispersion. Higher degree of aggregation for black population was recorded for May to early October, 1992 and April, 1993 as shown by Morisita’s index of patchiness, while relatively lower level of aggregation occurred during late October, 1992 to March, 1993. Brown population showed a somewhat similar trend of dispersion.
| Table 1: | Jackknife estimates of Lloyd's index. Morisita's index and ptchiness and the variance mean ratio |
For brown population greater degree of population aggregation was found during early May to early October, 1992 and lower level of aggregation was recorded for late October to late November, 1992 and March, 1993.
The equations of Taylor's power law for black, brown and total populations calculated were as follows:
S2 = 7.082 m1.169 R2= 0.6619black population
S2 = 2.329 m1.1447 R2= 0.7970brown population
S2 = 23.42 m0.907 R2= 0.4206total population
The brown population showed greater aggregation than did the black population. Differential aggregation patterns could be the result of differential mortality and/or differential predation by predators such as sea-gulls and crabs. The total population, however, showed a tendency towards random distribution. Once b is estimated a common transformation can be applied to the original counts (Healy and Taylor, 1962). The appropriate transformation for black and brown P. cingulatus populations are X0.415 and X0.206 respectively.
The differences in colour, size class structure and distribution pattern between the two groups of individuals of P. cingulatus mentioned above, suggest that they may be considered for its two different varieties or forms. However, further work especially cytogenetic studies are needed to establish this proposition.