Indian Journal of Geo Marine Sciences Vol. 47 (01), January 2018, pp. 127-134 Studies on the vertical distribution pattern in mangrove associated molluscs along the Karachi coast, Pakistan Ziaullah1, Itrat Zehra2 & Muhammad Asif Gondal*3 1Department of Tourism and Hospitality, Abdul Wali Khan University, Mardan, Pakistan. 2Centre of Excellence in Marine Biology, University of Karachi, Karachi, Pakistan 3*Department of Biosciences, COMSATS Institute of Information Technology, Park Road, Islamabad, Pakistan [Email: [email protected] ; [email protected]] Received 13 October 2015 ; revised 17 November 2016 Present study consists the vertical distribution pattern in mangrove associated molluscs (gastropods and bivalves) in two mangal areas: Sandspit and Korangi along the Karachi coast, Pakistan. The vertical distribution pattern delineated that the number of species decreased from high to low tide zone at two sites, however, marked differences were encountered with reference to the density distribution. At Sandspit, highest annual mean density was observed in the high tide zone followed by low and mid tide zones respectively, while, at Korangi the highest annual mean density was observed in the mid tide zone followed by high and low tide zones respectively. An important correlation was observed between the vertical distribution pattern of species and the sediments. [Key Words: mangroves, molluscs, distribution, Karachi, Pakistan] Introduction tolerance and desiccation rate are the primary Mangrove or Mangal forests cover about factors which limits the upper extent of zonation 617, 470 hectare in the Indus delta1. These of intertidal species within the supralittoral zone, wetlands provide both hatching as well as while the behavioral adoptions like restrictions nursery grounds for large number of to specific food resources or lack of upper shore commercially important fish and shell fish migratory behavior limited the distribution of species. Molluscs, especially the bivalves and species occurring in the lower intertidal. Several gastropods, form an important component of workers have their own observations regarding mangrove fauna2,3. However, the key ecological the distribution of molluscs in mangrove areas, factors that actually determine the fate of e.g. the factors that governs the vertical molluscs and govern their distribution pattern in distribution of gastropods and ranged out the mangrove habitat, is still a debatable question distribution zones for each species mainly based among the ecologist. The controversy exists upon the desiccation tolerance11. The intertidal about the totality of factors affecting the zonation seems maintained by the different distribution of these intertidal individuals. responses of organisms to change in physical Most studies on intertidal gastropods and biotic factors which vary from low to high suggest that the level of desiccation tolerated by tide zones12. The studies13-15 considered that a particular species generally increases with behavioral pattern, such as phototaxis, geotaxis, increasing height of the zone of occupation hydrotaxis and rheotaxis may be the main within the intertidal region4-10. Both desiccation determinants of vertical zonation in gastropods. 128 INDIAN J. MAR. SCI., VOL. 47, NO. 01, JANUARY 2018 Physiological characteristics of an animal may Korangi fish harbor. Geographically the site is govern its vertical distribution but movement located in the northern creek mangroves of the behavior is also important in leading a dislocated Indus delta. animal back to its preferred habitat13,14,16. It was For sampling each site was regularly observed while studying the ecology of molluscs visited, during low tide, at monthly intervals in mangroves the species abundance and from March 2001 to February 2002. The two distribution increase with salinity17. They also sites were thoroughly investigated for molluscan pointed out that vertical distribution of biomass diversity and their distribution pattern. At each remains highest in high tide zone and lowest in mangal area, a long transect of approximately low tide zone, however, density was calculated 180 meters of length was laid from the seaward highest in the mid tide zones. Various studies18 edge to landward. At each transect three recommend the importance of local habitat sampling stations were established with the first differences in species distribution and in the Low Tide Zone (LTZ) at the seaward edge mangrove associated gastropod fauna variation while the second (Mid Tide Zone: MTZ) and considerably between sites, both in terms of third (High Tide Zone: HTZ) stations were number of species and species composition. located at equal intervals within the mangroves Local habitat differences have major influences facing landward edge. At each station fifteen over species distribution both within and quadrates (each 1 m2) were randomly laid on between mangals. either side of the transect. At regional level the mangrove flora For sampling a fixed pattern was received due attention and is therefore well followed throughout the study period. The documented. On the contrary the mangrove samples were collected and observations were fauna, particularly the molluscan fauna, is not made from high (HTZ) to low (LTZ). The paid due attention and is therefore poorly collected samples were labeled and habitat notes known. The occurrence of some molluscs in the were made in the field. The epifaunal samples Indus mangroves is some decades old study were simply picked up by hand and collected in along the coast of Pakistan19. Occurrence of plastic bags while for the infaunal samples, molluscs in the mangrove area of Karachi coast sediment samples were obtained by sand corer. is also a significant study20-22. The The core samples were taken from a depth of 12 investigations23 on the benthic fauna of Sindh cms. Three replicates were made for each mangroves reported of five species of molluscs station. In laboratory, the sediment was passed e.g. Cerithidea cingulatus, Natica lamarckii, N. through a brass sieve (0.8 mm mesh size). didyma, Nodilittorina leucostica and N. picta Individuals collected were preserved in 70 % were regularly found in abundance. However, no alcohol. Molluscan friction was studied in detail detail work has been carried out on the and only live taken samples were used in distribution pattern and the various ecological subsequent analysis. factors that influence the occurrence and The preliminary investigations indicated distribution of molluscan fauna of the region. the predominance of epifaunal macromolluscs at the two sites. Telescopium telescopium, Materials and Methods Cerithidium cingulatus and Onchidium species Ecology and distribution of mangrove were observed as the permanent dwellers of the associated molluscs were investigated in two area and form an important component of the mangal areas: Sandspit (24 48'N, 66 59'E) and selected mangroves ecosystem. These species Korangi (24 48'N, 67 13'E) along the Karachi were selected for comprehensive studies, only coast, Pakistan. The study site at Sandspit was because of their abundance, to work with located within the mangrove habitat, quantitatively. representing backwaters of Karachi harbor. It The various physiochemical parameters forms the western most part of the Indus delta like salinity, pH, and temperature were recorded mangroves ecosystem, while, the study site at at each station. Impact of tidal effect was also Korangi was bounded on either side by channels investigated with reference to species of Korangi creek situated opposite to the distribution. Observations on behavioral responses of the individuals have also been ZIAULLAH et al.: VERTICAL DISTRIBUTION PATTERN IN MANGROVE ASSOCIATED MOLLUSCS 129 incorporated in the study. Whenever possible, tidal heights as the low, mid and high tide zones comparison was also made on population represent distinguished differences in the species dynamics of molluscan assemblages both before diversity as well as in abundance and and after low tide. distribution. Of the total 14 molluscan species recorded (Table-1), the number of species in Results general decreases from high to low tide zone. Vertical distribution of molluscan component varies significantly with reference to Table 1- List of Molluscs found in the Mangrove areas of Sandspit and Korangi along the Karachi Coast Pakistan CLASSIFICATION IDENTIFIED FROM CLASS GASTROPODA SUB CLASS: PROSOBRANCIA ORDER: ARCHAEOGASTROPODA Family: Turbinidae Turbo (Lunella) coronatus Zehra (1984)40 ORDER: MESOGASTROPODA Family: Potamididae Telescopium telescopium Zehra (1981)39 Cerithidea cingulatus Kira (1962)41 Family: Cerithidae Clypeomorus bifasciatum Dance (1974)38 Family: Littorinidae Littorina undulate Gray ORDER: NEOGASTROPODA Family: Thaididae Cymia carinifera Zehra (1981)39 SUBCLASS OPISTHOBRANCHIA Family: Haminoeidae Haminoe exarata *Gosliner (personal communication)44 SUBCLASS: PULMONATA ORDER: BASOMMATOPHORA Family: Ellobiidae Melampus taeniolatus (Hombron & Jacquinot) Habe (1964)42 Ellobium opportunatum (Gould) Habe (1964) 42 Melampus castaneus (Muhlfeld) Bosh et al. (1995)43 Family: Amphibolidae Salinator fragilis (Lamarck) Bosh et al. (1995) 43 ORDER: SYSTELLOMMATOPHORA Family: Onchididae Onhidium sp. Gosliner (personal communication)44 CLASS: BIVALVIA Family: Ostreidae Crassostrea madrasensis (Preston) Siddiqui (1998)45 Crassostrea gryphoides (Schlotheim) Siddiqui (1998) 43 *Dr Terrace M. Gosliner, Department of Invertebrate Zoology and Geology, California