Ecological Study of the Molluscan Fauna on Intertidal Artificial Rocky

1.0 INTRODUCTION 1.1 Background to the Study The Nigerian coastline is characterized by natural sandy and muddy beaches. There are no naturally occurring rocky shores along the Nigerian coast (Edokpayi et al, 2010). However, between 1908 and 1912, three moles were built seaward to forestall powerful East- West along shore currents that silt the entrance of Lagos harbour (Awosika, 2001; Nwankwo, 2004). These three moles at Tarkwa Bay in Lagos provide artificial rocky surfaces for the littoral molluscan communities (Lewis, 1964; Addessi, 1994; Edokpayi et al, 2010). Intertidal rocky shores contain critical habitats which include: migrating route, feeding, nesting, breeding, nursery areas for marine fauna and flora. It also serves as focal points for urban tourism, shell fisheries and scientific research (Asakura and Suzuki, 1987; Nwankwo, 2001; Clarke et al., 2002). Rocky intertidal communities develop in response to a wide range of interacting factors. The effect of the daily tidal cycles of submersion and emersion established basic vertical gradients of species richness and biomass (Menge and Daley, 1999; Murray, 2006). The other significant abiotic influences on these communities include the degree of wave action, substrate instability, substrate hardness and heterogeneity, sand inundation, desiccation stress, hydrographic variations i.e currents, upwelling, human induced and natural disturbance (Littler, 1980; Clarke and Grahame, 1999, 2002). The community structure are further and often profoundly, influenced by biotic interactions, especially by the presence of algae, seaweeds, filter feeders, grazing, competition and predation (Wood, 2001; Rose, 2003; Smith, et al., 2008). According to Chapman (2000), habitat, temperature, desiccation, salinity, food, predation and reproduction are the major factors limiting the distribution and abundance of littoral molluscs on global intertidal rocky 1 ecosystems. Coastal area can be regarded as the interface among three habitable media namely earth, air and sea. The ecological importance of the littoral zone in marine ecosystem is widely recognized. The intertidal or littoral portion of the shore, as it is variably described, covers the area between the high and the low tides (Thompson et al., 2002). Shores can be classified into three types based on substratum structure: sandy, rocky, and muddy. Rocky shores are more variable than other coastal habitats. Depending on the local geology they may range from steep, overhanging cliffs to wide, gently shelving platforms, from smooth uniform slopes to highly dissected irregular masses or even extensive boulder beaches (Ibe, 1987, 1989, GESAMP, 2001). McQuaid, et al., (1985) documented a decrease in number of species on each shore in an upshore direction probably owing to higher niche heterogeneity on the lower shore coupled with more extreme and less stable conditions at the top of the littoral zone. Littler (1980) observed a variation in the distribution and abundance of organisms on three tidal zones within the southern California bight. The greatest cover of both macrophytes and invertebrates occurred lower in the intertidal zone and decreased upshore direction. Globally, rocky shore ecosystems have been primary research sites for studies of land-sea interactions: morphological, physiological, and life history adaptations, as well as ecological interactions, and the experiments on intertidal shore do not require elaborate oceanographic equipment (Underwood, 2000; Folke, 2004). Information from such studies has been critical to understanding the role of competition, predation, and physical gradient on community dynamics (Wood, 2001; Halpern, et al., 2007; IPCC, 2007; Crain, et al., 2008). 2 Artificial rocky shores serve as habitats for preponderant molluscan fauna and flora. They are characterized by high biodiversity, productivity and disturbance in the artificial fractal geometry of the rocky shore assemblages of Tarkwa Bay have been linked to seasonal fluctuations in rainfall, hydrological regime, tides, wave action, desiccation, salinity, shoreline recession and temperature fluctuations (Connell, 1961; Lewis, 1964; Thompson, 1980; Edokpayi and Eruteya, 2012). The physico-chemical characteristics and the algae community of Tarkwa Bay have been described by (Nwankwo, 1993, 2004). The algal community dynamics was reported to depend on seasons, salinity, temperature regimes and depth gradient (Onyema et al., 2009). Intertidal shore animals evolve physiological and behavioural adaptations to withstand the fluctuating nature of the shore environment (Underwood, 2000; Clarke, et al., 2002; Somero, 2002; Rose, 2003). The wide and rapid changes of temperature and salinity that occur on the shore surface during low tide require high level of eurythermy and euryhalinity in the exposed population (Murray et al., 2006; Murawski, 2007; Riebesell, 2008). They must also be capable of making appropriate adjustments of behaviour in response to changes in their surroundings (Chapman, 2000). Appropriate changes of activities are required to meet the profoundly different conditions of submergence and exposure to air. Movement, feeding, or reproduction are only possible for many littoral molluscs during the periods when they are covered by waters; and when uncovered, they become more or less inactive (Chesson, 2002; Coleman, et al., 2006). Restriction of movement as the shore dries out during low tides may also help to confine free-living forms to their appropriate zones (Smith, et al., 2008). 3 Some shore animals when removed from the zone, displayed cyclical changes of activity having a tidal frequency, apparently controlled by endogenous rhythms (Underwood, 2000). Wood (2001) reported that a 12 hour-cycle of activities related to the tidal cycle has been observed in shored molluscs. Patella safiana feeds mostly, by scrapping the surface with its long, toothed radular, rasping off the microscopic film of algae which forms a slimy coating on the rocks (Noel, et al, 2009). Marine molluscs often exhibit distinct zonation, with the size of upper intertidal species increasing in an upshore direction, while the size of lower intertidal species decreases in an upshore direction (Vermeij, 1972, 1973, 1974; Pauly, et al., 2005). It has been reported that the upper limits of marine intertidal molluscs are limited by physical factors, while lower limits are constrained by biological factors (Noel, et al., 2009; Murawski, 2007). Differential growth rates and active migrations have also been proposed to explain size gradients within an intertidal community (Sivadas, et al., 2008). It was suggested that the migratory nature of these species was related to the increased tolerance of larger molluscan species to conditions of increasing desiccation stress (Rose, 2003; Stirling, 2007). There is dearth of scientific information on the diversity, distributions, growth, feeding, heavy metal distribution, and reproduction of the attached molluscs of the three moles of Tarkwa Bay. This study has been designed to provide a comparative ecology and taxonomic inventory of the resident molluscan communities of the three moles of Tarkwa Bay, Lagos, Nigeria. 4 1.2 Statement of Problem There is dearth of scientific information on molluscs worldwide and in particular on the resident molluscan communities on the shores of the three artificial breakwaters of Tarkwa Bay, thus leaving a gap in our knowledge of the resident molluscs on intertidal shores. It may be difficult to know when the shores are invaded by exotic species from trans-oceanic tankers‟ ballast waters. Intertidal molluscs are important food items of maritime communities in Tarkwa Bay because of their high protein content (Faulkner, 1992). Data on the biology and the general ecology of the molluscan communities of Tarkwa Bay that are of ecological, economic, and commercial interest that would stimulate research on mass production, sustainable exploitation are sparse. Globally, there is dearth of taxonomic experts, and this has affected the study of molluscs worldwide. Available literature on how environmental chemistry, and alterations related to human activities affect molluscan dynamic on the global scale (Lewis, 1964; Thompson, 1980; Edokpayi and Eruteya, 2012). Tarkwa Bay coast is frequently subjected to multiple sources of stress operating over several spatial and temporal scales (Ukwe, et al., 2003). Exponential population growth and anthropogenically-driven changes affect the molluscan community dynamic on the Tarkwa Bay shores (Ajao, 1994). Physical alteration and destruction of habitats through land reclamation and landfilling during coastal development, affects water quality. Changes in sediment morphology brought about by shoreline modifications i.e by dredging of harbour and shipping channels, construction of embayment and marinas as reported by (Ibe, 2005) affect intertidal molluscan population dynamics. 5 1.3 Aim: This study is aimed at investigating the bio-ecology of molluscan fauna of the three moles of Tarkwa Bay. 1.3.1 Objectives: The following were the specific objectives of the study: 1. Investigate the spatial and temporal variations in physico-chemical parameters of the three moles of Tarkwa Bay. 2. Determine the composition, abundance, diversity, distribution of the molluscan communities of the three moles of Tarkwa Bay. 3. Investigate the patterns of growth in molluscs of interest in the laboratory, and the three different tidal zones on the shores of the study area. 4. Study the temporal variations

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