Ecological Study Of The Molluscan Fauna On Intertidal Artificial Rocky Shore Of Tarkwa Bay Area, Lagos, Nigeria

Abstract

The molluscan community living in the three moles of Tarkwa Bay, Lagos was studied monthly between April 2008 and March 2010. These moles are made of rock known as the west, training and east. They provide attachment surfaces for the settlement of littoral communities. The study investigated the physico-chemical parameters, nutrients, heavy metals concentration levels in sea surface water, algae and resident molluscs of the three moles. Molluscan diversity, feeding, guts content analysis; growth and reproduction of selected species were assessed. Water samples were collected monthly at low tides from twelve geo-referenced network of sampling stations established on the littoral shorelines of the study sites. Sea surface water was collected just 0.5 metre below the surface using 2.01 non-metallic Hydrobios water sampler, kept in pre-labelled polyethylene 500ml bottles, and later transported to the laboratory for storage in deep freezer at 40C for further analyses. Algal samples were scraped from rock surfaces at low tides, and washed 3 times in seawaters and kept in pre- labelled 1000ml plastic containers for storage in the laboratory. Molluscs were collected at both subtidal and intertidal using quadrats of inert aluminium frame of size 1 m2 were used at low tides along transects fixed landward-seaward directions at 12 study sites. A total of 8486 individuals belonging to 69 species, 34 families and 4 classes were recorded. The four classes of Phylum Mollusca encountered were: Gastropoda 89.92%; Bivalvia 9.55%; Cephalopoda 0.3%; and Polyplacophora 0.2%. A total of 5794 individuals constituting 68.3% of the total population, were collected during the dry seasons, compared to 2692 individuals (31.70%) collected during the rainy season. The One - way ANOVA results showed that growth in Patella safiana and Nerita senegalensis were significantly difference among the three tidal zones on the shore (P < 0.05). The annual mean percentage of Chlorophyta in the gut of Patella safiana was 34.3 ± 7.51% with a range of 1.8% (July) to 44% (May). Rhodophyta had a minimum percentage gut content of 4% (April), reaching a maximum of 31% (July) with a mean percentage gut content of 14.58± 8.43%. Detritus accounted for a minimum of 17% (March) and a maximum of 32% (November) with annual mean value of 23.7 ± 4.31%. The mean value for the level of metal concentrations decreased in the order: Mn > Fe > Zn > Cu > Cr > Pb; in seawater, algae and molluscs. The level of heavy metal concentrations in the body parts of molluscs decreased in the order: Body tissues > foot > mantle > shell. The highest level of metal concentration was recorded in Buccinum undatum (266.6µg/g dry wt.) for Mn and lowest concentration was found in Nerita Senegalensis (0.01µg/g dry wt. of Pb).The assessment of Gonadosomatic Index (GSI) in male and female showed similar temporal patterns (t 0.05, 38= 2.02), however, but there was significance different among months (ANOVA; P < 0.05) for reproductive cycles of Patella safiana and Nerita senegalensis. The annual reproductive efforts for patella safiana (0.7048) male and female (0.5360), while Nerita senegalensis male (3.2436) and female (3.0190). The study generated a comprehensive checklist of molluscan communities of Tarkwa Bay which will be useful for future assessments and ecotourism development. Information on water quality, dietary preference and heavy metal concentration will be handy for shellfish industries. The established seasonal influence and reproductive patterns in selected molluscs, which will serve as valuable tools for breeding and conservation of intertidal molluscs in maritime communities in Nigeria.