Salinity Induced Longitudinal Zonation of Polychaete Fauna on the Bonny River Estuary

Abstract

The longitudinal zonation of polychaetes was studied across 13 stations on the Bonny River estuary. A total of eighty-eight (88) species of polychaetes, from 68 genera, belonging to 31 families, were identified. Based on salinity values, three major salinity zones were identified: The alpha-Polyhaline – beta-Polyhaline and Mesohaline. The dendrogram shows four polychaete associations which correlated with salinity variation. The first group comprises of species which occurred in a very narrow salinity range (0.9-2.0 gL-1) and are referred to as the true marine or the alpha-polyhaline inhabitants. The second were group of species that colonize the transition zone where salinity oscillates from alpha to beta-polyhaline (0.9-5.3 gL-1). The third group comprises of species that are specific to the beta-polyhaline which has a constant salinity of 5.3 gL-1. The fourth association was the euryhaline group which composed of species that occurred at all the stations from salinity variation of 0.9 to 15.4 gL-1. The study observed that some genera with more than one species have ecological divergence along the salinity gradient. The three species of the genus Aricidea namely (Aricidea sp.; Aricidea simplex; and Aricidea (Acrima) assimillis) occur over the range with Aricideasp. occurring at the alpha-polyhaline, Aricidea simplex occurring at both the alpha and beta polyhaline zone while Aricidea (Acrima) assimillis) occur strictly at the beta-polyhaline zone. Similarly the Lumbrineris aberrans and Lumbrineris fragilis occur only in the alpha-polyhaline while their sympatric species Lumbrineris latreilli and Lumbrineris tetraura occur in both the alpha and beta polyhaline zones. Similarly the salinity preferences for Eteone siphonodonta were the alpha-polyhaline zone while Eteone picta were in the beta-polyhaline zone only. The Scoloplos spp group was observed to be spaced with Scoloplos dayi,occurring in the alpha-polyhaline zone while two species, Scoloplos (Scoloplos) armiger and Scoloplos(Leodamas) johnstonei occur in both the alpha and beta polyhaline zones. The Notomastus spp group have two species Notomastus sp. which occur in salinity gradient of alpha and beta polyhaline zones while Notomastus aberrans were observed to occur in the alpha-beta polyhaline and mesohaline zones. Similar salinity spacing preferences in the Notomastus genus were observed for Glycera prashadi and Glycera tridactyla. Two species namely Phyllodoce mucosa and Phyllodoce tubicola were spaced between salinity gradients of alpha-polyhaline and beta-polyhaline respectively. This divergence suggests that other biotic and abiotic factors other than salinity may be responsible for the zonation. Therefore understanding observed polychaete distribution along the salinity gradient need additional studies that consider nonlinear abiotic and biotic variables.