Distribution and abundance of chaetognaths on the continental shelf of Tamaulipas and adyacent oceanic zone during april 1987

Abstract

An analysis of Chaetognatha species distribution and abundance is presented for the northwestern region of the Gulf of Mexico. Zooplankton samples were collected during April 1987 at 16 collection sites located in front of the State of Tamaulipas. Thirteen species were recorded. Specific richness was lower at northwestern sites in front of Laguna Madre. Species with the highest density (2000 to 6000 orgs.100m-3) were Flaccisagitta enflata y Sagitta tenuis which were found towards the north of the area. The most frequent species collected throughout the sites were F. enflata and Serratosagitta serratodentata, the last one showed its highest densities in the southern region. Three groups of species were detected by using the Importance Value (IV): the first set included F. enflata, S. tenuis and S. serratodentata. Also two groups were defined when density data were analyzed by Morisita´s similarity index. The first included 7 northerly sites and the second, the most southern site. The last one shows a low Shannon diversity value (1.3 bits individual-1) and an intermediate Simpson´s dominance value (0.51). The second group included the rest of the 8 southern stations, with an average diversity of 2.2 bits individual-1 and a mean dominance of 0.27. A Principal Component analyses ran on abiotic variables grouped the stations in two sets: oceanic and neritic, this in turn agreed with the results of the similarity index. Chaetognaths species distribution and abundance were probably influenced by salinity and temperature.