Holoplanktonic polychaetes from the Gulf of Tehuantepec, Mexico

Composition, abundance, and main assemblages of holoplanktonic polychaetes were analysed in the Gulf of Tehuantepec. This Gulf, in Mexican waters of the eastern tropical Pacific, is a very productive system due to an upwelling during the winter. Zooplankton samples were taken over a station grid covering neritic and oceanic waters during March and November 1978. Most species showed their highest densities in the oceanic region. In the western Gulf, the area sam- pled in both seasons, mean polychaete abundance was higher in November (549.2 ind.1000 m-3) than in March (143.8 ind.1000 m-3). A regression tree analysis showed that pH and dissolved oxygen were the main factors affecting the total polychaete abundance over the study area during March. Identified families were: Lopadorhynchidae (88.8%), Alciopidae (6.6%), Iospilidae (2.8%), Tomopteridae (1.1%) and Typhloscolecidae (0.7%). The dominant species, Pelagobia longicir- rata, accounted for 86.5% of the total polychaete abundance. A Bray-Curtis analysis identified two main polychaete assem- blages during March: "oceanic" and "neritic". Species richness and mean polychaete abundance were higher in the "oceanic" assemblage, and P. longicirrata, Plotohelmis capitata, Rhynchonerella gracilis and Lopadorhynchus henseni were the most frequent and abundant species in this assemblage. In the "neritic" assemblage, the iospilids Phalacrophorus uniformis and Iospilus phalacroides registered the highest relative abundance. It is suggested that the structure of holo- planktonic polychaete assemblages could be determined by the feeding habits of the species and their tolerance to the varia- bility in environmental conditions. Resume : La composition, l'abondance et les principaux assemblages de polychetes holoplanctoniques du Golfe de Tehuantepec sont analyses. Ce Golfe, situe dans les eaux mexicaines de l'est du Pacifique tropical, est un systeme tres pro- ductif en raison d'un upwelling hivernal. Le zooplancton a ete echantillonne sur un quadrillage couvrant les eaux neritiques et oceaniques en mars et novembre 1978. Les plus fortes densites sont obtenues dans la region oceanique pour la plupart des especes. Dans la partie ouest du Golfe, region echantillonnee pendant les deux saisons, l'abondance moyenne des poly- chetes est plus elevee en novembre (549,2 ind.1000 m -3 ) qu'en mars (143,8 ind.1000 m -3 ). L'analyse par arbres de regres- sion montre que le pH et l'oxygene dissous sont les principaux facteurs affectant l'abondance totale des Polychetes sur la zone d'etude en mars. Les familles identifiees sont: Lopadorhynchidae (88,8%), Alciopidae (6,6%), Iospilidae (2,8%), Tomopteridae (1,1%) et Typhloscolecidae (0,7%). L'espece dominante, Pelagobia longicirrata, represente 86,5% de l'a- bondance totale. Une analyse de Bray-Curtis permet d'identifier deux principaux assemblages en mars: un assemblage "oceanique" et un assemblage "neritique". La richesse specifique et l'abondance moyenne des polychetes sont plus elevees dans l'assemblage "oceanique", P. longicirrata, Plotohelmis capitata, Rhynchonerella gracilis et Lopadorhynchus henseni sont les especes les plus frequentes et les plus abondantes de cet assemblage. Les plus fortes abondances relatives des