Appendicularian distribution in the Río de la Plata estuary and adjacent neritic areas

Abstract

Appendicularians from the Rio de la Plata estuary and southern coastal areas from 34° to 41°S, Argentina were studied. Three species were found: Oikopleura fusiformis, O. dioica, and Fritillaria borealis. O. fusiformis showed a pronounced preference for outer shelf waters (33.5 %o salinity), being dominant at the Uruguayan coast. O. dioica was mainly found at coastal stations and its abundance was higher in waters with salinities between 25 and 27 %c. F. borealis appeared at very low densities, generally associated with O. dioica. No appendicularians were found at the mouth of the estuary with high concentrations of suspended sediments. Trunk lengths of individuals of each species from three areas with very different hydrological features, were compared to analyze size differences among animals showing the same maturity degree. The northern part of the estuary was characterized by the predominance of large individuals, forming a significantly different group from that of the southern area. Small individuals were predominant at the southern area, which showed marked stratifications of temperature and salinity. Histological sections of gonads of small animals with mature appearance from the southern area showed the same maturity degree as the larger mature specimens from the northern areas. The relationship between size differences and environmental factors is discussed.