IS SPIOCHAETOPTERUS COSTARUM REALLY AN ORGANIC POLLUTION INDICATOR? OR THE ROLE OF RETENTIVE STRUCTURES IN THE DISTRIBUTION OF THIS SPECIES
Bhaud, M. R.1 ; Lastra, M. C.2; Lopez-Jamar 3 ; Marcano, G. 4 ; Mora, J5. & Rey, L. R5.
1Observatoire Océanologique de Banyuls, Université P. and M. Curie-CNRS, BP 44, 66651 Banyuls-sur-mer Cedex, France. 2Departamento de Ecologia y biologia Animal Facultad de Ciencias, Universidad de Vigo, E-30000 Vigo, La Coruña, Spain. 3Instituto Español de Oceanografia, Laboratorio de La Coruña. Apdo. 130. La Coruña, Spain. 4 Universidade de Aveiro, Departamento de Biologia, 3810 Aveiro, Portugal. 5Departamento de Biologia Animal, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, La Coruna, Spain.
Within the framework of a review of the chaetopterid family, an examination of conditions required for the survival of several species in the genus Spiochaetopterus has been undertaken. For S. costarum, studies undertaken on the Spanish coastline of the Galician Rias (Atlantic Ocean) form the basis of the main body of data. This species is considered as the benchmark for such communities and has been proposed to act as an indicator of organic contamination of sediment. However this feature is questionable for several reasons: a) the animal is isolated from the sediment by a tube; b) the animal is not a sandivore but feeds above the sediment in the water column; c) other species classified by the international research community as pollution indicators are not present. These reasons lead to a reassessment of the environmental conditions for the survival of individuals from this species. The large number of benthic individuals (often close to 1000 specimens/m2 ) may be explained by the fact that topographical conditions are particularly favourable for retention in these closed areas which reduce dissemination. In contrast, in the open sea or close to the mouth of bays where retention is reduced or nonexistent, the density of benthic individuals is lower. This species reproduces by planktonic larval stages. The larvae are not spread by currents, while a balance between the source of larvae in the adult population, loss by lateral escaping and successful benthic recruitment is set up at a high quantitative level. The high density of adults is not linked to an elevated fertility per individual, but to a reduced loss of larvae. In the same way, the high density of benthic individuals does not necessarily reflect a healthier environment relative to an area with a lower density of individuals. Rather, this situation illustrates an extreme example of recruitment in which the physical conditions of relative confinement control the volume of dissemination and thus the density of benthic organisms.