Considerações em tôrno das hydromedusae da região de Fernando de Noronha

Abstract

During a seven days trip to the oceanic island of Fernando de Noronha, plankton was hauled from depths varying between 50 m and the surface. All the nets used were open nets, one was 1 m wide at the mouth, the other 25 cm wide. Due to lack of suitable equipment only the surface temperature and salinity were taken. Five holoplanktonic species wore collected, being Aglaura hemistoma the most abundant (145 specimens) and present at almost every station. Next in frequency of specimens and number of stations are: Liriope tetraphylla (27 specimens), Rhopalonema velatum (25 specimens), Solmundella bitentaculata (2 specimens) and finally Geryonia proboscidalis (1 specimen). Most of the specimens were immature. Of the meroplanktonic species, the most abundant was Eutima mira (7 specimens), next Obelia (perhaps Obelia geniculata). 4 specimens), Phialidium sp. (the medusa of the hydroid Clytia. 1 specimen). a fragment of Clytia cylindrica (hydroid). Sarsia sp. (the medusa of the hydroid Syncoryne, 1 specimen) and a few hydroids of Syncoryne sp. Numerous fragments of thecate hydroids, some taken alive, some not, were also present in the samples. On the whole, the present collection is characterized firstly by a great majority of oceanic forms both in number of species and number of individuals and secondly by relative abundance of benthonic specimens in the plankton. The author believes that the first fact may be explained by the geographical position of the island, placed 175 miles away from the edge of the continental shelf or north-eartern Brazil. The second fact might be explained by the strong impact of the South Atlantic current on the insular shelf that may be strong enough to tear fragments from the benthonic hydroid colonies. Due to the impact upon the island's submerged portion, upwelling or strong turbulence might insue, responsible for the presence of benthonic animals in the plankton. Benthonic nematodes were also present.