Spatial and temporal variation of Branchiostoma lanceolatum larvae (Cephalochordata) in a hypoxic bay

Abstract

Abstract A 12-month zooplankton survey (September 2008 to August 2009) in the hypoxic, stratified and seasonally eutrophic Amvrakikos Gulf (western Greece) revealed the presence of larvae of the Cephalochordate Branchiostoma lanceolatum. The larvae were found at almost all sampling occasions, except September and February, and presented their peak of abundance in April (426.8 ind. m−3). The oxygen depletion in the water column affected their vertical and horizontal distribution. Thus, their abundance was greater in the subsurface layers (10-20 m) which corresponds to the well oxygenated thermocline layer. An east to west increase of their abundance in the deeper layers was associated with the severity of oxygen depletion. The lower oxygen limit for their presence seems to have been 2 mg L−1 which corresponds to hypoxia. A size-specific depth distribution was noticed, with larger larvae residing in deeper strata. A diel vertical migration of the larvae in the summer was recorded, with a night ascent in shallower depths and a day descent in deeper strata. There was an apparent coexistence in space and time between the larvae and the dinoflagellate Ceratium sp., though this could not indicate a trophic interrelation between them due to the large size of the latter. The results suggest that, during their planktonic stage, the larvae utilize the water stratification within the Amvrakikos Gulf by inhabiting the depths within or close to the thermocline, where they can satisfy their oxygen and energy demands while being protected from predation.