Use of an intelligent CCD camera for the study of endogenous vertical migration rhythms in first zoeae of the crab Carcinus maenas

This paper describes an actographic system, based on video camera and image analysis software, which can detect small planktonic organisms and automatically determine their position in a bi-dimensional plane. This actograph was used to study swimming behaviour, under constant dark conditions in the laboratory, of the first zoea of the crab Carcinus maenas (L.), hatched from ovigerous females collected in the Ria de Aveiro, Portugal. An endogenous rhythm of vertical migration with a circatidal period was detected, in accordance with previous laboratory and field observations made on the species' first zoea from Wales and the Ria de Aveiro, respectively. The system also enabled the computation of dispersion indexes that supported the conclusion that crab zoeae use passive sinking during the descent phase of vertical migration. Mean swimming speed in the absence of light, irrespective of direction, was 0.0666 cm s–1. Vertical upward and downward components of swimming were 0.0437 and 0.0599 cm s–1, respectively. Although lower than values reported in the literature, these velocities are high enough to allow swimming throughout the water column of most estuaries during vertical migration in natural conditions at night.