This paper introduces a software system for detecting and tracking starfish in an underwater video sequence. The target of such a system is to help biologists in giving an estimate of the amount of starfish present in a particular area of the sea-bottom. The nature of the input images is characterised by a low ratio signal/noise and by the presence of noisy background represented by pebbles; this makes the detection a non-trivial task. The procedure we used is a chain of several steps that starting from the extraction of the area of interest ends with a classifier and a tracker, providing the necessary information for counting the starfish present in the scene.