Pyrosoma atlanticum (Tunicata, Thaliacea): diel migration and vertical distribution as a function of colony size

A large population of the colonial pelagic tunicate Pyrosomaatlanticum occurred in April 1991 in offshore waters of theLigurian Sea (Northwestern Mediterranean). The high numbersof colonies caught allowed their vertical distribution and dielmigration in the 0–965 m water column to be describedas a function of their size. Daytime depths and amplitudes ofthe migration were correlated with colony size. The amplitudeof the migration ranged from 90 m for 3-mm-length colonies to760 m for 51-mm-length colonies, with a mean amplitude of 410m for the whole population, all sizes pooled. The results ofhorizontal hauls at a given depth around sunrise and sunsetshowed a marked diurnal symmetry of the migratory cycle relativeto noon, and that migration of the population was not cohesive.For example, the larger the colonies, the later after sunsetthey reached the upper layers during their upward migration.