A view from below: new insights into the offshore swarm formation in <i>Velella velella</i> (Cnidaria: hydrozoa)

The pleustonic hydrozoan Velella velella, found worldwide in temperate and tropical regions, creates vast offshore swarms that occasionally reach the coastline, resulting in mass strandings. The life cycle of this species and the dynamics of its swarms remain partially unclear, and most available biological information has been gathered from strandings. This study provides the first characterization of the population structure of an offshore swarm. Sampling was conducted in the north-western Ligurian Sea on 15 April 2024, approximately 9.4 nautical miles from the nearest coast. A dense patch of 20 m2 was photographed from below by snorkelling; each image was analysed to determine colony density and size-frequency population structure. Results showed an average density of 23,815 ± 2315 colonies m−2, with a colony length ranging from 0.08 to 4.50 cm. Rataria stages (≤0.2 cm) dominated the sample (59.7%), while fertile colonies (>2 cm) were rare (8% of the total). Assuming a growth rate of 0.4–0.5 mm per day, the largest colonies likely formed in early January, while the rataria and the smallest ones, which constituted the majority of the swarm, appeared just days before the sampling; the timeline aligns with the coastal stranding data. The observations suggest that the swarm persists for months above deep larval reservoirs in submarine canyons, while being seeded from below.