Siphonophores are at times amongst the most abundant invertebrate zooplankton predators in
the oceans. Historically, siphonophores have been under-sampled and of the studies conducted there
has been a bias towards oceanic oligotrophic waters where they are considered to be more important.
In temperate coastal regions, comparatively less is known about the diversity and abundance of
siphonophores, where periodic blooms can restructure the plankton communities and have been
correlated with high mortalities in the salmon aquaculture industry. To address this lack of knowledge,
plankton samples were collected during two periods (March 2009-March 2011 and April 2014-
November 2015) from a coastal embayment in the southwest of Ireland. In total, three siphonophore
species were found, the calycophoran Muggiaea atlantica, and the physonects, Nanomia bijuga and
Agalma elegans. Muggiaea atlantica was the most abundant species (250 colonies m3), with densities an
order of magnitude higher than either physonect. Muggiaea atlantica displayed a distinct seasonality,
whereas the physonect species were sporadic in occurrence. Comparing siphonophores in Bantry Bay
and the Western English Channel (Plymouth Marine Laboratory’s L4 station) indicates both regions
share a similar pattern of inter-annual occurrence and provides novel information on the seasonality
and occurrence of siphonophores in Irish coastal waters.