Development and persistence of patches of Thaliacea

The distribution and abundance of Thaliacea on the south-eastern continental shelf of the U.S.A. (29°30′–31°N) was studied at 4- to 10-day intervals. During July, Thalia democratica and doliolids covered the same geographic area of the outer middle shelf (～2 800 km² on 27–29 July). By 8–12 August, these organisms occupied the entire middle shelf from 29°30′ to 30°50′N (～9 300 km²). The increase in area covered was due to the shoreward advection of water. Over a period of 10 days, concentrations of salps increased by as much as 25-fold in the thermocline, which amounted to a daily increase of zooids of 50 per cent. Doliolids increased five-fold during the same period. Both increases resulted from asexual reproduction and indicate that Thaliacea, through asexual reproduction, can respond almost instantaneously to favourable food conditions. Thaliacea seem to be the only metazooplankton taxon that can respond immediately, with large increases in population, to upwelling events.