Floating docks in tropical environments - a reservoir for the opportunistic ascidian Herdmania momus

The solitary ascidian Herdmania momus is considered native to the Red Sea, and invasive in the Mediterranean. Periodic surveys have revealed high recruitment and growth rates of this species on floating docks in the Gulf of Aqaba, Red Sea, following the annual vertical mixing event. In order to ascertain the length of time taken by H. momus individuals to settle on new artificial substrates, and the pace at which they grow and reach the reproductive stage, we monitored a newly-deployed floating dock for two years following its deployment. The number of individuals and their sizes were recorded weekly in March-June 2013 (spring-early summer), in June-August (summer), and re-visited each spring (April 2014, 2015). In addition, seven fixed quadrates were artificially cleaned and monitored in summer 2013. Ascidians were visible after eight weeks in the spring and five weeks in the summer. Growth rate in spring was twice as fast as in summer (0.275 and 0.106 cm/week respectively). Recruitment was higher during spring, reaching a density of 34.3 ± 7.1 in June, with fully reproductive individuals, dropping to 0.86 ± 0.46 individuals per quadrate in August. Field surveys of additional floating docks revealed significantly higher densities at the northern sites during spring. Following a nutrient enrichment event such an opportunistic filter-feeder can potentially release and distribute propagules to natural reefs, inducing a shift in community assemblage. In addition, its ability to colonize artificial substrates within its native range sheds light on its success as an invasive species in the Mediterranean.