Abstract
Vessel biofouling is the putative vector for many marine invasions. Biofouling communities are often dominated by organisms with sessile life history strategies that cannot themselves exit a transport vector, thereby requiring another means of dispersal such as reproduction, which releases mobile propagules. For many sessile organisms, reproduction and associated propagule delivery is tightly coupled with environmental conditions, which frequently vary along vessel transit routes or in ports. However, there are still major gaps in our understanding of specific environmental effects on the reproductive biology of most marine non-indigenous species (NIS), limiting our ability to predict when and where introductions by reproduction will occur. Here, we conducted a manipulative laboratory experiment to explore how variation in temperature and food availability - two conditions that vary among vessel transit routes and ports - affect reproductive output of a common marine biofouling species, the barnacle Amphibalanus improvisus. The experiment had two phases: (1) a simulated "route" portion where temperature and food availability were manipulated independently followed by (2) a simulated "port" portion where temperature and food co-varied. We found that temperature and food availability during both phases affected barnacle fecundity, with reproductive output varying by an order of magnitude between some treatments. Barnacles tended to produce more propagules when temperatures were moderate and food was more plentiful as well as when entering relatively warm ports with greater food availability. Our results show how reproduction by marine biofouling species can be regulated by environmental conditions, which vary predictably with transit route, such that some routes and ports may be especially susceptible to introduction via reproduction. Identifying triggers for reproduction in common marine NIS is therefore key to evaluating the risk of marine invasions.