Variation in growth and competitive ability between sexually and clonally produced hydroids

In controlled laboratory experiments, colonies of Podocoryna carnea typically overgrow and kill colonies of Hydractinia symbiolongicarpus. Generally, these experiments have used colonies grown from tissue explants (clonal replicates) surgically removed from mature colonies taken from natural populations. In contrast, experiments involving interspecific bouts between small, sexually produced colonies reveal that both the characteristics and outcomes of competition differ from previous studies. During competition between small sexually produced colonies, H. symbiolongicarpus exhibits directional growth toward P. carnea and produces nematocyte-rich hyperplastic stolons more readily than P. carnea does. Nevertheless, P. carnea can still overcome H. symbiolongicarpus if it initially grows away from the contact zone and subsequently flanks H. symbiolongicarpus. Overall, sexually produced colonies of H. symbiolongicarpus destroyed their P. carnea counterparts in more than 35% of competitive bouts, whereas P. carnea dominated their H. symbiolongicarpus counterparts in all similar encounters between clonally produced colonies. In natural populations, competition between small sexually produced colonies of H. symbiolongicarpus may predominate, and these results support the hypothesis that this species is adapted to competition early in colony development. More generally, studies of competition between sexually produced colonies should complement similar studies of clonally produced colonies.