Van Winkle, D.H.*; Blackstone, N.W.: Ontogenetic changes in competitive behavior & ability in colonial hydroids

In controlled laboratory experiments, colonies of Podocoryna carnea (PC) typically overgrow and kill colonies of Hydractinia symbiolongicarpus (HS). Generally, these experiments have used colonies grown from tissue explants surgically removed from mature colonies taken from natural populations. Recent experiments comparing interspecific bouts among juveniles and those among explants taken from mature colonies reveal that both the behavioral characteristics and outcomes of competition are significantly different. Among juveniles, HS colonies exhibit directional growth toward PC and more readily produce nematocyte-rich hyperplastic stolons than PC. Nevertheless, PC can still overcome HS if it initially grows away from the contact zone and subsequently outgrows HS. Overall, HS juveniles destroyed their PC counterparts in more than 35% of competitive bouts on limited substrata, while mature PC colonies dominated their HS counterparts in all similar encounters. In the field, both species occur on gastropod shells inhabited by hermit crabs. On these shells, competition among young colonies is a common phenomenon. The same factors that lead to differential larval settlement over the shell surface likely also lead to food gradients. The directional growth potential of juvenile HS colonies leading to the occupation and defense of these nutrient-rich zones may thus provide a significant advantage owing to increased growth rates in early ontogeny. The recently observed behavior and relative success of competing juvenile HS colonies may in part explain the considerably greater abundance of this species in natural habitats relative to PC.