Fitness Consequences of Allorecognition-Mediated Agonistic Interactions in the Colonial HydroidHydractinia [GM]

Abstract

In sessile and sedentary organisms, competition for space may have fitness consequences that depend strongly on ecological context. Colonial hydroids in the genus Hydractinia use an inducible defense when encountering conspecifics, and intraspecific competition is common in natural populations, often resulting in complete overgrowth of subordinate competitors. My goal in this study was to quantify the impacts of agonistic interactions in Hydractinia [GM] (an undescribed species from the Gulf of Mexico) in terms of three primary fitness components: colony survival, growth rate, and immature gonozooid production. The results demonstrate that the fitness consequences of intraspecific competition depend on the size at which competitive encounters are initiated and the growth form (an indicator of competitive ability) of the competitors. Moreover, some competing colonies consistently produced more immature gonozooids than the controls without competition, and they exhibited extremely low mortality even after 90 days of growth. These results have several ramifications. First, agonistic interactions do not always proceed to competitive elimination. Second, the increase in production of immature gonozooids--an investment in future reproduction--in response to intraspecific competition supports the hypothesis that indeterminately growing organisms increase sexual reproductive effort when growth becomes limiting. Lastly, in light of known ontogenetic variation in the ability of Hydractinia to differentiate among genetically related colonies, strongly size-dependent fitness consequences are consistent with an adaptive, kin-discriminating allorecognition system.

Gastropod shell size and morphology influence conspecific interactions in an encrusting hydroid

“…Delayed overgrowth or persistent standoffs between larger conspecifics influence sexual reproductive characters in Hydractinia (Ferrell 2004). Several authors (Abrahamson 1975, Hughes & Cancino 1985, Harvell & Grosberg 1988 have hypothesized that clones can maximize fitness by postponing the allocation of energy to sexual reproduction until extrinsic factors limit asexual growth.…”

“…Having an increased capacity for hyperplastic stolon production, more stoloniferous colonies dominate in conspecific agonistic encounters (Buss & Grosberg 1990). Despite frequent competition for space, Hydractinia colonies found in natural populations differ widely in growth form (Hauenschild 1954, Ivker 1972, McFadden et al 1984, Yund 1991, Ferrell 2004 and, hence, competitive ability.Competitive outcomes are size-dependent in Hydractinia (Yund et al 1987, Ferrell 2004. Competitive relationships among small Hydractinia colonies are essentially transitive, consistently resulting in rapid overgrowth of the inferior competitor (Yund et al 1987, Buss & Grosberg 1990, Ferrell 2004.…”

“…In contrast, contests involving larger colonies are significantly prolonged, and may or may not ultimately end in overgrowth of the inferior competitor. In those cases in which competitive exclusion occurs, competitive reversals may emerge (Yund et al 1987, Ferrell 2004. The competitive abilities of the colonies involved likely determine the prolonged duration or effective permanence of such encounters (Ferrell 2004).…”

“…In those cases in which competitive exclusion occurs, competitive reversals may emerge (Yund et al 1987, Ferrell 2004. The competitive abilities of the colonies involved likely determine the prolonged duration or effective permanence of such encounters (Ferrell 2004).Much of what is known regarding competition in larger Hydractinia spp. colonies involves interspecific competitors.…”

“…Competitive relationships among small Hydractinia colonies are essentially transitive, consistently resulting in rapid overgrowth of the inferior competitor (Yund et al 1987, Buss & Grosberg 1990, Ferrell 2004. In contrast, contests involving larger colonies are significantly prolonged, and may or may not ultimately end in overgrowth of the inferior competitor.…”