Early-life stress reduces senescence in the field for an indeterminately growing ectotherm

Theory predicts that aged-related declines in performance (senescence) depend on experiences during early life. Experimental field tests of this theory are rare: most manipulative studies have been done in the laboratory on short-lived invertebrates; while most field studies focus on longer-lived vertebrates and involve no experimental manipulation. Consequently, few studies manipulate early-life stress and test its effects on senescence in the field, especially in invertebrates. We manipulated early-life stress in the colonial marine bryozoan, Bugula neritina, by experimentally extending the larval period (an energetic stress), then monitored the survival, fecundity and growth of more than 500 individuals in the field across their lifetimes. We used these data to generate trajectories of actuarial and reproductive senescence and test how both components of ageing were affected by early-life stress. While colonial organisms with indeterminate growth such as B. neritina have been theorised to show negligible senescence, instead we saw strong declines in survival and reproductive output in the oldest individuals, regardless of early-life stress. We also found that adults originating from stressed larvae had lower fitness overall (in terms of lifetime reproductive success), but experienced weaker actuarial senescence. Our findings for reproductive senescence were more nuanced, with some evidence to suggest that early-life stress also decreases rates of reproductive senescence. Our study provides a rare demonstration of senescence for an invertebrate in the field and suggests that the relationship between life history and senescence may be more complex than has been anticipated.