One of the commonly assumed consequences of modularity is that senescence may be avoided, or at least delayed, at th e lgenet, with senescence frequently being expressed at the level of the individual module. Here we present evidence from a laboratory study of senescence at the zooid (= module) level in the bryozoan Electra pilosa. Post-larval ancestrulae of this species were collected intertidally in St Andrews Bay, SE Scotland, on the macroalga Fucus serratus, and reared under constant conditions in the laboratory. Polypides in this species undergo repeated cycles of regression and regeneration. Zooid senescence was indicated here by polypide life spans decreasing over successive polypide cycles, while the period required to regenerate new polypides increased over time. Senescence sensu stricto may have evolved either in a unitary ancestor of the Bryozoa, or in the phylum itself, but the proximal deterioration of physiological parameters at the zooid level might not constitute evolved senescence sensu stricto. Rather, it may result from selection for rapid distal colony growth, with a concomitant decrease in proximal zooid investment and provisioning. Additional key words: modular organisms, polypide life spans, polypide regression, regeneration time, proximal investment Senescence (aging) is defined as a decline in the agespecific fitness components of an organism due to internal physiological degeneration (Rose 1991). Modem evolutionary theory of aging goes back to Medawar (1952), who predicted that the frequency of mutations with age-specific deleterious effects will increase with age for a given cohort. Williams' antagonistic pleiotropy hypothesis (Williams 1957) extended existing theory; it predicts that mutations having negative effects late in life, as well as positive effects before reproductive age is reached, will be passed on because their net effect on fitness is still positive. The accumulation of these negative effects then will be reflected in an agespecific intrinsic decline in fitness. Both hypotheses are well supported by experimental evidence (reviewed by Stearns 1992). Modular organisms as defined by Harper (1977) comprise multicellular units which are either physiologically connected (as for the zooids of bryozoan colonies, or the polyps of corals) or separate, genetically identical individuals (e.g., ramets of a clone of plants). The range of modular organisms is extremely diverse and includes most plants, most protists, fungi, and members of 19 animal phyla. Modular organisms often a Author for correspondence. E-mail: mmb@st-andrews.ac.uk are able to avoid, or at least markedly delay, senescence at the level of the genet (Jackson & Coates 1986; Begon et al. 1990). More specifically, wholeorganism senescence may not evolve in species where reproduction is primarily clonal and sexual reproduction is rare (Caswell 1985) because the pattern of selective pressure on different stages in the life cycle may be very different from that found in non-clonal organisms. Senescence frequently occurs at the module level, with each module passing through the life-history phases characteristic of unitary organisms. In sharp contrast to the volume of work on senescence in mammals, published work on senescence in modular organisms is scant and often anecdotal. Many higher plants display organized senescence at the module level in the form of either sequential or simultaneous senescence and death of leaves (Hardwick 1986), while at the whole-organism level there are numerous strategies ranging from ephemeral (several weeks) to extended (>1000 yr) life spans (Woolhouse 1972). Partial, proximal senescence also has been reported for the bracken fern, Pteridium aquilinum (Watt 1947). Modular animals display a wide range of senescence strategies. In the colonial ascidian Botryllus schlosseri, zooids regularly degenerate and are replaced in a process known as takeover (Millar 1971), but deterministic, whole-organism senescence This content downloaded from 157.55.39.207 on Tue, 05 Jul 2016 04:20:33 UTC All use subject to http://about.jstor.org/terms