Evidence for zooid senescence in the marine bryozoan Electra pilosa

One of the commonly assumed consequences of modularity is that senescence may be avoided, or at least delayed, at the level of the genet, 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.