Growth or differentiation? Adaptive regeneration in the brittlestar Amphiura filiformis.

Amphiura filiformis is a burrowing brittlestar, which extends arms in the water column when suspension feeding. In previous studies, unexpectedly high variability was observed in regeneration rate between individuals even when experiments were performed under identical conditions. The aims of this work were to understand this variability and interpret the observed variability in terms of adaptation to sublethal predation. Our experiments on the dynamics of arm regeneration in A. filiformis revealed that the developmental program during regeneration is well adapted to its burrowing life style. We demonstrate that there is a trade-off between regeneration in length and functional recovery for feeding (differentiation index). The amount of tissue lost (length lost), which represents the quantity of tissue needed to completely regenerate an intact arm with no previous history of regeneration, determines whether the arm will invest more energy in growth and/or in differentiation, which must be a reflection of the ability to differentially regulate developmental programs during regeneration. We show that combining regeneration rate with differentiation index provides an ideal tool for the definition of a standard temporal framework for both field and laboratory studies of regeneration.