Year class splitting in the woodlouse Philoscia muscorum explained through studies of growth and survivorship

Abstract

Year class splitting has been observed in a population of the terrestrial isopod Philoscia muscorum (Scopoli) in a fixed dune grassland. In this population, individuals born in one season differentiate into two forms, one of which grows to maturity in one year, the other in two.Laboratory investigations of growth, fecundity and survivorship of individuals showed fundamental positive relationships between rate of growth and the ambient temperature/day light regime and between parental size and the number of offspring produced. Size also affected survivorship, with adults surviving better than juveniles. A link between survivorship and the temperature/day light regime was only obvious in small juveniles.No direct physiological cause for the year class split could be derived from the laboratory studies, but a trade‐off between the need for delayed or early breeding could result in the patterns of growth and reproduction observed in the field population. Delayed breeding allows continued growth of adults and hence larger numbers of offspring. Early breeding allows more growth of juveniles which greatly improves their chances of winter survival.