Morphology, life history and colour variability in the endemic New Zealand isopod, Isocladus armatus : a thesis in partial fulfilment for the requirements for the degree of Master of Science, Zoology at Massey University, Auckland, New Zealand

Abstract

Research into colour polymorphism has been central to understanding the evolutionary mechanisms that maintain variation within populations. This is because colour polymorphism is quantifiable and often associated with differential selective pressures. Work on colour polymorphic organisms requires detailed knowledge of both the natural history of the focal organism and of the specific mechanisms that generate colour variation. Isocladus armatus is a Sphaeromatid isopod crustacean endemic to New Zealand. This marine isopod is found in semi-sheltered shores across the country and is well known for its astonishing diversity of colour morphs. Although previous research into this species has been limited, I. armatus appears to have a considerably higher degree of colour polymorphism than most other isopod species. In this study, I document the developmental life history, sexual dimorphism and reproductive behaviour of this species and I explore the potential genetic factors influencing the expression and maintenance of colour polymorphism. The research described here required the development of methods for breeding and maintaining isopods under laboratory conditions, making this the first comprehensive investigation of reproductive biology in I. armatus. Because there have been no prior published studies on the I. armatus lifecycle or how and when females are sexually receptive, uncovering the reproductive biology of this species was a challenge. However, through trial and error and detailed observation of male and female morphology and development, this research describes the first successful multi-generation reproduction of this species in captivity. In addition, through these generations, I demonstrate a clear genetic basis to colour variability in this species. The work in this thesis will inform future studies on this species, isopod biology in general and provides necessary insight into the wider question of how colour variation is maintained in natural populations.