Island gigantism or dwarfism? Phylogeny and taxonomy of Madagascar's chirping Giant Pill-Millipedes, describing newly discovered, microendemic species

2010 REU Intern Stephanie Loria


Junior Environmental Studies: Ecology and Biodiversity major at Sewanee: The University of the South

REU Mentors: Dr. Petra Sierwald (Curator, Zoology, Insects) and Dr. Thomas Wesener (Postdoctoral Fellow, Zoology, Insects)

Symposium Presentation Title: Island Gigantism or Dwarfism? Phylogeny and Taxonomy of Madagascar's Chirping Giant Pill-Millipede

Symposium Presentation Abstract: The occurrence of island gigantism and dwarfism has not been extensively studied in invertebrates. On the island of Madagascar, both of these trends have been observed in the giant pill-millipede family Arthrosphaeridae (Sphaerotheriida).  In particular, the genus Zoosphaerium contains some gigantic species, while the genus Microsphaerotherium consists entirely of dwarfed forms. Within the endemic genus Sphaeromimus, only three species have been described and none are known to display gigantism or dwarfism. However, recent collections have discovered nine new populations of Sphaeromimus, whose taxonomic status was unclear. These findings include specimens from localities further north than previously known, from a cave, and from isolated, azonal rainforests, as well as the first gigantic Sphaeromimus specimensTo determine if gigantism in Arthrosphaeridae represents a plesiomorphy, or if it evolved independently in the different Malagasy genera, we constructed a molecular phylogeny. DNA of all Sphaeromimus populations, including all undescribed forms,  was extracted and fragments of the  CO1, 28S and 16s, as well as the entire 18S gene (>4000 bp in total) were sequenced. Our results indicate high microendemism within the genus, particularly in southeastern populations, with populations only 1 km apart belonging to distantly related species. Other southeastern Sphaeromimus species consist of a single population located in an extremely small area of distribution. Such sharp species boundaries are probably due to abrupt changes in habitat, including changes in soil type which suggests that speciation within the genus has occurred as a result of environmental change. Our phylogeny also reveals that our basal most species has the widest distribution and is found in the southwestern dry spiny forests, the only Sphaeromimus species adapted to such a habitat. Scanning electron microscope images taken of the antennae and endotergum of the various Sphaeromimus populations agree with our molecular data providing further support for our phylogeny. Finally, our results indicate that gigantism is an apomorphic character inside the genus Sphaeromimus and therefore has evolved multiple times on Madagascar.

Original Project Description: The giant pill-millipedes reaching the size of a baseball belong to Madagascar's most conspicuous invertebrates. Especially members of the genus Sphaeromimus (Latin for 'small ball') possess highly developed stridulation organs in both the male (the 'harp') and the female (the 'washboard') sex. Recent molecular and morphological phylogenies show that the members of this genus are closer related to pill millipedes from India than to other species from Madagascar. Nevertheless, they are poorly known. Recent inventory programs already lead to the discovery and description of one new genus and 36 new species. Now, around six new species of the genus Sphaeromimus were discovered, including one spectacular species which seems to display island gigantism (reaching a size larger than a golf ball). The aim of this study is to find out, if this gigantism is a recent development or an old trait inside the endemic genus Sphaeromimus.

Research methods and techniques: REU participants in this project will receive  training in general millipede morphology, and will handle and sort specimens from our and other museums' collections, image important morphological characters using light and scanning electron microscopy, collect data on the morphological differences of various species, as well as prepare descriptions of these species.