ROM Curator Defines New Order of Tapeworms

Paper describing study appears on cover of March 2009 issue of the International Journal for Parasitology

Collaborative research led by Royal Ontario Museum (ROM) curator Claire Healy has resulted in the establishment of a new tapeworm order, the Rhinebothriidea. The order, which includes five genera new to science, represents a significant step forward in terms of understanding the evolutionary interrelationships of tapeworms. An article proposing this new order, written by Healy and an international group of co-authors, will be featured on the cover of the March 2009 issue of the International Journal for Parasitology, the most highly cited journal covering original research in all aspects of parasitology. The issue is now available on-line.

“This study illuminates an important part of the backbone of the tapeworm tree of life,” said Claire Healy, ROM Associate Curator of Invertebrate Zoology. “It sets the stage for further research into the evolutionary relationships among tapeworms.”

The article, entitled Proposal for a new tapeworm order, Rhinebothriidea, began as part of a study aimed at describing comprehensively a group of tapeworms that parasitize batoid fishes (stingrays and their relatives). These study species were at the time classified within a subfamily of the Tetraphyllidea, an order that has lost credibility over the last two decades and is now viewed as a catch-all category for species that did not clearly fall within other orders. Healy and her colleagues obtained genetic sequences of 58 species of tapeworms, representing 30 genera in 5 tapeworm orders, the majority of these sequences having been obtained for the first time as part of this study. Analysis of these data resulted in an evolutionary tree that strongly supports a lineage of tapeworms, including the study species, that is distinct from the Tetraphyllidea. Using light and scanning electron microscopy, as well as histology, the research team found that the members of this lineage of tapeworms displayed physical characteristics that demonstrated a shared common ancestry. Specifically, the authors observed that the bothridia (muscular pads or cups that a tapeworm uses to grip the surface of the host’s intestine) in members of this lineage of tapeworms were borne on stalks, rather than being directly attached to the scolex (head part of body) proper. Based on this morphological feature, members of this lineage can be identified as belonging to the new order, the Rhinebothriidea, separate from the Tetraphyllidea.

The proposed new order includes 13 genera, five of which are new to science, having been discovered in the course of Healy and her colleagues’ research. More than 200 species, many of them undescribed, fall into this order with more being discovered as study continues. Future research on the Tetraphyllidea and its relatives may elucidate how these worms made the evolutionary transitions from marine to freshwater and terrestrial lifestyles.

Claire Healy joined the ROM in May 2006, after having received her MSc and PhD in Zoology from the University of Connecticut, U.S.A. She is Associate Curator of Invertebrate Zoology, and an Assistant Professor in the Department of Ecology & Evolutionary Biology at the University of Toronto.

Tapeworms (class Cestoda) are parasitic flatworms (phylum Platyhelminthes) that live, as adults, inside the digestive tract of their hosts, absorbing nutrients from their bodies. Adult tapeworms range in size from 0.5 millimetres to over 30 metres long, and they parasitize a variety of vertebrate animal hosts ranging from fish to mammals. Tapeworms are largely understudied, with thousands of new species awaiting discovery and description.

The authors on the paper are Claire J. Healy (ROM and University of Toronto), Janine N. Caira (University of Connecticut – Corresponding author), Kirsten Jensen (University of Kansas), Bonnie L. Webster (Natural History Museum, London), and D. Timothy J. Littlewood (Natural History Museum, London).