Two dimensions of biodiversity research exemplified by nematomorpha and gastrotricha

Integrative and Comparative Biology, Jun 2002 by Schmidt-Rhaesa, Andreas

Two Dimensions of Biodiversity Research Exemplified by Nematomorpha and Gastrotricha1

SYNOPSIS. Biodiversity research combines two dimensions, the horizontal one that contains species diversity, patterns among this diversity and its interconnections and the vertical one that deals with the history of biodiversity, ie., its phylogeny. With these tight interconnections, the importance of so-called "lesser known groups" such as Nematomorpha and Gastrotricha can be shown. Two examples are the life cycle of Nematomorpha and the phylogenetic position of Gastrotricha. The life cycle of Nematomorpha is only partially known and almost no conclusions can be made about the impact of Nematomorpha on their hosts. For the phylogenetic position of Gastrotricha, alternative hypotheses are available, mainly due to different results of morphological and molecular (18S rDNA) analyses. It is demonstrated how these different hypotheses influence character interpretation and reconstruction among Protostomia (Gastroneuralia).

INTRODUCTION

Besides direct importance to our human species such as medical or economic uses, the inherent value of nature itself has been increasingly realized. This is summarized in the keyword biodiversity. We recognize a diversity of living organisms in nature as the product of a historical process and whose parts interconnect with each other and with their environment. Knowledge of this diversity, its history and interconnections is essential in understanding and anticipating the effects of disturbances in this system such as habitat destruction or the exchange of organisms between different localities.

This leads to a concept for biodiversity research. Biodiversity is characterized by two dimensions: the horizontal and the vertical one (Fig. 1). In the center of the horizontal dimension is the recognition of primary units, usually species. This includes theoretical (such as species concepts) and practical aspects (such as recognizing polymorphic and sibling species).

Among the diversity of species, patterns may be recognized, such as biogeographical patterns or patterns of morphological, genetical, ecological and other similarities. Finally, interconnections between and within species as well as connections to the environment have to be understood. Current biodiversity is the result of a historical process and all extant species are connected phylogenetically through time. This is the topic of the vertical dimension of biodiversity research which covers phylogenetic relationships and other evolutionary research such as paleontology and population genetics.

To illustrate this approach to biodiversity, two examples are given from small, understudied taxa. These are the horsehair worms or Nematomorpha and the Gastrotricha. As parasites during one phase of their life cycle, nematomorphs have an impact on their hosts.

Can we estimate this impact? To answer this question it is necessary to review how much we know about the life cycle of nematomorphs. This is used as an example of the horizontal dimension of biodiversity research, because the nematomorph life cycle is a good example for parasite-host relationships and therefore for connections of one taxon with another. Gastrotrichs are likely to be very basal bilaterian animals. As such, they have an impact on the reconstruction of the bilaterian ancestor and on the interpretation of character evolution within Bilateria. To illustrate the vertical dimension of biodiversity research, I will review what we know about the position of Gastrotricha within the Bilateria. Although both Nematomorpha and Gastrotricha are "lesser known groups," it will be shown that detailed knowledge is important in understanding more general biodiversity patterns and processes.

THE LIFE CYCLE OF NEMATOMORPHA

The Nematomorpha is a monophyletic taxon of parasitic worms that develop in their hosts but reproduce in aquatic environments. About 300 species have been described from freshwater (taxon Gordiida, Fig. 2A) while only five species are marine (genus Nectonema).

Nematomorphs are dioecious, they reproduce with true copulation (Nectonema) or pseudocopulation (Gordiida) (Schmidt-Rhaesa, 1999). From eggs, tiny larvae (about 100 um) hatch (Fig. 2C). They are equipped with two or three rings of cuticular hooks and terminal stylets with which they are able to penetrate soft epithelia of their hosts.

Knowledge about the nematomorph life cycle is crucial for a number of topics such as their impact on host populations, but still there are numerous open questions. In spite of the fact that the nematomorph life cycle is similar to that of mermithid nematodes, details of the nematomorph life cycle are distinctly less well known. One reason for the advanced study of mermithids is that some species parasitize mosquitoes and have been tested as biological control agents against mosquito-transferred diseases such as malaria (Platzer, 1981; Yu, 1986; Gafurov et al., 1987; Rojas et al., 1987).

Observations concerning the life cycle of Nematomorpha are more or less sporadic and few systematic approaches have been made. There are numerous host reports for larval and juvenile nematomorphs (the term "juvenile" is used here for wormlike stages in the parasitic phase, the term "larva" only for the morphologically distinct earliest stage of development). Early reports, such as the lists by Siebold (1842) seem to be of limited reliability, because at that time gordiids and mermithids were often included into one taxon and not further distinguished. Hosts reported for gordiid larvae are quite different from hosts of juveniles: while almost any aquatic animal (e.g., insect larvae, molluscs, annelids, fishes, frogs) may contain encysted larvae, emergence occurs mostly in insects and less often in other arthropods or (in two cases) in leeches (see Schmidt-Rhaesa, 1997, 2001a). The fascinating aspect is that gordiids copulate and develop in water but parasitize almost entirely terrestrial hosts. In the following, if results are not explicitly assigned to Nectonema, they refer to gordiids.