Andreas Schmidt-Rhaesa

Broad phylogenomic sampling improves resolution of the animal tree of life.

Long-held ideas regarding the evolutionary relationships among animals have recently been upended by sometimes controversial hypotheses based largely on insights from molecular data. These new hypotheses include a clade of moulting animals (Ecdysozoa) and the close relationship of the lophophorates to molluscs and annelids (Lophotrochozoa). Many relationships remain disputed, including those that are required to polarize key features of character evolution, and support for deep nodes is often low. Phylogenomic approaches, which use data from many genes, have shown promise for resolving deep animal relationships, but are hindered by a lack of data from many important groups. Here we report a total of 39.9 Mb of expressed sequence tags from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data. Analysed in combination with existing sequences, our data reinforce several previously identified clades that split deeply in the animal tree (including Protostomia, Ecdysozoa and Lophotrochozoa), unambiguously resolve multiple long-standing issues for which there was strong conflicting support in earlier studies with less data (such as velvet worms rather than tardigrades as the sister group of arthropods), and provide molecular support for the monophyly of molluscs, a group long recognized by morphologists. In addition, we find strong support for several new hypotheses. These include a clade that unites annelids (including sipunculans and echiurans) with nemerteans, phoronids and brachiopods, molluscs as sister to that assemblage, and the placement of ctenophores as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. Many relationships between a stable subset of taxa find strong support, and a diminishing number of lineages remain recalcitrant to placement on the tree.

The position of the Arthropoda in the phylogenetic system

Traditionally, Panarthropoda (Euarthropoda, Onychophora, Tardigrada) are regarded as being closely related to Annelida in a taxon Articulata, but this is not supported by molecular analyses. Comparisons of gene sequences suggest that all molting taxa (Panarthropoda, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera) are related in a monophyletic taxon Ecdysozoa. An examination of the characters supporting Articulata reveals that only segmentation with a teloblastic segment formation and the existence of segmental coelomic cavities with nephridia support the Articulata, whereas all other characters are modified or reduced in the panarthropod lineage. Another set of characters is presented that supports the monophyly of Ecdysozoa: molting under influence of ecdysteroid hormones, loss of locomotory cilia, trilayered cuticle and the formation of the epicuticle from the tips of epidermal microvilli. Comparative morphology suggests Gastrotricha as the sister group of Ecdysozoa with the synapomorphies: triradiate muscular sucking pharynx and terminal mouth opening. Thus there are morphological characters that support Articulata, but molecular as well as morphological data advocate Ecdysozoa. Comparison of both hypotheses should prompt further thorough and targeted investigations. J. Morphol. 238:263–285, 1998.

The evolutionary relationships of rotifers and acanthocephalans

Advances in morphological and molecular studies of metazoan evolution have led to a better understanding of the relationships among Rotifera (Monogononta, Bdelloidea, Seisonidea) and Acanthocephala, and their relationships to other bilateral animals. The most accepted morphological analysis places Acanthocephala as a sister group to Rotifera, although other studies have placed Acanthocephala as a sister taxon to Bdellodea or Seisonidea. Molecular analyses using nuclear 18S rRNA and mitochondrial 16S rRNA genes support Acanthocephala as a sister taxon to Bdelloidea, although no molecular data is available for Seisonidea. Combining molecular and morphological analyses of Bilateria leads to a tree with Platyhelminthes, Rotifera, Acanthocephala and Gnathostomulida (and probably Gastrotricha) as a sister group to the annelid-mollusc lineage of the Spiralia (Lophotrochozoa).

Biology of the Phylum Nematomorpha

Compared with most animal phyla, the Nematomorpha, also known as hair worms, is a relatively understudied metazoan phylum. Although nematomorphs make up only 1 of 3 animal phyla specializing solely on a parasitic life style, little attention has been focused on this enigmatic group scientifically. The phylum contains two main groups. The nectonematids are parasites of marine invertebrates such as hermit crabs. The gordiids are parasites of terrestrial arthropods, such as mantids, beetles, and crickets. Members of both of these groups are free-living as adults in marine and freshwaters respectively. In recent years, large strides have been made to understand this group more fully. New information has come from collection efforts, new approaches in organismal biology, modern techniques in microscopy and molecular biology. This review will focus on the advances made in four main areas of research: (1) morphology, (2) taxonomy and systematics, (3) life cycle and ecology and (4) host behavioural alterations. Recent research focus on the structure of both nectonematids and gordiids has added new insights on the morphology of adult worms and juveniles. The nervous system of gordiids is now well described, including the documentation of sensory cells. In addition, the availability of material from the juvenile of several species of gordiids has made it possible to document the development of the parasitic stage. New collections and reinvestigations of museum specimens have allowed for a critical reevaluation of the validity of established genera and species. However, traditional taxonomic work on this group continues to be hampered by two impeding factors: first is the lack of species-specific characters; and second is the problem of intraspecific variation, which has likely led to the description of numerous synonyms. Modern molecular techniques have been used recently to support independently the broad relationships among gordiids. During the turn of the millennium, the study of the life cycle and general ecology of gordiids enjoyed a revival. The pivotal outcome of this research was the domestication of a common American gordiid species, Paragordius varius. This species was the first of this phylum to be laboratory-reared. Through this research, the life cycle of several distantly related gordiid species was investigated. Other work showed that gordiids persist in the environment in the cyst stage by moving through different hosts by paratenesis. These cysts have been shown to retain infectivity for up to a year. These factors have likely contributed to the finding that gordiid cysts are one of the most common metazoans in some aquatic environments. Finally, recent work has focused on elucidating the mechanism of how gordiids make the transition from terrestrially based definitive hosts to a free-living aquatic environment. It has been shown that hosts are manipulated by the parasites to enter water. Using this study system, and using histology and proteomic tools, the method of manipulation used by these parasites is being further investigated. This manipulation, and the reaction of the cricket to this manipulation, has been postulated to benefit both the parasite and the host. Although large strides have been made within the last 10 years in the understanding of nematomorphs, we make the case that a lot of basic information remains to be uncovered. Although seemingly a daunting task, the recent advances in information and techniques lay a solid foundation for the future study of this unique group of parasites.

Redescription and compilation of Nearctic freshwater Nematomorpha (Gordiida), with the description of two new species

Abstract We compiled information on freshwater Nematomorpha (horsehair worms) from North America including new records of more than 100 specimens from 66 new locations and the description of two new species. From the 30 species names that have been used for Nearctic nematomorphs, 15 are regarded as valid species. The descriptions of most of these species were supplemented with new material and documented by light and scanning electron microscopy. Three species, Gordius agassizi, Gordius fasciatus and Gordius hawaiiensis are synonymized with Gordius robustus. Gordius leidyi is not a valid species, but its assignment is uncertain. Gordius tenuis is not a nematomorph, but a mermithid nematode. Parachordodes densareolatus is a questionable species and might belong to Gordionus. New combinations are given for three species. The two new species described here raise the number of gordiids known from the Nearctic region to 17. A taxonomic key and a geographic list of species distributions are given in appendices. Species novae: Gordionus sinepilosus (British Columbia, Canada), Pseudochordodes texanus (Texas, U.S.A.).

Are the genera of Nematomorpha monophyletic taxa

Twenty‐two genera of horsehair worms (Nematomorpha) have been described to date, one (Nectonema) from the marine environment and the remaining ones (taxon Gordiida) from freshwater. The aim of this investigation was to test whether the genera represent monophyletic taxa or are likely to be paraphyletic assemblages. The recognition and delimitation of monophyletic taxa are prerequisites for further systematic analyses. From the 15 nematomorph genera containing more than one species, autapomorphies can be found for five genera (Chordodes, Nectonema, Beatogordius, Acutogordius, Noteochordodes). In three further genera (Paragordius, Parachordodes, Gordionus), the evaluation of characters as autapomorphies is likely, but relies on further investigations. For seven genera (‘Gordius’, ‘Paragordionus’, ‘Euchordodes’, ‘Neochordodes’, ‘Pseudochordodes’, ‘Spinochordodes’, ‘Digordius’), no autapomorphy could be found. Five genera (‘Digordius’, Lanochordodes, Progordius, Pantachordodes, Pseudogordius) are likely to be synonymous with other genera, and one genus (Chordodiolus) has been synonymized with Beatogordius recently. The importance of high quality documentation of characters in investigations of nematomorph species is stressed.

Horsehair Worms (Nematomorpha) as Parasites of Praying Mantids with a Discussion of their Life Cycle

Praying mantids (Mantoptera) are the most important hosts of horsehair worms (Nematomorpha) in tropical and subtropical regions worldwide. 82 reports of nematomorph-mantid relationships are reported, 11 of which are new to science. The majority of nematomorph species parasitizing mantids belongs to the genus Chor- dodes. The life cycle consists of a free-living stage in which copulation and early embryological and larval devel- opment take place and parasitic phases in an intermediate and a final host. The intermediate hosts, which are insects with aquatic larvae, serve as vectors between the aquatic and the terrestrial environment. Development can only be completed if the intermediate host is captured and eaten by the final host, praying mantids. Inside the host, the main development of the worms takes place, with a size increase from about 100 µm up to several centimeters. The factors influencing emergence from the hosts, which must be established close to water, remain not well understood.

The musculature of horsehair worm larvae (Gordius aquaticus, Paragordius varius, Nematomorpha): F-actin staining and reconstruction by cLSM and TEM

The musculature of larvae of Gordius aquaticus was investigated by laser-scanning microscopy and compared to transmission electron microscopic data for the larva of Paragordius varius. In the anterior portion of the body, the preseptum, four different muscle groups can be distinguished: (1) 12 anterior parietal muscles in the body wall, (2) six oblique muscles that function as retractors of the introvert, (3) six proboscideal muscles, which function as retractors for the proboscis, and (4) six muscles associated with spines of the outermost of the three rings of spines. The posterior portion of the body, the postseptum, possesses four pairs of longitudinal muscle strands in G. aquaticus, the postseptal parietal muscles, that are located dorsolaterally and ventrolaterally. These are not clearly visible in P. varius, where instead three pairs of parietal muscles are present. Additional small muscles are associated with the terminal spines and with the duct running from the pseudointestine to the body wall. All fibers show a cross-striated pattern although this striation is less obvious at the ends of the fibers.

Australian species of Chordodes (Nematomorpha) with a description of two new species, remarks on the genus Chordodes and its life history

Only seven species of freshwater Nematomorpha (Gordiida) are known from Australia, four of which belong to the genus Chordodes. This paper describes two new species: C. queenslandi and C. brevipilus. Chordodes queenslandi is characterized by a polymorphism of crowned areoles which appear as two types with distinct distribution patterns. Chordodes brevipilus is characterized by crowned areoles with very short apical filaments. One specimen previously determined as C. modiglianii (Camerano, 1892) belongs to C. queenslandi. Chordodes jandae Camerano, 1895 is redescribed according to the terminology introduced for C. queenslandi and C. brevipilus. Two further species, C. annulatus Linstow, 1906 and C. undulatus Linstow, 1906 do not belong to Chordodes. The newly reported specimens were collected emerging from praying mantids in the vicinity of water and escaped by contact of the hosts abdomen with water. The active movement of mantids towards water suggests an internal stimulus of the nematomorph parasite to its host.

Tardigrades — Are They Really Miniaturized Dwarfs?

Abstract Tardigrades are animals of small body size which is often regarded to be a secondary phenomenon. This interpretation makes sense in the traditional concept that tardigrades are closely related to Onychophora, Euarthropoda and Annelida. A large body size in the ancestor of this common taxon (Articulata) is probable. Small size and the absence of organs such as a dorsal heart, segmental coelomic cavities and metanephridia must then be interpreted as derived in tardigrades. However, when Cycloneuralia are taken as an outgroup instead of Annelida (taxon Ecdysozoa), an interpretation of small body size as a primary feature is plausible. This also accounts for the absence of heart, coelom and nephridia.The choice of outgroup influences hypotheses about sister-group relationships within Panarthropoda, with either Onychophora (Articulata-concept) or Tardigrada (Ecdysozoa-concept) being basal.