Superphylum: Platyzoa
Phylum: Gastrotricha
Phylogenic Classification
•Placement unclear
•They were first a class within the Aschelminthes
•Based on traditional morphology, they were considered close allies to Gnathostomulida, Rotifera & Nematoda.
•According to Nielson (2001), based on morphology emphasizing different structures, they form a clade with the Cycloneuralia
•Other studies focused on protonephridial ultrastructure associate Gastrotriches with Gnathostomulida.
•18S ribosomal DNA studies show them close to Platyhelminthes
•Todaro et al. (2006) concluded Gastroticha as a monophyletic group in Lophotrochozoa as part of a clade including Micrognathozoa, Rotifera and Cycliophora.
•No obvious consensus between morphological and molecular data
General Facts
•Gaster = Stomach, trix = hair
•~700 species
•Transparent
•80 μm to 3 mm in length
•Free living acoelomates
•Meiobenthic lifestyle
•Freshwater & Marine species
•One of the least understood groups of aquatic invertebrate animals
•Bilaterally symmetrical
•Have eutely
•Body is covered in cilia
•Motion by hydrostatics
•Live for about 3 days
•Have 2 terminal extensions with cement glands for adhesion purposes
Pysiology & Feeding
•Has more than 2 cell layers, tissues and organs
•No body cavity
•Have a complete gut with a subterminal anus
•Body covered in cuticle with many scales, spines or hooks
•Contain a nervous system with ganglia
•No circulatory system
•Reproduction by parthenogenesis & are hermaphroditic
•Using powerful sucking from the muscular pharynx to swallow their food, they prey on microalgae, fungi, bacteria & small protozoans
•Turbellarians and small macrofauna prey on Gastrotrichs
Ecology
•Are 3rd most abundant group in marine sediments, reaching a density of around 364 per cm²
•In freshwater ecosystems they are in the top five most abundant groups, usually found at densities around 158 individuals per cm²
•In aquatic environments the ecological role of the Gastrotrichs is as the microphagous, detritivorous, benthic community.
Annotated Bibliography
1. Todaro, M. A., Telford, M. J., Lockyer, A. E., Littlewood, D. T. J. 2006. Interrelationships of the Gastrotricha and their place among the Metazoa inferred from 18S rRNA genes. Zoologica Scripta 35: 25-259
In order to better understand the placement of the phylum Gastrotricha, small subunit (SSU) ribosomal DNA from 33 species of Gastrotriches were compared in an alignment of 50 metazoan taxa representing 26 phyla.
Gastrotricha were determined to be a monophyletic within Lophotrochozoa as part of a clade including Micrognathozoa, Rotifera and Cycliophora.
2.Clausen, C. 2004. Gastrotricha from the Faroe Bank. Sarsia 89: 423-458.
The purpose was basically to collect samples of Gastrotricha from the Faroe Bank (in the Norwegian sea), and describe them.
Sampling was carried out by a heavy triangular dredge, and with a meiofauna sledge. The samples were then brought to their lab and studied extensively. 20 species of Gastrotriches were recorded, along with six of them being new.
Detailed morphology descriptions of the species obtained were presented in the paper. The types of species found were compared with those found in other surrounding areas.
3. Petrov, N. B., Pegova, A. N., Manylov, O. G. , Vladychenskaya, N. S., Mugue, N. S., and Aleshin, V. V. 2007. Molecular Phylogeny of Gastrotricha on the Basis of a Comparison of the 18S rRNA Genes: Rejection of the Hypothesis of a Relationship between Gastrotricha and Nematoda. Molecular Biology 41: 445-452.
The purpose of this study was to compare 18S rRNA between species within Gastrotricha, and also across phyla to determine their phylogenic placement once and for all.
They used the same methods as Todaro et al. (alignment). They indeed showed that Gastrotriches are not closely related to Nematodes, however their placement differs from that of Halaynch.
They conclude that Gastrotrich should be closely associated with Gnathostomulida rather than Platyhelminthes.
Short clip on the early development of Gastrotrichs
Relevant links for Gastrotrich information:
Gastrotricha world portal - Information, pictures, videos, phylogeny etc.
Wikipedia Gastrotricha page
