Phylum Platyhelminthes
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Author's personal copy Chapter 10 Phylum Platyhelminthes Carolina Noreña Departamento Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain Cristina Damborenea and Francisco Brusa División Zoología Invertebrados, Museo de La Plata, La Plata, Argentina Chapter Outline Introduction 181 Digestive Tract 192 General Systematic 181 Oral (Mouth Opening) 192 Phylogenetic Relationships 184 Intestine 193 Distribution and Diversity 184 Pharynx 193 Geographical Distribution 184 Osmoregulatory and Excretory Systems 194 Species Diversity and Abundance 186 Reproductive System and Development 194 General Biology 186 Reproductive Organs and Gametes 194 Body Wall, Epidermis, and Sensory Structures 186 Reproductive Types 196 External Epithelial, Basal Membrane, and Cell Development 196 Connections 186 General Ecology and Behavior 197 Cilia 187 Habitat Selection 197 Other Epidermal Structures 188 Food Web Role in the Ecosystem 197 Musculature 188 Ectosymbiosis 198 Parenchyma 188 Physiological Constraints 199 Organization and Structure of the Parenchyma 188 Collecting, Culturing, and Specimen Preparation 199 Cell Types and Musculature of the Parenchyma 189 Collecting 199 Functions of the Parenchyma 190 Culturing 200 Regeneration 190 Specimen Preparation 200 Neural System 191 Acknowledgment 200 Central Nervous System 191 References 200 Sensory Elements 192 INTRODUCTION by a peripheral syncytium with cytoplasmic elongations. Monogenea are normally ectoparasitic on aquatic verte- General Systematic brates, such as fishes, whereas trematodes and cestodes are The Platyhelminthes historically contain organisms grouped exclusively endoparasitic, generally in vertebrates. in three major clades, Acoelomorpha, Catenulida, and Rhab- The Platyhelminthes, both parasitic and free-living taxa, ditophora, which include the Neodermata (Figure 10.1). share a set of characters: they are nonsegmented acolomates The Neodermata (Ehlers, 1985) comprises the group of the without an anus, which lack respiratory and circulatory sys- parasitic Platyhelminthes: Monogenea (Monopisthocotylea tems but possess an excretory system. In addition, all are and Polyopisthocotylea), Trematoda (Aspidogastrea and hermaphroditic and asexual (mainly paratomy) and sexual Digenea), and Cestoda (Amphilinidea, Gyrocotylidea and reproduction are present. Eucestoda) (Littelwood et al., 1999). They are character- The classical characteristics defining the phylum Platy- ized by the presence of a neodermis (hence the name of this helminthes are: bilateral symmetry, multiciliated epidermis, parasitic clade), which is a specialized epidermis formed absence of an accessory centriolus at the ciliary roots, more Thorp and Covich’s Freshwater Invertebrates. http://dx.doi.org/10.1016/B978-0-12-385026-3.00010-3 Copyright © 2015 Elsevier Inc. All rights reserved. 181 Ecology and General Biology, Fourth Edition, 2015, 181-203 Author's personal copy 182 SECTION | III Protozoa to Tardigrada FIGURE 10.1 A diagram showing the Turbellaria groups and their position among the Platyhelminthes. Based on Tyler et al. (2006–2012). Taxa in bold represent traditionally considered orders. than one cilia in the terminal cell of the protonephridia, Fecampiida/Urastomidae (symbiotic taxa) or from Neoophora stem cells as differentiation cells, presence of a male porus (Rhabdocoela + Prolecithophora) (Littlewood et al., 1999). and male copulatory organ, and dorsoventral and anteropos- Within the free-living Platyhelminthes, i.e., the “Turbel- terior polarity (Ehlers, 1985; Ax, 1995). These are shared laria,” apomorphies, such as the neodermis of the parasitic by all the groups of this phylum, with the exception of the classes, are not present. Therefore, this taxon is considered multiciliated epidermis. This character, only present during paraphyletic (Ehlers, 1985) or polyphyletic (Smith et al., larval stages in Neodermata, becomes lost at adult stages 1986), and hence its name is often written between quotation with the development of the neodermal syncytium. marks (usually dispensed with later in this manuscript for con- The transition from a free-living to a parasitic life form venience). Nevertheless, the unique combination of several involved a series of adaptations, which generated patent characters—mainly based on characteristics of the body wall differences between the parasitic and free-living taxa of and the position and function of stem cells—produce a group- Platyhelminthes. Among several morphological and physi- ing of the free-living species (including Acoelomorpha) within ological changes, the formation of the neodermis with peri- the Turbellaria (Tyler and Hooge, 2004; Egger et al., 2007). karya under the surface that effectively protects the external Based on their simple body structure (“Bauplan”), Tur- body layer from abrasions or immune reactions of the bellaria has been considered one of the most basal bilateria host is particularly noteworthy. Epidermal ciliary rootlets (Littlewood and Bray, 2001). This hypothesis is supported are also a striking feature that may be useful for anchor- by recent molecular phylogenetic analyses (Ruiz-Trillo ing cilia in the epidermis in free-living turbellarians, which et al., 1999, 2004; Baguñà and Riutort, 2004a,b). Neverthe- retain the ciliated epidermis throughout their life. However, less, “Turbellaria” has traditionally been considered a class rootlets may not be necessary in neodermatan larvae that consisting of a heterogeneous group of orders. They are lose their cilia following infection. The neodermis also has found in all aquatic environments, show a worldwide distri- an important role in nutrient acquisition by increasing the bution and can be found in the interstitium of marine, brack- exchange surface area (see Halton, 1997; Tyler and Tyler, ish, and freshwater environments, such as gliding on coral 1997; Littlewood et al., 1999). reefs, in the water column, or between the roots and leaves From an evolutionary point of view, the parasitic of aquatic plants. Most turbellarians are ∼1 mm in size, but classes arose from a primitive free-living flatworm, some orders like Tricladida and Polycladida (the so-called specifically from Rhabdocoelida (Rieger et al., 1991), or macroturbellaria) can reach body sizes of 0.2–10 cm. Ecology and General Biology, Fourth Edition, 2015, 181-203 Author's personal copy Chapter | 10 Phylum Platyhelminthes 183 The body shape is variable, spindle-like, vermiform or and the intestine is a blind sac without an anus. Excretion leaf shaped covered with the characteristic multiciliated epi- occurs through the protonephridial system. Turbellarians thelium. Sensory cells (e.g., eyes, ciliated pits, and rhabdite are hermaphrodites. The most conspicuous reproductive rods) and the central nervous system are concentrated in the structures are the different parts of the male copulatory anterior part, whereas the reproduction organs are located organ that form stylets of different shapes and complexity. in the posterior part. The position of the pharynx, whether The female gonads are paired or unpaired, and the eggs are of the simplex, plicatus, or rosulatus types, varies along the endolecithal or ectolecithal (for a detailed description orga- main body axis. The oral pore is always ventrally situated, nization of Turbellaria, see Figure 10.2). FIGURE 10.2 Schematic representation of the different orders of Turbellaria: (a) Catenulida, Catenulidae (e.g., Catenula lemnae) reproducing asexu- ally, approximately 1–2 mm length; (b) Catenulida, Stenostomidae (e.g., Stenostomun sp.), 1 mm length; (c) Macrostomida, Macrostomidae (e.g., Macrostomum sp.), 1 mm; (d) “Lecithoepitheliata,” Prorhynchidae (Prorhynchus stagnalis), ∼5 mm; (e) Rhabdocoela, “Dalyellioida,” Dalyelliidae (e.g., Gieysztoria rubra), ∼2 mm; (f) Rhabdocoela, Temnocephalida, Temnocephalidae (e.g., Temnocephala sp.), 10 mm; (g) Rhabdocoela, “Typhloplanoida,” Mesostomidae (e.g., Mesostoma sp.), ∼5 mm; (h) Rhabdocoela, Kalyptorhynchia, Polycystididae (e.g., Gyratrix hermaphroditus), 2 mm; (i) Bothrioplanida, Bothrioplanidae (Bothrioplana semperi), 5 mm; and (j) Tricladida, Continenticola, Dugesiidae (e.g., Dugesia sp.), 10–30 mm. Abbreviations: b, brain; cp, ciliated pits; eg, eggs; ey, eyes; i, intestine; m, mouth; o, ovary; oc, oocyte; ph, pharynx; p, proboscis; pr, protonephridial duct; r, rhabdite tracks; sc, statocyst; st, stylet; su, sucker; sv, seminal vesicle; t, testes; v, vitellaria; vc, vitellocyte; ♂, male pore; ♀, female gonopore; ♂ ♀, hermaphrodite gonopore. Ecology and General Biology, Fourth Edition, 2015, 181-203 Author's personal copy 184 SECTION | III Protozoa to Tardigrada Phylogenetic Relationships known with more than 800 described species (Noreña, 2012). The faunas for part of the USA (150 species; Kolasa The nature of the available morphological characters and and Tyler, 2010) and South America, mainly for Brazil (over the relative overall simplicity of body structures make it 100 species in South America; Noreña et al., 2003; Tyler difficult to resolve the phylogenetic relationship among et al., 2006–2012) are also known. Within the Neotropical the “Turbellaria” and to establish their relationship to other Region, the turbellarians of southeastern Brazil (mainly in groups of invertebrates, such as Eutrochozoa or Gnathosto- the state of São Paulo) are the best known. From this region, mulida. Recently, the systematic position of Turbellaria has representatives of the orders Prolecithophora, Lecithoepi- come under dispute