Miscellaneous Invertebrates

Andreas Schmidt-Rhaesa

• Publisher: De Gruyter
• Year: 2018
• eBook ISBN: 9783110488418
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Entomology
• Index: Biological Sciences

This volume of the Handbook of Zoology summarizes "small" groups of animals across the animal kingdom. Dicyemida and Orthonectida are enigmatic parasites, formerly united as "Mesozoa" and their position among the multicellular animals is still not known with certainty. Placozoa are small, flat marine animals which provide important information on metazoan evolution. Comb jellies (Ctenophora) are esthetically fascinating animals which cause considerable discussion about their phylogenetic position. Seisonida are closely related to rotifers and acanthocephalans. Cycliophora were discovered and described as one of the last higher taxa and surprise by their complex life cycle. Kamptozoa (= Entoprocta) are small sessile animals in the sea and sometimes also in freshwater. Arrow worms (Chaetognatha) play an important role as predators in the plankton, but they also include benthic forms. Pterobranchia and acorn worms (Enteropneusta) belong to the deuterostomia and are related to echinoderms. In particular enteropneusts play an important role in understanding deuterostome evolution. These chapters provide up to date reviews of these exiting groups with reference to the important literature and therefore serves as an important source of information.

Hidetaka Furuya

1 Dicyemida

1.1 Introduction

Bodies of animals provide an excellent food and life space for parasites and symbionts. In cephalopods, excretion is carried out by the renal complex (renal and pancreatic appendages) (Fig. 1.1 A) and the branchial heart complex (branchial heart and pericardial appendage). These fluid-filled renal organs, “kidneys”, of cephalopods are unique habitats for the establishment and maintenance of parasites (Hochberg 1982). There are phylogenetically distant parasitic organisms, trematodes, dicyemids, and chromidinids, in the kidney of cephalopods (Nouvel 1945; Hochberg 1983, 1990; Furuya et al. 2004a). Dicyemids (phylum Dicyemida) are the most common parasites of the renal organs of benthic cephalopods, octopuses, and cuttlefishes (Nouvel 1947; McConnaughey 1951; Hochberg 1990; Short 1991; Furuya 1999). Occasionally, dicyemids are found in the pericardium of decapods (Hoffman 1965; Furuya 2007).

The body length of dicyemid species ranges from 0.1 to 10 mm. The body of vermiform stages consists of a central cylindrical cell called the axial cell and a single layer of 8 to 40 ciliated external cells called the peripheral cells (Fig. 1.1 C), which are the fewest in number of cells in metazoans except for aberrant myxozoans. The axial cell is a large cell, which extends to 100 mm in length in the largest dicyemid. This organization does not correspond to metazoan two-layered construction of endoderm and ectoderm, and dicyemids have neither body cavities nor differentiated organs. Van Beneden (1877) regarded the dicyemids as intermediate in the body plan between Protozoa and Metazoa and, thus, gave them the name Mesozoa. This phylum included several other microscopical enigmatic organisms, which were not assignable to any other phylum: Trichoplax, Haplozoon, Neresheimeria, Salinella, and orthonectids. Most of these organisms subsequently belonged to the other phyla (Hyman 1949). Only dicyemids and orthonectids were often united into a single phylum Mesozoa. Later Hochberg (1990) and Kozloff (1990) treated them independently as separate phyla, Dicyemida and Orthonectida. However, they were still treated as the Mesozoa in many zoological textbooks because of their unclear relationships to other animals.

Several zoologists regard the simple organization of dicyemids to be the result of specialization for parasitism (Nouvel 1947; Stunkard 1954; Ginetsinskaya 1988). However, Hyman (1949), Lapan and Morowitz (1975), and Ohama et al. (1984) concurred that dicyemids are primitive multicellular organisms. Because dicyemids have several protozoan-like features, an affinity to the protozoans has been pointed out (Czaker 2006; Noto & Endoh, 2004). Current analyses of molecular sequences have revealed that, rather than truly primitive animals that deserve the name “mesozoan”, they probably belong to the lophotrochozoans (Katayama et al. 1995; Kobayashi et al. 1999; Aruga et al. 2007; Suzuki et al. 2010; Mikhailov et al., 2016). Despite their extremely reduced body plan, dicyemids still appear to exhibit some degree of cell differentiation (Ogino et al. 2011). For this reason, the name Mesozoa is not suited for their phylogenetic place, and Dicyemida, which is the first name of dicyemids introduced by Krohn (1839), has been used as the phylum name since 1999 (Furuya 1999).

1.2 Life cycle

The life cycle of dicyemids consists of two phases of different body organization (Fig. 1.2). The first phase includes vermiform stages, in which the dicyemid exists as an asexually formed vermiform embryo, and as a final form, the nematogen or rhombogen. The second phase is the infusoriform embryo that develops from a fertilized egg. A high population density in the cephalopod kidney may cause the shift from an asexual mode to a sexual mode of reproduction (Lapan & Morowitz 1975). Vermiform stages are restricted to the renal sac of cephalopods, whereas the infusoriform embryos escape from the host into the sea to search for a new host. However, it is not clear how infusoriform larvae develop into vermiform stages in the new host.

Dicyemids have a high prevalence in their host cephalopods and are usually found to be heavily infecting the renal organs (Fig. 1.1 B). No damage has ever been observed in the infected renal tissue, so dicyemids apparently do no harm to their cephalopod hosts. Lapan (1975a) has even suggested that dicyemids facilitate host excretion of ammonia by contributing to acidification of the urine. In addition to the normal muscular contraction of the renal appendages, the ciliary activity of dicyemids present in the kidneys maintains a constant flow of urine, and as a result dicyemids assist in removal of urine. Thus, dicyemids are symbiotic, rather than parasitic, in their relationship with cephalopods.

Fig. 1.1: (A) Schematic diagram of renal organs of octopus. (B) Microphotograph of renal appendages with dicyemids. (C) Whole body of Dicyema japonicum. Bars represent 100 µm in panel B and 50 µm in panel C. AX, axial cell; CL, calotte; D, diapolar cell; G, gill; PA, parapolar cell; RA, renal appendage; RC, renal coelom; RO, renal opening, RS, renal sac; UP, uropolar cell; VC, vena cava.

1.3 Body organization

Vermiform stages, vermiform embryos, nematogens, and rhombogens, are similar in shape (Fig. 1.2). The body surface of dicyemids has numerous cilia and a folded structure, which is considered to contribute to absorb nutrients more efficiently from urine (Bresciani & Fenchel 1965; Ridley 1968; Furuya et al. 1997). The number of peripheral cells is species specific and constant. At the anterior region, 4 to 10 peripheral cells form the calotte, of which cilia are shorter and denser than in more posterior peripheral cells (Fig. 1.1 C). The calotte shape varies, depending on the species, and adapts to attach to the various regions of renal tissues in the host kidneys (Furuya et al. 2003a) (Fig. 1.3).

Infusoriform embryos are ovoid and have both an anteroposterior and a dorsoventral axis. Embryos mostly consist of 37 or 39 cells (Short 1971; Furuya 1999), which are more differentiated than those of vermiform stages (Matsubara & Dudley 1976; Furuya et al. 2004b). Internally, there are four large cells called urn cells, each c...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• List of contributing authors
• 1 Dicyemida
• 2 Orthonectida
• 3 Placozoa
• 4 Seisonidae
• 5 Cycliophora
• 6 Entoprocta (Kamptozoa)
• 7 Chaetognatha
• 8 Pterobranchia
• 9 Enteropneusta
• Index