Ultrastructure of the tegument of the cestode Paraechinophallus japonicus (Bothriocephalidea: Echinophallidae), a parasite of the bathypelagic fish Psenopsis anomala

Abstract. The ultrastructure of the tegument in Paraechinophallus japonicus (Bothriocephalidea: Echinophallidae), a cestode parasite of the bathypelagic fish Psenopsis anomala , was studied using scanning and transmission electron microscopy. Paraechinophallus japonicus lacks a true scolex. Four different types of microtriches have been observed on the tegumental surface of P. japonicus. Capilliform (∼2.3-μm long) and blade-like spiniform (∼1.4-μm long) microtriches are intermingled on the surface of the pseudoscolex. Capilliform microtriches are distinct in possessing a short base and a long electron-lucent cap. The strobila is covered with two types of microtriches, namely filiform (∼2.1-μm long) and tusk-shaped microtriches (≤4.5-μm long). Tusk-shaped microtriches are limited to the posterior border of each proglottid and are characterized by a short and narrow base, and a large and wide, sharply pointed, electron-dense cap. Similar tusk-shaped microtriches were previously found in members of the family Echinophallidae and may represent an autapomorphy of echinophallid cestodes, all of them being parasitic in centrolophid fish. A unified terminology of microthrix parts is proposed.     

Functional ultrastructure of the hexacanth larvae in the bothriocephalidean cestode Eubothrium salvelini (Schrank, 1790) and its phylogenetic implications

Functional ultrastructure and its phylogenic implications in the bothriocephalid cestode Eubothrium salvelini (Schrank, 1790) are described and discussed. The infective hexacanth shows bilateral symmetry in cellular organization. The mature hexacanth is armed with three pairs of oncospheral hooks of a heterogeneous electron density. It is covered by a thin layer of the oncospheral tegument, possessing characteristic bubble-like processes at the surface. Within the infective hexacanth larva five cell types were distinguished: (1) a binucleated subtegumental cell; (2) the U-shaped, tetranucleated penetration gland; (3) two nerve cells; (4) three types of somatic cells represented by: i) myocytons of both somatic and hook musculature, ii) numerous degenerating micromeres with pycnotic nuclei and iii) a new oncospheral cell type, the interstitial cell, that has never been observed in any other hexacanth; and (5) large germinative cells with characteristic prominent nucleoli in their large spherical nuclei. Functions of all the cell types are described on the basis of the obtained ultrastructural characteristics and previously published reports. The mode of the penetration gland secretion is classified as apocrine. Flame cells have never been observed within the hexacanth of E. salvelini. The results of the present study, comparing the functional aspects of the ultrastructure of the hexacanths of E. salvelini with literature data on the oncospheres of other bothriocephallideans and diphyllobothriideans, suggest potential phylogenetic and evolutionary criteria for determining relationships among these groups of tapeworms.     

Early intrauterine embryonic development of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost Merluccius merluccius (L., 1758) (Gadiformes: Merlucciidae)

The early intrauterine embryonic development of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost Merluccius merluccius (L., 1758), was studied by means of light (LM) and transmission electron microscopy (TEM). Contrary to the generic diagnosis given in the CABI Keys to the cestode parasites of vertebrates, the eggs of C. crassiceps, the type of species of Clestobothrium Lühe, 1899, are operculate and embryonated. Our LM and TEM results provide direct evidence that an operculum is present and that the eggs exhibit various stages of intrauterine embryonic development, and in fact represent a good example of early ovoviviparity. The intrauterine eggs of this species are polylecithal and contain numerous vitellocytes, generally ∼ 30, which are pushed to the periphery and remain close to the eggshell, whereas the dividing zygote and later the early embryo remain in the egg centre. During early intrauterine embryonic development, several cleavage divisions take place, which result in the formation of three types of blastomeres, i.e. macro-, meso- and micromeres. These can be readily differentiated at the TEM level, not only by their size, but also by the ultrastructural characteristics of their nuclei and cytoplasmic organelles. The total number of blastomeres in these early embryos, enclosed within the electron-dense eggshells, can be up to ∼ 20 cells of various sizes and characteristics. Mitotic divisions of early blastomeres were frequently observed at both LM and TEM levels. Simultaneously with the mitotic cleavage divisions leading to blastomere multiplication and their rapid differentiation, there is also a deterioration of some blastomeres, mainly micromeres. A similar degeneration of vitellocytes begins even earlier. Both processes show a progressive degeneration of both vitellocytes and micromeres, and are good examples of apoptosis, a process that provides nutritive substances, including lipids, for the developing embryo.     

Ultrastructural study of the male gamete of Glossobothrium sp. (Cestoda: Bothriocephalidea: Triaenophoridae) a parasite of Schedophilus velaini (Perciformes: Centrolophidae) in Senegal

This paper describes the ultrastructure of the male gamete of Glossobothrium sp. (Bothriocephalidea: Triaenophoridae). The mature spermatozoon of Glossobothrium sp. is filiform and possesses two axonemes, a single helicoidal crested body, a parallel nucleus, parallel cortical microtubules and granules of glycogen. In Glossobothrium sp. we describe for first time a 200-250 nm thick crest-like body in the Bothriocephalidean. The anterior part of the spermatozoon exhibits a ring of 27 electron-dense cortical microtubules encircling the first axoneme. This structure persists until the appearance of the second axoneme. When the ring of electron-dense cortical microtubules disappears, the spermatozoon exhibits two bundles of thin cortical microtubules. The posterior part of the spermatozoon contains the posterior extremity of the second axoneme, the posterior extremity of the nucleus and few cortical microtubules. Soon nucleus disappears and the axoneme is disorganized. Thus the posterior extremity of the spermatozoon of Glossobothrium sp. exhibits only singlets produced by the disorganization of the doublets of the second axoneme and few cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in the Triaenophoridae.     

Suppression of the tapeworm order Pseudophyllidea (Platyhelminthes: Eucestoda) and the proposal of two new orders, Bothriocephalidea and Diphyllobothriidea

Pseudophyllidea van Beneden in Carus, 1863, a well recognised order of tapeworms (Platyhelminthes: Eucestoda), is suppressed because it is composed of two phylogenetically unrelated groups, for which the new names Bothriocephalidea and Diphyllobothriidea are proposed. The new orders differ from each other in the following characters: (i) position of the genital pore: on the dorsal, dorso-lateral or lateral aspects and posterior to the ventral uterine pore in the Bothriocephalidea versus on the ventral aspect of segments and anterior to the uterine pore in the Diphyllobothriidea; (ii) the presence of a muscular external seminal vesicle in the Diphyllobothriidea, which is absent in the Bothriocephalidea; (iii) the presence of a uterine sac in the Bothriocephalidea, which is absent in the Diphyllobothriidea; and (iv) the spectrum of definitive hosts: mainly teleost fishes, never homoiothermic vertebrates in the Bothriocephalidea, versus tetrapods, most frequently mammals, in the Diphyllobothriidea, with species of Diphyllobothrium, Spirometra and Diplogonoporus parasitic in humans. The Diphyllobothriidea, which includes 17 genera in four families (Digramma is synonymised with Ligula), is associated with cestode groups that have a range of plesiomorphic characters (Haplobothriidea and Caryophyllidea), whereas the Bothriocephalidea, consisting of 41 genera grouped in four families, is the sister-group to the 'acetabulate' or 'tetrafossate' cestodes, which are generally regarded as having derived characters.