Paranucleospora theridion n. gen., n. sp. (Microsporidia,Enterocytozoonidae) with a Life Cycle in the Salmon Louse (Lepeophtheirus salmonis,Copepoda) and Atlantic Salmon (Salmo salar)

ABSTRACT. Paranucleospora theridion n. gen, n. sp., infecting both Atlantic salmon (Salmo salar) and its copepod parasite Lepeophtheirus salmonis is described. The microsporidian exhibits nuclei in diplokaryotic arrangement during all known life‐cycle stages in salmon, but only in the merogonal stages and early sporogonal stage in salmon lice. All developmental stages of P. theridion are in direct contact with the host cell cytoplasm or nucleoplasm. In salmon, two developmental cycles were observed, producing spores in the cytoplasm of phagocytes or epidermal cells (Cycle‐I) and in the nuclei of epidermal cells (Cycle‐II), respectively. Cycle‐I spores are small and thin walled with a short polar tube, and are believed to be autoinfective. The larger oval intranuclear Cycle‐II spores have a thick endospore and a longer polar tube, and are probably responsible for transmission from salmon to L. salmonis. Parasite development in the salmon louse occurs in several different cell types that may be extremely hypertrophied due to P. theridion proliferation. Diplokaryotic merogony precedes monokaryotic sporogony. The rounded spores produced are comparable to the intranuclear spores in the salmon in most aspects, and likely transmit the infection to salmon. Phylogenetic analysis of P. theridion partial rDNA sequences place the parasite in a position between Nucleospora salmonis and Enterocytozoon bieneusi. Based on characteristics of the morphology, unique development involving a vertebrate fish as well as a crustacean ectoparasite host, and the results of the phylogenetic analyses it is suggested that P. theridion should be given status as a new species in a new genus.     

Evolutionary aspects of the arthropod heart

Evolution has led to changes in the gross anatomy of the arthropod hearts. Changes are also seen in the ultrastructural organization of the cardiomyofiber. Thus the myofilament organization and the membrane systems (T-system and SR) vary within both Chelicerata, Crustacea and Uniramia. Yet, the variation is not haphazard, but constitutes a pattern which cannot be deduced from the gross anatomy. In the three taxa the evolutionary tendency seems to be towards a more strict sarcomeral organization of the myofilaments. This is due to parallelism. The organization of the membrane systems and the spatial relation of the interior couplings are not identical for all arthropods. However, no variations has so far been detected within one and the same order, despite differences in adaptation and size. These systems are conservative and it is suggested that they could be useful in studies of arthropod phylogeny.     

The crustacean heart ultrastructure and its bearing upon the position of the isopods in eumalacostracan phylogeny

The present paper discusses the use of myocardial ultrastructure, and its merits, in studies of crustacean phylogeny. It is argued that different modifications of the heart do not affect the membrane systems of the myofibers and that the membrane systems are independent of size and/or adaptation of the species. Finally, the phylogenetic implications of the membrane systems are considered. Using the myocardial membrane systems in addition to the cephalothorax (carapace), compound eyes, respiratory system and heart anatomy, a new phylogenetic arrangement of the larger eumalacostracan orders (Anaspidacea, Amphipoda, Cumacea, Decapoda, Euphausiacea, Isopoda, Mysidacea, Tanaidacea) is suggested. The isopods are regarded as a sister group to the other eumalacostracans.     

The Myocardial Ultrastructure of Hemilamprops rosea and Leucon nasica (Crustacea, Cumacea) and its Phylogenetic Implications

The heart wall of Hemilamprops rosea and Leucon nasica is innervated and consists of a single-layered epicardium and a single-layered myocardium. Their heart ultrastructure does not differ. Large lipid-containing cells are found in the heart lumen. The membrane systems conform to the typical eumalacostracan condition, with the isopods as the only exception. The transverse tubular system at the Z- and H-levels is connected by longitudinal tubules. The sarcoplasmic reticulum, which forms a fenestrated sheath around the myofibrils, is continuous across the Z-band and modified at the H-level only. The interior and peripheral couplings are located at the H-level. The posterior and anterior aorta lack contractile material. The aorta valves are bicuspid.