Sense receptors in Lobatostoma manteri (Trematoda, Aspidogastrea)

Integrative approach for discovering of the new species within the genus Allocreadium Looss, 1900 (Trematoda: Allocreadiidae) and framing of biogeographical hypotheses for the genus

In July 2012 new Allocreadium species was isolated from Carassius gibelio caught in the Arsenyevka River, Primorsky region, Russia. Analyses on the morphometrics and internal organs' topology revealed that these worms are morphologically closest with A. isoporum but both species are independent according to high genetic distances based on the 28S gene fragment (5.434±0.0073%). Unlike A. isoporum found earlier in Europe, the new species named A. pseudoisoporum sp. nov. has a shorter body length and the vitellarium not reaching the posterior end of the body at some distance and its anterior border is on the level of the ventral sucker. Allocreadium pseudoisoporum sp. nov. differs from seven species previously found in the Russian Far East with the following features: smaller size of the body, suckers' ratio, range values, and topology of internal structures. Newly localities in the Pavlovka River and the Artyomovka River were discovered for A. khankaiensis. Morphological variability of the worms from the Pavlovka River was observed in comparison with A. khankaiensis from the Komissarovka River. Using scanning electron microscope, we examined external surfaces of three species (A. pseudoisoporum sp. nov., A. khankaiensis, A. hemibarbi) and observed structures reminiscent sensory receptors. This study was aimed to describe species diversity of allocreadiids inhabiting the south of Primorsky region, and to clarify phylogenetic relationships between the species from the genus Allocreadium Looss, 1900 using molecular genetic methods. The phylogenetic Bayesian tree based on the 28S gene showed a clear separation of ten Allocreadium species and confirmed the validity of A. pseudoisoporum sp. nov. Allocreadium pseudoisoporum sp. nov. is most similar to A. gotoi (genetic distances - 3.578±0.0051% in 28S, and 18.777±0.0149% in cox1), and represented the earliest divergent lineage in Allocreadium clade on the phylogenetic tree based on the 28S gene, thereby indicating its proximity to the ancestral node. Also, dichotomous keys for 25 Palearctic species of Allocreadium were prepared based on the morphology of the adult worms.

Ecology and biogeography of marine parasites

A review is given of (mainly recent) work on the biodiversity, ecology, biogeography and practical importance of marine parasites. Problems in estimating species numbers have been thoroughly discussed for free-living species, and the main points of these discussions are reviewed here. Even rough estimates of the richness of most parasite groups in the oceans are premature for the following reasons: species numbers of host groups, in particular in the deep sea and the meiofauna, are not known; most host groups have been examined only insufficiently for parasites or not at all; even in some of the best known groups, latitudinal, longitudinal and depth gradients in species richness are only poorly understood or not known at all; effects of hosts on parasite morphology and geographical variation have been studied only in a few cases; there are few studies using techniques of molecular biology to distinguish sibling species. Estimates of species richness in the best known groups, trematodes, monogeneans and copepods of marine fishes, are given. Parasites are found in almost all taxa of eukaryotes, but most parasitic species are concentrated in a few taxa. Important aspects of the ecology of marine parasites are discussed. It is emphasized that host specificity and host ranges should be distinguished, and an index that permits calculation of host specificity is discussed. The same index can be applied to measure site specificity. Central problems in ecology are the importance of interspecific competition and whether equilibrium or non-equilibrium conditions prevail. Marine parasites are among the few groups of organisms that have been extensively examined in this regard. A holistic approach, i.e. application of many methods, has unambiguously shown that metazoan ecto- (and probably endo-) parasites of marine fish live in largely non-saturated niche space under non-equilibrium conditions, i.e. they live in assemblages rather than in communities structured by competition. Nestedness occurs in such assemblages, but it can be explained by characteristics of the species themselves. There is little agreement on which other factors are involved in "structuring" parasite assemblages. Few studies on metapopulations of marine parasites have been made. A new approach, that of fuzzy chaos modelling, is discussed. It is likely that marine parasites are commonly found in metapopulations consisting of many subpopulations, and they are ideally suited to test the predictions of fuzzy chaos. Some recent studies on functional ecology and morphology--especially with regard to host, site and mate finding--are discussed, and attention is drawn to the amazing variety of sensory receptors in some marine parasites. Effects of parasites on hosts, and some studies on the evolution and speciation of marine parasites are discussed as well. A detailed overview of biogeographical studies is given, with respect to latitudinal gradients in species diversity, reproductive strategies and host ranges/specificity. Studies of marine parasites have contributed significantly to giving a non-equilibrium explanation for latitudinal diversity gradients. Recent studies on longitudinal and depth gradients are discussed, as well as parasites in brackish water, parasites as indicators of zoogeographical regions and barriers, and parasites as biological tags. The practical importance of marine parasites in mariculture, as monitors of pollution, agents of human disease, the use of parasites for controlling introduced marine pests, and some related aspects, are also discussed.

The origins of parasitism in the platyhelminthes

Symbiotic associations have arisen independently in several groups of the largely free-living turbellarians. Morphological adaptations of turbellarians to a symbiotic way of life include suckers and adhesive glands for attachment, elaborate systems of microvilli and other epidermal structures for absorption of food, glands for the formation of cysts, cocoons and cement material, and lack of a pharynx and intestine in some species. However, many species closely resemble their free-living relatives. Egg production is greatly increased at least in some species, and life cycles are always direct. Food of symbiotic turbellarians consists of host food and/or host tissue. Ectosymbiotes show fewer physiological adaptations than entosymbiotes. The major groups of parasitic Platyhelminthes (Trematoda Aspidogastrea, Trematoda Digenea, Monogenea, Udonellidea, Cestoda including Gyrocotylidea, Amphilinidea and Eucestoda), form one monophylum, the Neodermata, characterized by a neodermis (tegument) replacing the larval epidermis, epidermal cilia with a single horizontal rootlet, sensory receptors with electron-dense collars, spermatozoa with axonemes incorporated in the sperm body by proximodistal fusion, and protonephridial flame bulbs formed by two cells each contributing a row of longitudinal ribs to the filtration apparatus. The sister group of the Neodermata is unknown but is likely to be a large taxon including the Proseriata and some other turbellarian groups. Among the Neodermata, the Aspidogastrea is likely to be the most archaic group, as indicated by DNA studies, morphology, life cycles and physiology. Aspidogastreans can survive for many days or even weeks outside a host in simple media, they show little host specificity, and have an astonishingly complex nervous system and many types of sensory receptors, both in the larva and the adult. It is suggested that Aspidogastrea were originally parasites of molluscs (and possibly arthropods and other invertebrates) and that they are archaic forms which have remained at a stage where vertebrates represent facultative hosts or obligatory final hosts into which only the very last stages of the life cycle (maturation of the gonads) have been transferred. The complex life cycles of Digenea have evolved from the simple aspidogastrean ones by intercalation of multiplicative larval stages (sporocysts, rediae) in the mollusc host, and of cercarial stages ensuring dispersal to the now obligatory final host. Monogenea may have lost the molluscan host or evolved before the early neodermatans had acquired it. Cestoda either replaced the original molluscan with an arthropod host, retained an original arthropod host or evolved from an early neodermatan before molluscan hosts had been acquired, newly acquiring an arthropod host.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmission electron microscopy of sensory receptors of Echinostoma revolutum (Froelich 1802) cercaria (Digenea: Echinostomatidae)

Nonciliate, uniciliate, and multiciliate receptors were shown by transmission electron microscopy to occur in the body tegument of Echinostoma revolutum (Froelich 1802) cercariae. Detailed studies revealed three types of nonciliate, four types of uniciliate, and one type of multiciliate sensory endings. Most receptors were singly distributed. Only a few of them formed paired receptor complexes in three regions of the cercarial body. These receptor complexes were found to possess nonciliate, uniciliate, and multiciliate sensory endings.

Marine parasites: an Australian perspective

A review is given of major studies in marine parasitology in Australia. Aspects discussed include: geographical distribution of parasites in Australian coastal waters and their affinities to parasites of other zoogeographical regions; species diversity in Australian coastal surface and deep waters; use of marine parasites for stock discrimination; use of marine parasites as ecological models; ultrastructural and phylogenetic studies of marine parasites; and effects of marine parasites on their hosts.