Infection byParorchis acanthus(Trematoda) decreases grazing by the keystone gastropod,Littoraria irrorata

Parasites are well-known to alter the behavior of their hosts, but there is still a paucity of knowledge about how parasites modify the behavior of many ecologically influential host species. I studied the keystone grazer, the salt marsh periwinkle(Littoraria irrorata),to determine the influence of infection by the digenetic trematode,Parorchis acanthus,on its grazing behavior. Comparative laboratory grazing studies of wild-collected and experimentally infected snails revealed thatParorchisdecreased grazing on liveSpartinaby more than 80%. Because of the large ecological influence ofLittorariain southern U.S. marshes, parasite modification of snail grazing may have ramifications for marsh ecosystem stability if parasite prevalence is sufficiently high.

Molecular Cytogenetics in Digenean Parasites: Linked and Unlinked Major and 5S rDNAs, B Chromosomes and Karyotype Diversification

Digenetic trematodes are the largest group of internal metazoan parasites, but their chromosomes are poorly studied. Although chromosome numbers and/or karyotypes are known for about 300 of the 18,000 described species, molecular cytogenetic knowledge is mostly limited to the mapping of telomeric sequences and/or of major rDNA clusters in 9 species. In this work we mapped major and 5S rDNA clusters and telomeric sequences in chromosomes of Bucephalus minimus, B. australis, Prosorhynchoides carvajali (Bucephaloidea), Monascus filiformis (Gymnophalloidea), Parorchis acanthus (Echinostomatoidea), Cryptocotyle lingua (Opisthorchioidea), Cercaria longicaudata, Monorchis parvus (Monorchioidea), Diphterostomum brusinae, and Bacciger bacciger (Microphalloidea). Whilst single major and minor rDNA clusters were mapped to different chromosome pairs in B. minimus and P. acanthus, overlapping signals were detected on a single chromosome pair in the remaining taxa. FISH experiments using major rDNA and telomeric probes clearly demonstrated the presence of highly stretched NORs in most of the digenean taxa analyzed. B chromosomes were detected in the B. bacciger samples hosted by Ruditapes decussatus. Although the cercariae specimens obtained from Donax trunculus, Tellina tenuis, and R. decussatus were in agreement with B. bacciger, their karyotypes showed striking morphological differences in agreement with the proposed assignation of these cercariae to different species of the genus Bacciger. Results are discussed in comparison with previous data on digenean chromosomes.

Factors influencing cercarial emergence and settlement in the digenean trematode Parorchis acanthus (Philophthalmidae)

The trematode Parorchis acanthus is widely distributed and exhibits a broad host spectrum, with larval stages occurring in marine gastropods and adult flukes parasitizing on various seabird species. Unlike the majority of digeneans, its cercariae do not invade a second intermediate host but encyst on a substrate in the open. In a series of laboratory experiments, we investigated factors influencing cercarial emergence and settlement of P. acanthus. Emergence of cercariae from the gastropod intermediate host Nucella lapillus increased with increasing temperature, whereas cercarial encystment success decreased. Numbers of emerged cercariae were highest in the presence of light and water. However, although in much lower numbers, cercariae were also released from N. lapillus and successfully encysted under relatively dry conditions. Cercariae were found to settle predominantly on filtering blue mussels Mytilus edulis, suggesting that cercarial concentration on the valves is largely facilitated by the bivalve's inhalation current. After being inhaled, cercariae actively emerged from the mussels using their suckers and subsequently encysted on the shells, preferably along the rim of the valves. Barnacle epibionts Semibalanus balanoides were observed to impair cercarial settlement on mussels, by disturbing or preying on cercariae. Whilst settlement success of cercariae on M. edulis increased with increasing mussel density, intensity of encysted metacercariae decreased. Furthermore, cercariae settled more efficiently on small than on large mussels. Our findings suggest that the transmission of P. acanthus cercariae is regulated by a range of abiotic and biotic factors, with filtering activity of mussels playing a key role by enhancing accumulation of cercariae on a substrate suitable for transmission to the bird final host.

Stictodora cablei n. sp. (Digenea: Heterophyidae) from the royal tern, Sterna maxima (Laridae: Sterninae) from Puerto Rico and the Brazos County area of the Texas Gulf coast, U.S.A., with a list of other endohelminths recovered in Texas

During a study of the endohelminths of wading birds from the Texas Gulf coast, 3 immature specimens of Stictodora (Heterophyidae) representing the same species that had previously been identified as Stictodora acanthotrema from the royal tern, Sterna maxima, in Puerto Rico by Raymond M. Cable, Robert S. Connor, and Jan W. Balling in 1960 were recovered from a royal tern, collected from the Bryan Utility Lake, Bryan, Texas. An additional 17 slides (14 whole mounts and 3 slides of sections) of this species that had been collected from this same bird host in Puerto Rico by Dr. Raymond M. Cable were examined from the holdings of the Harold W. Manter Laboratory of Parasitology, University of Nebraska, Lincoln, Nebraska. This species of Stictodora has the characteristic of the subgenus Galactosomoides and does not conform to the original description of Stictodora (=Acanthotrema) acanthotrema from the royal tern in Brazil, and it is therefore described as Stictodora cablei n. sp. The new species can be distinguished from all the other species in the genus by its unique acetabulogenital complex in which the acetabulum is highly modified with a small pad-like structure at its base surrounded by 3 papilliform arms extending sinistrally from it, whose outer edges fuse with the wall of the acetabulogenital sac, supporting the walls of the sac and the genital opening, and where the acetabulogenital sac contains a second, larger pad-like structure (the gonotyl described by Raymond M. Cable, Robert S. Conner, and Jan W. Balling in 1960) that extends from the left wall of the sac. Eleven other endohelminths were found in royal terns from Texas, U.S.A.: 3 cestodes, Angularella sp. (Dilepididae), an unknown genus and species of Dilepididae, and an unidentified immature cestode; 1 nematode, Contracaecum sp. (Anisakidae); and 7 trematodes, Cardiocephaloides brandesii (Strigeidae), Cercarioides cochleariformis (Heterophyidae), Mesostephanus fajardensis (Cyathocotylidae), Natterophthalmus andersoni (Philophthalmidae), Parorchis acanthus (Philophthalmidae), Stephanoprora conciliata (Echinostomatidae), and Stephanoprora denticulata (Echinostomatidae). Angularella sp., Contracaecum sp., M. fajardensis, S. conciliata, C. cochleariformis, and P. acanthus represent new host records. A checklist of parasites previously reported from the royal tern is also provided.

Las comunidades parasitarias como expresión de distinto comportamiento trófico en aves del Mar Argentino

El objetivo de este trabajo es caracterizar las comunidades de helmintos de dos especies de aves marinas con marcada diferencia en su comportamiento alimenticio, teniendo en cuenta la influencia que las tramas tróficas ejercen sobre la diversidad y estructuración de las comunidades parasitarias. Se escogió como modelo de especie estenofágica al pingüino de Magallanes (Spheniscus magellanicus) y de especie eurifágica a la gaviota cocinera (Larus dominicanus). Veintisiete pingüinos y veintinueve gaviotas fueron colectados en las costas de Península Valdés, provincia de Chubut, Argentina. Adicionalmente, fueron analizados cuatro pingüinos procedentes de la costa de la ciudad de Mar del Plata, provincia de Buenos Aires, Argentina. Spheniscus magellanicus albergó 6 especies de helmintos: 2 cestodes (Tetrabothrius lutzi y Tetrabothrius n. sp-Tetrabothriidae-), 1 digeneo (Cardiocephaloides physalis-Strigeidae-), 2 nematodes (Cosmocephalus obvelatus-Acuariidae- y Contracaecum pelagicum-Anisakidae-) y 1 acantocéfalo (Corynosoma sp.-Polymorphidae-). La fauna de helmintos de Larus dominicanus estuvo constituida por 19 especies: 3 cestodes (Tetrabothrius sp., -Tetrabothriidae- y 2 Dilepididae indeterminados); 10 digeneos (Himasthla escamosa y Stephanoprora sp.-Echinostomatidae-, Odhneria odhneri, Maritrema sp. 1 y Maritrema sp. 2, -Microphallidae-, Bartolius pierrei y un género indeterminado-Gymnophallidae-, Parorchis acanthus -Philophtalmidae-, Brachylecithum (Brachylecithum) sp.-Dicroceoelidae- y un Diplostomidae indeterminado); 4 nematodes (Cosmocephalus obvelatus y Paracuaria adunca-;Acuariidae-, larvas de Contracaecum sp.-Anisakidae- y un Capillaridae indeterminado) y 2 acantocéfalos (Corynosoma sp. y Profilicollis sp.-Polymorphidae-). El análisis a nivel infracomunitario reveló que las especies parásitas de S. magellanicus en Península Valdés presentan una localización específica a lo largo del tracto digestivo y que su distribución no se ve afectada por relaciones inter ni intraespecíficas. Solamente en el caso de Corynosoma sp. se observó cierto grado de competencia intraespecífica. La comunidad estuvo caracterizada por la presencia de una especie central (T. lutzi), tres secundarias (C. physalis, C. pelagicum, Corynosoma sp.) y una satélite (C. obvelatus). La similitud entre las infracomunidades fue muy alta debido a la existencia de un núcleo de especies que co-ocurren frecuentemente. En el caso de Larus dominicanus, se registraron relaciones inter e intraespecíficas que sugieren la competencia por un mismo recurso. La comunidad estuvo caracterizada por un alto porcentaje de especies secundarias y satélites, no asociadas positivamente, confiriendo escasa similitud a las infracomunidades. En la comunidad de S. magellanicus las especies más prevalentes y abundantes fueron las que presentaron mayor especificidad hospedatoria, respondiendo al modelo de "intercambio" observado generalmente en los peces. Por el contrario, las especies de Larus dominicanus presentaron características intermedias entre el modelo de intercambio y el modelo de "recurso amplio" característico de las aves, en donde las especies más prevalentes y abundantes son las menos específicas. Las características observadas en S. magellanicus (e. g. baja riqueza, escasas interacciones) definen comunidades del tipo "aislacionista", comunes en hospedadores ectotérmicos. La comunidad de helmintos de Larus dominicanus, tuvo características mixtas, sin embargo la mayoría de ellas definen comunidades del tipo "interactiva" (e. g. alta riqueza, relaciones inter e intraespecíficas), lo cual es común en animales endotérmicos. El hábito estenofágico del pingüino de Magallanes limita la incorporación de especies parásitas y favorece la co-ocurrencia entre ellas, mientras que el hábito eurifágico de la gaviota cocinera se refleja en una mayor riqueza específica. Los resultados obtenidos indican que la dieta determina la estructura de las comunidades de helmintos, y que las infracomunidades son predecibles sólo cuando los hospedadores poseen una dieta poco variada (estenofágica) y estable en el tiempo. Al comparar los resultados del presente trabajo con los obtenidos por otros autores se observa que la riqueza específica de L. dominicanus en el área de estudio es mayor a la reportada para otras localidades, lo cual sugiere que las gaviotas de Playa Fracasso poseen una dieta más natural y diversa que aquellas procedentes de ambientes influenciados por la presencia humana (e. g. basurales, zonas de descarte pesquero), áreas continentales u otras costas marinas. La composición de la fauna parasitaria de los pingüinos colectados en la costa de Mar del Plata sugiere que estos ejemplares no provienen de la misma colonia reproductiva que el resto de los individuos, y que probablemente procedan de poblaciones más australes en donde la disponibilidad de ítems presa es diferente. El estudio de las comunidades de helmintos permite conocer las variaciones en el comportamiento trófico de una misma especie hospedadora a nivel poblacional.