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A total of 863 specimens of Gammarus spp. was examined for helminth parasites. The amphipods were collected in the summers of 1988-1990 from 5 places along the shore of the south-east Baltic. Five species of parasites were found: metacercariae of Maritrema subdolum, Spelotrema papillorobustum and Levinseniella propinqua, cysticercoids of Microsomacanthus abortiva and cystacanths of Polymorphus contortus.
The tegument of the microphallid digenean Maritrema feliui, examined by means of TEM, is described as a syncytial epithelium organised into two layers. The outer layer is an external anucleate, cytoplasmic region connected to a second region composed of nucleate perikarya (cytons) deeply embedded in the surrounding cortical parenchyma. The surface layer of the tegument is covered by a plasma membrane with many deep invaginations, which are apparently pinocytotic. This layer also bears numerous large, electron-dense spines of two types, which are intracellular and attached to the basal plasma membrane. Its cytoplasm is rich in free ribosomes, contains numerous mitochondria, disc-shaped granules frequently arranged in a rouleau, and several large, moderately electron-dense, membranous bodies. The subtegumentary perikarya and their nuclei, which are both flattened, are described in detail, as are their connections with the surface tegument. These perikarya appear to be the source of the disc-shaped granules and some of the other inclusions present in the surface layer. The main characteristics of the tegumental structure of M. feliui are commented upon in relation to the findings of previous publications and their suggested functions.
Maritrema formicae sp. nov. is described from the Patagonian coast, Argentina, based on adults obtained from the kelp gull, Larus dominicanus. The new species fits with the “eroliae complex” and can be distinguished from other related species mainly in shape and size of body, shape, size, and pattern of distribution of cirrus spines, uterus extension, number and size of eggs, vitellarium in a complete ring in all specimens, and its Neotropical distribution. The new species is sympatric with another species of the genus, Maritrema madrynense, which was recorded in the same host and locality.
Maritrema portucalensis sp. nov. (Digenea, Microphallidae) from Aveiro estuary, northern Portugal, is described on the basis of excysted metacercariae obtained from the gill lamellae of the shore crab Carcinus maenas (L.). M. portucalensis sp. nov. most closely resembles Maritrema subdolum Jägerskiöld, 1909, a species that also frequently uses C. maenas as second intermediate host. The new species differs from M. subdolum in having a narrower body at the level of testes and a smaller ovary and testes. Also, the digestive caeca presented various folds along their length, thickening in the terminal region. Moreover, the body surface was fully covered by tegumental spines provided with several teeth. To complement our morphological characterization and to identify M. portucalensis metacercariae, the ITS1 rDNA region of the cysts isolated from C. maenas was sequenced and compared with the corresponding available sequences of digenean trematodes belonging to the family Microphallidae. Alignments revealed 28 base-pair differences between the query-ITS1 and that of M. subdolum, and more importantly a 100% similarity with the sequence of Microphallidae sp. no. 15 cercaria from the snail Hydrobia ulvae. The last observation allowed us to establish an unequivocal association between cercaria known as Microphallidae sp. no. 15, and the metacercaria of M. portucalensis sp. nov. characterized in this work, contributing decisively to the clarification of its life cycle.
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