Amphipod intermediate host of Polymorphus minutus (Acanthocephala), parasite of water birds, with notes on ultrastructure of host-parasite interface

From November 1997 to June 1998, 3,118 specimens of Echinogammarus stammeri (Karaman, 1931) (Amphipoda) were collected from the River Brenta (Northern Italy) and examined for larval helminths. Larvae of Polymorphus minutus (Goeze, 1782) singly infected the hemocoel of 23 (0.74%) crustaceans; all these larvae were cystacanth stages. This is the first record of Polymorphus minutus in E. stammeri. Some cystacanths had their forebody and hindbody fully inverted. Parasites were bright orange in colour and each was surrounded by a thin acellular envelope. This envelope likely protects the developing parasite larva from cellular responses of the amphipod. Hemocytes were seen adherent to the outer surface of the envelope. The sex ratio among the parasitised E. stammeri was almost 1:1. All Polymorphus minutus larvae were central in the amphipod body, made intimate contact with host internal organs, and frequently induced a marked displacement of them. None of the infected females of E. stammeri carried eggs or juveniles in their brood pouch. In five hosts, Polymorphus minutus co-occurred with the cystacanth of another acanthocephalan, Pomphorhynchus laevis (Müller, 1776), a parasite of fish. Species of Polymorphus are parasites of the alimentary canal of aquatic birds which act as their definitive hosts (Nicholas and Hynes 1958). According to Amin (1992), this genus was divided in two subgenera, respectively, Polymorphus Lühe, 1911 with 36 species, and Profilicollis Meyer, 1931 with 10 species. The life cycle of a Polymorphus species was first worked out by Greeff (1864), working on P. minutus (see Nicholas and Hynes 1958). Later, Le Roux (1933) and Hynes (1955) provided data on the effects of this acanthocephalan on its intermediate host, freshwater amphipods. Larvae of helminths are known to have a variety of effects on arthropod intermediate host. Within the hemocoel of the arthropod host, larval acanthocephalans are encircled by an envelope or capsule. The origin of this envelope or capsule has been the subject of controversy among many researchers (see Wanson and Nickol 1973). The evidence from transplantation, cytochemical, and biochemical investigations suggests that the envelope protects the parasite from the arthropod immune response (Lackie and Holt 1988). This paper is the first record of an intermediate host for Polymorphus minutus in Italy. Herein we provide data on: parasite loads, infection rates, co-occurrence of P. minutus with Pomphorhynchus laevis larva (Acanthocephala) in the same host individual, and a description of the parasite larvae. Ultrastructural observations are also included. MATERIALS AND METHODS From November 1997 to June 1998, monthly samples of 310 to 543 specimens of Echinogammarus stammeri (Karaman, 1931) were collected from the River Brenta near Grantorto (Northern Italy, Province of Padua). Amphipods were sampled with a dip net (mesh size, 3 mm) in shallow waters near the bank, both from overhanging vegetation and below the waterline, then fixed in 6% formaldehyde for subsequent investigations. In the laboratory, the amphipods were placed in Petri dishes with lactic acid to clear the crustaceans, then measured and sexed. The number and position of parasites in the host hemocoel were noted and some infected crustaceans were dissected, and the parasites isolated and mounted in lactophenol for measurement and identification of larval stages. Twenty-three Polymorphus minutus larvae form the basis for the following description. In addition, in May 1998, seven adult specimens of an acanthocephalan were encountered in the intestines of two moribund wild ducks, Anas platyrhynchos, at Grantorto. On three occasions, some crustaceans were examined while alive, then processed for a host-parasite interface. Six E. stammeri infected with Polymorphus minutus and four uninfected ones were fixed in 2% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.2, postfixed in 1% osmium tetroxide in the same buffer for 1.5 h, dehydrated in a graded ethanol series and embedded in a Epon-Araldite mixture. Sections of 7μm in thickness were stained with azur A-methylene blue. Ultra-thin sections were stained with uranyl acetate and lead citrate and observed with a Zeiss EM9. Light micrographs were obtained using a Leitz photomicroscope. FOLIA PARASITOLOGICA 46: 117-122, 1999 Address for correspondence: B. S. Dezfuli, Dipartimento di Biologia, Università di Ferrara, Via Borsari, 46, 44100 Ferrara, Italy. Phone: ++39 532 291334; Fax: ++39 532 249761; E-mail: [email protected]