Field and laboratory studies of Ophryoglena sp. (Ciliata: Ophryoglenidae) infection in zebra mussels,Dreissena polymorpha (Bivalvia: Dreissenidae)

This study, conducted in the Dnieper-Bug Canal in Belarus, is the first to monitor the seasonal (June-November) dynamics of infection with the parasitic ciliate Ophryoglena sp. in a zebra mussel (Dreissena polymorpha) population. Mean population prevalence and intensity of infection varied, respectively, from 11 to 62% and from 0.9 to 24.1 ciliates/mussel. Mean prevalence was highly correlated with mussel length in mussels <20 mm (R(2)=0.97) and was lower in larger mussels. Mean infection intensity in mussels 1-25 mm long was similarly correlated with their size (R=0.98), reached a maximum in the 20-25 mm size-class, and then sharply decreased, thus providing evidence, albeit limited, that high intensity of infection might be lethal. Transinfection of zebra mussels by Ophryoglena sp. was achieved in the laboratory-a first for a protozoan parasite of D. polymorpha; from an initial complete lack of infection, mean prevalence and intensity rose, respectively, to 86.7% and 8.3 ciliates/mussel.     

Aggregata bathytherma sp. nov. (Apicomplexa:Aggregatidae), a new coccidian parasite associated with a deep-sea hydrothermal vent octopus

Aggregata bathytherma sp. nov. is described from the digestive tract of Vulcanoctopus hydrothermalis, a deep-sea octopus recently discovered associated with hydrothermal vents in the northeast Pacific Ocean. Oocysts typically are spherical in shape, sometimes irregular, 163 to 356 microm in length, and 219 to 313 microm in width. Each oocyst contains from 50 to over 200 sporocysts. Sporocysts measure 27 to 32 microm in longest diameter. The cyst wall is smooth and 1 microm thick. Each sporocyst typically contains 14 to 17 sporozoites, 49 microm in length. Histological lesions associated with the presence of A. bathytherma include rupture of the basal membrane and detachment of the epithelial cells. In heavily infected areas, most of the tissue of the host digestive tract is replaced by parasites. A. bathytherma is the first Aggregata species described from a host that lives in association with hydrothermal vents, and the third species of Aggregata from eastern North Pacific waters.     

Parvicapsula bicornis n. sp. and P. limandae n. sp. (Myxozoa, Parvicapsulidae) in Pleuronectidae (Teleostei, Heterosomata) from Denmark

Two species of Parvicapsula were found in the kidney tubules and the urinary bladder of 2 pleuronectid fish from the northern Oresund, Denmark. The coelozoic, spherical, disporic trophozoites of both species are 10 to 12 pm in diameter. The myxospores of both species are elongate, asymmetrical and slightly curved, and have spherical polar capsules. Parvicapsula bicornis n. sp. (6-8 x 5-6 microm, polar capsule 2.5 microm in diameter) occurs in Pleuronectes platessa. The polar capsules of P. bicornis are arranged symmetrically on either side of the longitudinal axis and its spores differ from other species of Parvicapsula in having two 2-3 microm long posterior processes of different length. Parvicapsula limandae n. sp. (8-11 x 4-5 pm, polar capsule 1.6 microm in diameter) is found in Limanda limanda. The polar capsules are arranged along the longitudinal axis. It differs from Parvicapsula unicornis Kabata, 1962, recorded from L. limanda, in the arrangement of the polar capsules and in the absence of a posterior horn-like projection. The phylogenetic relationship between P. bicornis n. sp., P. limandae n. sp. and other Parvicapsula spp. was examined with their partial small subunit rDNA (SSU rDNA) sequences. P. limandae n. sp. and P. asymmetrica appear to be closely related, while P. bicornis n. sp. and P. minibicornis are the most divergent members of the genus.     

ENDOSYMBIONTS OF DREISSENA POLYMORPHA IN IRELAND: EVIDENCE FOR THE INTRODUCTION OF ADULT MUSSELS

Although zebra mussels (Dreissena polymorpha) have invaded watersacross Europe for over 200 years, they colonized Ireland onlywithin the past decade. To test the hypothesis that Irelandwas colonized by adult D. polymorpha, we examined mussels fromdifferent sites along the Lower Shannon River system in Irelandfor the presence of host specific and generalist endosymbionts.Withinthe mantle cavity and/or associated with zebra mussel tissueswe found species specific-ciliates (Conchophthirus acuminatusand Ophryoglena hemophaga) and generalist symbionts (the ciliateAncistrumina limnica, nematodes, oligochaetes and chironomids).We found a significant difference in the prevalence of symbiontsamong sites, but all mussels at all sites harboured one specialistspecies C. acuminatus, and all of the mussels at three of thefour sites also had the second specialist, O. hemophaga. Thus,with the introduction of D. polymorpha into Ireland, at leasttwo additional species, their host-specific symbionts C. acuminatusand O. hemophaga, have also been introduced. The presence ofthese symbionts in Ireland supports the hypothesis that adultzebra mussels were introduced into Ireland, rather than larvalstages. This contrasts with the introduction of zebra musselsto North America, where adult zebra mussels are devoid of host-specificsymbionts. (Received 8 June 2005; accepted 7 November 2005)     

Species of coccidia (Apicomplexa: Eimeriidae) in shrews from Alaska, U.S.A., and northeastern Siberia, Russia, with description of two new species

Fecal samples (n = 636) from 10 species of shrews collected in Alaska (n = 540) and northeastern Siberia (n = 96) were examined for the presence of coccidia (Apicomplexa: Eimeriidae). Five distinct oocyst morphotypes were observed. Three types were consistent with oocysts of previously recognized coccidia species from other shrew hosts. These were Eimeria inyoni, E. vagrantis, and Isospora brevicauda, originally described from the inyo shrew (Sorex tenellus), dusky shrew (S. monticolus), and northern short-tailed shrew (Blarina brevicauda), respectively. We found 5 new host records for E. inyoni, 3 for E. vagrantis, and 3 for I. brevicauda. The 2 additional oocyst morphotypes, both from the tundra shrew (Sorex tundrensis), are putative new species. Sporulated oocysts of Eimeria beringiacea n. sp. are subspheroidal, 17.7 x 15.6 microm (14-24 x 13-20 microm) with a length (L)/width (W) ratio of 1.1 (1.0-1.4); these lack a micropyle (M), an oocyst residuum (OR), and a polar granule (PG). Sporocysts are ellipsoidal, 10.3 x 6.1 microm (7-14 x 4-8 microm), with a L/W ratio of 1.7 (1.3-2.3) and have a Stieda body (SB), Substieda body (SSB), and sporocyst residuum (SR). Oocysts of Eimeria tundraensis n. sp. are spheroidal to subspheroidal, 24.8 x 23.5 microm (23-26 x 22-25 microm), with a L/W ratio of 1.1 (1.0-1.2); these lack a M and OR, but a single PG is present. Sporocysts are elongate ellipsoidal, 15.4 x 8.3 microm (13-17 x 7-9 microm), with a L/W ratio of 1.9 (1.4-2.1) and have a SB, SSB, and SR.     

Ultrastructure of bacterized and axenic trophozoites of Entamoeba histolytica with particular reference to helical bodies

Electron microscopy of bacterized and axenic trophozoites of Entamoeba histolytica showed only slight differences in ultrastructure between the two. As with other species of Entamoeba so far studied, this species lacks typical mitochondrial structures and formed endoplasmic reticulum. Dense clusters of glycogen particles are especially characteristic in axenic amebas. Microtubular structures 360 A in diameter appear randomly oriented in both bacterized and axenic trophozoites. Ribonucleoprotein (RNP) bodies are of two typical forms—elongate, parallel arrays of helices (the classical chromatoid bodies), and short helical fragments. Both kinds of helix show a recurring pitch angle of 68–80° and an over-all diameter of 480 A. RNP particles comprising the helices average 180 A in diameter. The longitudinal axes of adjacent helices are 440 A apart. Following RNase digestion of water-soluble methacrylate sections, helices show a core approximately 60 A in diameter. Short helices are also associated with digestive vacuoles. Free RNP particles per se are never seen within digestive vacuoles, but intact short helices are frequently detected closely associated with the external membrane of digestive vacuoles. In some cases, continuation of externally intact helical forms could be related to filamentous material within the vacuole. Acid phosphomonoesterase activity could be demonstrated within digestive vacuoles where deposition of reaction product is especially intense on the filamentous material.     

Description of Eimeria motelo sp. n. (Apicomplexa: Eimeriidae) from the yellow footed tortoise, Geochelone denticulata (Chelonia: Testudinidae), and replacement of Eimeria carinii Lainson, Costa & Shaw, 1990 by Eimeria lainsoni nom. nov

Eimeria motelo sp. n. is described from faeces of the yellow-footed tortoise, Geochelone denticulata (L.). Oocysts are irregularly ellipsoidal or cylindrical, with slightly expressed lobed protrusions and irregularities at the poles, possibly caused by wrinkling of the oocyst wall, 17 (15-19) x 9.4 (8.5-11) microm, shape index (length/width) being 1.81 (1.45-2). The oocyst wall is smooth, single-layered, 0.5 microm thick with no micropyle. There are no polar bodies. Sporocysts are ellipsoidal, 8.9 (7.5-10) x 4.4 (4-5) microm, shape index 2.03 (1.7-2.5). A sporocyst residuum is present, composed of many granules of irregular size. The sporozoites are elongate, lying lengthwise in the sporocysts. Comparison with other species of the genus Eimeria parasitising members of family Testudinidae indicates that the presently described coccidium represents a new species. The name of Eimeria carinii Lainson, Costa & Shaw, 1990 is found to be preoccupied by a homonym, Eimeria carinii Pinto 1928 given to a coccidium from Rattus norvegicus. Therefore, it is replaced by Eimeria lainsoni nom. nov.     

Ultrastructural studies of Henneguya rhamdia n. sp. (Myxozoa) a parasite from the Amazon teleost fish, Rhamdia quelen (Pimelodidae)

Henneguya rhamdia n. sp. is described in the gill filaments of the teleost fish Rhamdia quelen, collected from the Peixe Boi River, State of Pará, Brazil. This myxosporean produced spherical to ellipsoidal plasmodia, up to 300 microm in diameter, which contained developmental stages, including spores. Several dense bodies up to 2 microm in diameter were observed among the spores. The spore body was ellipsoidal (13.1 microm in length, 5.2 microm in width, and 2.5 microm in thickness) and each of the two valves presented a tapering tail (36.9 microm in length). These valves surrounded the binucleated sporoplasm cell and two equal ellipsoidal polar capsules (4.7 x 1.1 microm), which contained 10-11 (rarely 12) polar filament coils. The sporoplasm contained sporoplasmosomes with a laterally eccentric dense structure with a half-crescent section. Based on the data obtained by electron microscopy and on the host specificity, the spores differed from previously described Henneguya species, mainly in their shape and size, number and arrangement of the polar filament coils, and sporoplasmosome morphology.     

Consumption rates of two rotifer species by zebra mussels Dreissena polymorpha

We demonstrated that zebra mussels Dreissena polymorpha collected from the Hudson River could consume two rotifer species that were common before the zebra mussel invasion. The clearance rates (volume of water filtered per hour) of zebra mussels differed when feeding on the two rotifer species but both decreased with an increase in rotifer density. The ingestion rates (biomass of rotifers per hour) for the two rotifer species also differed, but both increased with increasing rotifer density. This is the first experiment to measure zooplankton consumption by bivalve molluscs at different zooplankton densities. The feeding rates of mussels in this study were compared with those of phytoplankton and other zooplankton from previous studies. The diameter of the mussel inhalent siphon was correlated linearly with the shell length and tissue weight, and was usually an order of magnitude wider than rotifer sizes. It is concluded that bivalve suspension feeders not only act as food competitors but also as predators on zooplankton in the aquatic ecosystem.     

Sphaerospora epinepheli n. sp. (Myxosporea: Sphaerosporidae) observed in grouper (Epinephelus malabaricus)

Sphaerospora epinepheli n. sp. is described from grouper, Epinephelus malabaricus, in cage-cultured and wild fish collected from both coastal lines of southern Thailand. Subspherical to spherical spores and mono- or disporous pseudoplasmodia were observed in the lumen of kidney tubules. Pseudoplasmodia were round to elongate, size range 15.6-22.9 microns (length) x 8.4-21.6 microns (width). Spores were 7.8-10.0 microns (length) x 12.3-14.5 microns (thickness), and 7.0-9.5 microns (width) with two spherical polar capsules of equal size measuring 2.9-4.4 microns in diameter and containing polar filaments with six or seven windings. Two uninucleate sporoplasms showed iodine vacuoles. Blood stages, similar to C-blood protozoans observed from freshwater fish in Europe, were found from peripheral blood smears of grouper. Ultrastructural studies of blood stages showed a similar structure to unidentified mobile protozoans from the blood of carp. Electron dense bodies were observed in the cytoplasm of the primary cell blood stages. Infected proximal-tubular epithelial cells showed highly vacuolated cytoplasm and pycnotic nuclei.     

Africana telfordi n. sp. (Nematoda: Heterakidae) from the lizard,Enyalioides heterolepsis (Sauria: Iguanidae) from Panama

Africana telfordi n. sp. (Nematoda: Heterakidae) from the large intestine of the lizard Enyalioides heterolepsis collected in Panama is described and illustrated. Africana telfordi n. sp. represents the seventh species assigned to the genus and the second from the Neotropical Realm. It is distinguished from the other neotropical species, A. chabaudi, by the size and shape of the spicules. The spicules of A. telfordi are robust, analate, and 366-458 microm in length; those of A. chabaudi are narrow, alate, and 644-869 microm in length.     

Orectolobicestus n. g. (Cestoda: Tetraphyllidea), with the description of five new species and the transfer of Phyllobothrium chiloscyllii to the new genus

ORECTOLOBICESTUS N. G. (CESTODA: Tetraphyllidea) is erected for six cestode species parasitising bamboo sharks (Orectolobiformes: Chiloscyllium). Members of this genus differ from all other phyllobothriid genera in possessing modified maisiform spinitriches on their distal bothridial surfaces. In addition, they are easily recognised in that they share the following unique combination of characters: their vitelline fields are interrupted by the ovary, their necks are scutellate, and their bothridia bear an apical sucker and marginal loculi. Five new species of Orectolobicestus are described, including O. tyleri n. sp. from Chiloscyllium punctatum off Borneo, O. lorettae n. sp. from C. cf. punctatum off Australia, O. mukahensis n. sp. and O. kelleyae n. sp. from C. indicum off Borneo, and O. randyi n.sp. from C. hasselti also from off Borneo. In addition, Phyllobothrium chiloscyllii Subhapradha, 1955 is transferred to the new genus. O. chiloscyllii (Subhapradha, 1955) n. comb. is readily distinguished from all five new species in its greater total length. In addition to a number of proglottid features, O. kelleyae n. sp. and O. randyi n. sp. clearly differ from the other three new species in their possession of trifid, rather than fully serrate, spinitriches on their proximal bothridial surfaces. The latter two species conspicuously differ from one another in total number of proglottids (11-21 vs 27-38). O. tyleri n. sp. generally has fewer proglottids than O. lorrettae n. sp. (7-17 vs 13-23) and, like O. mukahensis n. sp., possesses scutes that are spathate rather than elongate. O. tyleri n. sp. is readily distinguished from O. mukahensis n. sp. in its possession of fewer proglottids (7-17 vs. 19-29). The five new species of Orectolobicestus share derived bothridial microthrix features with Phyllobothrium squali Yamaguti, 1952, Thysanocephalum sp., Orygmatobothrium sp., Ruhnkecestus Caira & Durkin, 2006 and species of Paraorygmatobothrium Ruhnke, 1994. Among these taxa, Orectolobicestus most closely resembles Paraorygmatobothrium and Ruhnkecestus in its vitelline fields being interrupted by the ovary and the possession of a scutellate neck.     

Steinernema akhursti sp. n. (Nematoda: Steinernematidae) from Yunnan, China

A new species of entomopathogenic nematode, herein described as Steinernema akhursti sp. n., was recovered from soil samples collected from Yunnan Province, the People's Republic of China. Both morphological and molecular data show congruently that S. akhursti sp. n. belongs to the Steinernema feltiae group. It can be separated from all described Steinernema species by the combined morphological and morphometrical characters of various stages of the nematodes. For the first generation male, the new species can be recognized by spicule length 90 +/- 4.6 microm, spicule tip blunt with an aperture on the ventral side, gubernaculum with a long and needle-shaped cuneus, and tail conoid with a prominent mucron on the tip and a concave on ventral side. For the infective juvenile, the combination of the following characters: body length 812 +/- 19 microm, distance from anterior end to excretory pore 59 +/- 1.5 microm, tail length 73 +/- 2.9 microm, E% 77 +/- 4.5, lateral field with six evenly distributed and identical ridges at the middle body portion, and tail with long and slightly constrict hyaline portion can be used to separate the new species from other nematodes. For the female, the new species is characterized by: tail conoid with a short mucron and slightly swelling anal portion and a symmetrical, slightly protruding vulva with conspicuous double-flapped epiptygma. The nematode can be separated from other described species of Steinernema by DNA sequences of either a partial 28S rDNA or the internal transcribed spacer regions of rDNA and from the closely related species S. feltiae and Steinernema oregonense by cross-breeding tests.     

Sphaerospora epinepheli N. Sp. (Myxosporea: Sphaerosporidae) Observed in Grouper (Epinephelus malabaricus)

Sphaerospora epinepheli n. sp. is described from grouper, Epinephelus malabaricus , in cage-cultured and wild fish collected from both coastal lines of southern Thailand. Subspherical to spherical spores and mono- or disporous pseudoplasmodia were observed in the lumen of kidney tubules. Pseudoplasmodia were round to elongate, size range 15.6–22.9 μm (length) × 8.4–21.6 μm (width). Spores were 7.8–10.0 μm (length) × 12.3–14.5 μm (thickness), and 7.0–9.5 μm (width) with two spherical polar capsules of equal size measuring 2.9–4.4 μm in diameter and containing polar filaments with six or seven windings. Two uninucleate sporoplasms showed iodine vacuoles. Blood stages, similar to C-blood protozoans observed from freshwater fish in Europe, were found from peripheral blood smears of grouper. Ultrastructural studies of blood stages showed a similar structure to unidentified mobile protozoans from the blood of carp. Electron dense bodies were observed in the cytoplasm of the primary cell blood stages. Infected proximal-tubular epithelial cells showed highly vacuolated cytoplasm and pycnotic nuclei.     

Strombidinopsis jeokjo n. sp. (ciliophora: choreotrichida) from the coastal waters off western Korea: morphology and small subunit ribosomal DNA gene sequence

The planktonic ciliate Strombidinopsis jeokjo n. sp. is described from Quantitative Protargol-Stained (QPS) preparations, and the sequence of the small subunit rDNA (SSU rDNA) from cultured cells is reported. This species is ovoid and bluntly tapered towards the posterior. The ranges (and mean +/- standard deviation, n = 31) of cell length, cell width, and oral diameter of the QPS-stained specimens were 100-190 microm (149 +/- 25), 60-105 microm (79 +/- 13), and 55-80 microm (64 +/- 5), respectively. Fifteen to seventeen external oral polykinetids had oral membranelle cilia 20-35 microm long. Twenty-six to twenty-eight somatic kineties were equally spaced around the cell body and extended from the oral to the posterior regions with 23-44 dikinetids per kinety. Both kinetosomes of each kinetid bore cilia 3-7 microm long. Strombidinopsis jeokjo had two ovoid macronuclei of 25-38 microm x 12-15 microm. When properly aligned, the sequence of the SSU rDNA of S. jeokjo (GenBank Accession No. AJ628250) was approximately 2% different from that of an unidentified Strombidinopsis species (GenBank Accession No. AF399132-AF399135), the closest species in the SSU rDNA sequence.     

The marine mites Hyadesia sp. and Copidognathus sp. Associated with the mussel Mytilus galloprovincialis

Two species of marine mites belonging to the families Hyadesiidae and Halacaridae, Hyadesia sp. and Copidognathus sp., respectively, were found associated with the mussel Mytilus galloprovincialis from Baja California in NW México. The first species was found inside the mussel gut with an intensity ranging from one to six mites per mussel and their prevalence was from 20.0 to 46.7%; this species was also found living free in the sediment at a density of 0.7 mite/100 ml. The second species was found on the mantle and gills of the host with an intensity ranging from one to three mites per host and their prevalence was from 3.3 to 6.7%; this species was abundant (4.5 mites/100 ml) and living free in the sediment around mussel clumps. Hyadesia sp. was found alive and attached in the gut of the mussel. A histological analysis revealed this species in the lumen of intestine surrounded by mucus and attached to the epithelial cells of the intestine, where some disorder of epithelial cells was associated. Moreover, this mite may be encapsulated by hemocytes inside the digestive diverticulum, the reproductive follicle, or the connective tissue surrounding the diverticulum. No damages to branches or gills resulting from the presence of Copidognathus sp. were observed. The results suggest that these mites are occasional invaders of mussels; however, as a result of this infestation, Hyadesia sp. may produce damage in the host's tissues. This is the first record of marine mites inside the gut, reproductive follicles, branches, and mantle of a marine bivalve.     

Zebra mussel adhesion: structure of the byssal adhesive apparatus in the freshwater mussel, Dreissena polymorpha

The freshwater zebra mussel (Dreissena polymorpha) owes a large part of its success as an invasive species to its ability to attach to a wide variety of substrates. As in marine mussels, this attachment is achieved by a proteinaceous byssus, a series of threads joined at a stem that connect the mussel to adhesive plaques secreted onto the substrate. Although the zebra mussel byssus is superficially similar to marine mussels, significant structural and compositional differences suggest that further investigation of the adhesion mechanisms in this freshwater species is warranted. Here we present an ultrastructural examination of the zebra mussel byssus, with emphasis on interfaces that are critical to its adhesive function. By examining the attached plaques, we show that adhesion is mediated by a uniform electron dense layer on the underside of the plaque. This layer is only 10-20 nm thick and makes direct and continuous contact with the substrate. The plaque itself is fibrous, and curiously can exhibit either a dense or porous morphology. In zebra mussels, a graded interface between the animal and the substrate mussels is achieved by interdigitation of uniform threads with the stem, in contrast to marine mussels, where the threads themselves are non-uniform. Our observations of several novel aspects of zebra mussel byssal ultrastructure may have important implications not only for preventing biofouling by the zebra mussel, but for the development of new bioadhesives as well.     

Seasonal dynamics of endosymbiotic ciliates and nematodes in Dreissena polymorpha

We report the results of a two-year study in the Svisloch River (Minsk, Belarus) on the dynamics of infection in Dreissena polymorpha by nematodes and three ciliate species Conchophthirus acuminatus, Ophryoglena sp., and Ancistrumina limnica. Although these endosymbionts were present in most of the samples, their prevalence and infection intensity differed significantly. C. acuminatus and A. limnica infection intensities in both years of the study had a maximum in summer and were positively correlated with water temperature. In contrast, Ophryoglena sp. and nematode infection intensities were considerably lower in summer versus winter and were negatively correlated with temperature. In the first long-term study to monitor the size and reproductive rate of C. acuminatus, we found that mean length was negatively correlated with temperature and that temperature was positively correlated with asexual reproduction, with a peak of cell division in April as water temperatures increased.     

Predation on exotic zebra mussels by native fishes: effects on predator and prey

SUMMARY 1. Exotic zebra mussels, Dreissena polymorpha, occur in southern U.S. waterways in high densities, but little is known about the interaction between native fish predators and zebra mussels. Previous studies have suggested that exotic zebra mussels are low profitability prey items and native vertebrate predators are unlikely to reduce zebra mussel densities. We tested these hypotheses by observing prey use of fishes, determining energy content of primary prey species of fishes, and conducting predator exclusion experiments in Lake Dardanelle, Arkansas. 2. Zebra mussels were the primary prey eaten by 52.9% of blue catfish, Ictalurus furcatus; 48.2% of freshwater drum, Aplodinotus grunniens; and 100% of adult redear sunfish, Lepomis microlophus. Blue catfish showed distinct seasonal prey shifts, feeding on zebra mussels in summer and shad, Dorosoma spp., during winter. Energy content (joules g⁻¹) of blue catfish prey (threadfin shad, Dorosoma petenense; gizzard shad, D. cepedianum; zebra mussels; and asiatic clams, Corbicula fluminea) showed a significant species by season interaction, but shad were always significantly greater in energy content than bivalves examined as either ash-free dry mass or whole organism dry mass. Fish predators significantly reduced densities of large zebra mussels (>5 mm length) colonising clay tiles in the summers of 1997 and 1998, but predation effects on small zebra mussels (≤5 mm length) were less clear. 3. Freshwater drum and redear sunfish process bivalve prey by crushing shells and obtain low amounts of higher-energy food (only the flesh), whereas blue catfish lack a shell-crushing apparatus and ingest large amounts of low-energy food per unit time (bivalves with their shells). Blue catfish appeared to select the abundant zebra mussel over the more energetically rich shad during summer, then shifted to shad during winter when shad experienced temperature-dependent stress and mortality. Native fish predators can suppress adult zebra mussel colonisation, but are ultimately unlikely to limit population density because of zebra mussel reproductive potential.