Endoparasitism in colonial hosts: patterns and processes

This study begins to redress our lack of knowledge of the interactions between colonial hosts and their parasites by focusing on a novel host-parasite system. Investigations of freshwater bryozoan populations revealed that infection by myxozoan parasites is widespread. Covert infections were detected in all 5 populations studied and were often at high prevalence while overt infections were observed in only 1. Infections were persistent in populations subject to temporal sampling. Negative effects of infection were identified but virulence was low. Infection did not induce mortality in the environmental conditions studied. However, the production of statoblasts (dormant propagules) was greatly reduced in bryozoans with overt infections in comparison to uninfected bryozoans. Overtly-infected bryozoans also grew more slowly and had low fission rates relative to colonies lacking overt infection. Bryozoans with covert infections were smaller than uninfected bryozoans. High levels of vertical transmission were achieved through colony fission and the infection of statoblasts. Increased fission rates may be a strategy for hosts to escape from parasites but the parasite can also exploit the fragmentation of colonial hosts to gain vertical transmission and dispersal. Our study provides evidence that opportunities and constraints for host-parasite co-evolution can be highly dependent on organismal body plans and that low virulence may be associated with exploitation of colonial hosts by endoparasites.

Temperature-driven proliferation of Tetracapsuloides bryosalmonae in bryozoan hosts portends salmonid declines

Proliferative kidney disease (PKD) is an emerging disease of salmonid fishes. It is provoked by temperature and caused by infective spores of the myxozoan parasite Tetracapsuloides bryosalmonae, which develops in freshwater bryozoans. We investigated the link between PKD and temperature by determining whether temperature influences the proliferation of T. bryosalmonae in the bryozoan host Fredericella sultana. Herein we show that increased temperatures drive the proliferation of T. bryosalmonae in bryozoans by provoking, accelerating and prolonging the production of infective spores from cryptic stages. Based on these results we predict that PKD outbreaks will increase further in magnitude and severity in wild and farmed salmonids as a result of climate-driven enhanced proliferation in invertebrate hosts, and urge for early implementation of management strategies to reduce future salmonid declines.

Low genetic diversity in a marine nature reserve: re-evaluating diversity criteria in reserve design

Little consideration has been given to the genetic composition of populations associated with marine reserves, as reserve designation is generally to protect specific species, communities or habitats. Nevertheless, it is important to conserve genetic diversity since it provides the raw material for the maintenance of species diversity over longer, evolutionary time-scales and may also confer the basis for adaptation to environmental change. Many current marine reserves are small in size and isolated to some degree (e.g. sea loughs and offshore islands). While such features enable easier management, they may have important implications for the genetic structure of protected populations, the ability of populations to recover from local catastrophes and the potential for marine reserves to act as sources of propagules for surrounding areas. Here, we present a case study demonstrating genetic differentiation, isolation, inbreeding and reduced genetic diversity in populations of the dogwhelk Nucella lapillus in Lough Hyne Marine Nature Reserve (an isolated sea lough in southern Ireland), compared with populations on the local adjacent open coast and populations in England, Wales and France. Our study demonstrates that this sea lough is isolated from open coast populations, and highlights that there may be long-term genetic consequences of selecting reserves on the basis of isolation and ease of protection.

Evaluation of malacosporean life cycles through transmission studies

Myxozoans, belonging to the recently described Class Malacosporea, parasitise freshwater bryozoans during at least part of their life cycle, but no complete malacosporean life cycle is known to date. One of the 2 described malacosporeans is Tetracapsuloides bryosalmonae, the causative agent of salmonid proliferative kidney disease. The other is Buddenbrockia plumatellae, so far only found in freshwater bryozoans. Our investigations evaluated malacosporean life cycles, focusing on transmission from fish to bryozoan and from bryozoan to bryozoan. We exposed bryozoans to possible infection from: stages of T. bryosalmonae in fish kidney and released in fish urine; spores of T. bryosalmonae that had developed in bryozoan hosts; and spores and sac stages of B. plumatellae that had developed in bryozoans. Infections were never observed by microscopic examination of post-exposure, cultured bryozoans and none were detected by PCR after culture. Our consistent negative results are compelling: trials incorporated a broad range of parasite stages and potential hosts, and failure of transmission across trials cannot be ascribed to low spore concentrations or immature infective stages. The absence of evidence for bryozoan to bryozoan transmissions for both malacosporeans strongly indicates that such transmission is precluded in malacosporean life cycles. Overall, our results imply that there may be another malacosporean host which remains unidentified, although transmission from fish to bryozoans requires further investigation. However, the highly clonal life history of freshwater bryozoans is likely to allow both long-term persistence and spread of infection within bryozoan populations, precluding the requirement for regular transmission from an alternate host.

Malacosporean-like spores in urine of rainbow trout react with antibody and DNA probes to Tetracapsuloides bryosalmonae

Tetracapsuloides bryosalmonae is the myxozoan parasite causing proliferative kidney disease (PKD) of salmonid fishes in Europe and North America. The complete life cycle of the parasite remains unknown despite recent discoveries that the stages infectious for fish develop in freshwater bryozoans. During the course of examinations of the urine of rainbow trout (Oncorhynchus mykiss) with or recovering from PKD we identified spores with features similar to those of T. bryosalmonae found in the bryozoan host. Spores found in the urine were subspherical, with a width of 16 micro m and height of 14 microm, and possessed two soft valves surrounding two spherical polar capsules (2 microm in diameter) and a single sporoplasm. The absence of hardened valves is a distinguishing characteristic of the newly established class Malacosporea that includes T. bryosalmonae as found in the bryozoan host. The parasite in the urine of rainbow trout possessed only two polar capsules and two valve cells compared to the four polar capsules and four valves observed in the spherical spores of 19 microm in diameter from T. bryosalmonae from the bryozoan host. Despite morphological differences, a relationship between the spores in the urine of rainbow trout and T. bryosalmonae was demonstrated by binding of monoclonal and polyclonal antibodies and DNA probes specific to T. bryosalmonae.

Post-haemorrhagic anaemia in sea bass, Dicentrarchus labrax (L.), caused by blood feeding of Ceratothoa oestroides (Isopoda: Cymothoidae)

The effects of the fish parasitic isopod, Ceratothoa oestroides (Risso), on haematological parameters of its cage-cultured sea bass host, Dicentrarchus labrax (L.), were studied. Analyses of blood parameters (cell counts, haemoglobin content and haematocrit) were carried out on parasitized and unparasitized sea bass from a fish farm in Turkey. Parasitized fish had significantly lowered erythrocyte counts, haematocrit and haemoglobin values and significantly increased leucocyte counts. Blood feeding by C. oestroides thus produces a post-haemorrhagic anaemia and the fish appear to mount an immune response to the presence of parasites.

Ultrastructure of Buddenbrockia identifies it as a myxozoan and verifies the bilaterian origin of the Myxozoa

The phylogenetic affinities of Buddenbrockia, a nematode-like parasite of freshwater bryozoans, have remained unknown since it was first reported in the nineteenth century. The discovery of Buddenbrockia parasitic in Hyalinella punctata in Ohio and Plumatella repens in France has provided material for the first ultrastructural study of this animal. This has revealed the presence of polar capsules, diagnostic myxozoan features, in the body wall. Other features, which place Buddenbrockia firmly among tetracapsulid myxozoans in the Class Malacosporea, are the unusual morphology of the polar capsules, the absence of the external tube in capsulogenesis, the body wall with its unusual cell junctions and utilization of freshwater bryozoans as hosts. The ultrastructural study has established the triploblastic organization of Buddenbrockia by confirmation of the presence of an inner layer of cells and 4 sets of longitudinal muscles. Our studies have, thus, simultaneously revealed that Buddenbrockia is a myxozoan and that the myxozoans are derived from bilaterians. The latter conclusion resolves the ongoing controversy over the triploblastic versus diploblastic nature of the Myxozoa. Our studies also provide evidence that bryozoans are ancestral hosts for the myxozoans and that loss of triploblast features has characterized the major radiation of the better known endoparasites of fish and worms in the Class Myxosporea.

Cymothoid isopod parasites in aquaculture: a review and case study of a Turkish sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus) farm

This paper reviews the occurrence of cymothoid isopod parasitism in aquaculture, reports the first case of infection by a cymothoid isopod (Ceratothoa oestroides) in Turkish aquaculture, and analyses its effects on sea bass Dicentrarchus labrax. Analyses revealed that C. oestroides negatively affects the weights and lengths of sea bass hosts. These effects have been previously underestimated because host age has not been accounted for. The analysis of condition factors as a means of assessing parasite effects is therefore likely to be misleading. Infection of fish of all ages by all cymothoid stages indicates that sea bass are not intermediate hosts but that C. oestroides has effected a complete host shift.

Evidence that infectious stages of Tetracapsula bryosalmonae for rainbow trout Oncorhynchus mykiss are present throughout the year

Proliferative kidney disease (PKD) is a hyperplastic condition of the lymphoid tissue of salmonids infected with the spores of Tetracapsula bryosalmonae, a myxozoan parasite formerly designated PKX, which has recently been described as a parasite of several species of bryozoans. The occurrence of PKD is generally associated with seasonal increase in water temperature, with research indicating that transmission of the disease does not occur below 12 to 13 degrees C. This suggested that the infectious stages are absent from about November to March/April. Here we document the transmission of PKD at water temperatures and seasons previously considered to be non permissive for PKD infection. The exposure of naive rainbow trout Oncorhynchus mykiss (Walbaum) to PKD-infected water ranging from 8 to 13 degrees C during the Autumn, Winter and early Spring, resulted in the infection of kidney interstitium once the trout were transferred to 16 degrees C. In addition, cohabitation studies were conducted with the bryozoan host Fredericella sultana collected from a river at times of low seasonal temperatures because this bryozoan species overwinters as living colonies. Cohabitation of trout with colonies of F sultana in parasite-free city water at 16 degrees C, also led to renal lymphoid tissue infection with the parasite and even to nephromegaly. Our results provide evidence that the infectious stages of T bryosalmonae for rainbow trout were present in the water throughout the entire year and that the impact of temperature on the development of PKD is primarily a result of the kinetics of Tetracapsula multiplication in bryozoan and fish hosts.

Induction of proliferative kidney disease (PKD) in rainbow trout Oncorhynchus mykiss via the bryozoan Fredericella sultana infected with Tetracapsula bryosalmonae

Proliferative kidney disease (PKD) is a serious infection of wild and farmed salmonids, affecting mainly the kidney and spleen but becoming systemic in most susceptible fish hosts. This report deals with the transmission of Tetracapsula bryosalmonae Canning, Curry, Feist, Longshaw & Okamura 1999 from naturally infected bryozoans Fredericella sultana Blumenbach 1779 to naive rainbow trout Oncorhynchus mykiss Walbaum 1792, thereby confirming the recent conclusion based on partial 18S rDNA sequence data that bryozoans are hosts of the myxozoan parasite T. bryosalmonae (formerly PKX organism) that causes the disease. Parasite transmission using T. bryosalmonae spores was successful by short-term exposure to disrupted bryozoans known to contain T. bryosalmonae spores and T bryosalmonae sacs liberated from the bryozoans, and by long-term cohabitation with infected bryozoan colonies. Infection was confirmed by examination of kidney imprints, detection of the parasite in stained tissue sections, PCR using T. bryosalmonae-specific primers, and comparison of amplified 18S rDNA sequences from the bryozoans and experimentally infected fish. Transmission was not apparent, nor was PKD induced, in fish challenged by intraperitoneal injection of spores isolated from F. sultana.

Patterns of occurrence and 18S rDNA sequence variation of PKX (Tetracapsula bryosalmonae), the causative agent of salmonid proliferative kidney disease

Recent progress in understanding the etiology of proliferative kidney disease (PKD) includes the identification of freshwater bryozoans as the natural hosts of the myxozoan parasite that causes the disease in salmonid fish and formal description of the parasite as Tetracapsula bryosalmonae. This paper presents data on patterns of occurrence of T. bryosalmonae and sequence variation among isolates. T. bryosalmonae infects bryozoans that range from primitive to more derived genera within the Phylactolaemata and that differ in growth form and habits. Infected bryozoans have been collected in diverse habitats including cold, clear streams and warm, eutrophic lakes. Temporal surveys reveal intra- and interannual variation in infection levels, and spatial variation in incidence of infection is implicit by the apparent absence of T. bryosalmonae from many bryozoan populations. The significance of minor variation in partial sequences of 18S rDNA requires further investigation. The information presented here provides the first significant insights into the ecology of T. bryosalmonae.

Gene flow and genetic diversity: a comparison of freshwater bryozoan populations in Europe and North America

We have used microsatellite and mitochondrial sequence data to gain insight into patterns of gene flow and genetic diversity among North American and European populations of the freshwater bryozoan Cristatella mucedo. Mitochondrial sequence data reveal numerous, widely distributed, divergent genetic lineages in North America that can be broadly categorized into two groups, one of which is genetically homogeneous and relatively similar to the European haplotypes, the other of which is more diverse. The maximum North American sequence differentiation translates into a divergence time of approximately 1.5 Myr BP. In contrast, European populations contained only three haplotypes that are all closely related. Microsatellite data reveal higher overall levels of genetic diversity in North America than Europe, although levels of within-population genetic variation are similar on the two continents. In North America, two of the three microsatellite loci show bimodal distributions of allele sizes which are significantly associated between the two loci. As a result, two microsatellite lineages are evident, and these are assortatively distributed between the mitochondrial haplotype groupings. The combined mitochondrial and microsatellite data suggest two distinct genetic lineages in North America that may represent cryptic species. Hybridization between the two presumptive species or subspecies may have contributed to the high levels of genetic diversity in North America. The overall lower levels of genetic diversity in Europe can be attributed to postglacial derivation of extant populations from a single mitochondrial lineage, and conformation to a metapopulation structure.

A new class and order of myxozoans to accommodate parasites of bryozoans with ultrastructural observations on Tetracapsula bryosalmonae (PKX organism)

Tetracapsula bryosalmonae, formerly PKX organism, is a myxozoan parasite that causes proliferative kidney disease in salmonid fish. Its primary hosts, in which it undergoes a sexual phase, are phylactolaemate bryozoans. It develops in the bryozoan coelomic cavity as freely floating sacs which contain two types of cells, stellate cells and sporoplasmogenic cells, which become organised as spores. Eight stellate cells differentiate as four capsulogenic cells and four valve cells which surround a single sporoplasmogenic cell. The sporoplasmogenic cell undergoes meiosis and cytoplasmic fission to produce two sporoplasms with haploid nuclei. Sporoplasms contain secondary cells. The unusual development supports previously obtained data from 18S rDNA sequences, indicating that species of Tetracapsula form a clade. It diverged early in the evolution of the Myxozoa, before the radiation that gave rise to the better known genera belonging to the two orders in the single class Myxosporea. The genus Tetracapsula as seen in bryozoans shares some of the characters unique to the myxosporean phase and others typical of the actinosporean phase of genera belonging to the class Myxosporea. However, it exhibits other features which are not found in either phase. A new class Malacosporea and order Malacovalvulida are proposed to accommodate the family Saccosporidae and genus Tetracapsula. Special features of the new class are the sac-like proliferative body, valve cells not covering the exit point of the polar filament, lack of a stopper-like structure sealing the exit, maintenance of valve cell integrity even at spore maturity, absence of hardened spore walls and unique structure of sporoplasmosomes in the sporoplasms.

Genetic diversity of North American populations of Cristatella mucedo, inferred from microsatellite and mitochondrial DNA

Research over the past 20 years has shown, with the help of molecular markers, that the population genetics and distribution patterns of freshwater invertebrates in North America are often more complex than was previously believed. Here we extend this research to an, as yet, unstudied but widespread and common group, the freshwater bryozoans. Colonies of the bryozoan Cristatella mucedo were collected from a number of lakes across central North America, and were characterized genetically by analysis of microsatellite loci and mitochondrial DNA (mtDNA) cytochrome b sequences. The microsatellites illustrate a pattern of generally diverse and highly differentiated populations that contain little evidence of recent gene flow. The mtDNA sequences yielded highly variable levels of divergence, ranging from 0.0 to 8.8% within populations, and 0.0 to 9.8% among populations. The multiple divergent mtDNA lineages within populations provide evidence for repeated colonization events. The lack of clustering of haplotypes by site suggests that there has been widespread dispersal of multiple genetic lineages since the last ice age. While some of the haplotype lineages may have evolved in disjunct glacial refugia, the maximum levels of divergence predate the time since the last glacial-interglacial cycles. It is likely that multiple factors including vicariance events, patterns of dispersal, localized extinction, and an unusual life history, explain the unique phylogeographic patterns evident today in populations of C. mucedo.

Molecular data implicate bryozoans as hosts for PKX (phylum Myxozoa) and identify a clade of bryozoan parasites within the Myxozoa

Proliferative kidney disease (PKD), a condition associated with high mortality in salmonid fish, represents an abnormal immune response to the presence of an enigmatic myxozoan, which has been designated simply as PKX organism because its generic and specific status are obscure. Phylogenetic analyses of partial sequences of the 18S rDNA of PKX and of myxozoan parasites infecting the bryozoans Cristatella mucedo, Pectinatella magnifica and Plumatella rugosa, including the previously named Tetracapsula bryozoides from C. mucedo, showed that these taxa represent a distinct clade that diverged early in the evolution of the Myxozoa before the radiation of the other known myxozoan genera. A common feature of the myxozoans in this clade may be the electron-dense sporoplasmosomes with a lucent bar-like structure, which occur in T. bryozoides and PKX but not in the myxozoans belonging to the established orders Bivalvulida and Multivalvulida. Variation of 0.5-1.1% was found among the PKX 18S rDNA sequences obtained from fish from North America and Europe. The 18S rDNA sequence for T. bryozoides showed that it is a distinct taxon, not closely related to PKX but some sequences from myxozoans infecting 2 of the bryozoan species were so similar to those of PKX as to be indistinguishable. Other sequences from the new myxozoans in bryozoans at first appeared distinct from PKX in a maximum likelihood tree but, when analysed further, were also found to be phylogenetically indistinguishable from PKX. We propose that at least some variants of these new myxozoans from bryozoans are able to infect and multiply in salmonid fish, in which they stimulate the immune reaction and cause PKD but are unable to form mature spores to complete their development.

Ultrastructure of Myxidium trachinorum sp. nov. from the gallbladder of the lesser weever fish Echiichthys vipera

Myxidium trachinorum sp. nov. is described from the gallbladder of the lesser weever fish Echiichthys vipera. Pseudoplasmodia attach themselves to the gallbladder epithelium by filose processes, which are inserted between host cells. Pseudoplasmodia undergo endogenous cell formation at the secondary and tertiary levels. In the proliferative cycle, primary and endogenous cells are packed with digestive vacuoles formed by phagocytosis. In the sporogonic cycle the pseudoplasmodium becomes a pericyte enclosing two secondary cells (lacking digestive vacuoles) in a vacuole. These give rise to five cells each two valvogenic, two capsulogenic and a binucleate sporoplasm, which mature into spores. Comparison of the disporic M. trachinorum with polysporic species of Myxidium revealed significant differences in plasmodial ultrastructure, especially their attachments to host cells, surface characteristics and mode of nutrition, and in formation of generative cells. These suggest that the genus Myxidium may require revision.

Suspension Feeding Adaptations to Extreme Flow Environments in a Marine Bryozoan

We describe the effects of extreme flow on the growth and morphology of a bryozoan, Membranipora membranacea, encrusting laminarian fronds in the Rapids of Lough Hyne (=Ine), County Cork, Ireland. An ultrasonic current meter was used to characterize ambient flow regimes at the level of the algal canopy over a complete tidal cycle at three sites within the Rapids. Colonies collected from sites exposed to different flows showed a trend towards miniaturization with increased flow: the zooids were less elongate, the lophophores were smaller in diameter and had fewer tentacles, and the distances between excurrent jets were shorter. These morphological changes probably place feeding surfaces into slower flow regimes of the boundary layer. Similar growth rates of colonies at sites differing in flow provide evidence that this miniaturization is adaptive and that bryozoans are capable of adopting appropriate morphological responses to varying environmental regimes. Such plasticity should be considered when assessing feeding from different flow regimes because particular colonies may be adapted to a limited and specific range of flow conditions.

Genetic variation in a freshwater bryozoan. I: Populations in the Thames basin, UK

In spite of increasing interest in metapopulation dynamics, the genetic consequences of a metapopulation structure remain poorly understood. Here we examine the metapopulation genetic structure of the colonial, facultatively sexual freshwater bryozoan Cristatella mucedo, in the Thames basin of southern England, UK. Populations from nine sites were sampled and colonies genetically characterized using random amplified polymorphic DNA (RAPD)-PCR. A total of 78 different clones was detected over all sites. Despite the large number of clones, genetic distances among clones both within and among sites were very small. Nonetheless, no clone was present at more than one site even though C. mucedo has an asexual dispersal propagule, and clones strongly clustered by sites. No consistent pattern of clonal structure was evident, with both the number and equitability of clones varying greatly among sites. Although sites were genetically distinct, population genetic regions were absent, and a Mantel test indicated that there was no relationship between geographical distances among sites and genetic distances among populations. Our results indicate that C. mucedo exists as a classical metapopulation in the Thames basin, with dispersal independent of distance and all sites contributing to the genetic diversity of the metapopulation.

A New Microsporidium, Nosema cristatellae n. sp. in the Bryozoan Cristatella mucedo (Bryozoa, Phylactolaemata)

A microsporidian infecting cells of the body wall of the phylactolaemate bryozoan Cristatella mucedo is described. All stages of the parasite are diplokaryotic and lie in direct contact with the host cell cytoplasm. Sporogony is probably disporoblastic. Spores measure 7.5 x 5.1 &mgr;m and have 22-32 coils of the polar tube arranged in several rows and a bell-like polaroplast of compact membranes. The parasite is assigned to the genus Nosema as a new species, Nosema cristatellae. It is differentiated from the previously described parasites of Alcyonella (=Plumatella) fungosa (Bryozoa), named Myxosporidium bryozoides and Nosema bryozoides, by spore characters and tissue specificity. Although it was found in a different species of bryozoan, it is not known whether N. cristatellae is infective to P. fungosa. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Temporal patterns of clonality and parasitism in a population of freshwater bryozoans

Clonal reproduction is commonly incorporated into the life cycles of many metazoans. However, whether and how such highly clonal animals persist in the face of natural enemies remains poorly understood. Here we report the first temporal genetic study of a clonal population, the freshwater bryozoan Cristatella mucedo, and the associated prevalence of a myxozoan parasite. High levels of both clonality and parasitism persisted over a 3 year period. Random amplified polymorphic DNA markers revealed four distinct clones of C. mucedo. The two most common clones varied in abundance with the significantly more common clone in the first year becoming the significantly less common by the third year. There was no evidence that the most common clone was disproportionately infected. These results are discussed in relation to predictions of the Red Queen and the metapopulation dynamics of clonal organisms.

Parent and larval RAPD fingerprints reveal outcrossing in freshwater bryozoans

The occurrence of outcrossing in benthic hermaphroditic colonial invertebrates has received much historical debate and little demonstration. Direct genetic study of this question using routine techniques has been limited by both the amount of material required and the detection of adequate DNA polymorphisms. However, the recent development of molecular techniques that require no a-priori sequence data provides new approaches to the characterization of both tiny and genetically similar individuals. Random amplification of polymorphic DNA (the RAPD assay) was used to amplify fragments of DNA (via the polymerase chain reaction) to obtain fingerprints of parental colonies and larval offspring of the hermaphroditic freshwater bryozoan Cristatella mucedo. Here we report the first positive and direct genetic evidence for outcrossing in bryozoans. However, we find that outcrossing generates only low levels of genetic variation in populations that are highly clonal.

Randomly amplified polymorphic DNA analysis of clonal population structure and geographic variation in a freshwater bryozoan

The randomly amplified polymorphic DNA (RAPD) assay was used to identify genetic polymorphisms in three clonal populations of the freshwater bryozoan, Cristatella mucedo, a species with few useful biochemical genetic markers. Of the 19 decamer oligonucleotide primers screened, 13 gave clear, reproducible RAPD profiles. Clonal population structure was evident, and one clone was dominant at each site. Cluster analysis grouped populations from more distant localities separately (Thames Valley and Norfolk), whereas populations from the Thames Valley clustered together. However, even at the regional scale a high degree of relatedness pertained. This work is one of the first RAPD studies of natural populations, and demonstrates the suitability of the technique for examining population structure and geographic variation in clonal taxa.

Patterns of Suspension Feeding in the Freshwater Bryozoan Plumatella repens

Feeding of large and small colonies of Plumatella repens was assessed under two flow conditions. Large colonies ingested greater numbers of particles than small colonies and feeding of colonies of both sizes increased with flow. However, the rate of increase depended on colony size. Small colonies increased feeding to a greater degree than large colonies. Mechanisms that may explain these patterns are discussed. These results contrast with an earlier study of feeding in a freshwater bryozoan. The conflicting results may reflect experimental conditions. In the previous study a small volume of still water likely entailed greater food depletion by large colonies. In our study food depletion did not occur and ambient flow carried away filtered water. We discuss how the relatively large, U-shaped lophophores of freshwater bryozoans function to produce powerful feeding currents that are suited to feeding in lotic and lentic habitats.