University of Colorado, BoulderCU Scholar

Undergraduate Honors Theses Honors Program

Spring 2014

Ecological Drivers and Species Interactions ofWhirling DiseaseJulie ByleUniversity of Colorado Boulder

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Recommended CitationByle, Julie, "Ecological Drivers and Species Interactions of Whirling Disease" (2014). Undergraduate Honors Theses. Paper 58.

Ecological Drivers and Species Interactions of

Whirling Disease

Julie Allyson Byle

Department of Ecology and Evolutionary Biology

University of Colorado Boulder

April 7, 2014

Dr. Robert Guralnick of the Department of Ecology and Evolutionary Biology

Dr. Barbara Demmig-Adams of the Department of Ecology and Evolutionary Biology

Dr. Diane McKnight of the Department of Environmental Science, Civil, Environmental, and

Architectural Engineering, and the Institute of Arctic and Alpine Research

2

Table of Contents

Abstract…………………………………………………………………………………………..3

Introduction………………………………………………………………………………………4

Background ……………………………………………………………………………………...5

Whirling Disease………………………………….……………………………………...6

Myxobolus cerebralis lifecycle………………………………….………………………..7

Study System Species

Didymosphenia geminata……………………………………….…………8

Tubifex tubifex…………………………………………………….……...10

Trout………………………………………………………………………………11

Species interactions………………………………………………………………………12

Materials and Methods…………………………………………………………………………...12

Patterns of oligochaete abundance and D. geminata cover……………………………...12

D. geminata as a refuge from predation for T. tubifex …………………………………..13

D. geminata as a stream flow refuge for T. tubifex……………………………….……...14

Results……………………………………………………………………………………………16

Oligochaete abundance sampling………………………………………………..16

Predator refuge experiment………………………………………………………18

Stream flow refuge experiment ………………………………………………….19

Driver of disease table….………………………………………………………..20

Discussion………………………………………………………………………………………..21

Acknowledgements………………………………………………………………………………25

Literature Cited…………………………………………………………………………………..26

3

Abstract

Whirling disease is on the rise since its introduction in the United States in 1958 and is a health

problem both in fisheries and in wild populations of salmonids. Prevalence of the disease is

dependent on ecological context and interactions among multiple other species, including algal

species such as the also invasive diatom Didymosphenia geminata, the oligochaete worm Tubifex

tubifex, and the myxozoan parasite Myxobolus cerebralis that is the causative agent of whirling

disease in salmonid fishes. D. geminata, a stalk-producing diatom has increased in frequency

worldwide and is now invasive across the United States. The stalks of D. geminata create an

environment suitable for oligochaetes such as T. tubifex. Based on existing data oligochaete

abundance is higher in areas with higher percent cover of D. geminata, and whirling disease

prevalence in trout is 3X higher in streams with blooms. My research further examined why

oligochaetes are more abundant specifically in streams with D. geminata blooms focusing on the

mechanisms that might promote T. tubifex density increases, including predator release. The

results provide some new insights into the multifaceted and complex ecological interactions that

can promote increased amounts of whirling disease in ecologically and economically important

salmonid fishes. I attempt to place these results in the context of a larger collection of ecological

drivers that explain interactions among all the actors involved.

4

Introduction

In the summer of 2013, I conducted a research project on direct and indirect effects of

algal blooms by the diatom, Didymosphenia geminata on the intermediate whirling disease host,

Tubifex tubifex, a freshwater oligochaete worm, at the Rocky Mountain Biological Laboratory

with Dr. Brad Taylor from Dartmouth College. To understand why oligochaetes are more

abundant in streams with D. geminata blooms, I aimed to experimentally test several hypotheses.

My first hypothesis was that the long filaments, or stalks, that D. geminata produces may be

providing a refuge for oligochaetes against predatory invertebrates such as the stone fly

Hesperopola pacifica. I hypothesized that oligochaete mortality from invertebrate predators

would be lower when D. geminata stalks are more abundant. Second, since D. geminata stalks

form dense, thick mats on the streambed, I aimed to test the hypothesis that D. geminata mats

engineer an ecosystem that is more suitable for T. tubifex who prefer slow-moving backwaters

habitats (Larned, 2011), which are rare habitats in most high-gradient Rocky Mountain streams.

My experimental work provided a snapshot view of how substrate may directly or

indirectly impact the abundance of T. tubifex, which appears to be a key regulator in whirling

disease prevalence (McMurtry, 1983). In order to better understand how important the latter

factor may be given the complex life-cycle of Myxobolus cerebralis and the actors with whom it

interacts, I also looked more broadly at the whirling disease literature in order to properly frame

my results. In particular, I performed a literature review on those other factors and assembled a

conceptual model on what might drive increases in whirling disease, and where gaps in our

knowledge exist of this system overall. This model could serve as a means to generate new

hypotheses and to integrate both my current and previous work.

5

I have organized this work to first provide a brief background on the history of whirling

disease and the methods of introduction of the parasite M. cerebralis into the United States.

Next, the life cycles and habitat preferences of each main species involved in whirling disease

are described and it is explained how each of these actors interact with each other in their

environment to make this disease so successful. Relevant literature is incorporated on

environmental conditions that influence availability of preferable habits, rates of distribution,

establishment in these habitats, and the susceptibility of trout to infection with whirling disease

in these habitats. Details on my experimental design are provided as well as results related to

my hypotheses that the diatom blooms both provide predator refuge and engineer a more

suitable habitat for T. tubifex, and then close by again broadly considering management

implications for my work.

Background

Disease ecology is an interdisciplinary field that utilizes ecological theory and practice to

understand living, biotic and non-living, abiotic drivers determining host and pathogen

interactions and their ultimate impacts on human-relevant issues such as health. Living systems

are dynamic and complex and their behavior may be hard to predict from the properties of

individual parts, requiring an integrative discipline that can take a systems view of emerging

diseases and their course. Studying the ecology of specific diseases and creating models to better

understand interactions among wildlife hosts, vectors, and pathogens, can be a tool to help

determine risk, manage, and possibly prevent disease. My research aim was to both better

understand the interactions among the pathogens, hosts, and their environment; both in the broad

picture, as well as perform a targeted set of field experiments. The interest here was meant to

clarify a complex system and many strands and pieces of knowledge found in the literature,

6

informed by my own fieldwork. Below I provide an overview of that literature as background

before describing my experimental work in the field.

Whirling Disease

Whirling disease is on the rise since its introduction in the United States in 1958 and is a

health problem both in fisheries and in wild populations of salmonids (Gilbert, 2003). In

Colorado alone, 14 out of 15 major drainages in Colorado tested positive for whirling disease

(Nehring, 2003). Also in Colorado, recreational fishing generates millions of dollars in economic

activity each year, and since whirling disease has been introduced, some locations in Colorado

(e.g., Gunnison River) have experienced declines in salmonid density and biomass by as much as

90% (Nehring, 2003; Nehring, 2006).

Whirling disease was first described in rainbow trout in Europe in 1893 when Bruno

Hofer recorded signs and symptoms such as whirling behavior and a blackened caudal region,

and detected microscopic parasitic spores that he named Myxobolous cerebralis (Spaulding,

2007). M. cerebralis, a myxozoan parasite, has been identified as the causative agent of

whirling disease that requires a vertebrate and non-vertebrate host to complete its life cycle. M.

cerebralis is likely to persist in North America and is found in new places every year (Elwell,

2009). When high numbers of parasites are around susceptible fish, there can be high mortality

rates in native trout (Spaulding, 2007).

Much is already know about M. cerebralis and its salmonid host, yet less is understood

with regard to M. cerebralis and its T. tubifex host (Gilbert, 2003). T. tubifex is a common,

native freshwater worm that serves as the intermediate host for the myxozoan parasite and thus

7

is required for the whirling disease parasite to complete its life cycle. Many environmental

variables, including substrate properties, can influence habitat selection by tubificids, which has

produced conflicting results (McMurtry, 1983). Changes that affect T. tubifex abundance and

density thus have the potential to strongly affect downstream outcomes of whirling disease

(Minchella, 1991). What is less well understood is how and why changes in substrate influence

T. tubifex density.

Myxobolus Cerebralis lifecycle and requirements

Myxobolus has a two-stage life-cycle, consisting of triactinomyxons(TAMs) that infect

salmonids and develop in T. tubifex, and myxospores that infect T. tubifex develop in salmonids

(Wolf and Markiw, 1984). The life cycle of M. cerebralis starts when myxospores are released

from infected fish (Figure 1). Next, myxospores must be ingested by T. tubifex. The germ cell in

the myxospore then migrates to the intercellular space of the intestinal epithelium where it

undergoes reproduction and development into a TAM over a (70-120 day period). TAMs are

released from feces of T. tubifex and then enter the water column. TAMs are short-lived and

attach to fish and penetrate the skin. Once in the fish, reproduction takes place in the epidermis

and the parasites migrate through central nervous system to the associated cartridge where they

mature to plasmodia containing vegetative nuclei and generative cells. Approximately 80 days

after exposure to TAMs, the generative cells initiate sporogenesis to produce myxospores at

which stage whirling disease is manifested. The only way fish can become infected is if T.

tubifex ingest the myxospores and shed TAMs (Gilbert, 2003).

8

Figure 1: M. cerebralis lifecycle. Discus Club Romania, 2007

M. cerebralis infection timing is strongly tied to water temperature where disease

outbreak is strongly related to summer increases in water temperature (Allen, 2002). The ability

of M. cerebralis to complete its life-cycle in both hosts in order to produce both parasite spore

stages is critical for the continuation of the disease ( El- Matbouli, 1999).

Didymosphenia geminata

Didymosphenia geminata, a stalk-producing diatom, has increased in frequency

worldwide. Blooms of this algae have been reported in North America, Europe, Asia, and New

Zealand (Figure 2) and the diatom is believed to be expanding its geographic range in North

America where research is now just starting to be conducted (Spaulding, 2007). These single-

celled algal blooms result from excessive extracellular stalk production by individual cells that

form a contiguous mat covering the stream bottom (Spaulding, 2007). D. geminata is now

9

causing concern because of the possible impacts on rivers where blooms occur and on the

salmonid fisheries in these rivers; in particular, D. geminata blooms have the potential to alter

stream ecosystems by impacting the ecosystem’s metabolism, nutrient cycling, hydraulics and

food web (Spaulding 2007). The thick mats (e.g., 2-5 cm) that can cover much of the streambed

lead to changes in invertebrate species diversity and population sizes that may propagate up the

food web to affect fish populations (Spaulding, 2007). D. geminata forms thick mats that cover

>75% of the stream area and extend for 1 km, and may persist for several months of the year

(Spaulding, 2007). D. geminata cells produce copious amounts of extracellular stalk material

that form thick benthic mats, or blooms.

D. geminata thrives in a wide range of conditions such as both low-nutrient and high-

nutrient lakes and slow-moving shallow water to waters with greater depth and increased flow.

D. geminata mats alter the water velocity along the stream bottom in ways that may be

important to invertebrates such as oligochaetes that prefer slow-moving water and areas with

stable sediments (Larned, 2011). Hiner (2011) found that water flow rate has an effect on the

propagation of M. cerebralis, and habitats with lower velocity were found to promote higher

prevalence of infection and greater proliferation of the parasites invertebrate host T. tubifex

leading to greater severity of infection of whirling disease in fish (Hallett, 2007). D. geminata

may also provide a food source for T. tubifex by trapping particulate organic matter that is

readily colonized by bacteria, and D. geminata mats may also provide refuge for T. tubifex by

seeking protection from invertebrate hosts in the long stalks that D. geminata produces. D.

geminata is a good resource for T. tubifex due to their preference for fine silt and clay

substratum (i.e., depositional areas) that are areas with abundant microflora, which are a source

of bacterial food for oligochaetes (Kreuger, 2006).

10

Figure 2: World-wide distribution of records for D. geminata. Past and recent records show the

range expansion of D. geminata (Whitton, 2009).

Tubifex tubifex

T. tubifex is a habitat generalist and is extremely tolerant of a wide range of

environmental conditions (Kerans, 2002). McMurtry (1983) found a significant correlation

between abundance of heterotrophic aerobic bacteria in sediment and tubificid preference and

believes worms were attracted to the leaves because of the microfloras associated and provided

11

bacterial food. T. tubifex prefers silt and clay substratum to coarser substratum and have been

found to reproduce faster in silt sediment than other substrates (Arndt. Et al, 2002).

Trout

M. cerebralis affects several species of trout and salmon (Hoffman, 1990; Hedrick et al.,

1998; Gilbert and Granath, 2003). The infection of M. cerebralis that gives rise to whirling

disease has caused reductions in populations of rainbow trout, cutthroat trout, Yellowstone

cutthroat trout, brook trout, Chinook salmon, and kokanee salmon (Hedrick et al., 1998,

Macconnell and Vincent, 2002) The main sign of infection of whirling disease in salmonids is a

tail chasing behavior that causes the infected fish to whirl. As the disease progresses in the fish

skeletal deformation, misshaped heads, jaws and gill covers, and spinal curvature can develop

(Hnath, 1993).

The severity of the infection is evaluated by presence of clinical signs of whirling and or

a darkened caudal region, prevalence of infection, severity of microscopic lesions, and spore

counts 5 months after exposure (Hendrick, 1999). The development and severity of whirling

disease is known to be dependent on the age of fish when first exposed to the infective

triactinomyxon stage of Myxobolus cerebralis and the density of TAMs to which the fish are

exposed (Ryce, 2005). The time at which a salmonid is infected with M. cerebralis is crucial in

determining the likelihood and severity of infection. Ryce (2005) found that Rainbow trout must

be both 9 week post-hatch or older and at least 40 mm in fork length at time of exposure to

exhibit enhanced resistance to whirling disease.

12

Species Interactions

This study involved interactions among the algae, D. geminata, the oligochaete worm T.

tubifex, and the parasite M. cerebralis that is the causative agent of whirling disease in salmonid

fishes. M. cerebralis has a complex life cycle with two hosts and two intermediary spore stages

(Kerans, 2002). T. tubifex is one of the hosts in the two stage life cycle of Myxobolus cerebralis,

a myxozoan parasite that causes whirling disease in salmonid populations in the United States

(Gilbert, 2003). The myxozoan is spread by the death of infected salmonids near susceptible T.

tubifex and salmonids and it has been found that the death of just one infected salmonid upstream

is sufficient for establishment of M. cerebralis in a stream (Hallett, 2007).

T. tubifex distribution is strongly correlated to the distribution of leaf litter (Lazim, 1987)

and the deposition of fine particles (McMurtry et al, 1983; Bartholomew et al, 2005). Thus, D.

geminata has the potential to increase the available habitat for oligochaetes such as T. Tubifex,

and thereby increase the prevalence of M. cerebralis. Further, T. tubifex has been shown to itself

to be more abundant in streams with D. geminata mats (Kilroy et al, 2009). Moreover, brook

trout in streams with D. geminata blooms exhibit a higher M. cerebralis prevalence than brook

trout in streams without blooms (B. Taylor unpublished data).

Materials and Methods

Patterns of oligochaete abundance and D. geminata cover

To measure oligochaete abundance in relation to D. geminata cover in the East River of

Gothic, Colorado (Figure 3), I measured the percentage of the stream covered by algae with a

13

square-meter quadrant placed over various sections of the river. Assessment of algal cover was

made from each grid for a total percent of 0, 2, 5,35,55,90, and 95% D. geminata cover. Once

the percentage of algal cover was determined, sections of the sediments in the river were placed

into plastic bags and immediately filled with alcohol and red dye. Samples were then taken back

to the lab to be processed. This process was done by pouring the sample into a sifter and

finishing the collection and then placing the contents into a plastic tub to be sorted.

Macroinvertebrates were picked out with tweezers and placed into glass jars filled with alcohol

and labeled according the amount of D. geminata percentage cover that correlated with the

collection. Oligochaetes were then counted from the collection macroinvertebrates.

The relationship between the percent of D. geminata cover and the number of

oligochaetes found along a D. geminata cover gradient was calculated via a regression analysis

with oligochaete abundance as the response (e.g y-axis) variable and D.geminata cover as the

predictor, in order to determine how well cover might explain variation in oligochaetes.

D. geminata as a refuge from predation for T. tubifex

To test the hypothesis that D. geminata mats provides a refuge from invertebrate

predators for oligochaetes such as T. tubifex, I performed an experiment testing the effects of D.

geminata and the predatory stonefly Hesperoperla pacifica on T. tubifex mortality in Gothic,

Colorado on the upper East River near the Rocky Mountain Biological Lab. I used 80 streamside

flow-through tanks where stream water was gravity fed into the tanks from a stream located 200

m upslope draining the side of Gothic Mountain into the tanks inside of a portable weatherport. I

randomly assigned four treatments to each tank where twenty tanks received substrata covered

with polyester pillow stuffing to mimic the structural properties of D. geminata mats and one H.

14

pacifica stonefly, twenty tanks received substrata without the mimic material and one H. pacifica

stonefly, twenty tanks received substrata covered with the mimic material and no predators and

the remaining twenty tanks received substrata without the mimic material and no predators,

which served as controls for losses due to factors other than stonefly predation. Three T. tubifex

collected from the East River were added to each tank in the morning of 29 July 2013 and H.

pacifica collected from Avery Creek were added to specific predator assigned treatments three

hours later. After ~18 hours, I removed the stoneflies and counted the number of T. tubifex

remaining in each tank and whether they had been eaten, were dead, or were found to be bitten,

which was indicated by a bite mark out of T. tubifex by the stonefly. Because no T. tubifex were

dead or missing from the control treatments, and there was no significant block effect associated

with tank arrangement, I used a t-test to test for differences in stonefly-induced mortality

between treatments with and without the D. geminata mimic.

D. geminata as a stream flow refuge for T. tubifex

To test the degree to which D. geminata mats provide a refuge from high water velocity for

oligochaetes such as T. tubifex, I covered individual substrata with an artificial mimic of D.

geminata stalks and placed these substrata as well as substrata without the mimic in fast and slow

water velocity areas of the stream. Substrata were deployed for 3-4 weeks with the aim that the

number of oligochaetes colonizing would be quantified. Treatment rocks (8 mimic and 8 non-

mimic covered) were placed along a water velocity gradient (16 sites), to test the hypothesis that

diatom stalks provide a critical habitat or refuge especially at the fastest water velocities. I

predicted that oligochaetes would be more abundant on rocks with the mimic material, and that

the difference in number of oligochaetes between rocks with and without the mimic material

would be greatest at the fastest water velocities. Although data is still be counted, I plan to use an

15

ANCOVA to test for differences in oligochaete abundance between substrates with and without

mimic material along a water velocity gradient with water velocity as the covariate. A

significant interaction between water velocity and substrate type would suggest that D. geminata

is an important refuge or habitat for oligochaetes in fast flowing streams.

Figure 3: East River and Copper Creek field sites at the Rocky Mountain Biological Laboratory

in Gothic, Colorado.

16

Results

Patterns of oligochaete abundance and D. geminata cover

Oligochaete abundance increased along D. geminata cover gradient in the East River

(Figure 4 and 5). A regression analysis was done to show the variation of invertebrates found in

various samples, and the results showed that there is a relationship in the percentage of D.

geminata and the oligochaetes found per square meter.

Table 1: Number of oligochaetes found in relation to the percent of D. geminata cover.

Percent of D. geminata

cover

N # oligochaetes

found

0% cover 1 0

2% cover 1 0

5 % cover 1 2

35 % cover 1 1

55 % cover 1 3

90% cover 1 9

95% cover 2 1.5

17

Figure 5: Relationship between percent of D. geminata cover and number of oligochaetes found

along a D. geminata cover gradient. (R^2=0.5321, P-value=0.0628). Given the order of

magnitude differences in the response values, the y- axis was log 10 transformed to show that the

oligochaetes found per square meter most closely follows an exponential increase with a linear

increase in a percentage of D. geminata cover. The p-value is marginal, and although we cannot

falsify the hypothesis of no relationship between cover and oligochaete abundance, we also lack

power given our sampling.

R^2=0.5321, P-value=0.0628

18

Figure 6: The stonefly H. pacifica consumed, killed, or injured nearly twice as few T. tubifex in

treatments with the D. geminata mimic relative to treatments without D. geminata mimic (t38 =

3.19, P = 0.0014; Fig. 4). This data shows that the rocks that were covered in D. geminata mimic

were beneficial to the survival of T. tubifex from the stonefly H. pacifica. The body size of H.

pacifica was not different between treatments (t38 = 0.43, P = 0.6).

T38=3.19, P=0.0014

19

Table 2: East River Flow Refuge Experiment Table

Multiple variables were measured in the East River for the flow refuge experiment.

(Mean: Depth: 0.26m, Velocity0.25 m/s, Temperature: 15.32 c, Mimic rock: 208.4cm, Non-

mimic rock: 200.93 cm)

Rock Number Water Depth

(Meters)

Water

Velocity

(Meters per

second)

Water

Temperature

(Degrees

Celsius)

Distance from

shore of

mimic rock

(centimeters)

Distance

from shore

of non-

mimic rock

(centimeters)

1 0.21 m 0.0983 m/s 15.41 50 cm 69 cm

2 0.22 m 0.0575 m/s 15.53 70 cm 83 cm

3 0.21 m 0.0001 m/s 15.54 112 cm 135 cm

4 0.22 m 0.022 m/s 15.92 110 cm 100 cm

5 0.22 m 0.4354 m/s 15.48 197 cm 161 cm

6 0.21 m 0.4511 m/s 15.34 335 cm 301 cm

7 0.19 m 0.0195 m/s 15.26 70 cm 46 cm

8 0.22 m 0.3325 m/s 15.23 283 cm 306 cm

9 0.22 m 0.4749 m/s 15.18 225 cm 210 cm

10 0.2 m 0.3379 m/s 15.15 335 cm 305 cm

11 0.23 m 0.7513 m/s 15.1 423 cm 423 cm

12 0.2 m 0.0398 m/s 15.12 38 cm 65 cm

13 0.24 m 0.1399 m/s 15.13 585 cm 565 cm

14 0.24 m 0.2847 m/s 15.2 148 cm 133 cm

15 0.2 m 0.3048 m/s 15.22 145 cm 112 cm

20

Table 3: Summary of disease drivers in main species involved in whirling disease. This table

lists species that play major roles in whirling disease, the top five ecological drivers that

influence the success of each species, which in turn could contribute to the prevalence of

whirling disease.

1-20 Spaulding, 2007, Larned, 2011, Whitton, 2009, Kilroy, 2009, McMurty, 1983, Kerans, 2004,

Lazim, 1987, Kruger, 2006, Hallett, 2007, Hiner, 2001, Hallett, 2007, Allen, 2002, Gilbert,

2003, El-Matbouli, 1999, Ryce, 2005, Hoffman, 1990, Schlister, 2002, Hendrick, 1999, Nickum,

1999, Nehring, 2006

D. geminata T. tubifex M. cerebralis Trout

Habitat preference

1,2

Habitat preference

2,4,5,6,7,8

Water flow

9

Age and size of trout

at time of infection

12,15

Season length

1,3

Susceptibility of

infection

9

Lifespan of spores

6

Prevalence of M.

cerebralis is water

1,15, 16

Range expansion

1,3,4

Presence of D.

geminata

4

Water temperature

12

Time and means of

stocking

15,17

Stalk length

1

Water velocity

10

Susceptible T. Tubifex

9

Severity of infection

18,19

Means of introduction

1,4

Heterotrophic bacteria

in sediment

5,8

Actinospore- Triactinomyxon

lifecycle

6,13,14

Drainage Location

20

21

Discussion

The impact of an invasive diatom on oligochaetes and their predators

This study focused on the effects of the diatom D. geminata on the intermediate host of

whirling disease, T. tubifex. I asked two questions – does D. geminata cover impact abundance

of T. tubifex and if so, is part of the explanation for this relationship based on predator avoidance

potential afforded by the diatom were tested experimentally at the Rocky Mountain Biological

Lab. Percent of D. geminata cover present did have an effect on abundance of oligochaetes. This

pattern of increased abundance of oligochaetes could be explained by the habitat preference of

oligochaetes to fine particles that D. geminata mats trap (Larned, 2011).

The predator refuge experiment data illustrates that treatments with the D. geminata

mimic did provide a refuge for T. tubifex from the predatory stonefly H. pacifica. The higher

mortality rate of T. tubifex in treatments without the D. geminata mimic shows that T. tubifex

was found eaten, dead, or bitten more often due to a lack of protective environment in which to

find refuge. The patterns observed in a controlled experimental setting illustrate the habitat

refuge that D. geminata may be providing for oligochaetes in East River from predatory

stoneflies.

Multiple variables were measured in the East River for the flow refuge experiment. Based

on my hypothesis that T. tubifex finds a refuge in D. geminata from fast moving streams, it was

important to measure variables of the rock sites to make conclusions about which areas of the

stream had a higher velocity and depth that could be contributing to the amount of variables that

could be contributing to T. tubifex finding refuge in D. geminata. Final data on oligochaete count

22

is currently being processed by Dr. Brad Taylor. The predicted results are that there will be a

higher number of oligochaetes found on mimic D. geminata rocks than non-mimic D. geminata

rocks

Prior to my work, and based on preliminary work in Rocky Mountain Biological Lab, it

was known that the continued spread of D. geminata may impact abundances of T. tubifex, but it

was not known if this was related to changing habitats, or reduced predation, or both. My work

shows that both are likely to be happening, at least at the site I chose to examine. One outcome

is that simple measures of cover percent in relation to oligochaete abundance was a somewhat

weak result, perhaps dependent on sampling, compared to the predator experiments. More

systematic experiments may be needed to properly determine how and how much oligochaete

abundance changes are related to different predictors

Ecological Drivers

While M. cerebralis has been found in many watersheds in the US, distribution and

severity of whirling disease on fish populations vary regionally and locally (Nickum, 1999). A

main finding from my literature review related to disease prevalence and severity is that habitat

factors act synergistically across different species – warming, for example, may lead to both

increased algal invasions and increased amount of D. geminata, while also benefiting the

myxozoan parasite. How these timing events play out with regard to resistance of the trout

remains one of many unanswered questions that deserve further study, perhaps using forecasting

approaches based on a more formal model that can be constructed based on my initial assembly

of drivers presented here.

23

The best current way to manage the disease is for fish to be raised in a M. cerebralis

spore-free source of water, or in a farm setting, they need to be constantly monitored for the

presence of spores (Hoffman, 1990). Another means of managing the disease is the elimination

of susceptible T. tubifex, the parasite's alternate host, and their habitat, which could interrupt the

parasite's life cycle and prevent fish infections.

Each of the species that interact in this system play a key role in whirling disease. Further

research on parasite, hosts, and habitats that support them will be necessary to better understand

whirling disease outbreaks and to meet the goals of management or eradication of the disease.

Also needed is further research on the genetics of M. cerebralis infection in relation to

susceptible T. tubifex, and whether ecological or genetic variation within oligochaete host

populations may be responsible for determining whirling disease risk in a body of water (Kerans,

2004). In addition, examination how D. geminata will expand its range with a changing climate

and continue to alter ecosystem structure will be critical for forecasting how T. tubifex

populations may grow. These efforts may provide a data and analysis driven basis for decision

about how to manage watersheds.

The Bigger Picture of Whirling Disease: Towards A More Conceptual Model

Although I only covered a small piece of the larger puzzle related to whirling disease,

part of my Honors work was developing a larger-picture of how the different players and their

interactions may drive disease emergence and persistence and the directionality (positive or

negative effects on the disease) with the goal of documenting best- case and worst-case scenarios

for the spread of the disease. The ecological drivers above and some information about strength

of interactions and impacts allow me to assemble some initial ideas about change dynamics. For

24

example, a worst-case scenario would involve warming temperatures that may allow D.

geminata to continue blooming, with its filaments reaching their maximum length (Spaulding,

2007). Abundance of D. geminata would create a yet more suitable habitat for T. tubifex (Lazim

and Learner, 1983). At the same time, warming temperatures would also benefit the myxozoan

parasite given its preference for activity in warmer conditions (Allen, 2002).

Management solutions that could implement best-case scenarios for avoiding whirling

disease outbreak would involve partially controlling D. geminata in order to limit T. tubifex

prevalence across sites. Further, year-round cooler water temperature would further limit both

D. geminata and M. cerebralis, as would stocking streams at 9 weeks old or older so they would

be less likely to be affected by M. cerebralis if there happened to be any left in the water (Ryce,

2005).

25

Acknowledgements

This research was conducted first and foremost thanks to my parents William and Allyson Byle

for introducing me to the Rocky Mountain Biological Lab. I would like to thank Dr. Brad Taylor

from Dartmouth College for the intellectual design of the field component of this study at The

Rocky Mountain Biological Laboratory. I would like to thank my primary advisor, Dr. Robert

Guralnick at the University of Colorado Boulder for all of his assistance in the conceptual model

of this project and his unconditional support during this process. I would like to thank Barbara

Demmig-Adams for her support through the honors program and Dr. Diane Mcknight for serving

on my honors committee. I would like to acknowledge Dr. Alexander Cruz and Dr. David Stock

for their recommendations to the Rocky Mountain Biological Laboratory where the field

component of this project took place. Thank you to Dr. Jennifer Reithel, Dr. Emily Mooney,

Shannon Sprott, and Billy Barr for all of their support at The Rocky Mountain Biological

Laboratory. Finally, last but certainty not least, a special thank you to the Rocky Mountain

Biological Laboratory donors for their assistance in the funding of this project.

26

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