DISEASES OF AQUATIC ORGANISMSDis Aquat Org

Vol. 71: 33–49, 2006 Published July 11

INTRODUCTION

Euphausiids, commonly known as krill, are a compo-nent of zooplankton and micronekton that dominatecrustacean productivity of the world’s oceans, espe-cially in wind-driven coastal upwelling regions and the

Antarctic Ocean. Several euphausiid species are fishedcommercially, and are an important link between pri-mary productivity and higher trophic levels in pelagicecosystems (Nicol & Endo 1999). The ecology and lifecycles of euphausiids have been extensively studied,but sources of mortality are poorly understood.

© Inter-Research 2006 · www.int-res.com*Email: jagomezg@ipn.mx

Discovery of a ciliate parasitoid of euphausiidsoff Oregon, USA: Collinia oregonensis n. sp.

(Apostomatida: Colliniidae)

Jaime Gómez-Gutiérrez1, 4,*, William T. Peterson2, J. Frank Morado3

1College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Administration Building, Corvallis,Oregon 97331-5503 USA

2NOAA/NMFS, Hatfield Marine Science Center, 2030 S. Marine Science Drive, Newport, Oregon 97365, USA3NOAA/NMFS, Alaska Fisheries Science Center, Resource Assessment and Conservation Engineering Division,

7600 Sand Point Way NE, Seattle, Washington 98115-0070, USA

4Present address: Centro Interdisciplinario de Ciencias Marinas, Departamento de Plancton y Ecología Marina, Av. IPN s/n,A.P. 592, C.P. 23096, La Paz, Baja California Sur, México

ABSTRACT: An apostome ciliate, Collinia oregonensis n. sp., is reported inhabiting the cephalotho-rax and abdomen of 3 euphausiid species from the Oregon–Washington coast: Euphausia pacificaHansen, 1911, Thysanoessa spinifera Holmes, 1900, and Thysanoessa gregaria G.O. Sars, 1883. Thisciliate is the 7th species described for the genus Collinia and the 2nd species known to infecteuphausiids. Disease progression and ciliate morphology are described using (1) modified protargolstain, (2) hematoxylin counterstained with Fast Green, and (3) Scanning Electron Microscopy (SEM).All endoparasitic developmental stages (trophont, tomont, tomitogenesis, protomite, and tomite) ofC. oregonensis are astomatous and possess between 14 and 22 kineties. C. oregonensis is smallerthan C. beringensis Capriulo & Small, 1986, which infects the euphausiid Thysanoessa inermisKrøyer, 1846 in the Bering Sea and which possesses between 24 and 80 kineties. The ciliate is a para-sitoid because it must kill the host to complete its life cycle. Infections and mortalities in multiple hostspecies likely reflect the virulent nature of the ciliate. Adult euphausiids infected with this parasitoidpossess a swollen and bright orange cephalothorax. C. oregonensis feeds and proliferates insideeuphausiids, producing fulminating infections that rupture the cephalothorax and release large num-bers of tomites into the surrounding water. After several hours in the free swimming stage undershipboard conditions in the present study, the tomites adhered to each other, forming filaments.Infection rates ranged between 3 and 20% within individual euphausiid aggregations, but infectedaggregations were randomly and sparingly distributed. Infected euphausiids were found at 6.7% of316 stations sampled during 3 summer cruises. No infected euphausiids were collected in winter.Because E. pacifica and T. spinifera account for about 90% of the euphausiid standing stock in thenorthern California Current System, this parasitoid ciliate may have a significant impact oneuphausiid population abundance, distribution and secondary productivity.

KEY WORDS: Euphausiids · Apostomatidae · Ciliates · Endoparasitoid · Colliniidae · Collinia ·Oregon coast

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Dis Aquat Org 71: 33–49, 2006

Gómez-Gutiérrez et al. (2003) and Gómez-Gutiérrez(2003, 2004) reported on an undescribed parasitoidciliate of the genus Collinia responsible for a massmortality of the euphausiid Euphausia pacificaHansen, 1911 in Astoria Canyon, Oregon, USA. A par-asitoid is an organism that must kill the host to com-plete its life history, sensu Lafferty & Kuris (2002) andalthough the term is most frequently applied to anarthropod that infects another arthropod, a parasitoidmay be any eukaryote meeting the above criterion.The present ciliate also infects 2 other euphausiid spe-cies along the Oregon–Washington coast: Thysa-noessa spinifera Holmes, 1900 and Thysanoessa gre-garia G.O. Sars, 1883, indicating that this parasitoidmay infect other euphausiid species from the region orthat similar ciliate-euphausiid relationships may occurworldwide. E. pacifica and T. spinifera are widespreadand are the most abundant euphausiids along thenorthern California Current System, accounting for~90% of euphausiid abundance (Gómez-Gutiérrez etal. 2005). The character of the euphausiid mortalityobserved in Astoria Canyon challenges the widelyheld notion that mortality in pelagic organisms resultsprimarily from predation and starvation (Gómez-Gutiérrez et al. 2003), and ultimately also questions thetraditional food web perspective (Dobson & Hudson1986). The significance of this discovery is that para-sitoids may drive significant changes in the dynamicsand secondary productivity of euphausiid populations.From a theoretical perspective, terrestrial parasitoidsare able to regulate/affect their host populations; how-ever, field results have been difficult to interpret (Has-sell & Godfray 1992). As a result, the impact of para-sitoids on the population dynamics of zooplankton andfood webs has been generally ignored or at leastgreatly overlooked by modern marine pelagicresearch.

The genus Collinia belongs to the order Apostom-atida. All apostome ciliates live in a symbiotic relation-ship with marine invertebrates and those with knownlife cycles have major features in common (Fig. 1 fromBradbury 1966). Crustaceans are the most common,and probably the ancestral, hosts to apostomes. Apos-tome life histories are closely tied to the molting cycleof their crustacean hosts and are complex, involving anobligatory, polymorphic series of stages (Chatton &Lwoff 1935, Trager 1957, Grimes & Bradbury 1992).Apostome ciliate infections have been discovered on orin copepods, amphipods, isopods, ostracods, decapods,leptostracans, ctenophores, anemones, cephalopods(Chatton & Lwoff 1935, Bradbury 1994, Morado &Small 1995, Ohtsuka et al. 2004) and more recently ineuphausiids (Capriulo & Small 1986, Capriulo et al.1991, Gómez-Gutiérrez et al. 2003). Endoparasitoid cil-iates of the genus Collinia are well recognized as being

among the most virulent of all apostome ciliates, hav-ing a fulminating lethal effect on the host (Morado &Small 1995). Thus, it is predicted that members of thisgenus should be able to infect more than 1 host speciesto be stable in a parasitoid-host relationship. Indeed,the single ciliate species described here infects at least3 common epipelagic euphausiids along the Oregon–Washington coast.

Infection of Thysanoessa inermis Krøyer, 1846 by asimilar endoparasitic apostome ciliate, Collinia berin-gensis Capriulo & Small, 1986, was previously discov-ered in the Bering Sea (Capriulo & Small 1986, Capriuloet al. 1991). Because these 2 reports were based on pre-served specimens, the appearance of infected euphausi-ids and the lethal effects of the ciliate could not bedetermined. The new ciliate species described here wasdiscovered within the cephalothorax of 3 euphausiidspecies off the Oregon coast while experiments werebeing conducted on live euphausiids (~1100 femalesincubated between 1999 and 2003) to estimate femaleegg production and molting rates (Gómez-Gutiérrez2003). Live, infected euphausiids were collected and ob-served during shipboard incubations in order to describethe signs and progression of this parasitoidism. Thepresent study is an extension of Gómez-Gutiérrez et al.(2003) and describes the morphology and life-stages ofC. oregonensis n. sp. It also provides biological informa-tion on the possible impact of this parasitoid on euphau-siid population abundance and distribution patternsin the California Current System.

MATERIALS AND METHODS

Sampling protocol. During an oceanographic cruiseon the night of July 9, 2000, a Euphausia pacifica sur-face swarm was visually noted in which some individ-uals had an unusual orange and swollen cephalothorax(n = 21). After collection of several euphausiids with a20 l bucket it was discovered that they were infectedwith an unknown ciliate species. As part of the NorthEast Pacific (NEP) US GLOBEC sampling program,more infected euphausiids were collected (see below)at night during 10 spring and summer oceanographiccruises between 2000 and 2003 along the OregonCoast. These cruises were carried out on board the RV‘Wecoma’ (Oregon State University, OSU) and the RV‘New Horizon’ (Scripps Institution Oceanography,SIO). Live, infected euphausiids were observed duringshipboard incubations at constant temperature and indarkness to study the progression of the ciliate infec-tion. Live samples were captured with 4 differentplankton nets during the night: (1) a 4 m long conicalnet with 333 µm mesh net and a 1 m diameter mouthhauled from 20 m depth to the surface and designated

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Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

‘live net’, (2) a multiple opening-closing net and envi-ronmental sensing system (MOCNESS) with 1 m2

mouth area and 333 µm mesh net, sampling typicallyin 10 strata from 350 m depth to the surface, (3) aBongo net with 200 µm mesh net towed obliquely, and(4) a 20 l bucket for surface swarms. Some infectedspecimens were preserved immediately upon collec-tion with Bouin’s fixative, 4% formalin buffered withsodium borate, 96% alcohol or 2% Lugol’s solution.Most infected euphausiids were kept alive to beobserved shipboard, and then preserved after deathwith Bouin’s fixative.

Incubation and description of the ciliate. Live,infected euphausiids, identified by their orange andswollen cephalothorax, were transferred into 1 l bottlesfilled with surface seawater and incubated on-boardship inside a cold room at 10°C ± 0.5°C for several daysuntil they died. This was the average temperature ofthe upper 15 to 20 m of the water column over the innerand middle shelf off Oregon. The euphausiids weremonitored every 2 to 4 h during the night and every 6 hduring the day for behavior, disease progression andtime and manner of death. The sex of the euphausiidswas determined by the thelycum (females) or thepetasma in the first abdominal pair of appendages(males), and individuals were measured for totallength (mm) from behind the base of the eye stalk tothe tip of the telson as soon as possible after collection.Sequential photographs of the external appearance ofthe host were taken using a digital camera (3.3 × 106

pixels resolution). After a euphausiid died, the ciliatesinside the carcass and/or those swimming in the sea-water were sampled every 2 to 4 h and preserved withBouin’s fixative to obtain ciliates in different develop-mental stages. To describe the different life-stages ofthe apostome ciliates inside the hosts we used (1) amodified Protargol staining technique (Tuffrau 1967),(2) hematoxylin counterstained with Fast Green (pre-pared by Stephen Landers, Troy University, Alabama),and (3) Scanning Electron Microscopy (SEM) of sagit-tal and transverse cuts of the euphausiids using asurgical knife after the specimen had dried. SEMmicrographs were taken with an Amray 3300FE fieldemission scanning electron microscope at the BotanyDepartment at Oregon State University (prepared byAl Soeldner, OSU).

Experimental infection. To determine the method ofinfection under laboratory conditions, groups of 10 to15 healthy and apparently uninfected adult euphausi-ids were placed in 2.5 l containers and exposed to (a)an infected, but live euphausiid, (b) a recently killedeuphausiid that had burst and released all free-swimming tomites into the container, (c) a deadeuphausiid that had not burst (but with ciliates remain-ing inside the body), (e) filaments of ciliates adhered to

each other in a mucus matrix formed under laboratoryconditions several hours after the euphausiid ruptured,and (e) control incubations without infected euphausi-ids to test if individuals from those particular samplestations were previously infected with the ciliates. Theeuphausiids were monitored at least every 6 h over aperiod of between 7 and 10 d to detect coloration. Pre-served animals were dissected in the laboratory tosearch for ciliates in the hemocoel.

RESULTS

Morphology of the life stages of Colliniaoregonensis n. sp.

The external morphological characteristics of the dif-ferent endoparasitoid life-stages of Collinia oregonen-sis n. sp. are described from ciliates obtained from thecephalothorax of Euphausia pacifica and Thysanoessaspinifera using SEM (Figs. 1 & 2 & Table 1). These arethe first SEM micrographs taken of any species of thegenus Collinia. The morphological characteristics ofthe parasitoid ciliates obtained from both euphausiidspecies indicate that these are the same ciliate species.

Trophont: Young trophonts measure about 35 µmlength in early infection. As each trophont grows (to58–82 µm length and 13–51 µm width, ~0.55 length/width ratio) a total of 18, 20 or 22 long, spiralingkineties develop (Figs. 1B & 2B, Table 1) that extendthe length of the cell. The cilia are so long that thosefrom proximal kineties appear to overlap. The maturetrophont stage is astomatous (no cytostome), with anoval anterior and rounded posterior end. As food accu-mulates within the cells, the trophonts become morerobust, expanding to 2 times their original length andwidth until the cilia of proximal kineties fail to overlap.Protargol staining of the trophont stage shows that thisstage possesses an elongate nucleus and very longcilia presenting a ‘hairy’ appearance (Fig. 3A,B). Theshape of the trophont is dictated by how tightly packedthe ciliates are in the cephalothorax (Fig. 3B). Aschematic diagram of the trophont stage based on pro-targol staining indicates that at this stage the kinetiesclosely join at the anterior end of the cell while they aremore removed at the posterior end (Fig. 4A,B).

Tomont: SEM (Figs. 1D & 2D) and silver impregna-tions (modified Protargol) (Fig. 5A) of the tomontreveal that it has a variable number of spiralingkineties, usually 20, but less frequently they havefewer (16 or 18). The tomont is astomatous, and cellsize ranges from 12 to 58 µm length and 12 to 33 µmwidth, with a length/width ratio of 0.54 (Table 1). Inlive euphausiids with advanced infections, it is com-mon to observe trophont and tomont ciliates swimming

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Dis Aquat Org 71: 33–49, 200636

Fig. 1. Collinia oregonensis n. sp. infecting Euphausia pacifica. Ciliate morphology and chronological sequence of the infection ofthe host. (A) Adult euphausiid in early infection stage (12 to 24 h after the animal was observed to have a healthy appearance).(B) Young feeding stage (trophont) in early infection. (C) Intermediate infection with pale yellow and orange cephalothorax (10 to15 h after pale coloration of carapace was observed). (D) Mature trophont stage. (E) Same specimen in an advanced infectionphase. (F) Reproductive ciliate (tomont stage) ready to start palintomy — the ciliate starts cell division to produce transmissioncells (tomites). (G) Euphausiid bursts after the ciliates digest the internal organs (35 to 55 h after pale coloration was observed).

(H) Dispersion stage (tomite) at first division. Scale bars = 10 µm

Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids 37

Fig. 2. Collinia oregonensis n. sp. infecting Thysanoessa spinifera. Ciliate morphology and chronological sequence of the infec-tion of the host. (A) Adult euphausiid in early infection stage. (B) Young feeding stage (trophont) in early infection. (C) Intermedi-ate infection with pale yellow and orange cephalothorax. (D) Mature trophont stage. (E) Same specimen in an advanced infectionphase. (F) Reproductive ciliate (tomont stage) ready to start palintomy; the ciliate starts the cell division to produce transmissioncells (tomites). (G) Euphausiid bursts after the ciliates digest the internal organs of the euphausiid. (H) Dispersion stage (tomite)

at first division. Scale bars = 10 µm

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in the abdomen and appendages that are relativelymore transparent than the orange colored carapace.The flexible ciliates can squeeze through small pas-sageways of core tissues, moving anterior end first.The tomont divides repeatedly within the euphausiid’sbody cavity until tomites are formed.

Protomite: Variable, even numbers of kineties (16,18, and 20) extend from pole to pole without spiraling.The length of the cells ranged from 21 to 36 µm and thewidth from 11 to 28 µm (Table 1). The cells have a lesselliptical shape (0.67 length/width ratio).

Tomitogenesis: The tomont undergoes palintomy,and with each successive division the kineties shortenand straighten. Daughter cells may or may not be ofequal size (Table 1) and it is not known how manytomites are produced from each tomont (Figs. 1F & 2F).However, since the tomites are large in comparison tothe trophont and tomont, it is likely that only few

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Ciliate stage Number n Length, µm (L) Width, µm (W) W/Lof kineties Average Range SE Average Range SE ratio

Trophont 18, 20, 22 14 35.2 26.9–82.0 3.8 19.3 13.2–51.0 2.5 0.55Tomont 16, 18, 20 88 27.4 12.2–58.3 1.2 14.9 12.0–33.3 0.7 0.54Protomite 18, 20 3 28.5 21.1–36.0 3.7 19.1 11.2–28.1 2.9 0.67Tomitogenesis 14 to 20 37 23.0 15.9–36.7 0.9 19.0 10.0–28.3 0.8 0.82Tomite 14, 16 4 31.8 19.0–37.5 4.3 25.8 15.5–30.0 3.5 0.81Burst (Tomite) 78 30.4 16.0–58.3 1.1 20.2 11.5–33.3 0.6 0.67Not burst (combination 70 24.3 12.2–82.0 1.4 12.9 12.0–51.0 0.8 0.53Tomont-Tomite)

Table 1. Morphometry of the parasitoid life-stages of the apostome ciliate Collinia oregonensis n. sp. (Apostomatida) which in-fects the euphausiids Euphausia pacifica, Thysanoessa spinifera and T. gregaria along the Oregon coast. Measurements are frommicrographs obtained using a Scanning Electron Microscope (SEM). Trophont = feeding stage, tomont = reproductive stage,tomitogenesis = cells in division (palyntomy), tomite = free living transmission stage. Ciliate sizes are also compared between

infected euphausiids that died with and without bursting. SE = standard error

Fig. 3. Collinia oregonensis n. sp. (A) Feeding ciliate stage(trophont) using the modified Protargol staining technique.(B) Highly packed ciliates in the hemocoel of the euphausiids

where the ciliates form rectangular shapes

Fig. 4. Collinia oregonensis n. sp. Feeding ciliate stage (tro-phont) using the modified Protargol staining technique.

(A) Lateral view, (B) posterior view of the cell

A

B

Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

sequential divisions occur. The tomites do not formchains of cells before division (serial division).Hematoxylin stain counterstained with Fast Green(Fig. 6A–D) reveals that the tomont is vacuolate, oftenwith a single central vacuole, its macronucleus is com-posed of distinctly clumped and swirling chromatin,and it possesses 2 micronucleii (Fig. 6A). A series ofmicrographs of ciliates reveal the division of themacronucleus (Fig. 6B–D).

Tomite: The living tomites are almost spherical (0.81length/width ratio), 19 to 37 µm long and 15 to 30 µmwide. They invariably possess reduced numbers ofkineties (16 or in a few cases only 14), which extendfrom pole to pole without spiraling, and bear short ciliaor small, rounded buds (Figs. 1H & 2H, Table 1).Tomites break free from the euphausiid body and usu-ally swim off when the euphausiid burst; however, in afew cases the tomites kill the host without bursting itand leave the exoskeleton once they have consumedthe complete euphausiid.

Diagnosis: Collinia oregonensis is distinguishedfrom C. beringensis (Capriulo & Small 1986), whichalso infects euphausiids, by the smaller number ofkineties (14 to 20). C. gammari is the only species inwhich the number of kineties overlaps with the countin C. oregonensis in the trophont stage, but apparentlythe former species is usually larger than the latter (seeTable 3).

Type host: Euphausia pacifica Hansen, 1911.Type location: North East Pacific, along the Oregon

coast, USA, 43° 13.20’ N, 124° 59.40’ W.Location on host: Endoparasitoid ciliate living in the

hemocoel of the euphausiids.Type of material: Two slides with trophonts and

tomonts of Collinia oregonensis (Holotype USNM1084004 and Paratype USNM 1084005) and 1 Euphau-sia pacifica female in advanced stage of infection(23.7 mm total length, Non-type USNM 1084006). Thematerial was deposited in the International ProtozoanType Slide Collection of the Department of Inverte-brate Zoology of the National Museum of Natural His-tory, Smithsonian Institution, Washington DC, USA.

Etymology: The specific name refers to the geo-graphical location where this parasitoid was dis-covered.

Life cycle of Collinia oregonensis and signs ofinfection in euphausiids

Uninfected live euphausiids have a semi-transparentappearance as all organs (i.e. stomach, intestine, he-patopancreas, heart, and gonad) are visible in thecephalothorax and the abdomen. In the initial stage ofinfection, the euphausiids have a pale, cream-orangespot below the gonad and posterior to the stomach(Figs. 1A & 2A). Under laboratory conditions, infectedeuphausiids turned yellow or cream-orange and devel-oped a swollen carapace between 12 and 24 h after ini-tial incubation (n = 2 euphausiids), and large numbersof trophonts, sometimes acquiring a rectangular shape,were observed tightly packed inside the cephalothorax(Figs. 1B, 2B & 3B). As trophonts increase in size andnumber, the euphausiid cephalothorax swells and

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Fig. 5. Collinia oregonensis n. sp. and C. beringensis. Mor-phological comparison of the reproductive ciliate stage(tomont) of (A) Collinia oregonensis n. sp. (this study) and(B) C. beringensis (Capriulo & Small 1986). Both wereprepared using silver impregnation (modified Protargol)staining technique. C. oregonensis has a smaller nucleus

and fewer kineties (20) than C. beringensis (34 kineties)

Fig. 6. Collinia oregonensis n. sp. Tomitogenesis cells stainedwith hematoxylin counterstained with Fast Green in differentstages of palintomy. The macronucleus is divided and eachdaughter cell possesses a micronucleus. All photographs in thisfigure are courtesy of S. C. Landers (Troy State University, AL)

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changes color from pale white to bright orange(Figs. 1C & 2C). In advanced phases of the infection,the tightly packed ciliates replace all the organs, in-cluding the lipid-rich gonads. All stages of Colliniaoregonensis are astomatous, indicating they possess anosmotrophic feeding mechanism. The exact nature ofhost tissue replacement/disintegration is uncertain,but may be a result of metabolites or enzymes releasedby the proliferating ciliate, which ingest the internaleuphausiid biomass by osmotrophy. As in other endo-parasitic apostome trophont stages (Chatton & Lwoff1935), the mature trophont feeds and increases in cellvolume, then begins to enter into a reproductive stage(tomont) (Figs. 1F & 2F). The tomont divides by palin-tomy (Figs. 1F & 2F), to form a non-feeding ovoid dis-persal stage (tomite) (Figs. 1H & 2H).

Ciliates of the genus Collinia severely affect hostbehavior prior to death. In shipboard experiments,euphausiids in late stages of infection stopped orgreatly reduced their swimming activity. When a highproportion of the ciliates are in cell division (SEMpictures of a sagittally cut euphausiid enabled us toestimate that >60% of the cells are in tomitogenesis),the parasites rupture the cephalothorax-abdomenjunction, killing the host, and release tomites that thenswim inside and outside the euphausiid carcass (Figs.1G & 2G). In about 15% of cases the euphausiid hostsdid not rupture, but the trophonts replaced the remain-ing body tissues. If C. oregonensis has a life cycle sim-ilar to other apostome ciliates, the free-swimmingtomite stage should encounter a new host to encyst asa phoront and repeat its life cycle; however, we wereunable to actually observe this part of the C. orego-nensis life cycle. After the host ruptured, the carcasscontent was completely consumed by the ciliates, leav-ing an empty exoskeleton in less than 12 h. Free-swimming ciliates in experimental bottles (seawaterpreviously sieved with a GF/F filter of 0.22 µm porediameter) usually formed long filaments where theciliates were densely clustered (under laboratoryconditions filament formation usually occurred be-tween 12 and 36 h after bursting, and SEM micro-graphs showed clusters of ciliates adhered bymucilaginous material of unknown composition). Itis not known whether such filaments are formedunder turbulent conditions in the sea.

SEM micrographs of an advanced infected eu-phausiid stage in sagittal (Fig. 7A) and transversal (Fig.7C) sections of the cephalothorax indicate that ciliatesprimarily reside in hemocoel. However, it is interestingthat infected euphausiids can remain alive for severaldays even when heavily parasitized and withstandinga great deal of tissue damage. It is suspected that thisdamage is a result of lysins or toxins produced by theproliferating parasite. After the euphausiid bursts, the

rest of the organism deteriorates (liquefaction of tis-sue). In uninfected euphausiids, the cephalothorax of amature healthy female is filled by vitellogenic ovary,digestive gland, and muscles (Fig. 7B,D). Fig. 7D alsoshows the male’s spermatophore attached to thefemale’s thelycum.

Laboratory experiments. The infection mechanismis not known for any of the Collinia species so fardescribed (Collin 1909, Summers & Kidder 1936, Puy-torac 1953, Puytorac & Lom 1962, Puytorac & Grain1975, Capriulo & Small 1986, Capriulo et al. 1991).Therefore, in the present study several experimentswere performed to determine the possible infectionmechanism. There is no observational or experimentalevidence to suggest that the ciliate can infect theeuphausiid by ingesting released free-swimmingtomites. However, this mechanism could explain thehigh numbers of trophont stages in the early and inter-mediate stages of infection. An optical microscopephotograph of the intestine of an infected euphausiidshows 2 sections with highly aggregated ciliatesaround it, while other sections of the gut are still unaf-fected (Fig. 7E). This was observed in several of theburst euphausiids showing a similar clustered ciliatepattern, which suggests that these are points wherethe ciliate originally infected the host (from the intes-tine duct toward the hemocoel). Another possibility isthat ciliates may be drawn to these points by a highconcentration of absorbed food (osmotrophs), thusC. oregonensis may be a conventional parasite in earlystages of infection and convert to parasitoid status asthe infection progresses toward the death of the host.

Healthy euphausiids exposed to free-swimmingtomites never became infected under shipboard labo-ratory conditions. Most of the healthy euphausiidsmolted on regular schedules and none of them becameinfected even after 7 to 10 d of observation, possiblyindicating that euphausiids may have an effectivedefense mechanism to prevent infection. This hypoth-esis is supported by observations of initially healthyhosts with experimentally induced wounds exposed tofree-swimming Collinia oregonensis from a rupturedinfected host. Ciliates clustered around the experimen-tally induced wounds, but the euphausiid died withoutshowing clinical signs of infection (swollen orangecephalothorax). Furthermore, dissection of originallyhealthy individuals exposed to infected euphausiidsdid not show evidence of ciliate infection. Future labo-ratory infection experiments should consider the possi-bility that (1) infection could require a longer exposureand/or maturation period than that of our shipboardexperiments and (2) that ciliates may use alternative orintermediate hosts before they infect euphausiids.

Host/parasitoid interactions. The total length ofeuphausiids infected with the endoparasitoid (trophont

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Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

to tomite stages) ranged from 16 to 25.5 mm forEuphausia pacifica (Fig. 8A) and 15 to 27 mm forThysanoessa spinifera (Fig. 8B). Prevalence increasedwith size of the host for E. pacifica but this trend wasnot clear for T. spinifera. Only 1 adult T. gregaria,infected by Collinia oregonensis was collected andmeasured 12.8 mm total length. Endoparasitic stagesof C. oregonensis were collected mostly from adultfemale euphausiids (91%, n = 51). No infected larvaeor juveniles were found during 68 shipboard moltingrate experiments (n = 1873 euphausiids incubated byTracy Shaw et al., Hatfield Marine Science Center,

Newport, Oregon, pers. comm.), suggesting that thosestages are seldom, if ever, infected by C. oregonensis.

The average time between early ciliate infection andthe death of the euphausiid by bursting was 41 h (SE ±9.3 h, n = 9), and those that did not burst died within77 h after infection (SE ± 7.7 h, n = 9). The time of thehost’s death after infection was significantly positivelyassociated with host biomass expressed as body car-bon weight (Fig. 8C). Thus, as host biomass increases,the time spent inside the host also increases, suggest-ing that the number of ciliate cells is proportional tobody biomass. Several infected euphausiids molted,

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Fig. 7. Euphausia pacifica. (A) Sagittal section of a euphausiidshowing the ciliate-infected region in the cephalothorax.Most of the Collinia oregonensis are found in high densities inthe gonad and hepatopancreas region. (B) Sagittal section ofa healthy female showing the gonad and muscles in a semi-empty hemocoel. (C) Transverse section of the infectedeuphausiid showing how the ciliates cluster in the corehemocoel of the cephalothorax. (D) Transverse section of ahealthy female euphausiid showing the gonad occupyingmost of the cephalothorax volume. The spermatophoreattached to the female’s thelycum can also be seen. (E) Lightmicroscopy picture of the intestine after the euphausiid hadburst as a result of infection. The ciliates are clustered along

certain sections of the intestine

Dis Aquat Org 71: 33–49, 2006

but without an increase in body length (measured fromthe difference of the uropod length from the molt andthe uropod of the dead animal), as observed in unin-fected individuals (n = 5). Mature infected femalesnever spawned, probably because they were less fitthan uninfected individuals (n = 46).

Table 2 shows the prevalence and geographic loca-tion of 28 sampling stations where infected euphausi-ids were collected (n = 73). Infected specimens were

collected at 6.7% of the stations (MOCNESS and 1 mdiameter nets, n = 313) during 10 oceanographiccruises conducted between 2000 and 2002. Stationswere distributed mostly near the shelf-break (150 to300 m depth). No infected euphausiids were recordedduring a winter cruise (January to February, 2003, n =41 zooplankton samples). The prevalence rangedbetween 3 and 20% of all euphausiids within individ-ual samples, but in 3 locations all the specimens col-lected were infected (Table 2). However, because theparasitoidism is easily identified only during the latestages of infection (orange and swollen carapace), theestimates of prevalence of infected individuals areconservative.

DISCUSSION

Taxonomy of the genus Collinia

The taxonomy of the genus Collinia is based primar-ily on the morphology of endoparasitic stages becauseother life history stages (e.g. protomite, tomite?)appear to be similar for all apostome ciliate species. Itis important to note that virtually nothing is knownabout their exoparasitic stages. This is in contrast toother apostome genera like the exuviotrophic ciliatesof the genus Gymnodinoides where the exoparasiticstages, such as the tomites, are frequently used to dis-tinguish among species (Bradbury 1994, Landers et al.in press). It is likely that parasitoid ciliates like Colliniasp. infect other euphausiid species of the world oceans.However, they may easily be overlooked becauseeuphausiids can only be recognized as infected whenthey are alive. In addition, zooplankton samples areusually preserved in formalin immediately after sam-pling, causing infected euphausiids to lose colorrapidly, and thus infected individuals could escapedetection. Capriulo et al. (1991) claimed that euphausi-ids densely packed with ciliates and fixed with forma-lin or Bouin’s fixatives have a ‘cloudy’ or opaque whiteappearance. Observations during dissection of femalesto stage gonad development (Gómez-Gutiérrez 2003,n = 625 females) indicate that after preservation evenhealthy adult euphausiids, particularly Thysanoessaspinifera, can have a swollen carapace with whitishcoloration not associated with Collinia parasitism.Therefore, caution must be exercised when looking forinfected individuals in preserved samples.

The family Colliniidae has 2 genera, Collinia andMetacollinia (Lynn 2002, Lynn & Small 2002). Thegenus Collinia currently has 7 described speciesincluding C. oregonensis, each of which infects differ-ent crustacean hosts (Capriulo & Small 1986, see theirTable 1). In addition, Capriulo & Small (1986) reported

42

Freq

uenc

y

EndoparasitoidsEuphausia pacifica (n = 42)

0

2

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EndoparasitoidsThysanoessa spinifera (n = 16)

0

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th a

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TAD = 4.54(BCW) + 37.30r2 = 0.317 (p < 0.056; n = 12)

9

28262422201816141210

28262422201816141210

876543220

40

60

80

100

Euphausiid BCW (mg)

Euphausiid total length (mm)

C

Fig. 8. Collinia oregonensis n. sp. Size frequency distributionof the total length (from the posterior part of the eye to thetip of the telson) of infected sub-adult and adult euphausiidsof (A) Euphausia pacifica (n = 42) and (B) Thysanoessaspinifera (n = 16) collected during 2 summer oceanographiccruises in 2002 along the Oregon coast. (C) Linear regressionbetween the time of death after infection (TAD, h) and theeuphausiid body carbon weight (BCW, mg C ind.–1) for

E. pacifica and T. spinifera combined

Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

the kinetal number, morphology and biometry of anundescribed Collinia species that infects a freshwatersubterranean crustacean, Bactrurus mucronatus, butthey did not assign it a formal scientific species name.Each species of Collinia has a different number ofkineties (Table 3). C. oregonensis n. sp. is the secondCollinia species known to infect euphausiids, and thesecond to be confirmed infecting multiple-host species(C. gammari was reported in Pallasea cancelloides andEchinogammarus maaki; Capriulo & Small 1986). Thefirst report of an endoparasitic apostome ciliate ofeuphausiids was C. beringensis from Thysanoessainermis in the Bering Sea (Capriulo & Small 1986).However, because their description was based on pre-served specimens, the deadly effect of the parasite onthe euphausiid host and the signs of infection in livehosts was then unknown. However, they suggestedthat the ciliate parasites could impair physiologicalfunctioning and potentially cause the death of the host.

The incubation of live infected euphausiids allowedus to observe the parasitoid nature of Collinia orego-nensis. The present study provides evidence that theciliates infecting the 3 euphausiid species along theOregon coast belong to a different species than C.beringensis. The most salient feature of the presentciliate is that it possesses fewer kinety rows than C.beringensis in all observed stages. Other relevant dif-ferences are that all stages of C. oregonensis lack a fal-ciform field and a rosette structure, while the protomite2 stage of C. beringensis has both, as well as a y (butnot the full x, y, z) kinetid arrangement. However, allthe described stages of C. oregonensis and C. berin-gensis are astomatous, indicating an osmotrophic feed-ing mechanism. This suggests that enzymes, toxins orother metabolites cause tissue disruption. Other apos-tome ciliates such as Vampyrophrya pelagica Chatton& Lwoff, 1930 seem to ingest fragments of tissues(histophagous) rather than fluids, using a cytostome

43

Date Location (°N, °W) Sample depth (m) Host Euphausiid Prevalence (%)dd/mm/yy and collection species abundance and (no. of hosts

method infected)

09/07/00 43° 13.20’, 124° 59.40’ Surface (B) Ep QS 100.0 (n = 21)13/07/00 44° 00.00’, 125° 00.00’ 20 (1-m) Ep QS 100.0 (n = 2)28/06/01 46° 21.24’, 124° 51.42’ 600 (ROPOS) Ep

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Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

(Grimes & Bradbury 1992, Ohtsuka et al. 2004).C. beringensis appears to reduce the number ofkineties through its endoparasitic stages starting with80 (trophont), followed by 34 (tomont), 30 (during tomi-togenesis), 28 (protomite 1), and 24 (protomite 2). Themature trophont of the endoparasitoid ciliate V. pelag-ica, which infects several copepod species, bears only10 long and 3 short (x, y and z) kineties and it is slightlylarger (50–90 × 30–50 µm) than trophonts of C. orego-nensis (Chatton & Lwoff 1935, Grimes & Bradbury1992, Ohtsuka et al. 2004).

Collinia oregonensis shows a smaller variability innumber of kineties than C. beringensis, maintaining 20or 22 kineties in the trophont, 16, 18 or 20 in the tomontand protomite stages, and reducing to 14 or 16 kinetiesin the tomite stage. However, in relatively few cells aprogressive reduction in the number of kinetiesoccurred as early as the trophont or tomont stages(Table 1). The reduction in the number of kinetiesthroughout the life stages, also known as reabsorptionof kineties, has been observed in other species of thegenus Collinia (Table 3). This phenomenon also occursin other apostome species; for example, the cilia ofHyalophysa chattoni Bradbury, 1966 are absorbed bythe cell in situ, followed by the absorption of the kine-tosomes without the withdrawal of the structures intothe cytoplasm (Landers 1997). Both C. oregonensis andC. beringensis presumably have a similar life-cycle,having, so far as we know, euphausiids as the defini-tive host. Also, the structure of the trophont, tomont,and protomite stages is known for both C. oregonensisand C. beringensis, whereas the tomite and the pho-ront stages of C. beringensis and the phoronts ofC. oregonensis are still unknown. Although the pho-ront stages of C. oregonensis were actually observedattached to the filaments under laboratory conditions,we did not use silver impregnation staining for thedescription of this stage because of problems with fila-ment preservation.

When Cépède (1910) proposed the genus Collinia,he included Collinia circulans Balbiani, 1885, C. bran-chiarum Stein, 1852 (synonyms Opalina branchiarumor Anoplophrya branchiarum), and C. neophargiCépède, 1910 (synonym Anoplophrya branchiarum).His identification key was based on the size of the cili-ate and the host that the apostome ciliate infected. Inthe past almost all known species of the genus Colliniawere in the past considered or originally described asmembers of the genus Anoplophrya, with the excep-tion of the most recently described species C. berin-gensis (Capriulo & Small 1986) and C. oregonensisnov. sp. Most species of the genus Collinia share thefollowing features: trophonts are small, ovoid, pointedat rear, with no ogival field or lateral canal. The dorsalsurface of several Collinia species is largely unciliated

and their tomites bear 9 kineties, characteristics notshared by C. oregonensis. Bradbury (1966, 1994) pro-posed that the smaller number of kineties (9) in thetomite stage of apostome ciliates is a primitive (ple-siomorphic) characteristic. Thus, the C. oregonensistomite stage with 14 or 16 kineties is perhaps a derived(apomorphic) condition within the order Apostom-atida.

Biology and life cycle of Collinia oregonensis n. sp.

Two remaining problems to be researched in con-nection with the life history of Collinia oregonensis areto discover (1) the phoront stage and (2) the infectionmechanism of this parasitoid. In the present study,apostome phoronts and encysted tomites were com-monly observed attached to the appendage setae of atleast 6 euphausiid species (Euphausia pacifica, Thysa-noessa spinifera, T. gregaria, T. longipes Brandt, 1851,T. inspinata Nemoto, 1963, and Nematoscelis difficilisHansen, 1911), with prevalence of about 80% in thejuvenile and adult populations, but silver stainingrevealed a ciliary pattern inconsistent with the genusCollinia and most likely belonging to the genusGymnodinioides (Landers et al. in press). Additionally,several euphausiids with phoronts attached to thesetae were intentionally wounded, but the phorontsdid not excyst or enter the wound like other apostomeciliates (e.g. Vampyrophrya pelagica) (Grimes & Brad-bury 1992, Ohtsuka et al. 2004), suggesting that theseeuphausiids are interacting with at least 2 differentkinds of apostome ciliates (1 exuviotrophic and 1endoparasitoid). Shipboard experiments with euphau-siids bearing encysted phoront stages incubated inwater previously GF/F filtered convincingly showedthat phoronts excysted and became trophonts of exu-viotrophic ciliates immediately after the euphausiidsmolted (S. C. Landers pers. comm.). Moreover, it wassubsequently shown that the abundant phorontsbelong to apostome ciliate groups that feed on theexuvial fluids trapped in the cast-off exoskeletonrecently described as Gymnodinioides pacifica nov. sp.(Landers et al. in press).

The resting cysts (phoront stage) of epibiontic apos-tome ciliates were frequently recorded attached toappendages (3 to 20% prevalence) of 9 euphausiids inthe North Atlantic (Euphausia hemigibba Hansen,1910, E. krohni Brandt, 1851, Meganyctiphanesnorvegica M. Sars, 1857, Nyctiphanes couchi Bell,1853, Nematoscelis megalops G.O. Sars, 1883, Thysa-noessa gregaria, T. inermis, T. longicaudata Krøyer,1946, and T. raschi M. Sars, 1864) by Lindley (1978).Distributions of the last 5 euphausiid species extend tothe North Pacific Ocean, so these species may have the

45

Dis Aquat Org 71: 33–49, 2006

same kind of epibionts and exuviotrophic apostomeciliates as described by Landers et al. (in press).

Regarding the infection mechanism of Collinia ore-gonensis, it is well known that the 2 greatest challengesany parasite faces are moving itself or its progeny fromone host to another (transmission), and overcoming thehost’s defenses. Since it appears that only adult eu-phausiids are infected, transmission must be amonghosts within a population and via the external environ-ment, rather than from parents to offspring. However, itis possible, although we think it unlikely, that C. orego-nensis infects juveniles and that infection accelerates asthe host matures. The parasitic ciliates of the orderApostomatida are known to infect their crustaceanhosts by breaching the cuticle or by entering throughwounds (Morado & Small 1995). In the first mode, secre-tion from the ciliate dissolves a passage through thecuticle (Bradbury 1994). Apostome ciliates such as Syn-ophrya or Terebrospira break through the host cuticleof the gill lamellae, causing extensive tissue damage(Bradbury & Goyal 1976, Johnson & Bradbury 1976).However, not all apostome ciliates are invasive or capa-ble of perforating epithelia. In the second mode, infec-tion can occur when the host has been wounded bypredators. This is the infection mechanism of the ciliateVampyrophrya pelagica on several calanoid, harpati-coid, cyclopoid and poecilostomatoid copepod species(Grimes & Bradbury 1992, Ohtsuka et al. 2004). Weconducted experiments to infect healthy or experimen-tally wounded euphausiids (>10 euphausiids per exper-iment with controls), but none of the experiments weresuccessful in inducing infections (that is, no experimentresulted in a euphausiid with an orange-colored,swollen cephalothorax).

Conventional knowledge about the life cycle of theapostome ciliates indicates that reproduction occursonly in the tomont stage, and that the trophont stage isexclusively a feeding stage. However, the SEM imagesof an infected euphausiid showed densely packedtrophonts in the hemocoel; therefore, if the trophontdoes not divide, this may indicate a massive infection ofthe host, otherwise it is not possible to explain the largenumber of this stage in the host carapace, indicatingthat Collinia oregonensis may use a different, hithertounknown infection mechanism. As a first approach, wepropose a hypothetical infection mechanism that mightbe used by C. oregonensis: the dispersal stages(tomites) may be eaten by the euphausiid and then pen-etrate the stomach or intestine to enter into the hemo-coel. However, we have as yet been unable to success-fully show this mechanism in exposing healthyeuphausiids to free swimming tomites recently releasedfrom a bursted host. Another hypothesis is that eu-phausiids may become infected after feeding on the fil-aments that form several hours after the euphausiid has

burst. Interestingly, Grimes & Bradbury (1992) reportedthat when the tomites settled during the encystationprocess, they appeared to attach by fine filaments to thecopepod (host) exoskeleton. Recently, for gregarines(Apicomplexa: Engregarinida) in the gamont stage, in-fection was discovered on the euphausiid Euphausiasuperba Dana, 1850. The gregarines infected the mid-gut gland and intestinal epithelium of the euphausiid,having a significant impact on the nutritional status ofthe host (Kawaguchi & Toda 1997, Kawaguchi et al.1999, Takahashi et al. 2003). In our study, recentlyruptured euphausiids frequently possessed portions ofintact intestine, with some areas densely packed withciliates.These may represent the points where theciliates originally infected the euphausiids (Fig. 7E), butthis remains speculative.

We found a biased sex proportion (more females) ofeuphausiids (Euphausia pacifica and Thysanoessaspinifera) infected by endoparasitic stages of Colliniaoregonensis. The fact that the ciliate more frequentlyinfects females might have an important effect on thepopulation’s sex ratio, decreasing the spawning stockand population egg production, since infected femalesnever produced eggs. However, the effect of infectionon sex ratio is ambiguous. Sex-biased mortality ineuphausiids has been reported, but not associated withparasitism. For example, mortality of E. superba ishigher for reproductive males (with low lipid levels)than for females (high lipid levels) (Virtue et al. 1996).However, other studies suggest that, in general, maleand female arthropods do not differ in prevalence orintensity of parasite infections due to similarities in thehost’s digestive physiology and foraging ecology(Sheridan et al. 2000). C. oregonensis may impose astrong energy constraint because infected euphausiidsthat molted did not increase in total length (measuredfrom the uropod length of the molt and the just moltedanimal). C. beringensis infection of the euphausiidT. inermis includes primarily adult stages (>3.8 mmcarapace length equivalent to >14 mm total length)also with a relatively higher proportion of females(Capriulo et al. 1991). The Collinia infection mecha-nism and why apparently the parasitoid ciliates preferto infect females are 2 questions to be addressed infuture studies.

The size of first maturity for Euphausia pacifica isabout 12 mm (Ross & Quetin 2000) and for Thysa-noessa spinifera about 15 mm (Gómez-Gutiérrez2003), thus all endoparasitically infected euphausiidswere most likely adults (see Fig. 8A,B). Yolk accumula-tion is the most energy-demanding process foreuphausiids, and is usually linked with high grazingrates and metabolism. The high energetic content ofthe female gonad (Pierce et al. 1969) certainly couldprovide a rich food resource that would allow for rapid

46

Gómez-Gutiérrez et al.: Novel apostome parasitoid ciliate infecting euphausiids

proliferation of Collinia oregonensis. However, para-sitoid ciliates may prefer to infect adults early in thegonadal maturation cycle since it is known thateuphausiids such as Meganyctiphanes norvegica losebetween 33 and 55% of their cephalothorax lipidsthrough spawning (Albessard & Mayzaud 2003). Froman evolutionary perspective, the ciliates may primarilyinfect adults, since this helps to maximize the foodavailable to support a high production of ciliates withwhich to infest other euphausiids. The infection ofmales may impose a lipid content limitation (see Virtueet al. 1996) and larvae and juveniles are usually notinfected, perhaps because of their low body biomass(usually 16 mm total length), preferring females.E. pacifica interbrood period is usually shorter than 5 d(Feinberg et al. 2003), thus it is unlikely that theCollinia life cycle, which is also of the order of days, isclosely connected to the reproductive cycle. This is infact a lethal infection that kills the host as soon as theparasites proliferate and invade the body cavity.

The prevalence of Collinia oregonensis on live in-fected euphausiids during 2002 cruises ranged be-tween 3 and 20% in individual swarms. However, massmortality events such as those observed in AstoriaCanyon, 46.2° N, near the mouth of the Columbia River(Gómez-Gutiérrez et al. 2003), indicate that under cer-tain conditions, infection can reach very high preva-lence. The analysis of videos recorded from a remotelyoperated vehicle (ROV) deep in the waters of AstoriaCanyon showed dense euphausiid aggregation nearthe sea floor during daytime (Gómez-Gutiérrez et al.2003). It is well known that euphausiids can become

abundant near topographic features like canyons, andcan achieve densities 2 to 3 orders of magnitude greaterthan background abundances (Greene et al. 1988).

The highest prevalence of infected euphausiids wasobserved near the shelf-break, where according toSwartzman (2001), Ressler et al. (2005) and Gómez-Gutiérrez et al. (2005) high densities of euphausiidaggregations are commonly recorded. Coevolution ofthe ciliate among multiple-host euphausiid species(Thysanoessa inermis, Euphausia pacifica, T. spinifera,and T. gregaria) should arrive at equilibrium, thus themore host species a parasite relies on, the more viru-lence it can attain without risk to its own survival. Thus,it is predicted that other Collinia species infect multiplehost species. The parasite kills the euphausiid in lessthan 72 h; this is considered very fast when comparedwith the life span of the host (1 to 2 yr). This disagreeswith the general trend that the lower the host back-ground mortality, the longer the parasite should wait tokill the host (Ebert & Weisser 1997). Under shipboardconditions the infected euphausiids swam in continu-ous circles for several hours. They then began to de-crease their swimming activity until the animal restedin the bottom of the bottle; death followed. Thus, in-fected animals in the ocean likely display impairedswimming ability and may sink to the bottom beforedeath. Schooling behavior studies on E. superba Danaindicate that sick or parasitized individuals (opaque orwhitish body color) are more abundant near the back ofthe schools and that some of them fall behind, unable tomaintain the pace (Hamner et al. 1983, Hamner 1984).When a diver touched several whitish E. superba indi-vidual they broke, showing a small cloud around thecarcass, presumably ciliates, thus giving observationalevidence that the Antarctic krill could also be infectedby parasitoid protozoans (W. Hamner, University ofCalifornia, Los Angeles, pers. comm.)

The effects of parasitism and diseases on the popula-tion dynamics of euphausiids are poorly known. Ebert& Weisser (1997) and Lafferty & Kuris (2002) reportedthat parasitoids such as Collinia oregonensis infect rel-atively small hosts, usually weighing no more than afew grams, with a relatively short life span. The longerlife spans of larger hosts might make the parasitoidstrategy a relatively inefficient life history. Para-sitoidism on euphausiids could be an important, previ-ously underestimated source of mortality that affectsthe trophodynamics of these crustaceans within theCalifornia Current upwelling ecosystem.

Further interpretation of the results must await spe-cific identification of the phoront stages and betterobservations and experiments on living, infectedeuphausiids in order to discover the infection mecha-nism and complete the knowledge of the parasitoid’slife cycle. The evaluation of the role of Collinia orego-

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nensis in the pelagic food web and the infection byapostome parasitoid ciliates of other euphausiid spe-cies around the world should be investigated in futurestudies.

Acknowledgements. We thank A. De Robertis, R. D. Brodeur,S. C. Landers, C. B. Miller, K. C. Jacobson, D. H. Lynn, W.Pearcy, E. Sherr and B. Sherr for helpful criticisms of earlierdrafts and the exciting discussions about this parasitoid fromthe Oregon coast. We particularly thank B. Boyer, a seamanand winch operator on the RV ‘Wecoma’ and the scientist A.Røstad, who first noticed the bright orange colored krill swim-ming in the water under the ship’s lights, on the evening ofJuly 9, 2000 during NEP US GLOBEC research sampling, andcalled us onto deck to observe and collect them. We are indebt to L. Feinberg, T. Shaw, J. Keister, A. Røstad, M. Vance,J. Lamb, and C. Vann from Hatfield Marine Science Centerand the crew of the research vessels ‘Elakha’ (OSU),‘Wecoma’ (OSU) and ‘New Horizon’ (SIO) for their collabora-tion in the collection of the live samples between 2000 and2003. We thank W. W. Wakefield and R. Embley for their lead-ership during the ROPOS/Brown operation in Astoria Canyonand A. Soeldner for SEM assistance. We thank 3 referees forthe criticism that substantially improved the content of themanuscript. Thanks to G. Keel for his help in the donationprocess of the specimens to the National Museum of NaturalHistory, Smithsonian Institution. This research was supportedby Mamie Markham Research Award from Oregon State Uni-versity, Hatfield Marine Science Center during 2001–2002and 2003–2004 to J.G.-G.. The NEP US GLOBEC program(NA860P0589) provided ship time and staff assistance. J.G.-G. was supported by an SNI fellowship, by COFAA–IPN, EDI-IPN, and a PhD CONACyT grant (122676) to study at OregonState University. This is Contribution No. 271 of the USGLOBEC program.

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Editorial responsibility: Timothy W. Flegel,Bangkok, Thailand

Submitted: September 15, 2005; Accepted: February 13, 2006Proofs received from author(s): June 7, 2006