Morphological features of the testis of freshwater blood flukes of the genus Sanguinicola Plehn, 1905, with consideration of the testicular patterns in the Aporocotylidae.

This is the first detailed study assessing the morphological features of the testis and testicular sperm of members of the freshwater blood flukes Sanguinicola sp. from Leuciscus idus (Cyprinidae) and for S. volgensis from Pelecus cultratus (Cyprinidae), and the marine blood fluke Aporocotyle simplex from Hippoglossoides platessoides (Pleuronectidae). The present study reports a unique feature in the distribution of germinal cellular components in freshwater Sanguinicola sp., showing the presence of the individual spermatocytes or their clusters in the testicular lobes, and the gathering of spermatid rosettes and spermatozoa within middle testicular lumen, which extends along the entire length of the testis. In contrast, each testis of marine A. simplex contains the usual mixed distribution of germ cells at various stages of development. The first TEM data on spermatozoon character of studied species has shown, unusual for digenean sperm structure, the absence of cortical microtubules in sperm principal region. Moreover, a variation in axoneme patterns is revealed in the studied aporocotylids, belonging to the different aporocotylid lineages a 9 + 0 axonemal type observed for freshwater teleost-infective species, Sanguinicola sp. and S. volgensis, and a 9 + '1' axonemal type revealed in spermatozoa of marine teleost-infective species A. simplex. The results discussed with the published data on the digenean sperm structure and the testicular patterns in the Aporocotylidae likely represent additional characteristics supporting the divergent evolutionary lineages of freshwater and marine aporocotylids. We anticipate future morphological studies of the sperm structure in aporocotylids of three lineages for an understanding of their phylogenetic relationships.

Interrelationships of vitelline and muscle cells within the vitelline follicles of the blood fluke Aporocotyle simplex (Digenea, Aporocotylidae) and morphological evidence for the modification of vitelline material for eggshell formation.

This study of the fish blood fluke Aporocotyle simplex represents the first detailed transmission electron microscopical (TEM) investigation of the vitellarium of an aporocotylid digenean blood fluke. It revealed some unusual characteristics in the cytoarchitecture of the vitelline follicles and demonstrated modifications of the vitelline granules for eggshell formation. The vitelline follicles consist of vitellocytes at different developmental stages surrounded by sarcoplasmic processes of myocytes which occur throughout each follicle. Sites of intimate contact occur between the vitellocytes and the myocytes. Individual vitelline globules (0.1-0.2 µm in diameter) accumulate in quite small clusters of 10-20 and have a dense, heterogeneous matrix possessing central and peripheral regions with a greater density. Modifications of the vitelline globules take place within the clusters and are first apparent when the vitellocytes reach the lumen of the vitelline duct and vitelline reservoir. Globules within the clusters become confluent, and, when the vitellocytes reach the lumen of the oviduct and proximal ootype, these consolidated clusters contain a shapeless, loosely packed, dense material which is released from the vitellocytes by exocytosis. This investigation has provided morphological evidence for shell formation from modified vitelline globules in the form of a discontinuous, thin layer (~ 0.07 µm in thickness) of electron-dense shell material around the fertilized ovum and associated vitellocytes in the proximal ootype. The eggshell of intra-uterine eggs acquires an additional thin, heterogeneous outer layer, increasing its thickness to ~ 0.1 µm. The cytoarchitecture of the vitellarium, modifications of the vitelline globules within the clusters and the structure of the eggshell of A. simplex may prove to be of value in studies examining relationships between the three distinct lineages of aporocotylid digeneans.

Gyrodactylus triglopsi n. sp. (Monogenea: Gyrodactylidae) from the Gills of Triglops nybelini Jensen, 1944 (Teleostei: Cottidae) in the Barents Sea.

INTRODUCTION: Monogeneans of the genus Gyrodactylus were found on the gills of specimens of the bigeye sculpin Triglops nybelini Jensen, 1944 caught by trawl in the Barents Sea in January-February 2016. METHODS: Morphological preparations of the parasites were examined and photographed under a microscope at magnifications of × 100-1000 and morphometric analyses were carried out on 22 specimens using ImageJ2 software. Eight of the specimens used for the morphological comparisons were also subjected to molecular analyses by sequencing a region of the ribosomal DNA spanning partial 18S, the internal transcribed spacers 1 and 2 (ITS1 and 2), 5.8S and partial 28S and comparing this with other species through a BlastN-search in GenBank and through phylogenetic analyses. RESULTS: The morphology of the species from T. nybelini was markedly different to that of any of other species of Gyrodactylus. It is characterized by having relatively long hamulus roots, a character that it shares with two other species described from marine sculpins (Cottidae); G. armatus and G. maculosi. It also has a narrow rectangular ventral bar membrane with a posterior notch which it shares with G. maculosi only. Compared with all the seven species from marine Cottidae described so far, it has the smallest opisthaptoral hard parts. A comparison of the internal transcribed spacer (ITS) rDNA sequence with available sequences in GenBank and a phylogenetic analyses also showed it to be highly divergent from other sequences. Therefore, a new species is proposed, Gyrodactylus triglopsi n. sp. CONCLUSION: Both the morphological and molecular analyses support the status of G. triglopsi as a new species. This is to our knowledge the first species of Gyrodactylus described from Triglops nybelini and the description extends the list of Gyrodactylus species found on fish in the Barents Sea to 17.

Unique ultrastructural characteristics of the tegument of the digenean blood fluke Aporocotyle simplex Odhner, 1900 (Digenea: Aporocotylidae), a parasite of flatfishes.

This paper includes the first transmission electron microscopical (TEM) study of the tegument of a member of the basal digenean family Aporocotylidae. Scanning electron microscopical investigations of the fish blood fluke Aporocotyle simplex show that each boss on the lateral body surface bears 12-15 simple, uniform spines which extend from 0.5-2.7 µm above the surface of the boss. TEM observations revealed that these spines reach deep beneath the distal cytoplasm of the tegument for much of their length (9-12 µm) and are surrounded by a complex of diagonal muscles in each boss. This is the first record of any digenean with so-called 'sunken' spines. The results suggest that aporocotylid spines arise from within the sarcoplasm of the boss diagonal muscles. The sunken cell bodies (perikarya) of the tegument are connected to the distal cytoplasm via ducts (specialised processes lined by microtubules); this in contrast to other digeneans studied, where they are connected via non-specialised cytoplasmic processes. Within the distal cytoplasm, the tegumental ducts of A. simplex are surrounded by invaginations of the basal membrane and release their cytoplasmic inclusions into the distal cytoplasm. These apparently unique morphological features of the tegument, especially the deep origin of the spines, may represent useful characteristics for understanding aporocotylid relationships, especially in view of the known variation in the spine patterns of aporocotylids.

Pseudalataspora vanderlingeni n. sp. (Myxosporea: Bivalvulida) from gall bladders of the Cape hakes Merluccius capensis Castelnau, and M. paradoxus Franca (Teleostei: Merlucciidae).

A new species of myxosporean parasite is described from the gall bladders of the hakes Merluccius capensis Castelnau and M. paradoxus Franca (Pisces: Teleostei) caught off the west and south coasts of South Africa. The new species, Pseudalataspora vanderlingeni, is described morphologically and molecularly and compared with the 15 other species of Pseudalataspora previously described from marine fish. Although a molecular description is available on GenBank for only one of these 15 species, the morphological description supports the status of P. vanderlingeni as a new species. Earlier reports, without detailed descriptions, of Leptotheca sp. and Ceratomyxa sp. from the same hosts caught off Namibia were very likely to have been of P. vanderlingeni. These earlier studies reported high prevalences of infection, similar to those of >60% described in the present study. The effects of fixation and freezing on the dimensions of spores of Pseudalataspora spp. are described, and the status of the genus Pseudalataspora within the family Ceratomyxidae is discussed.

The first ultrastructural observations of the egg-forming complex of Calicotyle affinis, an endoparasitic monocotylid monogenean which lacks a uterus.

This ultrastructural study of the female reproductive system of Calicotyle affinis, a monogenean without a uterus, demonstrates, for the first time in a monogenean, the presence of accessory cells located in the peripheral region of the ovary and a single large cell whose surface is penetrated by deep invaginations which restrict the passage of oocytes through the lumen of the distal extremity of the ovary. The cytoarchitecture of the epithelial lining of the two vaginae is a syncytium formed by an anucleate epithelial lining with sunken epithelial perykaria. The wall of the seminal receptacle is an enlarged prolongation of the vaginal epithelial lining. Fertilization occurs in the fertilization chamber and fertilized oocytes retain cortical granules within their cytoplasm, a characteristic known to occur in free-living flatworms. Our study also highlights concentrations of two distinct groups of Mehlis' gland cell ducts on either side of the proximal end of the ootype distinguished by ultrastructural characteristics of their secretory granules. The epithelial wall of the ootype is formed by a single layer of regular, columnar, glandular epithelial cells; these cells are closely adjacent, conjoined towards their apical region by septate junctions and produce rounded, electron-dense granules which are discharged into the ootype lumen via a merocrine or holocrine mechanism. Released granules concentrate around the eggshell and form an additional fibrous coat. The morphological diversity observed in the female reproductive system of the Monogenea is commented on it relation to clarifying patterns in monogenean evolution and for understanding the phylogeny of the Neodermata.

Ultrastructural characteristics of the male ducts and terminal genitalia of an endoparasitic monogenean, Calicotyle affinis Scott, 1911 (Monopisthocotylea: Monocotylidae), with the first detailed description of a copulatory stylet in a monogenean.

This investigation of Calicotyle affinis is the first ultrastructural description of the male reproductive system of a monocotylid monogenean. It has revealed numerous characteristics which are either new or clarify those determined from previous light microscopical observations on the morphology of Calicotyle spp. These include numerous individual testes, each surrounded by its own basal lamina; an ejaculatory bulb delimited by a basal lamina surrounded by two independent internal and external bands of muscles; a bipartite, but not fully divided, muscular chamber (an internal seminal vesicle) in the proximal region of the ejaculatory bulb; well-developed unicellular prostatic glands localised outside the ejaculatory bulb, the cytoplasmic extensions of which pass into the ejaculatory bulb, to form the ducts which open into the lumen of ejaculatory duct; and a male copulatory organ in the form of a coiled penis tube containing throughout its lumen a long, hollow stylet. The penis tube appears to be a modified prolongation of the ejaculatory duct and is supported by a basal lamina and muscle layers. The stylet wall consists of a homogenous, dense material with a narrow, denser inner lining; it is funnel-shaped proximally but narrows imperceptibly towards the distal end of the penis tube, and appears to be a derivative of the basal laminae of the ejaculatory bulb and distal ejaculatory duct. A comparative analysis of the nature of the stylet in relation to those found in free-living platyhelminths forms part of a search for characters useful for understanding monocotylid origins and relationships.

The unique uterine structure of the basal monogenean Chimaericola leptogaster (Monogenea: Polyopisthocotylea), an ectoparasite of the relictual holocephalan fish Chimaera monstrosa.

Ultrastructural studies of the monogenean uterus are few in number and no non-polystomatid polyopisthocotyleans have been investigated. The uterus of Chimaericola leptogaster, a basal polyopisthocotylean monogenean, has several unusual features, including six reflexed loops comprising four ascending and three descending, longitudinally oriented, linear sections. At the ultrastructural level, three readily distinguishable uterine regions and other distinctive characteristics are apparent. One novel feature occurs in the proximal uterus, where the lining forms a so-called 'single-layered multi-rowed cellular epithelium', which includes two types of cells, tall (ca. 14-19 µm in height) and short (ca. 6-9 µm in height) cells, both lying on the basement membrane. Although known from other bilaterian groups, this is the first record of this type of epithelium in the Neodermata. The lining of the middle uterine region comprises a single regular layer of columnar glandular epithelial cells, which produce numerous rounded, electron-dense granules associated with Golgi complexes. The presence of the uterine glands in the middle region of the uterus is an unusual feature for a monogenean, having previously been described only for basal orders of the Cestoda, i.e. the Gyrocotylidea, Caryophyllidea and Spathebothriidea. Seen in cross-section, the epithelium of the distal uterus contains three areas of tall single-layered columnar epithelium (ca. 30 µm deep) interspersed by three areas of flattened epithelium (ca. 0.2-0.9 µm deep). Such a pattern is quite different from those reported for other monogeneans and, indeed, other neodermatan groups. The investigation has shown that the outer layer of the fully formed eggshell is assembled from epithelial secretions in the middle uterine lumen, but is modified in terms of its shape in the distal uterus. Possible phylogenetic implications arising from the unusual features described are discussed in relation to other neodermatan groups and recent molecular phylogenies of the Bilateria.

Ultrastructural observations of the attachment organs of the monogenean Rajonchocotyle emarginata (Olsson, 1876) (Polyopisthocotylea: Hexabothriidae), a gill parasite of rays.

The present study uses scanning and transmission electron microscopical observations to examine the haptoral features of the hexabothriid polyopisthocotylean monogenean Rajonchocotyle emarginata from the gills of the elasmobranch Amblyraja radiata. The haptor possesses six equal, pedunculate suckers, each armed with a large, curved, hooked sclerite, and an appendix bearing two small, terminal suckers and a pair of minute hamuli. The outer side of the distal region of the hooked sclerite forms a large bulge along the antero-lateral side of each sucker and the proximal region forms a small bulge on the opposite postero-lateral side; part of the large bulge, which covers the small sclerite hook, surmounts the sucker rim. The sucker sclerite is situated outside the main sucker muscle complex of the sucker wall and is surrounded by its own muscular envelope; close to the sclerite itself, long, tightly packed lamella-like processes of sarcoplasm are present. A muscular sphincter surrounds the distal region of the sclerite shaft. The sclerite consists of three heterogeneous layers, but its hook region is distinguished from the shaft by the presence of an additional homogeneous layer. Hamuli within the appendix consist of three distinct layers loosely surrounded by sarcoplasmic lamellae. The luminal surface of the appendicular suckers is covered with unusual, long, thin, interconnected surface protrusions. In all other regions of the haptor, the tegumental surface is smooth, with numerous shallow pores penetrating the "terminal web" of the syncytial cytoplasm. Unicellular gland cells are localized close to the base of the appendix, with their ducts, containing non-homogenous secretory bodies, opening onto the anterior haptoral surface. Two types of sense receptors are visible on the haptoral surface. The ultrastructural features of the haptor are discussed in relation to our understanding of their function and the evolutionary relationships of the basal polyopisthocotylean monogenean groups Hexabothriidae and Chimaericolidae.

Surface ultrastructural characteristics of Dictyocotyle coeliaca Nybelin, 1941 (Monopisthocotylea: Monocotylidae), an endoparasitic monogenean of rays.

Scanning and transmission electron microscopical observations were made of the surface topography of Dictyocotyle coeliaca (Nybelin, 1941) (Monopisthocotylea: Monocotylidae), a unique endoparasitic monogenean from the body cavity of the ray Amblyraja radiata (Elasmobranchii: Rajidae). Scanning investigation show the presence on the ventral side of the anterior body of smooth areas with pit-like depressions and shallow ridges, whereas the tegument of the middle and posterior regions of the body is extensively folded, and on the dorsal surface the tegument is smooth but interrupted by deep depressions. Transmission observation revealed the presence of invaginations of varying irregularity and depth, which form the various depressions of the tegumental surface. Non-ciliated, dome-shaped papillae occur singly or in groups and are common around the mouth and both the genital and vaginal pores, but exhibit no particular orientation. The haptor is much reduced, compared with that of ectoparasitic monocotylids, and is covered ventrally with an irregular array of 40-70 shallow loculi. Internal differences occur in the thickness of the syncytial tegumentary layer of the haptoral loculi and septa and also in the number of cytoplasmic inclusions. The locular surface has distinct ultrastructural characteristics, the most obvious of which are a honeycomb arrangement of small pockets measuring 0.6 × 0.75 to 1.0 × 1.7 µm in diameter, a terminal web beneath the surface plasma membrane and large outgrowths on the surface of the loculi filled with lysosome-like bodies. The surface specializations of the body and haptoral tegument of D. coeliaca are discussed in relation to their being adaptations to an endoparasitic environment.

Ultrastructural characteristics of the caeca of basal polyopisthocotylean monogeneans of the families Chimaericolidae and Hexabothriidae parasitic on cartilaginous fishes.

Ultrastructural differences are shown between the caecal organization in three blood-feeding polyopisthocotylean monogeneans, i.e., the chimaericolid Chimaericola leptogaster from the holocephalan Chimaera monstrosa and two hexabothriids, Callorhynchocotyle callorhynchi from the holocephalan Callorhynchus capensis and Rajonchocotyle emarginata from the elasmobranch Amblyraja radiata. In C. leptogaster, digestive cells and connecting syncytium, joined close to the luminal surface by septate junctions, are arranged alternately along the caecal epithelial wall; the nuclear regions of both cell types are sunk below the general level of the caecal epithelium; a concave depression on the apical margin of the digestive cells bears lamellae; and this depression is covered by a lamellate bubble formed by thin projections emanating from the connecting syncytium. The luminal surface of the connecting syncytium is covered with outgrowths terminating in the form of long, narrow processes. In R. emarginata and C. callorhynchi, the predominant digestive cells are at different stages of development and occur in groups, developing digestive cells bulge into the caecal lumen from the connecting syncytium with contact sites present close to the luminal surface, and the luminal surface structures of both the connecting syncytium and the digestive cells are short lamellae. In these two hexabothriids, a holocrine (or apocrine) process for the elimination of digestive product is assumed via the detachment of fully differentiated, bulging digestive cells. Free, apparently sloughed digestive cells and residual bodies are present within the caecal lumen, and replacement digestive cells are numerous in the connecting syncytium. In the chimaericolid, free bubbles containing residual bodies and portions of digestive cells filled with degenerating digestive vesicles occur in the caecal lumen along with large amounts of male and female reproductive material. The usefulness of characteristics of the caecal ultrastructure as taxonomic traits at the family level is discussed.

Clamp ultrastructure of the basal monogenean Chimaericola leptogaster (Leuckart, 1830) (Polyopisthocotylea: Chimaericolidae).

The ultrastructure of the haptoral clamps of the chimaericolid monogenean Chimaericola leptogaster, a basal polyopisthocotylean from the gills of a holocephalan fish, is described. These clamps are characterized by the presence of two muscle blocks interrupted mid-anteriorly and mid-posteriorly and different kinds of hard structures: a single median and paired lateral sclerites embedded in the clamp wall; six spine-like structures directed towards the clamp lumen; and electron dense surface structures along the internal surface of the anterior clamp lips and along the luminal surface of the tegument of the clamp lumen. The lateral sclerites are situated deep within muscular tissue and are closely bounded by radial myofibrils, possessing a uniform electron dense matrix within which are hollow areas of different sizes. The median sclerite occupies an area between the clamp wall myofibrils and the luminal epithelium, is surrounded by a basement lamina and is composed of a heterogeneous matrix comprising two different morphological layers related to variations in the type and concentration of fibrils. Four of the spine-like structures are extensions of the margins of the two spindle-like muscle blocks in the clamps, i.e. the two anterior and two posterior structures, and the two others are situated at the lateral constrictions of the left and right muscle blocks. The electron dense surface structures are derivations of the clamp tegument or, to be more precise, its outer, densely fibrous region. These results are discussed in relation to the evidence that the haptoral clamps of C. leptogaster are apparently ancient origin.

Ultrastructural characteristics of the vaginae of the basal monogenean Chimaericola leptogaster (Leuckart, 1830).

In the first study of the vaginal ultrastructure of any monogenean, the paired vaginae of Chimaericola leptogaster, a basal polyopisthocotylean from the gills of a holocephalan, are described. Each vagina opens laterally. A unique feature of this relict parasite is a short vagino-intestinal connecting duct, the lining of which is separated by septate junctions from the linings of the vagina and the intestine. After giving rise to this vagino-intestinal duct, each vagina travels in a posterior direction and opens into the vitelline collecting duct. The lining of each vagina close to the vaginal pore resembles the body tegument, the syncytial lining of which measures about 2.5 µm in thickness and has a dense surface layer 0.2 µm in thickness and different apical projections. Proximal to this and before the entrance to the vaginal-intestinal duct, the vaginal lining is characterised by: uniformly distributed electron-dense, lanceolate, spine-like surface structures of about 1.7 µm in length; three kinds of surrounding glands with three kinds of secretory granules (abundant oval or rounded electron-dense granules 0.7-2.5 µm in length, much less numerous spheroidal, vesicular granules 0.3-0.4 µm in diameter, and tightly packed, spheroidal granules 0.8 µm in diameter and containing fine particulate material of low density); and four kinds of sensory receptors (three uniciliate and one aciliate). These features are absent from the rest of the vagina. The likely roles of the vagino-intestinal connection, the different gland cells, sensory structures, and the armament of the distal vaginal regions are discussed. Considering the suggested polyopisthocotylean ancestor for the Neodermata, the relict parasite fauna of holocephalans and the unique vagino-intestinal connection in C. leptogaster, which is also known in turbellarians, an ancient origin for chimaericolids is supported.

Gammaherpesvirus infection in semidomesticated reindeer (Rangifer tarandus tarandus): a cross-sectional, serologic study in northern Norway.

Malignant catarrhal fever (MCF) is caused by a group of gammaherpesviruses that primarily affect domestic and wild ruminants. Using competitive-inhibition enzyme-linked immunosorbent assay, we screened 3,339 apparently healthy, semidomesticated reindeer (Rangifer tarandus tarandus) from Finnmark County, Norway, sampled during slaughter. The overall antibody prevalence was 3.5% and varied among reindeer herding districts in Finnmark (0-6.7%), the largest reindeer herding region in Norway. The risk of exposure to gammaherpesvirus (i.e., seroconversion) was significantly higher for adult reindeer than it was for calves ≤1 yr, for reindeer in east Finnmark (3.8%) compared with west Finnmark (3.3%), and with increasing population density. No evidence of disease associated with this virus was detected in reindeer sampled for this study, but because samples were collected at slaughterhouses, one cannot discard the possibility of these events happening in the field. The low antibody prevalence could indicate occasional infection of reindeer with another ruminant gammaherpesvirus or the presence of a yet-unknown, specific, low-pathogenic reindeer gammaherpesvirus. Further studies should aim at characterizing the virus circulating in reindeer and address the potential clinical impact of this virus.

Two new species of myxosporean parasites (Myxosporea: Bivalvulida) from gall bladders of Macruronus magellanicus Lönnberg, 1907 (Teleostei: Merlucciidae).

Two new species of myxosporeans are described from the gall bladders of hoki, Macruronus magellanicus Lönnberg, 1907, caught in the Southeast Pacific off Chile and in the Southwest Atlantic off the Falkland Islands. Pseudalataspora kovalevae n. sp. is described morphologically and genetically. Of the 12 species of Pseudalataspora previously described from the gall bladders of marine fish, P. kovalevae is most similar in morphology to P. umbraculiformis Gaevskaya and Kovaleva, 1984. The 18S rRNA gene sequence from P. kovalevae is the first for a member of the genus Pseudalataspora. Based on currently available myxosporean 18S rRNA gene sequence data, P. kovalevae shares greatest sequence identity with species of the genus Ceratomyxa (C. anko and C. pantherini). Palliatus magellanicus n. sp. is described morphologically only; it differs considerably in morphology, host species and locality from the five other Palliatus species described from marine fishes. A third species from the gall bladder is also described morphologically and genetically. On the basis of its morphology it is tentatively identified as Myxidium baueri Kovaleva and Gaevskaya, 1982, for which M. magellanicus is a new host record. Molecular analysis indicates that, of those species for which data are available, M. baueri is most closely related to Myxidium coryphaenoideum Noble, 1966 based on 18S rRNA gene sequence data, though support for a phylogenetic grouping is low.

Trypanosome infections of marine fish in the southern Barents Sea and the invasive red king crab Paralithodes camtschaticus.

The red king crab Paralithodes camtschaticus was introduced to the Barents Sea from the North Pacific in the 1960s. A previous study concluded that it may be indirectly responsible for increased transmission of Trypanosoma murmanense to cod Gadus morhua in the southern Barents Sea by promoting an increase in the population of the leech vector Johanssonia arctica. Eleven species of fish, totalling 681 individuals, caught in October 2002 along the coast of Finnmark, were examined for trypanosome infections. The aims were to investigate changes in levels of infection in cod since 1999-2001, and to extend the sampling to other fish species. Relatively high infection levels were found in cod, haddock Melanogrammus aeglefinus and long rough dab Hippoglossoides platessoides, while other species were lightly infected or uninfected. In cod, no significant geographical differences in levels of infection were found, but haddock were significantly more heavily infected in western Finnmark.

Crabs, leeches and trypanosomes: an unholy trinity?

The red king crab Paralithodes camtschaticus was deliberately introduced to the Barents Sea in the 1960s and 1970s from its native area in the North Pacific. The carapace of these crabs is a favoured substrate for the leech Johanssonia arctica to deposit its eggs, and the leech is a vector for a trypanosome blood parasite of marine fish, including cod. We examined cod for trypanosome infections during annual cruises along the coast of Finnmark in North Norway over three successive years from stations along a gradient of over 1000 km. In every year the level of trypanosome infection in cod was significantly highest in the area with the greatest density of king crabs. We propose the hypothesis that the burgeoning population of red king crabs in this area is indirectly responsible for increased transmission of trypanosomes to cod by promoting an increase in the population of the leech vector.