Photographic images by Gabriel Harp and Britt Koskella

Relating to biology conducted in the Lively Lab, Department of Biology, Indiana University, Bloomington, USA

The following information regarding trematode identification was taken from:

MAURI ORA, 1973, 2: 17-30
Department of Zoology, University of Canterbury, Christchurch, New Zealand

Larval forms of 11 species of Trematoda parasitising the New Zealand species of Potamopyrgus (Gastropoda:Hydrobiidae) are described. Fourteen trematodes are now known to use the freshwater species, P. antipodarum, as an intermediate host and two of these species have also been found parasitising the brackish water species, P. estuarinus and P. pupoides. Metacercarial infections were more frequently found than infections by active larval stages and they also produced the highest intrapopulation infection rates. Highest cercarial infection rates were produced by monostome species.





Cercaria Ml
Body length 0.43 - 0.56 mm; width 0.09-0.14 mm; tail length 0.11-0.15 mm.
Body transparent, granular, with a pair of dark, longitudinal bands following the courses of the main antero-lateral excretory canals; oral sucker inconspicuous; eye spots absent. posterior locomotor pockets (adhesive papillae) weakly developed; posterior angles of body smoothly rounded; excretory vesicle in death circular; tail less than half length of body. Locomotion by a looping action.

Length up to 1.1 mm; width up to 0.29 mm
Pharynx prominent; active, capable of limited contraction.

Metacercarial cysts
Roughly spherical, diameter 0.11 - 0.18 mm. No morohological structures are distinguishable through the cyst wall. Cysts are found attached to the operculum of P. antipodarum or to the shell or other cysts close to the aperture. The attached side of the cyst is flattened to fit the curvature of the surface. All snails examined with cysts attached to their shells, contained cercariae. As with the incidence of infection within a snail population was never high, it seems probable that cercariae habitually enyst on the shell of the snail they have just left.

Molluscan host: P. antipodarum

Collection localities: Submerged littoral vegetation Lake Pupuke, Takapuna; stony stream near Bunnythorpe, Manawatu.

Cercaria M2
Body length 0.34 - 0.44 mm; width 0.19 - 0.21 mm; tail length 0.22 - 0.39 mm.
Body opaque, granular, contractile, oval when relaxed, the posterior margin shallowly concave; oral sucker inconspicuous;
excretory vesicle circular in death; adhesive papillae prominent; a pair of dark eye spots present anteriorly behind the oral sucker, in some individuals each "eye" appearing to be made up of two adjoining spots; conspicuous band of dark pigment between and around the eyespots; tail simple and contractile. Inactive.

Similar to that of Cercaria M1. Length up to 0.95 mm; width up to 0.38 mm. Containing about 10 cercariae in various stages of development.

Metacercarial cysts
Diameter 0.17 - 0.23 mm. They occur on the operculum and shell of Potamopyrgus spp., often in large numbers, e.g., individual snails from Lake Pukaki had over 300 cysts.

Molluscan hosts: P. antipodarum and P. pupoides Hutton

Collection localities: a slow moving, muddy stream near Linton, Manawatu; Hokowhitu Lagoon, Palmerston North; Avon River, Christchurch; marginal vegetation. Lake Pukaki, Canterbury; Makara River, Wellington; littoral vegetation. Lake Tutira, Hawkes Bay; tidal reaches of the Kahao Stream, Porirua Harbour, Wellington.

Cercariae M3
Body length 0.37 - 0.44 mm; width 0.11 - 0.19 mm; tail length 0.33 - 0.55 mm.
Body grey-brown with two opaque, longitudinal bands as in cerariae M1; tail more transparent; three eyespots present , two lateral, one median immediately posteriorto the oral sucker; two pronounced adhesive papillae at the posterior body angles; tail with a regular "cellular" appearance. Cercariae are very active compared with other monostome species found, swimming being accompianied by strong contractions of both body and tail. When attached to a flat surface, the two posterior adhesive papillae are in contact with the substratum and appear to act as substitutes for a ventral sucker.

Indistinguishable from those of Cercariae Ml and M2.

Metacercarial cysts
Diameter 0.17 - 0.21 mm. Found attached to shells and opercula of host snails. Encystment was observed on the bottom of a
glass dish within minutes of cercariae being teased from a snail host. During encystment the tail thrashed frantically while still attached to the outside of the cyst, and the body of the cercaria rolled continuously within the developing cyst.Initially the three eyespots were clearly visible through the cyst wall but later the wall became more opaque and the internal structures could no longer be distinguished. The cyst wall was built up of two layers, an inner one 7-11 pro thick, initially with a golden appearance which was laid down first, and a thicker (15-23 pm) outer layer which at first was clear. On completion of encystment the inner layer assumed a grey, somewhat fibrous appearance, and the outer layer appeared more golden when viewed with transmitted light.

Molluscan host: P. antipodarum

Collection localities: Hokowhitu Lagoon, Palmerston North; muddy slow-moving tributary to Kahuterawa River, Manawatu.

Cercaria M4
Body length 0.29 - 0.43 mm; width 0.11 - 0.21 mm; tail length 0.29 - 0.56 mm.
Body colourless, opaque; tail more transparent, with a finrunning its full length ventrally and three quarters of its length dorsally; two prominent eyespots present; longitudinal bands of pigment extend the length of the body to the clear, excretory vesicle; oral sucker not easily seen; posterior margins of body "flanged" to form adhesive papillae. The body, but not the tail, exhibits considerable contractility, changing' from a roughly circular to a narrow, elongated form. In life the tail is normally about twice as long as the body, but at death it contracts, and approximates the body length. Cercariae swim actively by thrashing movements of the tail which is held vertically above the body.

Length up to 2.0 mm; width up to 0.19 mm.
Translucent with a short, broad pharynx. Inactive. Containing about 11 cercariae in various stages of development.

Metacercarial cysts
Indistinguishable from those of Cercaria M3. The process of encystment was observed in the laboratory and is also identical to that described for Cercaria M3. Found on shells of Potamopyrgus Spp.

Molluscan hosts: P. antipodarum and P. estuarinus Winterbourn.

Collection localities: muddy, slow flowing stream near Linton,
Manawatu; Avon River, Christchurch; littoral vegetation. Lake
Pupuke, Takapuna; tidally exposed mudflats at mouth of Huia
River, Manukau Harbour; stony stream near Waitomo, King Country.



Cercaria Fl
Body length 0.16 mm; tail stem length 0.08 mm; furcal length 0.12 mm (one specimen measured).
Colourless, translucent, shape rigidly defined; ventral sucker posterior to mid-point of body; excretory vesicle small, V-shaped; tail stem half the length of the body and containing 2 rows of clearly visible caudal bodies; furcae slightly longer than tail stem, tapering towards their apices. An active swimmer.

Sporocyst *
Length up to 0.44 mm, width up to 0.12 mm.

Molluscan host: P. antipodarum

Collection locality: muddy littoral zone, northern end of Lake Wairarapa.

Cercaria F2
Body length 0.06 - 0.10 mm; width up to 0.04 mm; tail stem length up to 0.15 mm; furcal length up to 0.03 mm.
An apharyngeal, brevifurcate, monostome cercaria. No normal suckers but with a protrusible anterior organ; body with a narrow dorsal fin; tail stem divided into 2 furcae at about three quarters of its length.

Length 0.26 - 0.28 mm; width 0.12 - 0.14 mm.

Molluscan host: P. antipodarum

Collection localities; muddy, slow flowing, stream near Linton, Manawatu; littoral vegetation. Lake Tutira, Hawkes Bay.


Cercaria XI
Body length 0.15 mm; width 0.06 mm; tail length 0.07 mm (one specimen measured).
Body colourless, opaque; oral sucker larger than ventral sucker, stylet and virgula organ prominent; excretory vesicle V-shaped with a long narrow posterior duct; body surface covered with small spines. Movement by muscular contractions and side to side lashings of the tail.

Length up to 0.44 mm; width up to 0.19 mm. Containing up to 6 cercariae.

Molluscan host: P. antipodarum

Collection locality: littoral vegetation. Lake Tutira, Hawkes Bay.


Cercaria Cl
Body length 0.28 mm; width 0.09 mm (one specimen measured).
Body translucent, white; body surface spinose; oral sucker and stylet prominent; large penetration glands present, one group either side of the oral sucker and extending some distance behind it; pharynx small, inconspicuous; ventral sucker notobvious from above, centrally positioned, almost as large as oral sucker; posterior margin of body drawn in medially to form a shallow V at the excretory pore; excretory vesicle a narrow, 2 branched, Y-shaped structure; tail absent. Cercariae are very
active, moving with a looping action.

Length up to 0.80 mm; width 0.25 mm. Containing about 12 cercariae.

Molluscan host: P. antipodarum

Collection locality: stony stream at Waitomo, King Country.



Cercaria G1
Body length 0.20 - 0.26 mm; width 0.07 - 0.09 mm; tail length 0.17 - 0.20 mm.
Body opaque, oval, non-contractile; tail strongly developed with no fin; oral and ventral suckers the same diameter, the latter slightly posterior to the mid point of the body; a pair of eyespots just posterior to the pharynx. Inactive.

Length up to 1.44 mm; width up to 0.38 mm.
Sac-like, narrowing anteriorly; pharynx small and inconspicuous. Containing up to 11 developing cercariae.

Molluscan host: P. antipodarum

Collection localities: muddy, slow flowing stream near Linton,
Manawatu; marginal vegetation. Lake Tutira, Hawkes Bay.

Cercaria G2
Body length 1.00 - 1.10 mm; width up to 0.30 mm; tail length 0.75 - 0.87 mm.
The largest cercaria parasitising Potamopyrgus species. Body colourless, elliptical; ventral sucker slightly posterior to middle of body, extremely protrusible and larger than oral sucker; bifurcation of the gut clearly visible just anterior to the ventral sucker; tail almost as long as body, a well developed fin running its full length ventrally and along the posterior third dorsally.

Length up to 1.50 mm; width up to 0.25 mm.
Pharynx prominent. Containing up to 6 developing cercariae. In young rediae, the gut posterior to the pharynx is clearly visible as a simple sac extending almost half the length of the body.

Molluscan host: P. antipodarum

Collection locality: marginal vegetation. Lake Tutira, Hawkes Bay.



Metacercaria A
Cyst spherical, diameter 0.10 - 0.12 mm; thickness of cyst wall 3-6 ym. Two dark patches, apparently not structural features of the encysted cercaria are clearly visible through the walls of many viable cysts. Metacercaria with a spinose body surface; anterior sucker 30-36 ym diameter; ventral sucker posterior to mid-point of body, 27-36 pm diameter; pharynx length 18-24 pm. Infected snails contain hundreds of cysts which pack the spire of the shell and replace much of the digestive gland and gonad, as well as occupying spaces between the viscera.

infected host

Molluscan host: P. antipodarum

Collection localities: see Table 1.

Infection rates were examined in collections of p. antipodarum from 18 localities and in P. estuarinus and P. pupoides from 6localities (Table 1). All snails examined had shell heights greater than 3.5 mm (antipodarum and estuarinus) or greater than 2.0 mm (pupoides) and were therefore adult or almost adult individuals. Little evidence of parasitism was found in p. estuarinus or P. pupoides and only one infected individual of each species was seen. The parasites concerned (Cercariae M2 and M4) were both monostomes.

Metacercaria A parasitised P. antipodarum most frequently (14 out of 18 collections) and also showed the highest intrapopulation snail infection rates (86% in a collection from Lake Pukaki). Cercariae were found in 13 collections and in 12 of these monostome species were present. The highest cercarial infection rates (8.0 and 8.6%) were also produced by monostomes (Table 2). No infections of a snail by two or more species of cercariae were found.


Larval Trematoda Not Described by Winterbourne


Coitocaecum anaspidid



Telogaster opisthorchis


Stegodexamine anguillae


All larval trematodes were found in the region of the digestive gland and gonad of the host snail and infection probably rendered the snail infertile. This was indicated by the observation that no infected individuals of the ovoviviparousp. ant-ipodarum contained embryos in their brood pouches, whereas in all cases a high percentage of uninfected snails did so. Although infections by passive parasite stages (metacercariae) were high in several populations, infections by active stages (rediae, sporocysts, cercariae) were low compared with those recorded in species of Hydrobiidae elsewhere. For example, in populations of the European estuarine hydrobiid, j, Hydrobia ulvae (Pennant), Rothschild (1941) found up to 70% of the snails over 3.75 mm high were parasitised by cercariae and in a Welsh freshwater lake, Probert (1966) found that 45% of ;Bithynia tentaculata (L.) were infected by active stages. Probert also recorded 27% of B. tentaculata infected by metacercariae. The numbers of snails found to be parasitised by active stages of individual trematode species in my study were similar to those recorded in New Zealand by Macfarlane (1939) for Coitocaecum anaspidls (0.5 - 1.0% in Heathcote River), Stegadexamene anguillae (0.7% in Hut River) and Telogaster opisthocchis plus s. anguillae (2.7% in Heathcote River). The highest snail infection rate recorded by Macfarlane (1952), 15.8% for all trematode species (number and identities not given) in the Heathcote River, however, exceeded the highest cercarial infection rates that I found. Most species of mollusc that have been examined, act as intermediate host for less than 10 trematode species (Ewers 1964), although there are a number of exceptions including the pulmonate Lymnaea natalensis Krauss, the natural host of at least 43 known species of fluke. The use of P. antipodarum as first intermediate host by a relatively large number of trema todes (at least 14 species), may have come about because suitable alternative snail hosts are lacking in New Zealand. Probert (1966) found that trematodes which have a prosobranch snail as intermediate host never parasitise pulmonate snails, and P. ant-ipodarum is the only prosobranch which is widely distributed in New Zealand freshwaters. The presence of a large number of host parasite relationships also indicates that p. antipodarum has probably been present in New Zealand freshwaters for a long time and contrasts strikingly with the situation found in Europe and Britain where Potamopyrgus jenkinsi (Smith), which may be P. antipodarum recently introduced from New Zealand (Winterbourn 1972), is parasite free (Fretter and Graham 1962).


DAWES, B. 1946. The Trematoda with special reference to British and other European forms. Cambridge University Press, Cambridge.

EWERS, W.H. 1964. An analysis of the molluscan hosts of the trematodes of birds and mammals and some speculations on host-specificity. Parasitology 54: 571-578.

FKETTER, V. and GRAHAM, A. 1962. British Prosobranch Molluscs. Ray Society, London. 755 pp.

HOPKIRK, J. 1927. Intermediate host of liver-fluke in New Zealand. Recorded as the common water snail. New Zealand Journal of Agriculture 35: 175-177.

MACFARLANE, M.V. 1937. Liver fluke in New Zealand sheep. New Zealand Journal of Agriculture 55: 274-279.
______ 1939. Life cycle of Coitocaecum anaspidis Hickman, a New Zealand digenetic trematode. Parasitolosry 31: 172-184.
______ 1945. The life cycle of the heterophyoid trematode Telosraster opisthorchls n.g. , n.sp. Transactions of the Royal Society of New Zealand 75: 218-230.
______ 1951. The life cycle of Stegadexamene anguillae n.g., n.sp., an allocreadiid trematode from New Zealand. Parasitology 41-1-10.
______ 1952. Bionomics of two trematode parasites of New Zealand eels. Journal of Parasltology 38: 391-397.

PROBERT, A.J. 1966. Studies on the incidence of larval trematodes infecting the fresh water molluscs of Llangorse Lake, South Wales. Journal of Helminthology 40: 115-130.

ROTHSCHILD, M. 1941. Observations on the growth and trematode infections of Peringa ulvae (Pennant) in a pool in the Tamar Saltings, Plymouth. Parasitology 33: 406-415.

WINTERBOURN, M.J. 1970. The New Zealand species of Potaroopyrgus (Gastropoda:Hydrobiidae). Malacologia 10: 283-321.
______ 1972. Morphological variation of Potamopyrgus jenkinsi (Smith) from England and a comparison with the New Zealand species, Potamopyryus antipodarum (Gray) . Proceed-ings of the Malacological Society of London 40: 133-145.

Back to Poster