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Zoology Publications from Victoria University of Wellington—Nos. 49 to 51

A New Species of Paeonodes (Therodamasidae, Cyclopoida, Copepoda) Parasitic on New Zealand Freshwater Fish, with a Re-examination of Paeonodes Exiguus Wilson

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A New Species of Paeonodes (Therodamasidae, Cyclopoida, Copepoda) Parasitic on New Zealand Freshwater Fish, with a Re-examination of Paeonodes Exiguus Wilson

Publication of this paper is assisted by a grant from the Victoria University of Wellington Publications Fund


A new species of copepod is described from brown trout (Salmotrutta) and reported from salmon (Oncorhynchus tschawytscha) in New Zealand freshwater; from information gained from an examination of the type of Paeonodes exiguus the new species is placed in this genus, and the genus transferred to the family Therodamasidae. Since most, if not all, New Zealand salmonids were introduced as fertilized ova, the new parasite may later be found on endemic fish species.

Family Therodomasidae Tripathi, 1960

Cyclopoida. FEMALES: Body consisting of a head, with or without processes, and a long neck which are embedded in the tissue of the host, a trunk which may be indistinctly segmented, a small, separate genital segment, and a one segmented abdomen with a very small pair of caudal laminae; eggs multiserriate, egg strings club-shaped, arising ventro-laterally from the genital segment; first antennae five-segmented, second antennae subchelate, mouth parts small, structure as in Ergasilidae, three or four pairs of very small biramous pereiopods confined to the trunk, rami of the first three pairs three-segmented, the fourth, when present, one-segmented. Male: unknown. Parasitic on teleosts.

Type genus: Therodomas Kröyer.

Paeonodes Wilson, 1944 (modified)

Cyclopoida; Therodomasidae. FEMALE: Head, with dorsolateral, rounded processes, continuous with the neck, which is continuous with the trunk, which may have transverse striations but has no sign of segmentation; one pair of pereiopods placed medially on ventral surface of trunk, the other three pairs close together on ventral posterior margin of trunk. Male: unknown.

Type species: Paeonodes exiguus Wilson.

Paeonodes nemaformis n.sp.


Four females, three bearing eggs, taken from the gills, gill covers and base of the pectoral fins of a brown trout (Salmo trutta L.) captured in the Haupiri River near Kopara, Greymouth, South Island on 2 November 1963, and forwarded by the Chief Field Officer of the Wildlife Branch, Department of Internal Affairs to the Fisheries Laboratory, Marine Department, Wellington. The parasites were dissected out, and passed on to me by Mr. C. L. Hopkins of the Fisheries Laboratory who reports that the parasites had their heads and necks buried in the host tissue, causing a lession at the site of attachment.

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Mr. Hopkins also reports that he has seen similar copepods on a salmon (Oncorhynchus tschawytscha (Walb.)) from the Queenstown region, sent to his office by the Fisheries Officer of the Office of the Conservator of Wildlife, Queens-town, on 18 December 1962.


Female only (figs. 1-13).

Overall length, excluding egg strings, 3.74 mm-4.46 mm.

Head a little wider than long (0.48 mm-0.52 mm), narrowing to half this width anteriorly, anterior margin rounded, with a small median rostrum; posterolateral angles of head expanded into broad, bilobate processes. First and second antennae carried near anterior margin, mouthtube and remaining cephalic appendages reduced in size, and carried one-quarter distance from anterior margin.

Neck very long and narrow (2.24 mm-3.05 mm × 0.17 mm-0.25 mm), narrowest anteriorly (0.10 mm-0.15 mm) very gradually becoming wider near the trunk.

Trunk width two-thirds length (0.78 mm-1.14 mm × 0.54 mm-0.72 mm), widest posteriorly, narrowing rapidly to fuse with neck over anterior third, with a further slight narrowing at the midpoint; posterior angles broadly rounded, posterior margin with dorsal and ventral swellings over median third, so that in posterior view the trunk is almost +-shaped, the branches short and rounded.

Genital segment small, as wide as long (0.09 mm-0.11 mm × 0.09 mm-0.12 mm), the lateral margins broadly rounded, the egg strings arising ventrolaterally about half way along the segment.

Abdomen as wide as long (0.06 mm-0.07 mm × 0.06 mm-0.08 mm), widest anteriorly, narrowing to two-thirds this width posteriorly, posterior angles rounded; on ventral surface of segment there is a transverse row of very short spines one-third distance from anterior margin, a further row with a median discontinuity two-thirds distance from this margin, and a further transverse row at the base of each caudal lamella.

Caudal lamella carried postero-ventrally on the posterior angles of the abdomen, a little longer than wide (0.020 mm × 0.014 mm), angles rounded, with two setae carried on posterior margin.

Egg strings club shaped, width one-quarter length (1.12 mm-1.33 mm × 0.25 mm-0.31 mm), the eggs multiserriate.

First antenna of five subcylindrical segments, ratios of length from the most proximal 6:4:3:3:2; first segment with eleven setae mostly in the outer distal area; second segment with six setae near outer margin; third segment with five setae, two on outer distal angle, one on outer margin, two on distal margin near inner distal angle; fourth segment with three setae, two on outer distal angle, one on inner distal angle; fifth segment rounded distally, with five setae on distal margin.

Second antenna of three segments; first segment a solid base, as long as wide, with a small spine near outer distal angle; second segment as long as wide, somewhat rounded distally, with a stout blunt spine, one-quarter length of segment, medially on inner margin; third segment as long as second, width two-thirds length, narrowing distally to a blunt point, curved medially, forming a claw which closes against the second segment.

Labrum a scale-like structure 0.06 mm in length, width one-third length, situated immediately anterolateral to mouthtube.

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Paeonodes nemaformis n.sp., female—Fig. 1: dorsal aspect; fig. 2: head, dorsal aspect; fig. 3: head, ventrolateral aspect; fig. 4: first antenna; fig. 5: second antenna; fig. 6: mouth parts, lateral view; fig. 7: mandible; fig. 8: maxilliped (?).

Paeonodes nemaformis n.sp., female—Fig. 1: dorsal aspect; fig. 2: head, dorsal aspect; fig. 3: head, ventrolateral aspect; fig. 4: first antenna; fig. 5: second antenna; fig. 6: mouth parts, lateral view; fig. 7: mandible; fig. 8: maxilliped (?).

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Paeonodes nemaformis n.sp., female—Fig. 9: trunk, ventral; fig. 10: genital segment, abdomen and caudal laminae, ventral; fig. 11: first pereiopod; fig. 12: second pereiopod; fig. 13: fourth pereiopod.

Paeonodes nemaformis n.sp., female—Fig. 9: trunk, ventral; fig. 10: genital segment, abdomen and caudal laminae, ventral; fig. 11: first pereiopod; fig. 12: second pereiopod; fig. 13: fourth pereiopod.

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Mandible 0.027 mm in length, situated on a small wide base, basal width of mandible one-third length, narrowing slightly immediately distal to base, then widening but narrowing again over distal sixth to form a blunt point, a series of small transverse striations over distal half, a branch, one-quarter length of mandible, arising from midpoint, with a row of fine spines on its outer margin, a further jointed branch (palp?) arises near base, length three-fifths mandible length, pointed distally, with transverse striations over distal two-thirds of inner margin.

Mouthtube 0.045 mm in length, subcylindrical, somewhat rounded distally.

Maxilla (?) a lobate structure situated immediately posterior to mandible, two-thirds mandible length, with two setae on rounded distal margin.

Maxilliped (?) 0.039 mm in length, three-segmented, second segment as long as first, third segment half as long again, first segment a broad base; second segment basal width twice length, narrowing to half this width distally; third segment basal width two-thirds length, rounded distally, its distal margin covered by a clump of what appear to be stout spines which are rounded distally.

First pereiopod biramous, each ramus of three segments; protopodite width one-quarter length, rounded distally, median two-thirds of outer margin swollen ventrally, the swelling with a few transverse striations; exopod second segment half length of first, as long as third; first segment with plumose setae on outer distal angle and inner distal angle, third segment with six setae on distal margin; endopod second segment two-thirds length of first, third segment three-quarters length of second; first segment with a small seta on outer distal angle and a long seta on inner distal angle, second segment with a row of cilia on distal half of outer margin and on rounded outer distal angle, and a seta on inner distal angle, third segment rounded, with a row of fine spines on outer margin, and five setae on distal margin.

Second pereiopod similar in form to first, except that proximal segments of rami are proportionately longer, there is a seta on the inner margin of the second segment of exopod, the setae on the distal segment of exopod are proportionately longer, second segment of endopod has two setae on inner distal angle, and row of spines on outer margin of third segment of endopod are missing.

Third pereiopod similar to second.

Fourth pereiopod biramous, each ramus of one segment; protopodite width three-quarters length, narrowing distally, proximal two-thirds of outer margin swollen ventrally; exopod width two-thirds length, subrectangular, with four setae on distal margin; endopod width half length, subrectangular, with three setae on distal margin.


Since the specimens of Paeonodes nemaformis described here bore a superficial resemblance to Wilson's description (1944, p. 550) of P. exiguus I obtained his type specimen from the United States National Museum (Cat. No. 79642). As in my material, the body in Wilson's specimen (fig. 14) consists of head, neck and trunk, not separated by joints, and a small genital segment and abdomen bearing caudal laminae; the head is expanded dorsolaterally into rounded processes; the first antenna (fig. 16) is five-segmented, some setae present but in damaged condition, others presumably lost; the second antenna (fig. 17) is three-segmented, subchelate, with spines on the first and second segments. All of this agrees with Wilson's description. I was unable to locate the mouth parts. The pereiopods were in poor condition, covered in mucus, dust, and crystalline material which could not safely be removed since this is the only recorded specimen of this species. However, with the aid of phase contrast microscopy, I was able to determine that the structure described by page 37
Paeonones exiguus Wilson, female—Fig. 14: whole view; fig. 15: posterior part of body, showing positions of pereiopods 2-4, broken lines indicate the position of structures seen from another aspect; fig. 16: first antenna; fig. 17: second antenna.

Paeonones exiguus Wilson, female—Fig. 14: whole view; fig. 15: posterior part of body, showing positions of pereiopods 2-4, broken lines indicate the position of structures seen from another aspect; fig. 16: first antenna; fig. 17: second antenna.

page 38 Wilson as the first two pairs of pereiopods are lacking. The first pereiopods are situated medially on the ventral surface of the trunk and the remaining three pairs on the ventral posterior margin of the trunk (fig. 15) as in P. nemaformis. The last two pairs were mistaken by Wilson for a dwarf male of the shyriid type. This mistake is not surprising considering the condition of the specimen. As in P. nemaformis the first three pairs of pereiopods are biramous, the rami three-segmented, the last pair biramous, with one-segmented rami. There is also some similarity in the relative lengths of the segments of the rami.

The similarities between P. exiguus and P. nemaformis are so great that they must certainly be assigned to the same genus.

However, the facts that P. nemaformis has its cephalic processes bilobate, not fully rounded, lacks the transverse striations found on the base of the neck of P. exiguus and has different body proportions are sufficient to make it a separate species.

Paeonodes exiguus badly needs redescribing from fresh material. Wilson states that it was taken from near the eye of an unidentified fish. However, the label in the tube reads "near eye, Verillida grayula, May, 1911, locality unknown". It is possible that further specimens may be obtained from members of the family Verillidae.

The occurrence of P. nemaformis on New Zealand salmonids is something of an inigma. From the records available to me New Zealand salmonids have been introduced into New Zealand as fertilised ova (Thomson, 1922, Chapter 6; Stokell, 1955; Scott, 1964) which could not have brought this parasite with them. In any case, if it occurred at all commonly on salmonids in Europe or North America one would expect that it would already have been recorded.

Salmonids have apparently become infected by other parasites from native fishes in the short time since they were introduced, e.g. the nematodes Hedruris spinigera Baylis and Eustongylides sp. (Stokell, 1937, p. 80). However, these nematodes have been recorded by Stokell from a variety of endemic freshwater and esturine fish species in addition to their salmonid host, illustrating the adaptability which one would expect on the part of a parasite which has infected an introduced species in 100 years or less. Dix (1968, pp. 371-372) reports that a number of helminth parasites have adapted to the brown trout in this way after it lost its natural parasites during importation. P. nemaformis has not, as yet, been recorded from any of the endemic fish species. Further, Dr. R. M. McDowell, Fisheries Laboratory, Marine Department, Wellington, has examined in detail thousands of specimens of the families Retropinnidae, Galaxiidae and Eleotridae without sighting parasitic copepods (pers. comm.). This suggests that the most likely endemic hosts are the shark bully (Cheimarrichthys forsteri Haast), anguillids or the tidal flounder (Rhombosolea retiaria Hutton). Even should this prove to be the case, it is hard to see why this parasite has been able to adapt so quickly to the introduced salmonids when it has apparently failed to parasitise endemic species in the same habitat.

It is possible that P. nemaformis is normally parasitic on a marine host, but this would require that the parasite has, in less than 100 years, adapted to a new host family and also to a freshwater or anadromous existence.

As described here, the genus Paeonodes has much in common with Therodomas Kroyer and Mugilicola Tripathi. In particular it shares the same body divisions, it has antennae and mouth parts that are very similar, and the pereiopods are similar in form and restricted to the trunk region. The genera are separated in a key given below.

Thomsen (1949, pp. 34-35) placed the genus Therodomas in the family Ergasilidae (Cyclopoida) because of the similarity between the mouth parts found in this genus page 39 and in other genera usually assigned to the Ergasilidae. Tripathi (1960, p. 545) described Mugilicola and pointed out its similarity to Therodomas. He agreed with Thomsen that the mouth parts indicated a relationship with the ergasilids but argued that the modification of the body to a parasitic habit was so profound as to necessitate their separation as a new family (Therodomasidae). This view is accepted in this paper and Wilson's genus Paeonodes becomes the third genus placed in this family.

Key to the females of the family therodomasidae
1 (2) Three pairs of pereiopods present Mugilicola Tripathi
2 Four pairs of pereiopods present 3
3 (4) All pereiopods well separated, trunk with indistinct segmentation Therodomas Kröyer
4 Posterior three pairs of pereiopods close together, trunk without segmentation Paeonodes Wilson.

Type Material

The type and one other specimen is deposited with the Dominion Museum, Wellington (type Z.Cr. 1856, paratype Z.Cr.1857). The other two paratypes are returned to the Fisheries Laboratory, Marine Department, Wellington.


I wish to thank Mr. C. L. Hopkins for the loan of the specimens of Paeonodes nemaformis, Dr. Roger F. Cressey, United States National Museum, for arranging the loan of the type of Paeonodes exiguus, and Dr. R. W. Balham, Zoology Department, Victoria University of Wellington, for constructive criticism of the manuscript.


Dix, T. G., 1968. Helminth parasites of brown trout (Salmo trutta L.) in Canterbury, New Zealand. N.Z. Jl mar. Freshwat. Res. , 2 (2): 263-274.

Scott, D., 1964. The migratory trout (Salmo trutta L.) in New Zealand. 1—The introduction of stocks. Trans. roy. Soc. N.Z., Zool. , 4 (17) : 209-228.

Stokell, G., 1937. The nematode parasites of Lake Ellesmere trout. Trans. Proc. roy. Soc. N.Z. , 66: 80-96.

—, 1955. Fresh Water Fishes of New Zealand . Simpson and Williams, Christ-church. 145 pp.

Thomsen, R., 1949. Copépodos parásitos de los peces marinos del Uruguay. Comun. zool. Mus. Hist. nat. Montev. , 3 (54) : 1-41, 14 pls.

Thomson, G. M., 1922. The Naturalisation of Animals and Plants in New Zealand . Cambridge University Press. 607 pp.

Tripathi, Y. R., 1960. Parasitic Copepods from Indian fishes II. Two new families Therodamasidae and Catlaphillidae. Sobretiro del libro Homenaje al Doctor Eduardo Caballero y Caballero, pp. 543-548.

Wilson, C. B., 1944. Parasitic copepods in the United States National Museum. Proc. U.S. natn. Mus. , 94: 529-582.

G. C. Hewitt