Náttúrufræðingurinn

						SUMMARY

The Biology of arctic charr

(Salvelinus alpinus (L.)) in

Lake Þingvallavatn, Iceland. II.

Parasites: Plerocercoids of the

genus Diphyllobothrium

(Cestoda).

by

Hilmar J. Malmquist,

Sigurður S. Snorrason

and Skúli Skúlason.

Insíitute of Biology

University of Iceland

Grensásvegur 12

108 Reykjavík

Lake Þingvallavatn (66° 10' N and 22°

10' W) is the largest lake in Iceland, 83.7

km2 and 2860 mill. tonns. It is a deep

oligotrophic lake (mean depth 34.1 m,

max depth 114 m) and cold (min in Jan.,

0-2°C, max in Aug.-Sept., 10-12°C).

Four dífferent charr morphs inhabit the

lake; dwarf charr (dvergbleikja), snail

charr (kuðungableikja), pelagic charr

(murta) and piscivorous charr (síla-

bleikja). The charr morphs are quite easily

distinguished on the basis of outer morp-

hology and colour pattern and the diet of

each charr morph is quite distinct (Hilmar

J. Malmquist et al. 1985). Dwarf and snail

charr feed on zoobenthos in the littoral.

Snail charr feed almost exclusively on

Lymnaea peregra. Dwarfs feed mainly on

Lymnaea, but the diet is more heterogene-

ous and includes chironomid larvae and

pupae. Pelagic charr feed mostly on zoo-

plankton, mainly Cyclops abbyssorum and

Daphnia longispina. Piscivorous charr

feed almost solely on three-spined stickle-

back (Gasterosteus aculeatus).

Diphyllobothrium spp. (D. ditremum

and D. dendriticum) are common parasites

in Icelandic lake-resident populations of

arctic charr. A relatively high percentage

of the arctic charr in lake Þingvallavatn

was infected by Diphyllobothrium spp.

The main species involved is probably D.

ditremum.

1299 charr were examined for infection

of Diphyllobothrium spp. plerocercoids on

intestines (stomach, pylorus and liver). In-

fection (percentage cover of plerocerco-

ids) was estimated and grouped into four

classes (Fig 1). Formation of connective

tissue around and between intestines was

estimated and grouped into four classes

(Fig. 2).

There was a clear difference in frequ-

ency of infection between the charr

morphs (Fig. 1): Only 0.3 % of the dwarfs

were infected and just 6.2 % of the snail

charr. On the other hand 76.9 % of the

pelagic charr were infected and 90.1 % of

the piscivorous charr.

Formation of connective tissue around

intestines followed a very similar pattern

as plerocercoid infection (Fig. 2).

Intensity of infection increased with age

(Tables 1—2). Heavily infected piscivorous

charr with intestines adhering in a bundle

of connective tissue were common (Fig.

3). Pelagic charr were seldom found in this

condition. On the contrary, intestines of

dwarf and snail charr were invariably well

segregated.

Both copepods (e.g. Cyclops) and the

three-spined stickleback are well known

intermediate hosts for Diphyllobothrium

spp., but the gastropod Lymnaea is not.

Thus, Diphyllobothrium infection of arctic

charr in lake Þingvallavatn reflects very

clearly the specialized feeding habits of the

charr morphs.

An increase in intensity of infection with

age probably results from an accumalation

of plerocercoids. A higher intensity of in-

fection in piscivorous charr compared with

pelagic charr may be traced to fish eating.

The lifecycle of D. ditremum in lake

Þingvallavatn is discussed in relation to the

feeding habits of the charr morphs (Fig.

5).

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