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Antarctic fish parasites

It has been estimated that more than 50% of the species on earth are parasites, exploiting the remaining species acting as hosts. Parasites may interfere with almost any aspects of their host’s life; familiar to most are the effects on appetite and growth caused by tapeworms, but parasites may also cause disease and death (epidemics).

Above:

Figure 1. Single-celled fish parasites. A. Coccidians are spore producing cingle celled parasites, that produce small cysts in groups of 4. The picture shows such cysts (called sporocysts) of a new species of Eimeria infecting deep-sea fishes. B. Each Eimeria sporocyst is shaped like a small bottle, with both neck and cap, and each contain two small sausage-shaped parasites. C. Also the Antarctic krill is parasitized, every Euphausia superba examined was infected in their gut by a particular type of spore-producing parasites called gregarines. D. The stomach of certain deep-sea fish homes single celled parasites (flagellates) of the genus Cryptobia.

Parasites and disease in fish are by some perceived as anomalies related to human activities, such as fish farming or pollution. The AKES research expedition represent an opportunity to collect fish from the perhaps most unaffected parts of the world-oceans. This allows us to examine the occurrence of some viral and bacterial diseases, causing troubles in aquaculture worldwide in these nearly untouched seas. These results, however, must await the analysis of our samples at the laboratory back in Bergen. Some of the larger fish parasites are studied on-board and a selection of these is presented below.

Why study Antarctic fish parasites?

We need a better understanding of the ecosystems in the Southern Ocean, not least to be able to detect effects of climatic change and to allow environmentally sustainable harvesting of resources such as krill. Parasites and disease cannot be neglected in ecosystem studies, despite being small and inconspicuous parasites may be important in regulating the size of their host populations. A problem in Antarctic waters is our incomplete knowledge of the local parasite faunas. Considering fish parasites, there seems to be a clear relationship between the size of the parasites and the level of our knowledge, the smaller they are the less we know. Therefore the AKES surveys have discovered several new microscopic fish parasites, ranging from single celled small creatures (flagellates) swimming in the stomach fluid, spore-producing animals (coccidians) infecting the gut-wall or larger intestinal flukes (see Figure 1).


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Figure 2. Many of the deep-sea fishes collected share big mouths with numerous sharp teeth. In the picture a specimen of Anoptopterus pharao is displayed, which represents the largest fish individual collected during the (77 cm). Also this relatively ‘large’ fish had few parasites, only eight tapeworm larvae...

The examined fish represent species living in open waters above great depths (Fig 2). Elsewhere in the world such fish have been found to harbour few parasites, mostly larval types. This pattern is representative also for the situation in the Antarctic region, where the most common parasites are tapeworm-larvae, representing several types. The adult tapeworms of the recovered types either infect seals, whales, seabirds or other fish. A clue to which types of larvae that belong in respectively warm bodied hosts and fish was obtained when freed larvae where heated to room temperature in small vials with physiological saline water. Some types survived and became increasingly active, and hence likely occur as adults in ‘warm guts’, other died and may represent fish parasitic types (Fig. 3). Further insight into the lifecycles of these tapeworms may be obtained when comparing their DNA sequences with those of adult tapeworms from different host-types.


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Figure 3. Tapeworm larvae, the most common parasites in Antarctic deep-sea fishes. A. Diphyllobothrium larva in the musculature of the belly in a barracudina (Notolepis coatesi), visible externally. B. Small Diphyllobothrium-larva in the stomach wall of a lantern-fish. C, D, E, freed, live tapeworm larvae (same magnification). C. Diphyllobothrium larva from the musculature of a barracudina, D. Scolex pleuronectis from the intestine of a barracudina. E. Unknown type tapeworm larva from the stomach wall of Bathylagus. E. Freed tapeworm larvae either becomes very active (Diphyllobothrium, left jar) or die (unknown type, right jar) in room-temperature. These probably represent tapeworms with adults in respectively warm-bodied animals and fish.


The most conspicuous fish parasites encountered so far are some very transformed types of crustaceans (copepods), appearing more or less maggot-like. Two types infect deep-sea fishes of the genus Bathylagus (Fig 4 A). While representing rather large aliens to be carried around by their hosts, these parasites are in addition anchored deeply inside their hosts bodies, either in the muscle (Sarcotretes), or in the vertebral column with their mouth in the kidneys (Paeonocanthus) (Fig. 4 B,C).


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Figure 4. Crustacean fish parasites (copepods) from deep-sea fishes in the Antarctic area. A. Bathylagus specimens parasitized by the yellowish Paeonocanthus antarcticensis (3 fish to the left) or blood-red Sarcotretes sp. (3 fish to the right). B, C. Paeonocanthus is attached deep in the hosts tissues, with the mouth in the kidneys of the fish (see forceps-tip in C). D. Barracudinas (Notolepis coatesi) may also be infected with Sarcotretes sp., here a rare double infection. E. Sarcotretes infection in a barracudina, the green arrows indicates the position of the anchoring head-ends of the parasites in the muscle.

Unfortunately we did no fishing in the Bouvet Island region, where we had expected to be able to sample typical Antarctic fish species such as the mackerel-icefish (Champsocephalus gunnari). A short distance to the bottom (e.g. shelf) introduces additional groups of fish parasites that have bottom-living intermediate hosts. Also, the occurrence of colonies of fur-seals, sea-elephants, penguins and other marine birds on the island suggests that larval stages of the parasites of these hosts may be particularly common. Such larval parasites may actually facilitate the capture of their fish host by a correct final host such as a fur-seal. It may be concluded that there is much left to be discovered down here, subjects for future research cruises.

 

 

Written by

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Egil Karlsbakk (dr.scient.) works as a scientist at the Institute of Marine Research in Bergen, Norway. Egil is involved in several projects on fish parasites and other marine organsisms.