Discocotyle sagittata, from here.

Belongs within: Polyopisthocotylea.

Gill graspers
Published 2 May 2023

The class of parasitic flatworms known as the flukes can mostly be divided between two major assemblages, known as the Digenea and Monogenea. The better-studied of these assemblages is certainly the Digenea, by virtue of including those species that infect terrestrial vertebrates such as humans. However, the diversity of the Monogenea should not be underestimated.

Discocotyle ohridana, ventral view with opisthaptor upwards, from Stojanovski et al. (2005).

In contrast to the digeneans, which infect a wide range of vertebrates and are often endoparasitic, the monogeneans are invariably ectoparasites, usually of fish. Whereas digeneans typically have a pair of distinct sucker-style attachment organs on the underside of the body, one near the head and one further back, monogeneans have the rear end of the body broadened into a large attachment organ called the opisthaptor. Anterior attachment organs (prohaptors), if present, are less prominent than those of digeneans. The opisthaptor is typically a much more complex apparatus than the simple suckers of digeneans, often incorporating multiple pairs of adhesive suckers or grasping clamps. Monogeneans also differ from digeneans in their simpler life cycle, incorporating only a single host rather than successive larval and adult hosts. As a rule, monogeneans are hermaphrodites (Dawes 1956).

Discocotyle sagittata on gills of brook trout, copyright Chris Blanar.

Among the numerous families of the Monogenea may be listed the Discocotylidae, blood-feeders found attached to the gills of their host (Dawes 1956). In this family, the prohaptors are two small buccal suckers within the mouth opening. The opisthaptor comprises four pairs of clamps that each grip onto the host gill filaments. Clamps are supported by a series of cuticular sclerites whose arrangement is characteristic of the family. Typically, this is a massive middle sclerite, two pairs of lateral sclerites and an additional basal sclerite, though the lateral and basal sclerites may be variously fused or modified. The opisthaptor as a whole may be symmetrical or one side may be larger than the other. A vagina is typically present but may have one or two openings (e.g. Dechtiar 1967); it is absent in members of the subfamily Tagiinae (Dillon & Hargis 1965).

Species of Discocotylidae have been found on both marine and freshwater hosts. A number have been recorded from multiple (albeit related) host species, such as Discocotyle sagittata on trout and salmon (Dawes 1956). This last species has been recorded as potentially causing the death of its host, with more than one hundred flukes found on the gills of a single host individual. Ending your time with your gills gummed up by worms would not be a pleasant way to go.

Systematics of Discocotylidae
Discocotylidae [Discocotylinea]
|--Chimaericola Brinkmann 1942 [Chimaericolidae, Chimaericolinae]D56
| `--*C. leptogaster (Leuckart 1830) (see below for synonymy)D56
| |--Diplozoon Nordmann 1832 [=Diplozoum; incl. Diporpa Dujardin 1845]D56
| | |--*D. paradoxum Nordmann 1832 [incl. Diporpa dujardinii Diesing 1850]D56
| | `--D. nipponicum Goto 1891D56
| `--Discocotyle Diesing 1850 [=Discotyle; incl. Placoplectanum Diesing 1858]D56
| |--*D. sagittata (Leuckart 1842) (see below for synonymy)D56
| |--D. sybellae (Scott 1909)D56
| `--D. thyrites (Hughes 1928)D56
|--Octomacrum [Octomacrinae]DH65
| `--O. europaeumDH65
|--Bicotylophora Price 1936D56 [BicotylophorinaeDH65]
|--Anthocotyle van Beneden & Hesse 1863 [Anthocotylidae, Anthocotylinae]DH65
| `--*A. merluccii Beneden & Hesse 1863 [incl. A. merluccii americanus MacCallum 1916]D56
|--Allocotylophora Dillon & Hargis 1965 [Allocotylophorinae]DH65
| `--*A. polyprionum Dillon & Hargis 1965DH65
|--Tagia Sproston 1946DH65
| |--*T. ecuadori (Meserve 1938) [=Heterobothrium ecuadori]DH65
| `--T. gempylli Dillon & Hargis 1965DH65
|--Hemitagia Sproston 1946DH65
| `--H. galapagensis (Meserve 1938)DH65
`--Allotagia Dillon & Hargis 1965DH65
`--*A. otolithis (Yamaguti 1953)DH65 [=Kuhnia otolithisP61, Tagia otolithisDH65]

*Chimaericotyle leptogaster (Leuckart 1830) [=Octobothrium leptogaster, Discocotyle leptogaster, Neoheterobothrium leptogaster, Octocotyle (Octobothrium) leptogaster, Placoplectanum leptogaster]D56

*Discocotyle sagittata (Leuckart 1842) [=Octobothrium sagittatum, Mazocraes sagittatum, Placoplectanum sagittatum; incl. Cyclocotyla lanceolata Zäringer 1829 (n. n.), Discocotyle salmonis Shaffer 1916]D56

*Type species of generic name indicated


[D56] Dawes, B. 1956. The Trematoda with special reference to British and other European forms. University Press: Cambridge.

Dechtiar, A. O. 1967. Neodiscocotyle carpioditis n. gen., n. sp., monogenetic trematode (Discocotylidae: Neodiscocotylinae subfam. n.) from the gills of the quillback, Carpiodes cyprinus (Le Sueur) of Lake Erie. Canadian Journal of Zoology 45: 473–478.

[DH65] Dillon, W. A., & W. J. Hargis Jr. 1965. Monogenetic trematodes from the southern Pacific Ocean. 2. Polyopisthocotyleids from New Zealand fishes: the families Discocotylidae, Microcotylidae, Axinidae, and Gastrocotylidae. In: G. A. Llano (ed.) Biology of the Antarctic Seas II pp. 251–280. American Geophysical Union.

[P61] Price, E. W. 1961. North American monogenetic trematodes IX. The families Mazocraeidae and Plectanocotylidae. Proceedings of the Biological Society of Washington 74: 127–156.

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