Overview

Brief Summary

Taxonomy

Derogenes varicus (Müller, 1784) belongs to a large group of species known as the Hemiuroidea, many of which also occur in the stomach of fish. The details of the parasites relationship with the fish are as yet unknown.

Adult morphology
The parasite may reach about 3mm long and 0.8mm wide.At the anterior or front end is the mouth (M) surrounded by the oral sucker (OS).In the middle of the worm is a large attachment sucker, the ventral sucker (VS).Behind this sucker are sex organs, the two testes (T), the ovary (O) and the two yolk or vitelline glands (V).There are many eggs (E) spread throughout the body.
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Introduction

Derogenes varicus is a trematode flatworm that lives parasitically in the stomach of many species of marine fish in all cold water areas of the world. It has no common name, but it is one of the most widely spread and common marine species.It lives in at least three different hosts during different stages of its lifecycle,
  • a marine snail
  • another invertebrate
  • fish
During its free-swimming stage as a cercaria, the parasite uses its tail for protection, propulsion and as a way to infect its next host.
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Comprehensive Description

Biology

The body - or soma - of Derogenes varicus has two suckers and an inverted U-shaped intestine.

Lifecycle
Derogenes varicus typically has a three host life-cycle and lives in
  • a marine snail
  • another invertebrate, then
  • a fish, in which the adult develops
In some cases other hosts may be fitted into the cycle.In the stomach of the fish, the worms develop into fully mature hermaphroditic adults, with testes and ovary, and continuously produce a flow of eggs which are shed into the fish gut and pass out in the host’s faeces.These eggs are consumed by the snail where they hatch into a motile larva - miracidium.The miracidium moves to the digestive glands of the snail and transforms into a sac-like stage called the mother sporocyst.The mother sporocyst produces - by asexual reproduction - another generation called rediae. These are similar to the sporocyst, but have a gut and pharynx.Each redia gives rise to many cercariae - motile larvae with a tail - which emerge from the redia and then the snail.
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Distribution

Gulf of St. Lawrence (unspecified region), Laurentian Channel (bathyal zone) to the northeast of Anticosti Island (=Jacques Cartier Strait), Laurentian Channel (bathyal zone) (=Esquiman Channel), Saguenay Fjord, southern Gaspe waters (Baie des Chaleurs, Gaspe Bay to American, Orphan and Bradelle banks; eastern boundary: Eastern Bradelle Valley), lower St. Lawrence estuary, Magdalen Islands (from the eastern Bradelle valley to the west, as far as Cape North, including the Cape Breton Channel, Prince Edward Island (from the northern tip of Miscou Island, N.B. to Cape Breton Island south of Cheticamp, including the Northumberland Strait and Georges Bay to the Canso Strait causeway), middle North Shore (from Sept- Iles to Cape Whittle, including the Mingan Islands and the southwestern slope of NL.
  • North-West Atlantic Ocean species (NWARMS)
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Derogenes varicus has been reported in over 250 fish species from shallow cold waters of both northern and southern hemispheres. It is also occasionally reported in squid.There is evidence to suggest it is also common in cold deep waters in equatorial regions, exhibiting a phenomenon known as ‘equatorial submergence’.
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Ecology

Habitat

parasitic
  • North-West Atlantic Ocean species (NWARMS)
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Associations

Known prey organisms

Derogenes varicus preys on:
Anguilla anguilla
Ciliata mustella
Pholis gunnellus
Myxocephalus scorpius
Platichthys flesus
Copepoda

Based on studies in:
Scotland (Estuarine)

This list may not be complete but is based on published studies.
  • Huxham M, Beany S, Raffaelli D (1996) Do parasites reduce the chances of triangulation in a real food web? Oikos 76:284–300
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Life History and Behavior

Behavior

Diet

parasitic on host
  • North-West Atlantic Ocean species (NWARMS)
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Growth

Motile larvae

Cercariae
Cercariae are motile larvae with a tail which are produced asexually by the redia (see lifecycle).The cercariae are a type called cystophorous. The tail is used in at least three ways to assist the cercaria’s passage:
  • it is enlarged and when swimming freely, the body of the cercaria is withdrawn within it
  • the narrow posterior part of the tail is forked, and beats vigorously to keep the cercaria afloat
  • when ingested by copepods or larvae of barnacles and crabs, the cercariae turn part of their tail inside out to produce a delivery tube which
    • passes up into the predator’s gut and perforates the gut wall and,
    • provides a tunnel for the body of the cercaria to pass along into the invertebrate’s body-cavity
    • in the body-cavity the larva develops into a metacercaria -a stage where the larva grows, but does not usually become mature
The tail is in two parts:
  • a spherical part which contains a coiled delivery tube and into which the body can be withdrawn
  • an elongate forked swimming or flotation organ
Metacercariae, in relatively large crustaceans, can grow large enough to infect fish when the crustacean is swallowed.In smaller crustaceans the metacercariae may first need to get into a larger host - such as an arrow worm - to grow large enough to infect fish. In some cases these metacercariae produce eggs.
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