Interactive Parasitology

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Parasitic Copepoda

By Tim Goater

Haemobaphes diceraus

There are about 1,700 species of parasitic copepods infecting fish. Some parasitic copepod species share many of the morphological adaptations of their ancestral free-living counterparts. For example, the common sea lice, e.g. Lepeophtheirus spp., possess locomotory appendages and are able to move freely over the surface of the fish host and from fish to fish.

Other species are permanently anchored, some penetrating deeply into the tissues of their host. In such species, jointed appendages and external segmentation are greatly reduced and the adult organism loses virtually all of the diagnostic copepod features.

The bizarre morphology and life histories of this fascinating group of parasites will be apparent in the material which follows.

Female pennellid copepods are relatively large and, as you can see from this specimen, have lost much of the free-living arthropod features such as external segmentation and swimming appendages.

Extending from the trunk of this individual can be seen the greatly modified u-shaped neck, which traverses the gills and heads directly into the bulbous arteriosus of the heart of the fish. This invasion of the circulatory system of the fish host is typical of several pennellid copepod species.

The most noticeable feature is the presence of large spirally coiled egg sacs which extend from the trunk of the organism and protrude from the gill arch of the fish host. Such large body sizes and enlargment of the reproductive regions is typical of parasitic copepods. The tremendous reproductive potential of this parasite is obvious.

In most parasitic copepod species nauplius larvae hatch and leave the egg sacs. In water they undergo a series of molts comprising several nauplius stages and up to five copepodid stages. The fifth copepodid stage searches out a host, molts to the adult and attaches to a host. Pennellids are unique among copepods in that they require an intermediate host (usually another fish species) for development. The copepodid larvae becomes a chalimus larva stage on the intermediate host and male and female chalimus individuals mate on this host. Males are short-lived and female individuals leave the intermediate host and search out the definitive host where the astonishing metamorphosis to this large, gravid female occurs. The intermediate host for Haemobaphes diceraus is unknown.

This photograph shows an intact, metamorphosed female of H. diceraus, removed from the gill region of shiner perch, Cymatogaster aggregata. This species is a representative of the Family Pennellidae, one of the most highly modified families of copepod parasites infecting fish.

This photograph shows a living, mature female specimen attached to the gill arch. Again, this is only the greatly modified trunk region and egg sacs of the parasite. Note that the egg masses appear as two clusters of spirally coiled filaments. This photograph also shows an unusual morphological feature of this genus; the presence of outgrowths or processes extending from the trunk and at the base of the egg sacs. Their functional significance is unknown.

The large red trunk region with outgrowth, yellow egg sac and long and narrow neck leading to the cephalothorax/holdfast region are obvious morphological features.

This is a living specimen
removed intact from a shiner perch's
circulatory system.

Video footage obtained during a
field excursion on the research boat
from the Bamfield Marine Station.
One of the shiner perch is infected
with this copepod species,
and shows dramatically the location
and large body size of this species
on the fish's gills.
627 KB AVI File

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Mytilicola orientalis

While parasitic copepods are most common on fish hosts, several species infect invertebrates. This unusual species parasitizes the intestine of bivalves; such as blue mussels, Mytilus trossulus.

Mussels are infected by contact with infective copepodid larvae which enter the mussel while filter-feeding. Gravid females brood egg sacs on the trunk region indicated and nauplii larvae are released into the plankton. They pass through two nauplii molts and five copeopodid stages before the life cycle is completed. The reduction in nauplii stages may help transmission since fewer free-living stages reduce the chances that the parasite will be swept away from the host habitat. This host-parasite system is patchily distributed and confined primarily to muddy, estuarine habitats.

In an ecological study conducted at the Bamfield Marine Station by student, Amy Weber, it was found that abundance of Mytilicola was positively correlated with mussel body size and density. Larger mussels and dense mussel populations from mudflats harboured larger populations of this copepod species than smaller individuals and those mussels from exposed sites.

Whether or not M. orientalis is the cause of parasite-induced mortality is still a matter of controversy. In Europe, Mytilicola has been implicated as a major pathogen of mussels, interfering with bivalve feeding by destruction of host intestinal mucosa lining.

A young female of M. orientalis.

This video gives an impression of a young, living Mytilicola removed from the intestine of a mussel. At the anterior end you will see the organism's antennae. You will also see digestive contents coursing through the intestine of the animal.
750 KB AVI File

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Last modified: April 22, 1996 nfm