''Lernaeocera branchialis'', sometimes called cod worm, is a

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

of marine fish, found mainly in the

North Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

. It is a marine

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

which starts life as a small

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

crustacean larva. It is among the largest of copepods, ranging in size from when it matures as a copepodid larva to more than as a sessile adult. ''Lernaeocera branchialis'' is ectoparasitic, which means it is a parasite that lives primarily on the surface of its hosts. It has many life stages, some of which are motile and some of which are sessile. It goes through two parasitic stages, one where it parasitizes as a secondary host a

flounder Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuary, estuaries. Taxonomy The name "flounder" is used for several only distantly related speci ...

or lumpsucker, and another stage where it parasitizes as a primary host a

cod Cod (: cod) is the common name for the demersal fish genus ''Gadus'', belonging to the family (biology), family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gad ...

or other fishes of the cod family ( gadoids). It negatively impacts the

commercial fishing Commercial fishing is the activity of catching fish and other seafood for Commerce, commercial Profit (economics), profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice ...

and mariculture of cod-like fish.

Life stages

The life-cycle of a cod worm involves a complex progression of life stages, including two successive hosts. It comprises "two free-swimming nauplius stages, one infective copepodid stage, four chalimus stages and the adult copepod, each separated by a moult". The cycle begins with the females laying eggs which hatch into a nauplius, the usual early larval stage of crustaceans. This nauplius I moults about 10 minutes after hatching to produce nauplius II, and 48 hours later, nauplius II moults to a copepodid stage. At this point the copepodid is pelagic and free-swimming with an average length of about 0.5 mm. The next stage is finding a secondary or intermediate host, a demersal fish like a

or lumpfish which is often stationary and therefore easy to catch. The copepodid has only a day to find such a fish and attach itself to the fish's gills. When they locate such a fish, they capture it with grasping hooks at the front of their body. They penetrate the fish with a thin filament which they use to suck its blood. The nourished cod worms then progress via four moults from the naupliar stage to the mature chalimus stage. At this point the males transfer sperm to the females. Both sexes develop swimming

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e, detach from the flounder or lumpfish and again swim freely as pelagic organisms. The female cod worm still resembles a copepod and is 2 to 3 mm long. Females undergo another pelagic quest, searching this time for a definitive or primary host. With her fertilised eggs, she looks for a

or a fish belonging to the same family as cod, such as a haddock or whiting. When a suitable definite host is located, females enter the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

chamber. There, while attached to a gill, the female develops a plump, sinusoidal, worm-like body, with a coiled mass of egg strings at the posterior end. Females now measure about 20 mm long, but can grow up to 50 mm. The oral end of the female copepod penetrates the body of the cod until it enters the rear bulb of the host's

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

. There, firmly rooted in the cod's circulatory system, the front part of the parasite develops in the shape of antlers or branches on a tree, reaching into the main

artery An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...

. In this way, while safely tucked beneath the cod's gill cover, the female's deeply embedded oral end can feed on blood while eggs develop and are released into the water column from the posterior end.

Behaviour

It is not known how ''L. branchialis'' searches for its fish hosts, but it probably uses

chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...

s and

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...

s, and follows physical clues in the water column such as those provided by

halocline A halocline (or salinity chemocline), from the Greek words ''hals'' (salt) and ''klinein'' (to slope), refers to a layer within a body of water ( water column) where there is a sharp change in salinity (salt concentration) with depth. Haloclin ...

s and thermoclines.

Effects on fisheries

The most serious parasitic crustaceans among fish in general are sea lice. However, ''L. branchialis'' is probably the most serious parasitic crustacean among cod. Infestation reduces the efficiency with which food can be utilised, delaying the development of the

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s. Up to 30% loss in weight can occur, with increases in mortality because of open lesions with loss of blood, and possibly occlusion of vessels or aorta. These can have commercial impacts on wild fisheries, making cod-like fishes more expensive to market. Gadoids, particularly cod, are emerging marine aquaculture species in some North Atlantic countries. ''L. branchialis'' present potential problems for their successful mariculture.Fisheries Research Services (2005
Final report of the Aquaculture Health Joint Working Group sub-group on disease risks and interactions between farmed salmonids and emerging marine aquaculture species
Page 29. Scotland.

Life stages

Behaviour

Effects on fisheries

See also

References

Further reading