Copepods are a diverse group of small crustaceans found in nearly all aquatic environments, from the deepest ocean trenches to temporary puddles. While many species are free-living and form a foundational part of aquatic food webs, approximately half of the estimated 14,000 described species have evolved to become parasites. These parasitic copepods exhibit remarkable adaptations, allowing them to live on or within a wide array of other aquatic animals. Their presence can significantly impact the health of their hosts and the ecosystems they inhabit.
Identifying Parasitic Copepods
Free-living copepods possess a teardrop-shaped body, prominent antennae, and a single median eye. Their bodies are divided into two main segmented regions: the prosome, which includes the head and legs, and the urosome, containing the abdomen and reproductive organs. In contrast, parasitic copepods often display dramatically altered forms, appearing shapeless or lacking many features seen in their free-living relatives. They often lose their swimming limbs and develop specialized structures for attaching to their hosts.
These modifications include hooks and claws for adherence to host tissues. An example is the “anchor worm” (Lernaea), which embeds a distinctive branching “anchor” structure deep into the host’s skin and muscle. The visible portion of an adult female Lernaea can appear as a slender, white, thread-like body, reaching up to 1 centimeter in length, often with paired egg sacs trailing from its posterior end. Some siphonostomatoid copepods use a “frontal filament,” a mass of filamentous tentacles, to adhere to their hosts.
The Parasitic Life Cycle
The life cycle of parasitic copepods begins with eggs, which are carried in sacs by the female. Once hatched, these develop into free-swimming nauplius larvae. This initial stage is non-parasitic, characterized by a simple body plan: a head and tail without a distinct thorax or abdomen. These larvae then undergo several molts, progressing through various naupliar stages, which can take about four days in species like Lernaea.
After the naupliar stages, they transform into copepodid stages, the first parasitic forms. During these copepodid stages, lasting approximately seven days, the young copepods must locate and attach to a suitable host, often initially on the gills. They continue to develop through several copepodid stages, remaining attached but not always permanently embedded. Upon reaching maturity, after multiple molts, they develop into the adult parasitic form, with the female embedding herself more deeply into the host.
Once attached to a host, parasitic copepods inflict detrimental effects. They feed by consuming the host’s blood, mucus, or tissue, causing direct physical damage at the attachment site. This feeding can result in visible lesions, inflammation, and tissue erosion, sometimes causing frayed fins, gill hyperplasia, or patchy epidermal necrosis. For instance, the anchor worm, Lernaea, can burrow deeply into a fish’s muscle, and in severe cases, its anchor structure may even penetrate the host’s skull or body cavity.
Beyond direct physical harm, infestations lead to indirect consequences for the host, including increased stress, reduced growth rates, and anemia. The compromised integument at attachment sites also makes the host susceptible to secondary bacterial infections, such as Aeromonas hydrophila, and fungal infections, like Saprolegnia species.
In aquaculture, parasitic copepods, particularly “sea lice” (Caligus and Lepeophtheirus genera), pose a substantial economic challenge. These parasites cause significant losses in salmon farming globally, estimated to exceed US$100 million annually, due to treatment costs, reduced growth, and carcass downgrading. Outbreaks can result in high mortality rates for farmed salmon. The spread of sea lice from fish farms also negatively affects wild salmon populations, impacting their overall productivity.
Notable Examples and Host Range
Among recognized parasitic copepods are “anchor worms” (Lernaea). These freshwater crustaceans are known for their modified adult females that embed into various freshwater fish, including common carp, goldfish, and koi. They can also infect amphibians like tadpoles and salamanders. They attach to the skin, fins, gills, or even the oral cavity of their hosts.
In marine environments, “sea lice” (Caligus and Lepeophtheirus genera) are prominent. Lepeophtheirus salmonis is a parasite of farmed Atlantic salmon in North America and Europe, while Caligus clemensi affects various Pacific salmon.
The host range of parasitic copepods demonstrates their adaptability, extending from the gills of small fish to the skin of whales. For example, Pandarus rhincodonicus parasitizes whale sharks, feeding on their blood and regenerating tissue. A striking example is Ommatokoita elongata, a pinkish-white copepod that permanently attaches to the corneas of Greenland and Pacific sleeper sharks, causing visual impairment. Other pandarid copepods are also found on the external surfaces of pelagic sharks, including blue sharks, shortfin mako sharks, and great white sharks.