Marine parasites may be small in size, but they can be present in very high numbers and put together can weigh even more than all the top predators in an estuary or bay ecosystem! They play an important role in keeping their host population from growing out of control—allowing them to exert power over food webs and ecosystem function. High parasite diversity is even an indicator of a healthy ecosystem.

What makes parasites fascinating to study is that they have had to evolve complex strategies that allow them to live both inside a host and outside in the environment.

Here are a few examples of some crazy (and really cool) marine parasites:


Hans Hillewaert

    1. What kind of creature can live on land in cats and in the ocean in seals? The parasite Toxoplasma gondii causes a disease called toxoplasmosis and is known for infecting cats on land. Recently, researchers have found this parasite infecting, and in some instances killing, a variety of marine mammals including California sea otters, Hawaiian monk seals, and Pacific harbor seals. However, researchers still don’t know how the parasite is getting into the ocean or how it’s infecting marine mammals.
    2. What parasite changes its look as often as its host? Digenetic trematodes are flatworm parasites with really complicated life cycles. These involve multiple life history stages that look completely different from each other, such as a swimming stage and a cyst stage, where it lies sleeping in host tissues. Their life cycles can include two, three, or even four different hosts, such as Ribeiroia ondatrae, which first infects a snail, then a fish or tadpole, then a mammal or bird at different points in its life cycle!
    3. How can you make an otherwise scrumptious crab taste like aspirin? The parasite Hematodinium infects a variety of different crustaceans including crabs, hermit crabs, and lobsters. In cold water-dwelling crab species, such as the snow crab, Hematodinium causes Bitter Crab Disease, which makes the crabmeat taste like aspirin. Researchers aren’t sure what makes the meat of these cold-water hosts taste badly and why the meat of infected crustaceans from temperate or tropical waters doesn’t taste like aspirin.
    4. How do you make a male crab behave like a female? It takes more than lipstick. Rhizocephalans are barnacles that castrate the crabs they’ve invaded. These parasites essentially take over the crab’s behavior, destroy the crab’s genitals, and then create external sacs that look like crab eggs. Even though male crabs don’t carry eggs, they’re snagged by the parasitic trick as well, caring for the parasite as if it were its own young!
    5. What causes oysters to lose the strength to go on? Parasites can be particularly problematic when they manage to invade a new geographic location. The parasite Haplosporidium nelsoni, which causes Multinucleated Sphere Unknown (MSX) disease, was accidentally introduced when Pacific oysters were imported to the Chesapeake Bay (and MSX can be found as far north as Maine). Oyster populations can survive with low levels of MSX present, but an outbreak of the parasite can affect the reproduction and feeding of the shellfish, and ultimately weakens the oyster to the point where they can’t hold their shell shut. Unfortunately, after its introduction, MSX disease killed a large number of eastern oysters in the Chesapeake Bay and is now infamous for its role in decimating oyster populations, effectively changing the entire ecosystem of the Bay.

Although we know about some of the wacky ways that marine parasites can survive and thrive, there is still so much that we don’t know. Most of the research that has been conducted thus far on marine parasites has focused on those that impact commercially important marine species, like edible crabs, shrimp, lobsters, oysters, and clams. However, there are still a large number of marine species that aren’t commercially important and probably harbor parasites. We can only imagine what crazy, cool parasites have yet to be discovered!