Counter-intuitively, leeches that attach themselves and their eggs to crabs are often regarded by humans as being a sign of a good-quality crab.
A heavy buildup of leeches and cocooned leech eggs on a crab strongly indicates that the crab molted months ago is now fleshier.
Do leech eggs on crabs harm humans?
Leeches and their eggs on crabs do not cause diseases to humans and are not known to be harmful to humans for any other reason either. Well-cooked crabs do not present a health risk to humans from leech eggs or leeches.
Leech eggs and humans
Leech eggs can be so tiny that they cannot be seen. Humans who get them in the wrong water can accidentally swallow hundreds of leech eggs.
Even when they don’t get into humans’ stomachs, leech eggs can still get into humans’ noses, ears, and eyes.
Once inside, the eggs will hatch, and the leeches will feed on the blood of their unfortunate victims.
Leeches on crabs–cause and effect
Leeches go into the zoological family of segmented worms. There are over 500 species of terrestrial, marine, and freshwater worms.
‘Johansonia Arctica’ is a common North Pacific leech.
It lives parasitically off cod and several other types of marine fish, and serves as a transporter for ‘Trypanosoma Murmanensis’, yet another parasite that feeds on the blood of its host.
‘Notostomum Cyclostoma’, another leech species, is a parasite that can be found on several fish species, including cod, pollock, and flatfishes. It is a transporter for ‘Cryptobia sp.’, a hemoflagellate parasite.
‘Malmiana sp.’, a smaller fish leech, is a relatively rare leech.
How do leech eggs affect host crabs?
None of the leeches mentioned above are parasites on crabs.
They choose to attach themselves and their eggs simply because the shells of crabs act as a stable and reliable surface.
By moving about rather than being stationary like rock, crabs unwittingly help the leeches to spread from place to place.
Meanwhile, host crabs are never harmed by “guest” leeches.
Which crabs carry leech eggs?
Leeches hitch a ride and/or lay their eggs on several crab species.
These are mainly the Canadian Snow crab, Red King crab, Blue Crab, Tanner crab, Golden King crab, Giant King crab, and Horseshoe crab.
Horseshoe crabs inhabit the sandy and muddy bottoms in shallow coastal water.
Their ten legs and pointed, hardened exoskeletons make them easy to spot at the beach.
Horseshoe crabs are often called “living fossils” because they haven’t changed much throughout the history of the fossil record for which they have been around.
Signs that crabs are carrying leech eggs
The best way to tell that crabs are carrying leech eggs is by visually inspecting the crabs.
When crabs are carrying leech eggs, dark brown or black cocoons will be observable in various places of their shell, especially on the undersides of their legs.
Leeches seem particularly prevalent with Tanner and King crabs.
Smaller types of adult leeches on these crabs can reach about 0.8 inches in length, while larger leeches can reach up to about 4 inches.
Cocoons of the smaller types of leeches attain around 0.06 inches, while cocoons of larger-sized leeches attain around 0.3 inches.
How leech eggs infection spreads between crabs
Leeches’ eggs hatch directly on crabs’ shells. The freshly hatched leeches then find a host fish to feed on parasitically to get a meal of blood.
Leeches are hermaphrodites and can engage in sexual reproduction to fertilize their eggs.
When ready to lay eggs, leeches leave the host fish and find a hard object with a suitable surface on which to lay their eggs.
A single egg is enclosed in each cocoon.
Leech eggs and the Horseshoe crab
Horseshoe crabs are a valuable prey animal for many organisms, including leeches.
Horseshoe crabs age slower and molt less as they get older, making them pretty attractive to leeches.
Alongside leeches, many snails, bivalves, and sponges can be found living on the undersides and backs of Horseshoe crabs.
However, the eggs of the Limulus leeches are often found around the leg joints and book gills of Horseshoes and mainly target old female crabs.
Leeches are not always bad for humans
Not at al. Leeches can help increase blood circulation and break up blood clots.
They can also be used to treat cardiovascular disease and circulatory disorders.
Various medicines can be made from chemicals extracted from the saliva of leeches.
These medicines have proved to be effective in treating hypertension.
What leeches look like
Leeches are segmented worms. One major departure from other worms is that they have suction cups at each of their ends.
Leeches’ bodies are far more flattened and thicker than other types of worms.
Leeches are always dark shades of their color, which can be green, black, or brown.
Some leech species have stripes or spots, while others don’t have any markings at all.
The deadliest species of leech
The deadliest species of leech is the Diamondback leech.
Its saliva is such a powerful anticoagulant that it can prevent the human body from clotting, which is a life-threatening condition.
When humans’ blood can’t clot, it causes an illness that some unlucky people are born with called hemophilia.
People suffering from hemophilia who sustain what would normally be an ordinary cut or bruise find that the injury can quickly escalate into an emergency due to copious loss of blood.
Lifecycle of leeches
Once laid, leech eggs typically hatch in about a fortnight. At this point, fully formed juveniles emerge from their cocoons (around five juveniles to each cocoon).
It takes about a year for the young leeches to reach sexual maturity. Depending on the species, leeches live for between two and eight years.
Frequently Asked Questions About Leech Eggs on Crabs
Why do leeches lay their eggs on crabs?
Marine leeches lay their eggs on the hard shells of crabs like Snow crabs and all variants of King crabs, in order to protect the cocooned eggs and assist their distribution.
Are leeches dangerous to humans?
Leeches are very dangerous. Any leeches that are found on a person’s body should be removed as soon as possible. Leeches can take a lot of blood if they are attached to a person for a long time, and they can also carry deadly diseases.
Afterword: Do leech eggs on crabs harm humans?
Since humans only eat crab meat and do not generally consume crab shells, and since crabs are not eaten uncooked, it is exceedingly unlikely that anyone ever gets to eat leech eggs.
Consequently, leeches and their eggs cannot cause humans any harm.
Finally, crabs are wrapped in ice before transportation, and although neither leeches nor their eggs can survive being frozen, if they do by some exceedingly unlikely chance, they will not survive the cooking process.