Why do male crabs experience "false pregnancy" after being parasitized?

When it comes to parasites, humans always condemn them from a moral or health perspective: they are a small group of creatures that will do anything to survive, grabbing cruelly and exploiting greedily.

But from the perspective of nature, parasites are neither a small group nor simple predators. Parasites account for at least 40% of all biological species. From whales to insects, they all have parasites on their backs. Parasites calmly control the density of the parasitized populations and prevent them from over-reproducing. They are also important controllers of the balance of the ecosystem and even a benchmark for ecological diversity. Almost all organisms are occupied by parasites to varying degrees without knowing it.

A vicious parasite

Parasitism contains endless secrets. In order to survive, parasites have evolved a variety of parasitic strategies that would put science fiction to shame: For example, Ophiocordyceps sinensis invades the brains of ants and turns them into "zombies", which then leave their nests and become fertilizer for fungi. Another example is the Cymothoa exigua, which parasitizes in the mouths of fish and slowly eats away at the tongues of fish, sucking the fish's blood and mucus while performing the work of the tongue. The larvae fluke invades the eyes of snails, turning the snail's eye stalks into colorful colors, and is eaten by birds as if they were bugs to complete their life cycle.

A fish louse found in a fish's mouth | Elkin Fricke / Wikimedia Commons

If all this isn't scary enough, there are some parasites that can actually "castrate" their hosts.

Some parasites eat the reproductive organs of their hosts and use the empty space as their own residence. For example, Echiodon rendahli, a member of the family Cryptozoology, has a slender and smooth body that can slip into the body of a sea cucumber through the anus. It uses the sea cucumber as its home and also enjoys the sea cucumber's sexual organs and even other organs as food.

Carapus acus | Mercedes González-Wangüemert et al. / SPC Beche-de-mer Information Bulletin (2014)

There are also parasites that affect the host's reproductive behavior and use the saved energy for their own use. For example, female ground bees (Andrena.spp) often collect pollen as "food" for their children, and there is a "pollen basket" structure on their hind legs. But once they are parasitized by a twisted wing insect (Stylops melittae), they lose their fertility, no longer prepare food for their children, and the pollen basket disappears. They also behave more like adult drones, and completely become slave workers of the twisted wing insect.

The existence and continuation of organisms as a group depends on reproduction. Reproduction itself consumes a lot of energy and nutrition of organisms. If these resources can be controlled, parasites can not only prolong the life of the host, but also take the energy used for reproduction for themselves. It's really the best of both worlds! The castration strategy can be said to be the cruelest and most successful of all parasitic strategies.

Crabs become slaves of barnacles

Barnacles belong to the arthropod crustacean subclass and are close relatives of lobsters and crabs, but most barnacles seem to choose to "lie flat" willingly. They stick to the bottom of ships, docks, and whales, filtering edible plankton from the flowing seawater to feed themselves.

There is a type of barnacle that is not content with such an ordinary life. They are members of the class Rhizocephala, and the parasitic crabs of the genus Sacculina are called crab sacs. When they are young, crab sacs are not much different from their independent relatives, and look very much like the larvae of shrimps and crabs. But when they mature, they take a different path and live a wonderful life in the bodies of others.

Sacculina carcini on the body of Pachygrapsus marmoratus | vmoser / inaturalist

When young, female crab slaves look for target crabs and take root in the weak spots of their bodies, such as the base of the crab's sensory bristles or gill filaments. The crab slave larvae bite into these gaps, use their mouthparts as needles, and inject the soft parts into the crab after molting. The process of the crab slave looking for a new home only takes a few seconds, and the crab's nightmare begins before it knows it. Just like the roots of a big tree, the crab slaves also grow thin root-like tissues that spread all over the crab's body.

Peltogaster species (also known as crab slaves) parasitize the hermit crab Pagurus pubescens, with root-like tissues burrowing into the crab's body | Christoph Noever et al. / Journal of Sea Research (2016)

On the surface, the crabs are still the same as usual, walking around under the water every day, looking for shellfish and other food. But the inside of the crab is very different. From the tip of the crab legs to the tip of the crab claws, they are gradually occupied by crab slaves. The crab slaves deprive the crabs of nutrients to repair their bodies, making it impossible for the crabs to shed their shells. If they lose their claws or legs, they have no extra energy to regenerate, and can only continue to maintain their broken bodies.

If a female crab slave chooses a female crab, she will occupy the crab's egg-carrying position (abdominal segment, also known as the crab navel) for her own use; if she chooses a male crab, it will affect the male crab's sex hormone secretion, damage the reproductive system, and form a "castration" effect. The male crab's originally triangular abdomen becomes flat, and its behavior becomes more and more feminine. Normal male crabs may even mistake the castrated crab for a female crab and ask for mating.

If the crab slaves are removed, the female crab still has a chance to develop ovaries and start over, but the male crab's sex hormones will be permanently affected and he will never be able to become a normal "man" again.

Normal and parasitized common shore crabs Carcinus maenas. From top to bottom: normal common shore crabs carrying juveniles, female common shore crabs parasitized by S. carcini, male common shore crabs parasitized by S. carcini. Both sexes of the parasitized crabs are shown holding eggs | David Rees & Henrik Glenne / Ecology and Evolution (2014)

The mature crab finally reveals its shape. A yellowish bulge similar to crab eggs forms at the "navel" of the crab, waiting for mating. The male crab swimming in the sea is attracted by the female crab, takes off its shell, passes through a narrow channel, enters the female's body, and produces sperm.

Each female crab slave has two tubes, so she usually invites two lucky males to live with her for life. The two husbands fertilize her eggs almost endlessly, and she produces thousands of new larvae every few weeks, controlling the pulsed fluctuations of the crab's plastron to send these larvae out of the house and start the next wonderful adventure. The crab, who has long lost his mind, treats the crab slaves as "lovingly" as his own larvae, stirring the surrounding seawater with his claws to help the offspring of these parasites go on a longer journey.

A game of mantis catching cicada

Barnacles are a terrible pain for marine life, and whales and sea turtles are particularly vulnerable. The densely packed barnacles on whales can cause intensive phobia. To avoid being scraped off and washed away by running water, whale barnacles prefer to choose the wrinkles on the whale's body, such as around the eyes, nostrils, and genitals. Sometimes whales will even "itch" on the boat in an attempt to scrape off the barnacles on their bodies.

Another shark-parasitic barnacle, Anelasma squalicola of the order Perithrax, is even more sinister. It parasitizes the black-bellied black shark (Etmopterus spinax), and instead of growing a hard shell, it forms a hard, non-calcified purple-black covering. Unlike whale barnacles, which simply take up space, the stalks of shark-parasitic barnacles penetrate deep into the shark's muscles, and the root-like filaments absorb nutrients from the shark. Although "castration" is not a special strategy of A. squalicola, under the intrusion of parasitic barnacles, sharks also lose their ability to reproduce due to malnutrition. This is the first known case of a parasite "castrating" a vertebrate.

Purple A. squalicola parasites on sharks | Christoph Noever/Universitet i Bergen

The first person to discover and name A. squalicola was Darwin, the biggest fan of barnacles. Between returning from his trip around the world and publishing The Origin of Species, Darwin spent eight years immersed in studying barnacles. Some people jokingly call it "Darwin's procrastination."

The world is almost made up of parasites and parasites. Barnacles have so many parasitic pathways and strategies, but they can't escape the fate of being parasitized. There is a kind of soft-shelled isopod called Hemioniscus balani, which specializes in parasitizing adult barnacles, such as the Atlantic barnacle (Semibalanus balanoides). H. balani's host is a hermaphroditic barnacle. It feeds on the ovaries of the barnacle and "castrates" it, but the male reproductive system of the barnacle is still retained, so it can still produce sperm and become "male", which is exactly the opposite of the crabs that the crab slaves parasitize.

Things will change, and the tricks used on barnacles will one day be used on barnacles.

Author: Ǒphelia

Edit: Red Queen

This article comes from the Species Calendar, welcome to forward

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