The arrival of the fishing season has brought abundant and succulent swimming crabs to dinner tables.
As one of China’s significant economic crab species， swimming crabs are now readily available， even in inland regions far from the coast， thanks to the increasing convenience of fresh logistics.
For those less familiar with swimming crabs， questions might arise: How should one choose a swimming crab? Is it true that swimming crabs are full of parasites? Are swimming crabs with patterns poisonous? And， how can swimming crabs be prepared?
Commonly Consumed Swimming Crabs in China
The name “swimming crab” is quite descriptive， derived from the shape of their carapace， which is broad in the middle and tapers at both ends， resembling a shuttle used in ancient weaving.
Members of the swimming crab family lack sharp tips on their fourth pair of legs; instead， the segments have evolved into flattened， paddle-like structures， aiding their rapid movement through the water.
To blend in with their environment， swimming crabs exhibit variations in shell color depending on their habitat. Those living on sandy seabeds are typically lighter in color， while those found among algae are darker.
(Image courtesy of Wikipedia)
Swimming crabs are omnivorous and are mostly found in shallow seas. Common species in China include the three-spotted swimming crab (Portunus trituberculatus)， the red-spotted swimming crab (Portunus sanguinolentus)， the fleshly-swimming crab (Portunus pelagicus)， and mud crabs (Scylla spp.).
The three-spotted swimming crab， commonly known as “flying crab” or “white crab，” is the most widely distributed and accounts for the largest market share. Its name comes from the three distinct warty protuberances on its carapace.
Three-spotted swimming crab | (Image courtesy of Wikipedia)
The red-spotted swimming crab is slightly smaller than the three-spotted swimming crab and is easily identifiable by the three dark circular spots on its carapace， arranged in a triangle， giving the shell an almost facial appearance. It is colloquially known as “three-spot crab.”
The fleshly-swimming crab has a distinctive shell color. Males are deep blue or dark purple with white patterns， commonly called “blue flower crab” or “flower crab” (though some regions refer to the rusty blenny crab by the same name).
When cooked， the fleshly-swimming crab’s shell turns orange-red， but the white markings remain visible.
Females are greenish-brown or grayish-brown and can be easily confused with the three-spotted swimming crab. However， they lack the protuberances on their back and have four sharp teeth on the frontal carapace， whereas the three-spotted swimming crab typically has two.
Male fleshly-swimming crab | (Image courtesy of Wikipedia)
Within the larger family of swimming crabs are “mud crabs” (genus Scylla). Commonly encountered species include the green mud crab (Scylla serrata)， the Indonesian mud crab (Scylla paramamosain)， the purple claw mud crab (Scylla olivacea)， and the orange mud crab (Scylla tranquebarica).
The most common mud crabs in the Chinese market are the green mud crab and the Indonesian mud crab. Sanmen County in Zhejiang Province has a long history of mud crab aquaculture and is known as the “hometown of Chinese mud crabs，” with “Sanmen Mud Crab” being a protected geographical indication product.
The green mud crab’s name is apt， with its shell edge featuring sharp serrations. Its claws and the last pair of legs display dark green reticulated patterns， leading to its common coastal name “flowery leg crab.” Ancient texts refer to it as “蝤蛑 (yóu móu).”
Green mud crabs are large and have a fierce temperament. This fighting prowess may contribute to their particularly plump claw meat， making them highly favored by crab enthusiasts.
(Image courtesy of Wikipedia)
The Indonesian mud crab is similar in color to the green mud crab but lacks the patterns on its claws and legs. It is also smaller and is a more commonly farmed species.
(Image courtesy of Wikipedia)
However， while swimming crabs with patterns and unique colors， like the fleshly-swimming crab， are edible， it does not mean all similar-looking sea crabs are safe to consume.
Some poisonous crabs， such as the toxic crab (Zosimus aeneus)， the ornate box crab (Calappa heptancathus)， and the mosaic porcellanid crab (Neopetrolisthes maculatus)， are adorned with vibrant patterns. Ingesting them can be life-threatening. Therefore， when visiting coastal areas， do not casually consume sea crabs that are marketed as “wild-caught” and significantly differ in appearance from common edible crabs.
Ornate box crab | (Image courtesy of Wikipedia)
How to Choose Swimming Crabs? Are Dead Swimming Crabs Edible?
When selecting a swimming crab， observe its liveliness， check if its carapace is hard and glossy， and smell it to ensure it only has a briny sea odor without any signs of decay or strange smells.
If permitted by the vendor， hold the crab to feel its weight. A lighter crab suggests it has been out of the water for some time， and its nutritional content has been depleted.
You can also gently press the sides near the abdomen to check for firmness. Gently squeeze the joint between the leg and the carapace; if it feels hollow and is easily compressed， the crab is not plump enough.
Similar to hairy crabs， the sex of a swimming crab can be determined by its “belly button.” A triangular abdomen indicates a male crab， while a rounded one signifies a female.
Healthy swimming crabs have a white underside with distinct markings. If the “belly button” protrudes and you can see the color of the roe or tomalley beneath the shell， it indicates readiness. For female crabs， if the two outermost corners of the abdomen show an orangey-red hue， it signifies a particularly roe-filled and succulent crab.
It’s important to wear gloves when handling and preparing swimming crabs to avoid being pricked by their sharp shells.
Seafood surfaces may carry Vibrio vulnificus. If it enters a skin wound， it can cause infection， leading to superficial fasciitis， and in severe cases， necrotizing fasciitis and sepsis.
If you experience skin pain， swelling， or fever after contact with seafood or seawater， seek medical attention promptly.
Dead swimming crabs generally have a diminished flavor and may produce harmful substances， increasing food safety risks.
Therefore， it is best to cook and consume live swimming crabs as soon as possible after purchasing them. Avoid eating dead ones， especially if they have a strong off-odor (such as a rotten smell).
If you cannot finish them in one sitting， cover them with a damp cloth and refrigerate them， or steam and then freeze them. Attempting to freeze live crabs at home may not achieve the necessary low temperatures to prevent spoilage.
Do not submerge them in saltwater to create an “ocean environment，” as this will only hasten their death.
Given that swimming crabs have difficulty surviving outside their marine environment， some caught crabs are rapidly flash-frozen before death to inhibit bacterial growth， and then sold as chilled seafood.
This type of crab is safe to eat， but ensure it is consumed promptly after thawing and avoid repeated thawing， as this increases the likelihood of spoilage.
Why Do Swimming Crabs Sometimes Have Strange Objects Inside?
Many netizens have shared images of finding “parasites” inside swimming crabs. Upon opening the gills， they observe numerous tiny， millimetre-long pink organisms wriggling， inducing trypophobia and concern about the crab’s edibility.
In reality， these are animals called Capitulum attached to the swimming crab’s appendages and gills. They are related to barnacles but have no culinary value.
During their larval stage， they enter the crab’s body through water flow related to respiration， attaching to the gills and growing. However， they do not absorb nutrients from the crab.
Capitulum do not produce or accumulate toxins and will not parasitize humans. They are certainly safe to consume after cooking.
(Image courtesy of Wikipedia)
Additionally， some people have purchased swimming crabs with a visible mass of “roe” protruding from their abdomen; this is actually a berried crab.
Fishermen often release these crabs to promote the reproduction of young swimming crabs. Currently， they are under fishing prohibition， but “oversight” can occur.
Because berried crabs are about to lay eggs， their meat content is reduced， and the exposed “roe” is not particularly palatable.
When steaming swimming crabs， sometimes upon opening the lid， you may find clumps of white， foamy substances on the crab. This is the result of tissue fluid leaking out and solidifying during high-temperature cooking and does not indicate spoilage.
To prevent this， place the crab belly-up in the steamer. The leaked liquid will then be contained within the crab shell.
Creative Ways to Prepare Swimming Crab
While many appreciate the unique flavor of hairy crabs， there’s no denying the robust satisfaction of eating swimming crab， with its tender white meat and rich roe offering a truly gratifying experience.
Autumn marks the peak breeding season for crabs， during which they accumulate energy， leading to more robust bodies and well-developed reproductive glands. Thus， swimming crabs are considered to be at their best quality after the start of autumn.
Steaming， salt baking， stir-frying， or stewing – all methods yield delicious results. The freshest swimming crabs are exquisitely tasty when simply steamed， their meat sweet and tender， providing immense satisfaction.
How to judge the steaming time for swimming crabs?
Coastal dwellers have shared a handy tip: after the water boils， crabs weighing around 150 grams (three taels) or less require 10 minutes of steaming. For every additional 50 grams (one tael) in weight， increase the steaming time by 2 minutes. Crucially， avoid opening the lid prematurely during cooking.
For a slightly more elaborate preparation， try the popular Zhejiang dish， “Scallion Oil Swimming Crab.” Steam the crabs， cut them in half， and then pour hot scallion oil over them. The hot oil intensifies the crab’s natural aroma， blending with the fragrance of the scallions to create a more layered scent while also reducing any fishiness.
Carbohydrate enthusiasts might enjoy swimming crab stir-fried with rice cakes. Cut the swimming crab into large pieces and stir-fry them with softened rice cake slices， then thicken with sauce. This method not only preserves the plump texture of the crab meat but also allows the rice cakes to absorb the crab’s fresh flavor.
In Ningbo， Zhejiang， a major swimming crab producing region， “Red Roe Brined Crab” is a local delicacy. Live swimming crabs are washed and then marinated in a seasoned brine with a small amount of white wine， chilled for several hours. Though no longer alive， the crabs retain their deliciousness. Served cold and cut into pieces， the crab meat and roe have a jelly-like， smooth texture.
In Wenzhou， Zhejiang， raw， cut swimming crabs are seasoned with wine， vinegar， and pepper to create the local specialty “Jiang Xie Sheng” (raw river crab).
However， while raw marinated crab is delicious， its health risks cannot be overlooked. It can lead to infections from pathogenic bacteria such as Vibrio parahaemolyticus and Salmonella， and even introduce parasites like Anisakis. It is advisable to try this preparation with caution!
References
[1] Xiang， J. F.， Lin， L.， Jiang， S. T.， et al. (2022). Research Progress on Nutritional Quality and Processed Products of Swimming Crabs. Journal of Food Safety and Quality Inspection， 13(08).
[2] Guokr.com (Reprinted from Dingxiang Life Research). Ask coastal people: If crabs turn black/produce white foam/have small particles… can they still be eaten? September 7， 2024.
[3] Guokr.com (Nature). Can eating crabs also lead to hospitalization from poisoning? There are quite a few “poison masters” in crabs. April 7， 2021.
[4] Olive Pictorial. “First Wind” Swimming Crabs went on sale last week， did you get yours? August 14， 2023.
[5] Xue， J. Z.， Shao， X. Y.， & Wu， H. X. (2003). Attachment and Population Characteristics of Capitulum on Portunus trituberculatus and Scylla serrata. Science and Technology Information， (02).
