What Are Soft-Shell Crabs?

Images of crabs adorn the signboards and logos of most seafood restaurants, and who does not love to eat crabs? Crabs taste like other crustaceans, such as shrimp and prawns. However, due to their larger body size, crabs contain more meat compared to smaller crustaceans like shrimp and prawns. In most restaurants, crabs are consumed by cracking their hard outer shells to get to the delicious flesh underneath. However, crabs are also served in a soft-shell condition. In general, soft-shell crabs fetch a much higher price compared to hard-shell crabs. This is because soft-shell crabs have a higher nutritional value and, most importantly, they can be eaten whole! This avoids the hassle of prying open the hard outer shell to savor the juicy flesh inside. Therefore, soft-shell crabs are gaining popularity over hard-shell crabs as a healthy and convenient food choice. Are soft-shell crabs a totally different crab species? Where do they come from?

To Grow Is to Molt

In some ways, crabs are similar to us! As we grow, we become taller and bigger. As a result, our clothes eventually do not fit us anymore and we need to get a larger size. Crabs increase in size as they grow, too! However, unlike us, crabs have hard shells called exoskeletons on the outsides of their bodies, which act like protective armor. The exoskeleton cannot expand as a crab grows, so crabs must shed their old shells so they can develop bigger shells, through a process called molting (Figure 1). The molting process is complex. Before molting, a new tissue layer forms underneath the crab’s old exoskeleton. Molting is controlled by a group of hormones called ecdysteroids, which increase to a peak level and then drop drastically in concentration right before molting. This drop signals the crab to take in water so that it expands, cracking open its old exoskeleton so that it can emerge from it (Figures 1A–C). The process of expanding and emerging from the old skeleton typically takes several hours. After emerging, the crab is covered in a new, soft exoskeleton that will harden within a few hours (Figure 1D) [1]. During this short period, the crab expands its soft shell by taking in water like a balloon. This ensures that the new exoskeleton will be much bigger than the old one when it hardens, so the crab will have plenty of room to grow before the next molting. Crabs are exhausted and immobilized when they are in the soft-shell condition. Therefore, molting is extremely dangerous, as the newly molted crab is vulnerable to predators.

Soft-Shell Crab Production

So now you know that soft-shell crabs are crabs harvested right after they molt, while they are still in a soft-shell condition (Figure 2). Producing soft-shell crabs to sell, however, is a long process—it normally takes weeks after crabs are collected. In general, soft-shell crabs can be produced from any edible portunid crab species, including Atlantic blue crabs, blue swimming crabs, and mud crabs [2–4]. Crabs that have not yet matured (called sub-adult crabs) are commonly collected from the wild to generate soft-shell crabs. The size of sub-adult crabs may differ among species. For example, mud crabs with a body size of 5–8 cm, or Atlantic blue crabs of 4–5 cm are commonly captured for soft-shell crab production. Crabs are normally raised in individual cages throughout the production period, so that it is easier to see when they are molting (Figure 2D). They are also raised individually to avoid cannibalism, especially in mud crabs, which are prone to fight and feast on their friends when they are hungry or too crowded. Monitoring the crabs every 4–5 h is important so that crabs can be identified and harvested when they have recently molted. After they are harvested, the soft-shell crabs are immediately cleaned using fresh water, individually packed, and frozen at -20°C to ensure their freshness and quality. Frozen soft-shell crabs can then be easily packed and transported [5].

Soft-shell crabs have a higher market value than do hard-shell crabs. The value of soft-shell crabs also depends on the time of harvest after molting—the calcification (hardening) process of the new exoskeleton begins immediately. An exoskeleton that has a high water content and low levels of calcification is considered truly soft. This type of soft-shell crab is often pricier and can be turned into mouth-watering dishes for consumers.

Limitations of Soft-Shell Crab Production

Soft-shell crab production is becoming a profitable business. Although the process of soft-shell crab production is well-established and can be carried out on a large scale, farmers must still collect a lot of sub-adult crabs from the wild. Crab farmers cannot produce sub-adult crabs because of the low survival during the larval stages. However, wild sub-adult crabs are limited and seasonal. Additionally, collecting sub-adult crabs greatly reduces the overall crab numbers in the wild because those crabs are harvested before they can reproduce and repopulate the wild population (Figure 3). Therefore, uncontrolled crab fishing is not sustainable, as it leads to overharvest and threatens the health of the wild crab population. Overcoming the low survival of crabs during the larval stage is now the main priority, and scientists from various countries are working together to solve this mystery.

Another limitation is that crabs tend to molt at their own pace—sometimes it takes more than a month! Therefore, farmers must grow and monitor them for a long period, which increases the production cost. Researchers are currently experimenting with various methods to help sub-adult crabs molt faster and in a more synchronized way, including the use of ecdysteroid hormones. If this is successful, farmers will be able to produce more soft-shell crabs easily, in a shorter time.

Conclusion

Soft-shell crabs are regular edible crabs that have recently completed the molting process. Soft-shell crab production is a promising money-making industry with high market demand due to the excellent taste of the crabs and the unique eating experience. However, the reliance of the soft-shell crab industry on the capture of wild sub-adult crabs is still unsustainable, and it puts tremendous pressure on the currently decreasing wild population. For soft-shell crabs to remain a sustainable food source, it is important to only use farm-produced sub-adult crabs in the soft-shell crab producing process.

Glossary

Crustaceans: ↑ A large class of aquatic animals with exoskeletons, such as crabs, shrimp, lobsters, and krill.

Exoskeleton: ↑ The hard, external covering of the body found in some invertebrates.

Molting: ↑ The shedding of an old shell by most invertebrates with exoskeletons.

Ecdysteroids: ↑ A type of hormone that controls molting and reproduction.

Portunid: ↑ A swimming crab of the Portunidae family, characterized by appendages shaped like flattened paddles, for swimming.

Cannibalism: ↑ The act of consuming another organism of the same species.

Calcification: ↑ The hardening of a tissue due to calcium build-up.

Sustainable: ↑ Using natural resources in a way that will enable us to use them for a long time.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.