Ranina ranina

The *Ranina ranina* is a unique species of crustacean found in warm ocean waters.

People call this animal by many names. It is known as the spanner crab, the red frog crab, or the Huỳnh Đế crab. This species is highly valued as a food source in many tropical and subtropical regions. Because it is a popular seafood, it is commercially exploited in several parts of the world. Understanding its biology and habitat is important for managing the fisheries that rely on it.

This crab has a very specific way of living and hunting. It is primarily a nocturnal animal, which means it is most active during the night. During the daylight hours, the crab seeks out smooth, sandy seafloors to hide. It uses the sand to bury its body almost completely. However, it does not hide entirely. The crab leaves its eyes and its mouthparts poking out above the sand.

This allows the crab to detect movement and sense food nearby. When a small, bottom-dwelling fish swims close enough, the crab attacks from its hidden position under the sand.

The physical appearance of *Ranina ranina* makes it easy to identify. It has a reddish-brown carapace, which is the hard upper shell that protects its body. This carapace is wider at the front than at the back. It also features ten distinct white spots. Another key feature is its elongated midsection, which sets it apart from other crab species in its environment. These physical traits help it blend into its sandy habitat while it waits for prey.

These crabs inhabit a wide range of coastal waters across the globe. In Australia, they live along the east coast from Yeppoon in Queensland to the north coast of New South Wales. There is also a population located north of Perth in Western Australia. Beyond Australia, they are abundant in the waters of southwestern Mindanao in the Philippines. Their range extends across the Indian Ocean to the east coast of Africa. They can also be found in Indonesia, Japan, and Hawaii. They require environments with ample sand to flourish and use their burying method.

Fisheries play a massive role in the life of the spanner crab. The largest fishery for this species is located in Australia. In Queensland, there are specific rules to protect the population. For example, only adult crabs above a certain carapace length may be landed by fishers. In Australia, 96% of female crabs are not fished. This helps keep the species from disappearing. However, at Tallows Beach, the decrease in hunted females does not apply during the spawning season. This suggests that Tallows Beach is a specific location for spawning migrations.

In many cultures, *Ranina ranina* is considered a culinary delicacy. In the Philippines, it is known as curacha and is a regional specialty. It is often prepared as *halabos*, which means it is steamed. It can also be cooked in coconut milk in a dish called *ginataan*. A famous version of this is the *curacha Alavar* from Zamboanga City.

In Vietnam, the name Huỳnh Đế literally means "emperor crab." This name comes from the fact that these crabs were favorite foods for historical Vietnamese monarchs. In the provinces of Bình Định and Quảng Ngãi, it is hailed as the "monarch of all the crab."

Even though they are important for food, scientists are still learning about them. In 2008, the price for these crabs in the Philippines was around 200 to 300 pesos per kilogram.

While they are a target for commercial fishing, very little is known about their exact biology or population dynamics. Scientists have tried to grow the crabs in captivity, but these attempts have seen little success. Currently, surveys show that the populations remain stable. Researchers continue to monitor them to prevent overfishing and to better understand their complex ecology.