7-11 Crab (Spotted Reef Crab)

The 7-11 crab is named for its distinctive spots on its shell, seven spots on the top and another four on the bottom (although it has more than that). Based on the most visible spots, it should be called a 7-4 crab, but since 7-11 had a nicer ring to it and its characteristic spots totaled 11, the name 7-11 stuck. This crab adapts quickly to aquarium life. However, it has a destructive nature, so it is not a crab recommended for home aquariums, even if it does look cute and appealing when it's small.

Characteristics

Origin and Distribution

These crabs are widely distributed in the Indian Ocean, the South Pacific, the Hawaiian Islands, Australia, Red Sea, South Africa, and it is also found in areas to the south of mid-Honshu in Japan. Most are found on coral and rocky reefs moving slowly along sandy bottoms.

Colors and Markings

Its body is an oval shape, its surface is smooth and convex, and it has an orangish-light rust color. It has seven large dark reddish-maroon colored spots on its back. These spots can appear to look more of dark brownish color at times. Its shell is extremely thick and heavy and lacks spines.

Hawaiian legend says that this crab has its spots because a sea god tried to capture it and eat it, but the crab drew blood. The sea god kept on trying to catch the crab with his bloodied fingers and that's why he has so many red-brownish spots.

Although most think this crab has 11 spots, which are the most obvious, it actually has a total of 18 spots. It has nine large violet -to-maroon spots on the dorsal surface of its carapace or shell, three on the median region, two on the posterior region, two on the anterolateral region, and two around its eyes. Like most other crabs, one claw grows bigger than the other.

Tankmates

This crab has very strong pinchers. Once it clamps on to something, it is difficult to try and get it to let go or forcibly open the pincers. It will attack and eat other crustaceans and invertebrates, and if given the chance, it will grasp on to sleeping fish. Males commonly fight for females, with the larger one usually winning. It is not recommended to keep this crustacean with other animals.

Habitat and Care

The 7-11 crab's great strength and strong armor make it a destructive animal in an aquarium. Its hard shell allows it to act like a bulldozer moving rockscapes and corals around in an aquarium.

Diet

In the wild, this crab feeds on marine snails. However, like most crabs, the 7-11 is a scavenger and will eat just about anything. This crab has been observed in nature carrying sea urchins and cowrie shells, suggesting a possible food preference. It mostly comes out at night to feed, hiding during daylight hours.

Sexual Differences

The 7-11 crab is gonochoric, which means it stays one sex, unlike other marine animals that have the ability to switch sex based on the lack of mating partners available. For most crabs, you can tell the difference if it is male or female by looking at the underside of the crab at its abdominal flap. A male crab has a small triangular flap, while a female crab has a broad oval-shaped abdominal flap.

Breeding

This crab does have a mating courtship ritual, which it uses to woo the target of its desire through olfactory (pheromone) and tactile cues. The male usually waits until the female molts. The male can sense through pheromones released by the female that her molt is imminent. Then, to ensure that he will be the mating partner, he holds her in a close embrace known as amplexus, which literally means “embrace” in Latin until she molts.

This pre-molting embrace may last up to a week, with each crab sternum to sternum, until the female molts. At some point during the later stages of the amplexus the female flips right-side-up, which is preceded by her pinching the male’s eyestalks to make him relax his hold. No matter how much eye pinching goes on, at no time does the male completely release the female during her molt. Actual copulation occurs about an hour and a half after the female molts, when the new exoskeleton has firmed somewhat, and with the mating pair back in a sternum-to-sternum position.

When a male and female crab copulate, the female crab gets a male sperm package, which she stores in her abdominal cavity until her eggs are ready to be released. When the eggs are released, the stored sperm flows over them and they become fertilized. The female crab holds the fertilized eggs in a big spongy mass between its abdominal flap and the body. The eggs are cemented to the pleopods, which are small legs, creating the "berried" appearance. To keep the eggs healthy, the female crab continually "waves" water over the eggs with the pleopods.

When the eggs hatch into zoea larvae, they drift away in the ocean currents as plankton. As the crab grows in size, it goes through a series of molts and finally, a process of metamorphosis. Each larval stage, it changes form and function. At each molt, more segments are added to the end (posterior), and the feathered limbs are replaced by the clawed limbs. When it is a megalops, the last stage before metamorphosis and before it becomes a juvenile crab, it most closely resembles the adult crab.

More Pet Fish Breeds and Further Research

If the 7-11 crab appeals to you, and you are interested in maintaining a saltwater aquarium, check out other saltwater fish. These fish are great for saltwater tanks but not suitable for living with a 7-11 crab:

• Tangs
• Wrasses
• Gobies

Check out additional fish breed profiles for more information on other freshwater or saltwater fish.