When you think of a crab, the iconic image that probably comes to mind is that of a red crab with two large front claws, a hard rounded shell, and legs designed for scuttling sideways along the seafloor. This is the quintessential “true crab” body plan that belongs to the taxonomic infraorder Brachyura. However, not all crabs conform to this familiar body shape. There are several other groups of crustaceans that are also commonly referred to as “crabs” due to their resemblance, even though they are not true crabs. So what kinds of crabs look like crabs, but aren’t actually true crabs? Let’s take a closer look at some of these crab imposters.
Hermit Crabs
One of the most well-known non-true crabs are hermit crabs. There are over 800 species of hermit crabs belonging to the superfamily Paguroidea. Unlike true crabs, hermit crabs have a soft, spiraled abdomen that is not armored. To protect their vulnerable hindquarters, they take up residence in abandoned seashells, hollow pieces of coral, or even manmade objects like glass bottles or tin cans. The shell provides safety and allows the hermit crab to curl up inside for protection from predators.
While hermit crabs may appear crab-like from the front with their two claws, multiple legs, and eyestalks, their body shape and lifestyle sets them apart from true crabs. Hermit crabs are actually more closely related to squat lobsters. They belong to the taxonomic infraorder Anomura along with mole crabs, king crabs, and coconut crabs.
Horseshoe Crabs
Horseshoe crabs are another marine arthropod that look like true crabs but belong to a completely different subclass of crustaceans. Horseshoe crabs are chelicerates which puts them more closely related to spiders and scorpions than to true crabs. There are only 4 living species of horseshoe crabs which belong to the family Limulidae.
Horseshoe crabs have a hard semicircular shell, or carapace, that covers the cephalothorax and gives them their distinctive horseshoe shape. Under the shell, they have a long spear-like tail used for flipping themselves upright if they get overturned. Horseshoe crabs also have a cluster of compound eyes on the top of the shell along with a pair of leg-like pedipalps for feeding. So while they may bear some similarities to true crabs, horseshoe crabs represent a much more ancient and primitive lineage of marine arthropods.
Mole Crabs
Mole crabs, also known as sand fleas, are unusual looking crustaceans that live along sandy beaches around the world. They belong to the infraorder Anomura along with other crab-like creatures including hermit crabs and king crabs. There are around 20 species of mole crabs which are placed into two families – Hippoidea and Albuneidae.
Mole crabs are well adapted for burrowing through the sand, giving them their mole-like appearance. They have a globular body enclosed in a hard exoskeleton and flattened shovel-like front pincers designed for digging. Powerful hind legs propel them backwards through wet sand. Mole crabs spend nearly their entire life buried underground, only venturing onto the surface at high tide to feed. So while mole crabs certainly don’t look like your average crab, they do share some common crab-like features.
King Crabs
King crabs comprise a family of large edible crabs called Lithodidae. There are around 20 species divided between two subfamilies – Lithodinae and Hapalogastrinae. King crabs are widespread in cold, deep waters of the world’s oceans. Some of the most commercially important species include red king crab, blue king crab, golden king crab, and brown king crab.
Unlike true crabs which fold their abdomen under the thorax, king crabs have a reduced abdomen that is permanently curled up and locked in place under the cephalothorax. This gives king crabs a chunkier, triangular profile compared to the oval shape of a typical crab body. The legs are also thicker than most crabs and typically spiny. So while king crabs don’t quite match the body plan of a true crab, their imposing size and armored appearance still lend them a quintessential crab-like look.
Porcelain Crabs
Porcelain crabs belong to the infraorder Anomura, along with other non-true crab species. They comprise over 120 species within the family Porcellanidae. Porcelain crabs are small, flattened crabs that often live communally in large groups. They have a triangular body enclosed in a smooth, porcelain-like carapace. Unlike true crabs, porcelain crabs fold their legs under the body rather than splaying out to the sides.
The pincers of porcelain crabs are very small, forcing them to use their longer walking legs for grasping food. While porcelain crabs don’t have the typical body shape of true crabs, their sideways scuttling locomotion and shared habitat on rocky shores still gives them a crab-like resemblance. Many species adorn themselves with sponges or anemones for further camouflage.
Comparison of Features
| Type of Crab | Body Shape | Front Pincers | Number of Legs |
|---|---|---|---|
| True Crab | Broad oval carapace, folded under abdomen | Large, asymmetry | 10 |
| Hermit Crab | Spiral abdomen, lives in borrowed shell | Asymmetry | 10 |
| Horseshoe Crab | Semicircular shell, long tail | Pedipalps for feeding | 10 |
| Mole Crab | Globular, shovel-like pincers | Symmetry | 8 |
| King Crab | Triangular, curled under abdomen | Asymmetry | 10 |
| Porcelain Crab | Flattened triangular | Small pincers | 10 |
Conclusion
While several types of marine arthropods have common names containing “crab”, not all of them actually belong to the taxonomic infraorder Brachyura which encompasses true crabs. Hermit crabs, horseshoe crabs, mole crabs, king crabs, and porcelain crabs are examples of crab-like creatures that display some similarities to true crabs in terms of appearance, behavior, and habitat. However, they represent distinct evolutionary lineages that have converged on the crab body plan for its advantages in mobility, defense, feeding, and camouflage. Examining their divergent anatomical features helps illustrate the diversity of forms that are all commonly referred to as “crabs”. So next time you see a crab, take a closer look to determine if it’s a true crab or one of these other fascinating crab impersonators!
