Thin-Shelled Rock Crab
ExploreAnimal
Thin-Shelled Rock Crab
grapsus tenuicrustatus
The Thin-Shelled Rock Crab, scientifically known as Grapsus tenuicrustatus, is a highly active and incredibly agile crustacean native to the rocky coastlines of the Indo-Pacific region. Commonly referred to as the Natal lightfoot crab, this species is famous for its astonishing speed and ability to leap across gaps in tidal rocks to evade incoming waves and hungry predators. It occupies the high intertidal zone, navigating the harsh, wave-swept splash zone with ease using its specialized gripping legs. As an ecological generalist, this crab plays a vital role in maintaining the health of coastal ecosystems. It grazes on algae covering the rocks and scavenges on organic debris, acting as both a cleanup crew and a critical link in the marine food web. Watching one of these crabs navigate the slippery, vertical faces of volcanic shorelines is a mesmerizing display of natural adaptation and athletic prowess. Can you find a Thin-Shelled Rock Crab? Log it and verify its identity with the Snappit app.
Habitat: Found on exposed, wave-beaten rocky shores, cliffs, and jetties in the intertidal and splash zones.
Appearance
This medium-sized crab features a highly flattened, circular carapace measuring roughly 5 to 8 centimeters in width. Its coloration ranges from a dark, mottled olive green to brownish-black, often highlighted with fine pale yellow or white speckles that match the texture of wet basalt rocks. It possesses long, flattened legs equipped with sharp, needle-like tips at the dactyls, which allow it to secure a firm grip on slick rock surfaces, alongside short but powerful claws.
KingdomAnimaliaPhylumArthropodaClassMalacostracaOrderDecapodaFamilyGrapsidaeGenusGrapsus
Category
AnimalRarity
Common
Danger
1/5 · Very low
Snaps
Be the first to snap!
Interesting facts
Their flattened bodies are a specialized adaptation that minimizes drag from crashing ocean waves and allows them to squeeze into tight rock crevices.
This species is known as the lightfoot crab because it can run so fast that it appears to glide effortlessly across the surface of water and wet rocks.
To breathe while out of the water, they run water over their gills from a reservoir kept in their gill chambers, continuously aerating it.
Special abilities
Ability
Agile Rock Leaping
This crab can leap or jump across wide gaps between wet coastal rocks to rapidly escape predators or heavy wave impacts.
Ability
Gripping Dactyls
Its legs terminate in sharp, spine-like tips that act like crampons, allowing the crab to cling securely to sheer vertical cliffs amidst crashing waves.
Ability
Sacrificial Autotomy
It can voluntarily drop its limbs when grabbed by a predator and later regenerate them during subsequent molting cycles.
Measurements & details
- Length
- 5-8 cm
- Weight
- 0.05-0.15 kg
- Lifespan
- 2-4 years
- Top Speed
- 8 km/h
- Clutch Size
- 5000-20000
- Incubation
- 14-28 days
Diet & Feeding
Primarily grazes on marine algae growing on intertidal rocks, but opportunistically scavenges animal matter and preys on small invertebrates.
Primary Foods
- Filamentous algae
- Sea lettuce
- Marine detritus
- Small gastropods
- Carrion
Foraging Method
- Foraging And Grazing
Ecological connections
eaten by
Day Octopus
Octopus cyanea
Preys upon the crab in tidepools and lower rocky shelves.
eaten by
Yellow-edged Moray
Gymnothorax flavimarginatus
Hunts the crab when they venture too close to submerged rocky crevices.
hunts
Sea Lettuce
Ulva lactuca
Serves as a primary food source for the crab as it grazes on rock surfaces.
Traits
No trait badges are assigned for this object yet.
Also known as
No aliases listed yet.
Collections
Collections for this object will appear here as more themes are added.
Safety
Danger
1/5 · Very low
No special safety notes yet.
Frequently Asked Questions
How to identify Thin-Shelled Rock Crab?
The easiest way to identify Thin-Shelled Rock Crab is to use the Snappit nature identifier app.
How long is Thin-Shelled Rock Crab?
5-8 cm
How much does Thin-Shelled Rock Crab weigh?
0.05-0.15 kg
How long does Thin-Shelled Rock Crab live?
2-4 years
How fast can Thin-Shelled Rock Crab move?
8 km/h
What does Thin-Shelled Rock Crab eat?
Primarily grazes on marine algae growing on intertidal rocks, but opportunistically scavenges animal matter and preys on small invertebrates.
Where is Thin-Shelled Rock Crab usually found?
Found on exposed, wave-beaten rocky shores, cliffs, and jetties in the intertidal and splash zones.
How does Thin-Shelled Rock Crab hunt?
Foraging And Grazing
Snap Map
Zoom in to split clusters and explore where this object has been snapped.
Loading map…
Recent Snaps
Recent snaps will appear here as new observations are added.
Where to spot
Park12 sightings12 species
Hawaii, US
Hawaii Tropical Botanical Garden
You might spot Gold Dust Day Gecko, Star Of Bethlehem, and Spanish Shawl.
View guide →
Park14 sightings12 species
US
Lahaina Historic District
You might spot Brown Anole, Zebra Dove, and Red-Crested Cardinal.
View guide →
Aquarium3 sightings12 species
Hawaii, US
Sea Life Park Hawaii
You might spot Typical Boobies, Red-Tailed Tropicbird, and Yellow ʻilima.
View guide →
Zoo2 sightings12 species
Hawaii, US
Sea Life Park Hawaii
You might spot Red-Tailed Tropicbird, Typical Boobies, and Yellow ʻilima.
View guide →