Conservation Status:  Data deficient

Climate Change:  Vulnerable

AquaticCoral crab

FamilesTrapeziidae and Tetraliidae Arthropods

Aquarium of the Pacific - Online Learning Center - Species Print Sheet

Coral crab
Trapezia wardie: image taken in Micronesia waters | Permission of Katrina Adams, Kosrae Village Ecolodge, Kosrae, Micronesia

Species Overview

Reseach has shown that coral crabs play a role in the maintenance of a healthy coral ecosysem, corals that may be a source of future medicines of the sea.

Pairs of small and colorful coral crabs inhabit the branches of selected species of stony corals with which they establish a mutually beneficial symbiotic relationship. The corals and crabs are dependent on each other for survival, a relationship called obligate coral symbionts. The crabs use specialized feeding appendages on the tips of their legs to stimulate their host colony to produce excess lipid-rich mucus, the primary source of nutrition for the crabs. The coral protects the crabs from predators while the crabs clear their colony of excess debris and sediment that might impact the coral’s zooanthellia.

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Species In-Depth | Print full entry

At the Aquarium

Several species of coral crabs have a habitat in the Tropical Pacific gallery. They feed on the mucus excreted by their coral host.

Geographic Distribution

Throughout the Indo-West Pacific.

Habitat

Selective about the species of corals they select for a host, coral crabs establish habitats on stony branching corals in the genera Acropora and Pocillopora.

Physical Characteristics

Coral crabs are decapods. They have five pairs of legs with pinching claws on the first pair. The carapace (exoskeleton or shell) is flattened. Some members of the Tetraliidae family have a hairy pit located on lateral side of major cheliped (claw). These crabs occur in a variety of colors and some have pigmented patterns that are unique to the species and provide identification clues.

Size

About 2.54 cm (1 in)

Diet and Feeding

These crabs primarily feed on coral mucus which has high lipid content and on detritus trapped in the coral mucus. They use specialized feeding appendages on the tips of their claws to stimulate their host colonies to produce excess mucus.

Research has shown that corals contain large amounts of lipids in their tissues. In fact, 30 to 40 percent of a coral’s dry tissue mass is lipid. The majority of these lipids are passed from the symbiotic algae in the coral to the coral’s tissues, with the rest being manufactured by the coral from metabolites of the algae.

Reproduction

Sexes are separate in these crabs that appear to form monogamous pairs. Little is known about their reproduction cycle; however, most females collected in the wild were brooding eggs, suggesting there is no seasonal breeding time. Laboratory research indicates the gestation period is approximately one month.

Behavior

The crabs live as mated pairs on coral host colonies; pairs usually stay on coral host and defend their territories from members of their own species (conspecifics). Scientists believe that species recognize each other based on the unique colored pattern that each species has. When crabs of the same species try to invade an already occupied coral colony, fights result with one of the groups of crabs being driven away. However, crabs of a different species are sometimes allowed to stay providing the capability to maintain coral crab species diversity among coral reefs.

Adaptation

These crabs have adapted to living in the confining branches of stony corals by developing a flattened carapace and small size. Members of the Tetraliide family also have a hairy pit on the lateral side of their major cheliped (claw) to aid in the collection of coral mucus.

Conservation

Coral crabs may be affected by climate change because they rely on healthy coral for survival. If there is bleaching of their host stony coral species as a result of warming of tropical waters or changes in ocean chemistry as impacts of climate change, disease, or pollution, the crabs populations may be affected; however, there is no scientific data as yet to support such an impact taking place.

Amazing Facts

At one time coral crabs belonging to the genus Trapezia, Tetralia, and Domecia were thought to be ectoparasites, living off of the tissues of the coral. Then it was discovered that these little crabs actually protected the coral in which they lived from predators, such as the crown-of-thorns sea star (Acanthaster planci). When the sea star begins to climb over a branching coral, these little crabs move to intercept it, pinching the sea star’s tube feet. The pain forces the sea star to crawl away, defeated.

When coral crabs are removed from their host coral, the lower branches become covered with algae, sponges, and tunicates. It has been speculated that movements of the crabs lower down in the coral colony increase water circulation and prevent detritus from accumulating in and on the coral’s branches.

Additional Images

Coral crab
ourtest of NONAA