Swimming Crabs in Dominica

Swimming crabs are the paddle-legged crabs you see sprinting over sand and seagrass, or burying with only the eyes showing. They prefer calm, soft-bottom shallows, mud and sand inside Prince Rupert Bay, creek mouths around the Indian River, and protected edges near Cabrits Marine Reserve. Many forage at dusk and at night, and move toward higher-salinity water to spawn near inlets.

Species Confirmed from Dominica

A quick guide to swimming crabs in Dominica: how to identify them, their sizes and habits, and the easiest local places to watch them safely.

• Callinectes Marginatus: Olive to gray-brown with subtle marbling; nine lateral spines per side, the last spine distinctly longer. Adults typically around 10 cm carapace width. Opportunistic hunter–scavenger of bivalves, worms, small crustaceans, and fishes. Buries rapidly with eyes showing; most active at dusk. Look in sheltered shallows of Prince Rupert Bay, the Indian River mouth, and gently sloping beaches near creek inlets.
• Callinectes Bocourti: Olive-brown with reddish tones; long side spines and granular ridges on the “wrist” segment of the claw. Adults commonly 8–11 cm wide. Forages over sand and thin seagrass, pouncing on small invertebrates and carrion. Swims strongly on oar-like hind legs, then digs in when disturbed. Often encountered inside Prince Rupert Bay and along protected margins near Cabrits Marine Reserve.
• Callinectes Ornatus: Usually smaller (to ~9 cm). Six distinct frontal teeth across the front of the shell; slender blue-tipped claws, especially in males. Tolerates a wide salinity range from creek mouths to open nearshore. Hunts tiny crustaceans and worms along the edges of seagrass beds, retreating into shallow scrapes at low tide. Check quiet sand tongues and lagoon edges.
• Cronius Ruber: Red-brown swimmer without the exaggerated side spines of Callinectes. Flattened rear legs still give quick bursts across sand, rubble, and algae-coated rock. Carapace commonly up to ~8 cm. Patrols the boundary between rock outcrops and soft bottom, scavenging and nipping small prey. Good spots include mixed sand-rubble patches near Cabrits Marine Reserve and the rocky approaches to Scotts Head.

The above list support field identification and viewing; please tread lightly on soft flats, avoid trampling burrows, and replace lifted wrack so the habitat remains intact.

Life Cycle & Behavior (Quick Guide)

• Feeding. Opportunistic predators/scavengers, bivalves, polychaete worms, small crustaceans and fishes; carrion when available.
• Movement. Oar-like last legs power fast swimming; many species bury rapidly in sand with eyes exposed.
• Breeding. In tropical settings, ovigerous females occur much of the year, often peaking seasonally; mating typically precedes the female’s hardening after molt, with spawning near inlets where larvae drift offshore before returning inshore as megalopa/juveniles.

Field Tips for Readers

• When: Dusk/night at low to mid-tide is best for foraging activity on inner flats of Prince Rupert Bay and at the Indian River mouth.
• How: Scan for quick lateral sprints, then a shallow “dig” that leaves the eyes showing. A small dive light helps; keep to firm sand to avoid trampling burrows.
• Telling species apart:
  • Callinectes Marginatus: last side spine much longer than the one before it; subtle marbling.
  • Callinectes Ornatus: six frontal teeth; smaller overall; blue-tipped claws.
  • Callinectes Bocourti: two small central frontal teeth and granular ridges on the claw “wrist.”
  • Cronius Ruber: red-brown, lacks the extra-long lateral spine of Callinectes; often around algae-coated rock and rubble.

A handheld light, a falling tide, and a slow approach are all you need to watch swimming crabs at work. Note the long outer spine on Callinectes Marginatus, the smaller size and six frontal teeth on Callinectes Ornatus, and the quick bury-and-freeze behavior they share. Share sightings with guides at Indian River Tourism Facility or crews at Portsmouth Fisheries Complex, keep to firm sand, replace any lifted wrack, and never disturb egg-carrying females.