Cronius Ruber

Cronius Ruber is an agile swimming crab of warm, shallow coasts. It works on mixed sand, rubble and algae-coated rock from the intertidal to several tens of meters. It favours patchy bottoms where rock outcrops meet soft sediment, retreating under stones by day and roaming open sand at dusk. On the northwest coast, it turns up along the margins of Cabrits Marine Reserve and inside Prince Rupert Bay; on the south, check rocky approaches around Scotts Head and similar surge-washed ledges near popular beaches.

Ecology and Identification

Adults commonly reach about 7–9 cm in carapace width. The shell is reddish to chestnut brown, often mottled, with a smoother outline than the long-spined Callinectes species; anterolateral teeth are present but not exaggerated into a single very long spine. The last pair of legs are flattened like oars for quick bursts across sand and short swims between rocks. A capable predator and scavenger, it prises small bivalves and gastropods from the substrate, snaps up amphipods, shrimps, polychaete worms, and takes fish scraps when available. Activity peaks from late afternoon into night, with crabs burying lightly in sand or wedging among stones when disturbed. Courtship and mating follow the typical portunid pattern around the female’s molt; in tropical waters, egg-bearing females can be found in many months, releasing planktonic larvae that develop offshore before returning inshore as megalopa and juveniles.

Sightings and best viewing areas

Calm evenings with clear water are ideal. In shin-deep zones along the inner edge of Prince Rupert Bay, watch for quick lateral sprints followed by a shallow dig that leaves only the eyes showing. Around Cabrits Marine Reserve, scan the boundary where algae-coated rock gives way to sand; crabs patrol these seams and pause under rubble when lights approach. At Scotts Head, look between wave sets along low ledges and tidepool rims, then track individuals as they move from rock cover onto nearby sand tongues to feed.