Plagusia Depressa (Sargassum Crab)

The Plagusia Depressa (Sargassum Crab) is an agile shore crab that is closely tied to drifting Sargassum rafts and the wrack they leave on beaches. It clings to floating seaweed, buoys, and driftwood, then moves ashore with fresh strandlines after onshore winds. Around rocky points and splash zones, it slips across wet ledges and tidepool rims. Good places to look include the headland at Scotts Head, the lee waters of Prince Rupert Bay, and the rocky margins near Cabrits Marine Reserve, where Sargassum often accumulates before or after calm spells.

Ecological and Cultural Significance

Flattened legs and hooked tips give strong grip on slick weed and rock, while mottled olive-brown markings provide camouflage among Sargassum fronds. The crab scavenges small invertebrates, fish eggs, and algae on floating mats, cleaning the seaweed and recycling nutrients when wrack reaches shore. In turn it is taken by near-surface fishes and shorebirds, linking offshore rafts to coastal marine ecosystems and the everyday web of wildlife. Its presence during seasonal Sargassum arrivals helps explain why beach wrack can be a living habitat, not just debris.

Sightings and Best Viewing Areas

Check fresh, still-wet Sargassum at the waterline and the first meter of strand above it, where individuals tuck into fronds and dash sideways when disturbed. Calm mornings make spotting easier along the Scotts Head shore path and the edges of Prince Rupert Bay; after light seas, scan rock shelves near Cabrits Marine Reserve for quick runners between pools. If you lift a small clump to look, replace it gently so the wrack community remains intact.