Our ghost crabs are large active scavengers that scurry rapidly about at night. During the day, they hide in burrows dug into the sand on the higher shore. Juvenile create burrows with a smaller entrance. The study focused on the crabs' burrow density as well as sand particle size and sand compaction on various stretches of beaches on Pulau Hantu and East Coast Park.
Beach disturbances can affect the number of these crabs on our shores. The study considered disturbances such as boating and windsurfing, where equipment is dragged over the sand. As well as trampling by shore visitors, beach cleaning, fishing, beach games and "beach excavation by children".
Beach cleaning at East Coast Park is relentless.
I was particularly disturbed to read about this in the study: "On weekdays, mechanical beach sweepers were used at sites with large bays and high volumes of sea wrack [this refers to natural marine stuff that washes up such as seaweeds, mangrove leaves] and rubbish. Rubbish was cleared by dragging through the sand and this resulted in a lot of disruption of the sandy habitat. Sediment originally found below the surface might be exposed, with moisture from the sand evaporating, thus drying out the habitat. Organisms on the surface of the sand and those that live deeper in the sand can be crushed or brought to the surface. The dry sediment formed from the dragging might not be suitable for burrow construction by the ghost crabs. The removal of sea wrack during beach cleaning removes a food source and habitat for some intertidal organisms that could be preyed upon by ghost crabs."
Oh dear, that sounds terrible.
The study found that on more disturbed beaches, burrow density was lower, sand particles smaller and sand more compacted. On beaches without boating activities, burrow density was higher regardless of whether there was high or low disturbance.
The study also found that "Pulau Hantu and East Coast Park had only 0.1% and 7.3% of juvenile burrows, respectively, indicating a low recruitment rate of juvenile ghost crabs." It also noted that intense recreational activities such as trampling, beach sports, and the use of off-road vehicles can result in dwindling ghost crab populations.
The study suggests that the abundance of ghost crabs can potentially be used as a bioindicator of human impact on Singapore's sandy beaches.
Thus it seems that we should rejoice when we see lots of Ghost crabs on the shores!
The study is: The Potential of Ocypode Ceratophthalmus (Pallas, 1772) as a Bioindicator of Human Disturbance on Singapore Beaches by Yong, Adeline Y.P.1; Lim, Shirley S.L.2 in Crustaceana, Volume 82, Number 12, 2009 , pp. 1579-1597(19). Abstract on IngentaConnect.
Thanks to Tan Swee Hee of the Raffles Museum of Biodiversity Research for sharing this fascinating paper.
Marine life on Pulau Hantu: blog posts and photos and info for visitors.
Marine life on East Coast Park: blog posts and photos