Weekly Sea Discovery: Delicate Porcelain Crabs of the Intertidal

While porcelain crabs may look like true crabs, they are more closely related to squat lobsters and hermit crabs. They have flattened bodies so they can squeeze between rocks and crevices on rocky shores. One identifying feature is their large, broad claws, that can sometimes be even larger than their body! It may seem like these claws would be used for feeding, but porcelain crabs are filter feeders and use their claws for battling each other. Check out this video to see how these crabs feed on micro-organisms and dead material floating in the water.

Porcelain crabs are as delicate as their name suggests, and readily release their legs and claws when they feel they are in danger. But don’t worry! Porcelain crabs can regenerate these lost limbs in a matter of weeks.

Flat Porcelain Crabs bustling under some rocks in the rocky intertidal.

Two common species of porcelain crabs live in Mayne Island’s intertidal: the flat porcelain crab (Petrolisthes cinctipes) and flattop crab (Petrolisthes eriomerus). It can be hard to differentiate between these crabs, with their similar colouring and shape. But when searching along the shore, you will likely find them in different sections of the intertidal. 

Flattop Crab. Photo: Derek Hozapfel.

Like many organisms in these areas, they are subject to periods underwater and periods in the air and sun, throughout the daily tidal cycles. Depending on how well a species can survive without water and in the heat, it will live either lower, closer to the deeper water, or higher up, closer to the shoreline where water does not reach, to avoid competition. The flat porcelain crab lives in the upper and middle intertidal, where they are more exposed to heat and water loss. Flattop crabs must live in the lower intertidal because they cannot survive in warm temperatures like the flat porcelain crabs.  

Diagram of the different intertidal zones and some organisms found within them. Credit: Carey 2010.

With temperatures rising due to climate change, even species that can tolerate warm temperatures will have to move closer to deeper water to avoid drying out. While they escape the risk of drying out, they are entering a highly competitive zone with many more predators they have to avoid.

Locally, the rocky southeast side of Piggott Bay is a great place to find porcelain crabs at very low tides.

Resources:

Harbo, R. M. (2011). Whelks to whales: coastal marine life of the Pacific Northwest. Harbour Publishing Company.

Jensen, G. C., & Armstrong, D. A. (1991). Intertidal zonation among congeners: factors regulating distribution of porcelain crabs Petrolisthes spp. (Anomura: Porcellanidae). Marine Ecology Progress Series73, 47-60.

E-fauna BC: Infraorder Anomura (mole crabs, hermit crabs, king crabs, porcelain crabs, and squat lobsters) of British Columbia.


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