Crabs of Whidbey and Fidalgo Islands

Crabs pee from their face. Dr. Gregory C. Jensen, author of Crabs and Shrimps of the Pacific Coast, writes, “Since the term urine was used long before urea was discovered, the liquid coming out of a crab’s face is still technically called urine” (25). According to Hide or Get Eaten, Urine Chemicals Tell Mud Crabs, researchers at Georgia Tech found that the silver-dollar sized mud crab prey of hand-sized blue crabs “react most strongly when blue crabs have already eaten other mud crabs.” “The fact that blue crab urine scares mud crabs was already known. Mud crabs duck and cover when exposed to samples taken in the field and in the lab, even if the mud crabs can’t see the blue crabs yet. Digestive products, or metabolites, in blue crab urine trigger the mud crabs’ reaction, which also makes them stop foraging for food themselves.”

Crabs are skilled smellers, “Hairlike structures on crustaceans…technically called setae…are far more complex structures than hair and perform myriad, often specialized, functions…in fact, they are involved in nearly everything that the animal does,” including finding and tasting food (4, 60).

Setae are a distinguishing characteristic between two common silver-dollar sized shore crab species found in the Salish Sea, the Purple (Hemigrapsus nudus) and the Green (Hemigrapsus oregonensis). Nudus means “naked.” According to Invertebrates of the Salish Sea, that’s because, “Its legs are not covered with abundant setae,” and “Its chelipeds [the legs that bear the pincerlike organ or claw] have prominent purple spots and white tips.” Although Purple shore crabs tend to be slightly larger than Green, due to color variations and their small size, distinguishing between the two can be tricky, although they dwarf the Indo-Pacific-dwelling Sand bubbler, which is smaller than a dime.  

The Northern kelp crab (Pugettia producta) in an omnivore that eats several types of kelp, including Bull kelp (Nereocystis luetkeana), when available.

Photographing a small, skittish, moving target is challenging, as is distinguishing between similarly-sized and shaped larger ones, especially when “obscured by sponges and bryozoans” (55), like some of those shown below. Crabs tend to engage in one of two behaviors when encountered in the wild: scurry away and hide, or maintain a stationary, defensive posture, often with claws wide open, accompanying the stock-stillness with an expression that states, “Whatareyoulookingat?”

Some uninformed tide poolers believe the small crabs they see under rocks along the shore to be juvenile-size versions of more familiar species like Red rock (Cancer productus) and Dungeness (Cancer magister) crabs, members of the Cancridae family. These species start small but quickly outgrow their shore crab cousins, “[Dungeness crabs] can live between 8–13 years and reach a size of 10-plus inches,” “At 4-5 years, they have grown to the legal harvest size [6-1/4″].” They “can only grow by periodic shedding of their shell in a process called molting, and individuals typically increase in size by about 15 to 25 per cent with each molt.”

Even before I knew what I now know about crab mating behaviors, when I observed a large Red rock front-hugging a smaller one, I had my suspicions. According to Dr. Jensen, “Females approaching molt apparently release a scent or pheromone that signals their receptivity to males…When a male encounters a receptive female, he grasps her in a precopulatory embrace…When the female molts, the male loosens his grasp and may even assist her in working free of the old exoskeleton before repositioning her for mating” (5,6). I’ve seen (and photographed) occasional reproduction-related crab actions over the years, like: a male Red rock abandoning his attempt to “embrace” a prospective mate, a female checking out two prospective suitors within a 24 hour span, both Dungeness and Red rock crabs engaging in the precopulatory embrace, and once trapped a heavily-barnacled, berried Dungeness.

Animals Network claims that there are 1,100 species of hermit crabs, and that both terrestrial (like these shell exchangers from the BBC’s Life Story) and marine are omnivores. With their shell obscuring everything except their head, eyes, antennae and front legs, these tiny creatures give an amateur classifier very little to go on.

In his war story Peace, playwright Aristophanes wrote, “You will never make the crab walk straight.” Blogger Samuel Rodenhizer states, “The point of the Aristophanes analogy is that some things cannot be changed. They just ARE. No amount of reason, argument, proposition, complaint, wishing, hoping, or disgust will change them.” Not all crabs walk sideways, and even if they did, I wouldn’t dream of trying to change them. In Skeletal adaptations for forwards and sideways walking in three species of decapod crustaceans, researchers explain that they do so with good reason, “Walking sideways allows legs a greater stride length, as individual legs are not impeded by their neighbors. Sideways-walking provided crab the additional and unique benefit of being equally fast in op-posite directions.” The tracks of this Purple shore crab indicate that it’s a sideways walker.

Purple shore crab
Crescent Bay (Port Angeles)
18 June 2018
Purple shore crabs
Libby Beach (Whidbey Island)
23 June 2008

Reminiscing about tide pooling trips on Whidbey and Fidalgo Islands as I sifted through my collection of crab-containing digital photos reminded me of the importance of scrutinization over observation. Looking back, I wish I’d investigated the compliant crabs more thoroughly.

Thank you to Dr. Gregory C. Jensen, whose beautiful, informative books Crabs and Shrimps of the Pacific Coast (MolaMarine, 2014, pp. 4, 25, 55, 60) and Pacific Coast Crabs and Shrimps (Sea Challengers, 1995, pp. 5-6, 17-29, 60-68) inspired this post.


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