Crabs | The Marine Detective

Posts from the ‘Crabs’ category

Crabs Making Bad Choices

[Update: Species corrected thanks to Greg Jensen. I initially posted that the crab in the first 3 photos was a Moss Crab].

How do crabs make bad choices?

Let me show you via my photos and a “conversation” with the crab in the next three photos.

Oh hello mature male Sharpnose Crab. I almost didn’t see you there!

Please may I take a photo of how you have fabulously decorated yourself to camouflage against predators, using bits of algae, sponges, tunicates and hydroids?

It’s fascinating how your species, and others who decorate themselves, have little hooks (setae) on your exoskeleton to which attach life from around you AND that you change outfits when your change backgrounds. Do you sometimes also use the camouflage as easy-to-reach snacks?

Oh, oh! Wait!

You don’t know you are walking onto the head of a Red Irish Lord, an ambush hunter who is extraordinarily camouflaged too.

Careful! You are on the menu for this fish species.

The Red Irish Lord will try to grab you, ideally from the back of your shell. That’s what happened to the crab in the next two photos.

Indeed, that’s the same species of fish. Red Irish Lords have incredible diversity in colour to blend in so that you, and I, have great difficulty detecting them.

When the fish does not have the advantage of a sneak attack, you can defend yourself by spreading out your claws really wide. Like what you see below.

Then, it’s difficult for the Red Irish Lord to fit you into his / her mouth.

Yes, I too imagine the crab in the above two photos saying, “You want a piece of me?!”

It’s said of your species that you “put little effort into decoration”. Such judgement!

In another species, the Moss Crab, a correlation has been found between size and how much decoration there is. Once big, especially with claws spread wide, mature male Moss Crabs cannot easily be gulped up whereby there is less need for camouflage. But mature male Moss Crabs are huge! Up to 12.3 cm just across their carapace. Your species, the Sharpnose Crab (Scyra acutifrons) is only up to 4.5 cm across the carapace. Mature males of your kind have a far greater reach with their claws than mature females.

By the way what’s with the posturing with mature males of your kind when they do what is shown in the photo below?

Yours is NOT the only crab species that can be gulped up. I think it might be a Graceful Kelp Crab who has been engulfed by the Red Irish Lord below.

Below is another crab in danger of making a fatal choice as it advances down the face of the Red Irish Lord. See how precarious this is? The fish will remain motionless, waiting, waiting till you are in the ideal position to ambushed from behind. Then your claws are of little use to you.

There you go dear human readers.

I do not know the fate of either of the crabs on the heads of the Red Irish Lords. I had to return to the world where we humans can also make really bad choices.

Why no, my referencing human bad choices on November 4th 2020 is purely coincidental. Insert innocent eye batting here. What choices could I POSSIBLY be referencing? ☺️

Be kind. Be colourful. Be careful. Be truthful. Be safe. 💙

Regarding the photo above, see the Red Irish Lord and the two crabs with outstretched claws?

Related TMD Blogs:

Sources:

Drake, Catherine Anne, “Decorating Behavior and Decoration Preference in the Masking Crab, Loxorhynchus Crispatus” (2016). Capstone Projects and Master’s Theses. 74.

Jensen, Gregory. (2014). Crabs and Shrimps of the Pacific Coast: A guide to shallow-water decapods from southeastern Alaska to the Mexican border.

Wicksten, M. (1978). Attachment of Decorating Materials in Loxorhynchus crispatus (Brachyura: Majidae). Transactions of the American Microscopical Society, 97(2), 217-220. doi:10.2307/3225595

Hermit Crabs with Sponge Homes

Please see photo below.

You are looking at two animals, not one.

Alaskan Hermit Crab = Pagurus ochotensis to 5.3 cm long. The yellow eyes and “sheen” on the legs help in IDing this species. See how uniquely reflective the legs are? This species often lives in a shell made by a Northern Moon Snail (until a suberites sponge dissolves it away 😉 ).

This is an Alaskan Hermit Crab (who resides in front of Port McNeill, not Alaska).

Living on his/her back is a “Hermit Crab Sponge”.

This sponge species (Suberites latus) settles on the shell home of some hermit crab species and can completely dissolve the shell away.

Having a sponge home has its advantages. It is light. Also, the sponge will grow whereby the hermit crab need not find a new home as would be the case if it outgrew a shell home.

But, it can be awkward to tote around when it gets really big. See an example below.

Bering Hermit with a huge suberite home relative to its size. (Pagurus beringanus to 2.6 cm).

Yes, the hermit crab could leave the sponge and get another home if one were available. But, there is risk when outside your home, be it ever so brief.

Another disadvantage is when you have unwelcome house guests.

See below to get a sense of the inconvenience when a sponge predator crawls on your back.

Close-up on the inconvenienced Mud Hermit Crab.

From top to bottom: the big yellow animal is a Monterey Dorid (nudibranch species – gills are on left). This nudibranch is feeding on the Hermit Crab Sponge (tan colour) and then, see the tiny face? That’s a Mud Hermit Crab who isn’t going anywhere for a little while (Pagurus capillatus to 4 cm).!

Here’s another Mud Hermit Crab. See the bite out of the sponge? I initially found this individual upside down. The resulting photo of the underside of the sponge gives you a sense of how the sponge is shaped to the hermit crab’s body.

Note too how all the hermit crabs included in this blog have one claw bigger than the other? This is the case for many hermit crab species and it allows them, when they retreat into their home, to seal off the opening to the shell or sponge with the bigger claw. They close the door to their home.

In the photo below, see how the larger claw seals off the hole for the hermit crab on the right? I suspect this interaction captured in this photo more about mate selection that it is about home envy.

I hope this “who is sponging off who” interaction provides some wonder for you at a time when safety in homes is such a reality for our species too (re COVID-19).

Be safe whatever, and wherever, your chosen home. 💙

The Hermit Crab Sponge is Suberites latus to 20 cm long, 6 cm wide and 4 cm high.
Source for these dimensions is “Beneath Pacific Tides” by Greg Jensen.

Bluespine Hermit with sponge home (Pagurus kennerlyi to 3.5 cm long)

Juvenile Alaska Hermit who will benefit from the sponge growing bigger.

Bering Hermit Crab interaction. This too is more likely about dragging around a potential mate.

Decorator Crabs! The best-dressed in the NE Pacific Ocean.

Are you ready? I’ve been collecting these photos for a long time. Now, finally, I think I have enough to deliver this marine fashion show to you – the best dressed of the NE Pacific Ocean!

Decorator crabs are camo-crabs. They pluck bits of life from their surroundings and attach it to themselves. AND, if their surroundings change, they change their outfit.

Graceful Decorator Crab covered with hydroids including the “Raspberry Hydroid” which was only recognized as a new species in 2013 with the area near Telegraph Cove (Weynton Pass) being one of the few areas these colonies are known to live. ©Jackie Hildering.

This is highly functional fashion. Not only does this covering of life allow the crabs to hide from potential predators, it also apparently changes the way the crabs feel and taste in a way that deters their predators. Sponges taste bad or are even toxic to many predators and animals like hydroids and other “cnidarians” have stinging cells. Thereby, if you cover yourself with sponges or cnidarians, predators be gone!

Indeed, even though decorator crab species look like walking gardens, often what they attach are not algae but other animals – hydroids, sponges and bryozoans.

Additional bonuses of carrying other organisms on your back may be:

You have potential snacks within a pincher’s reach.
Your camouflage allows you to get closer to your prey.
You are carrying weapons!

From A Snail’s Odyssey: “Apart from passive camouflage from potential predators, other functions of the behaviour may include disguise for closer approach to prey, and provision of tools for active defense, such as a branches of hydroids containing functional stinging cells or pieces of sponges or tunicates containing toxic chemicals.”

Graceful Decorator Crab with snippets of sponge attached to his/her carapace (Hooded Nudibranchs in the background). This individual realized it had been seen and switched to the defence strategy of looking big since “so many fish predators are limited by the size of their mouths” (Source: Crabs and Shrimps of the Pacific Coast); ©Jackie Hildering.

Note too that not all growth on the back of crabs is the result of decorating and remember that crabs moult, crawling out the back of their shells in order to grow. Also from A Snail’s Odyssey: “In some cases these camouflagings result from settlement of spores and larvae . . . . Passive buildup of growths is greater with increasing age as moulting frequency decreases. Also, in many species there is a final or terminal moult which, if the species’ exoskeleton is receptive to settlement of larvae and spores, leads to an even greater build-up of cover.”

Graceful Decorator Crab adorned with (and atop of) Glove Sponge. ©Jackie Hildering.

“Spider crab” (superfamily Majoidea) species are the ones that most often adorn themselves. From Greg Jensen‘s Crabs and Shrimps of the Pacific Coast: “Many spider crabs . . . mask themselves with algae or encrusting organisms so that they can hide in plain sight. The decorator crabs are equipped with curved setae much like the hook part of Velcro fasteners: after shredding material a bit with their mandibles, they press it into place. The largest species tend to stop actively decorating once they outgrow most of their predators.”

Crab predators include the Giant Pacific Octopus and fish species like Cabezon, some rockfish, Surfperch, Wolf-Eel and the Staghorn Sculpin. Of course, at low tide, birds and mammals are also predators.

Hoping this adds to the wonder, connection and respect for our marine neighbours. Enjoy the rest of the show!

[For research on decorator crabs with great diagrams explaining how how attachment occurs see this link.]

Try not to smile!

Another Graceful Kelp Crab adorned with Raspberry Hydroids.

Here you can even see where the Graceful Decorator Crab has clipped off bits of sponge. AND s/he’s in the act of attaching clippings. ©Jackie Hildering.

Heart Crab (I THINK) – not likely to have decorated itself but rather this is the result of the settlement and accumulation of organisms = a walking ecosystem. ©Jackie Hildering.

Graceful Decorator Crab in front of a Painted Sea Star. S/he had just moved from being camouflaged among kelp to moving in front of the sea star. ©Jackie Hildering.

This Graceful Decorator Crab has adorned him/herself with bits of Barnacle Nudibranch egg masses for camouflage. You can see the egg masses behind the crab.

Decorator crab species in the NE Pacific Ocean include:

Graceful Decorator Crab – Oregonia gracilis
Graceful Kelp Crab – Pugettia gracilis
Longhorn Decorator Crab – Chorilia longipes
Other species too will sometimes put a bit of camouflage on their rostrum e.g. Northern Kelp Crab – Pugettia producta

Challenge – Find the Crab!

Typical shape of members of the kelp crab family. Species in this family are usually from 5 to 9 cm across the carapace.

This week I bring you the “Where’s Waldo?” of the marine invertebrates. There is a decorator crab in each of the images at the link below. But first, here are some clues for you.

Most of the species of crabs that decorate themselves to be masters of camouflage are in the spider crab family (Majidae family – also known as “kelp crabs”). The image to the right shows you an undecorated kelp crab with the typical long legs and distinctly shaped shell (“carapace”) of this family.

Some crabs only partially camouflage themselves, especially when they are juveniles. Others “plant” so many marine neighbours onto themselves that you can’t tell them apart from their environment until they move.

Although they look like walking gardens, the organisms they attach to the stiff, curved hairs on their legs and backs are algae and animals, not plants. The animals can be soft corals, sponges or unique creatures like “bryozoans” and “hydroids”.

Not only does this covering of life allow the crabs to hide from predators, it also changes the way the crabs feel and taste. For example, sponges taste bad or are even toxic to many predators so, if you cover yourself with sponges, predators be gone! The bonus of carrying other organisms on your back is that you also have a food supply within a pincher’s reach.

It is truly astounding how well the decorator crabs match their immediate surroundings which added another mystery to my list: Is the range of decorator crabs really small so that they always match their background OR do they know to “adjust” their camouflage when they move to an area where they no longer blend in?

I have learned that the latter appears to be the case. Experiments with captive decorator crabs have shown that, if moved to a background that no longer offers camouflage, the crabs will “adjust” their decorations!

Click here to find the decorator crabs in my images or view gallery below.