Today on Animal Fact Files we’re discussing spider crabs. There is no one distinct taxonomic group of
animals commonly known as spider crabs. There are different families with "spider crabs" classified
in them. For example, there is the well known Japanese spider crab and the common spider crab,
however, these animals are currently classified in different families. This isn't surprising.
Crab classification is notoriously debated among scientists. So this episode will be
more general and we'll get more specific in future episodes, so be sure to subscribe!
Spider crabs are true crabs. As we discussed in our king crab episode, not all animals known as
"crabs" are true crabs. In very simple and generalized terms, "true" crabs can be told
apart from "false" crabs by their legs. True crabs have four pairs of walking legs and false
crabs only have three. True crabs also usually have a hard, chitinous shell, a pair of pincers,
and a symmetrical body plan. They also have a small tail. Thus, spider crabs possess all
these traits. Fascinatingly, spider crabs are able to walk sideways like other true crabs,
but they crawl forward, too! Also, we feel like this goes without saying,
but spider crabs aren't spiders; they get that name because they have a spider-like appearance.
In fact, there are also spiders known as crabs spiders, so it's mutual!
Spider crabs have long spindly legs, like a spider, and they are generally slow moving. This
suits them because these crabs are scavengers. Spiders crabs feast on decaying remains of
other animals, such as a whale fall at the bottom of the ocean. In fact, many spider crabs live in
deep waters. They can be found more than 1,000 feet (300m) below the waves, but other species
are coastal. Sailors used to tell tales of these crabs dragging people below the waves
and consuming them whole. While it's possible spider crabs would eat decaying human flesh,
they aren't even strong enough to pull a person underwater, so these are all fish
tales. Aside from carrion, spider crabs may also predate other slow moving animals like
sea stars as well as plants like kelp and algae. They live in oceans around the world.
These crustaceans have a beak-like shape to their face and have hairlike projections on
their legs and shell. Many species gather algae, sponges, and other material on their back.
This helps camouflage them in coral, rocks, seagrass beds, or anywhere else they may live.
There may even be small animals living on them! They are their own entire ecosystem!
When they need to molt, they actually move the organisms from their old shell to the new one
by sticking them on with a glue-like substance they produce from their mouth.
While the largest known spider crab might seem formidable at it's over
ten foot (3m) leg span, the smallest species don't even reach an inch (2.5cm) at full size!
So they make easy prey for octopuses and larger fish like grouper and stingrays.
Many spider crabs are nocturnal for this reason, only coming out at night when
they're more difficult to spot. It should be noted, however, that larger adults are less
impacted by predation and may change their behavior accordingly. Some bigger species don't
cover themselves in living decorations as adults because they don't need to!
Spider crabs are vulnerable after molting when their shell is shed and their body is squishy.
For this reason, when females are ready to molt a male will hold onto her and keep her safe.
During this time, the pair will mate. This can happen in gatherings that number in the
thousands! Spider crab mothers brood their eggs on their body for about a week.
These hatch into planktonic larvae that settle to the seafloor as benthic juveniles after a month or
two. It's unknown how long these crabs live but some estimates put them at fifty years or older!
For more facts on spider crabs, check out the links below. Give a thumbs up if
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